Patent Publication Number: US-11033071-B2

Title: Sole structure with progressively adaptive stiffness

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/814,778, filed Nov. 16, 2017, which claims the benefit of priority to U.S. Provisional Application No. 62/424,898, filed Nov. 21, 2016, and both of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present teachings generally include a sole structure for an article of footwear. 
     BACKGROUND 
     Footwear typically includes a sole structure configured to be located under a wearer&#39;s foot to space the foot away from the ground. Sole structures in athletic footwear are typically configured to provide cushioning, motion control, and/or resiliency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration in exploded perspective view of an embodiment of a sole structure for an article of footwear with a piston inverted. 
         FIG. 2  is a schematic illustration in perspective view of the sole structure of  FIG. 1  showing a foot-facing surface. 
         FIG. 3  is a schematic illustration in perspective view of the sole structure of  FIG. 1  showing a ground-facing surface. 
         FIG. 4A  is a schematic illustration in fragmentary perspective view of the sole structure of  FIG. 1  in dorsiflexion with the piston removed. 
         FIG. 4B  is a schematic illustration in cross-sectional fragmentary side view of the sole structure of  FIG. 1  in dorsiflexion with the piston in a first position. 
         FIG. 4C  is a schematic illustration in cross-sectional fragmentary side view of the sole structure of  FIG. 1  is dorsiflexion with the piston in a second position forward of the first position. 
         FIG. 5  is a plot of torque versus flex angle for the sole structure showing a bending stiffness of the sole structure with the piston in the first position of  FIG. 4B , and a bending stiffness of the sole structure with the piston in the second position of  FIG. 4C . 
         FIG. 6A  is a schematic illustration in cross-sectional fragmentary view of an engagement feature of the piston sliding up a tooth of a track of the sole plate during dorsiflexion of the sole structure. 
         FIG. 6B  is a schematic illustration in cross-sectional fragmentary view of the engagement feature of the piston of  FIG. 6A  after moving over the tooth. 
         FIG. 6C  is a schematic illustration in cross-sectional fragmentary view of the engagement feature of the piston of  FIG. 6A  sliding back toward the tooth following dorsiflexion. 
         FIG. 6D  is a schematic illustration in cross-sectional fragmentary view of the engagement feature of the piston sliding up a subsequent tooth of the track of the sole plate during a subsequent dorsiflexion of the sole structure. 
         FIG. 7  is a schematic illustration in exploded perspective view of an alternative embodiment of a sole structure showing a foot-facing surface of a sole plate. 
         FIG. 8  is a schematic illustration in exploded perspective view of another alternative embodiment of a sole structure showing a foot-facing surface of a sole plate. 
         FIG. 9  is a schematic illustration of an alternative pivotable tooth and post for the sole structure of  FIG. 8 . 
         FIG. 10  is a schematic illustration in exploded perspective view of another alternative embodiment of a sole structure showing a foot-facing surface of a sole plate. 
         FIG. 11  is a schematic illustration is a schematic illustration in exploded perspective view of another alternative embodiment of a sole structure showing a foot-facing surface of a sole plate. 
         FIG. 12  is a schematic illustration in perspective view of an alternative embodiment of a piston for a sole structure. 
         FIG. 13  is a schematic illustration in fragmentary plan view of a sole structure with the piston of  FIG. 12 . 
         FIG. 14  is a schematic illustration in perspective view of another alternative embodiment of a piston for a sole structure. 
         FIG. 15  is a schematic illustration in fragmentary plan view of an alternative embodiment of a sole structure with the piston of  FIG. 14  and a sole plate. 
         FIG. 16  is a schematic illustration in fragmentary perspective view of the sole plate of  FIG. 15 . 
     
    
    
     DESCRIPTION 
     A sole structure for an article of footwear has a sole plate and a piston that is moved by dorsiflexion relative to the sole plate, causing the stiffness of the sole structure to change as the piston progresses along the sole plate. The dorsiflexion and hence the change in stiffness is entirely human-powered (i.e., powered entirely by the movement of the wearer), and is referred to as a progressively adaptive stiffness. The progression of the piston and the corresponding change in stiffness can be tuned for a specific number of steps (i.e., number of dorsiflexions) that an athlete is expected to take in an athletic event of a given distance, and during different portions of the event. 
     The sole plate and piston can be configured so that the change in stiffness under bending along a longitudinal axis of the sole plate can increase and/or decrease with successive dorsiflexion, and/or the change in stiffness under bending in the lateral direction can increase and/or decrease. The progressive adaptive stiffness can thus be correlated with a particular race, including a race around a curved track, where increasing stiffness is desired. In this and other embodiments described herein in which the piston progresses along teeth or other protrusions of the sole plate, the number of teeth or protrusions can be correlated with a number of steps a person wearing the sole structure is expected to take when utilizing the sole structure for a predetermined event, such as participating in a race of a particular distance and/or on a track or course of a known route. In this manner, the change in bending stiffness can aid the wearer by varying the cushioning characteristic in a manner advantageous to the wearer, such as by increasing or decreasing longitudinal or transverse bending stiffness in correlation with various stages of the race. The expected number of steps can be specific to a particular athlete, or may represent a population average for the expected population of wearers. 
     For example, the sole structure may be configured to progressively increase in bending stiffness in the longitudinal direction (such as along a longitudinal midline of the sole structure) after a predetermined number of steps and corresponding number of dorsiflexions expected toward the end of a race of a known distance. The increased stiffness may help to maintain proper form when the foot is fatigued. The sole structure may be configured to progressively increase in stiffness after a predetermined number of steps and corresponding number of dorsiflexions expected when a runner is on a curved portion of a track or course. At the curved portion, increased bending stiffness in a lateral direction (i.e., perpendicular to the longitudinal midline) may be desired to support the side of the foot nearer the outside of the curve, such as at the lateral side of the sole structure on the right foot (assuming the race progresses in a counter-clockwise direction around the curved track). The sole structure may be configured to progressively increase and decrease in stiffness in the longitudinal and transverse directions multiple times over the course of progression of the piston along the sole plate. For example, the transverse stiffness may increase along two curves of an oval track, and decrease on the straightaway between the curves. 
     In an embodiment, the sole plate has a foot support portion with a foot-facing surface and a ground-facing surface. An opening in the sole plate extends through the foot support portion from the foot-facing surface to the ground-facing surface. The sole plate has a bridge portion underlying the opening and secured to the foot support portion fore and aft of the opening. The piston has a body and a support arm extending transversely from the body. The body extends through the opening. The support arm is supported on the bridge portion, and is trapped below the ground-facing surface by the foot support portion, extending under the ground-facing surface at medial and lateral sides of the opening. 
     With the support arm above the bridge portion and below the ground-facing surface, the distance of the bridge portion from a neutral axis in the sole plate and the resulting bending stiffness of the sole structure are dependent on the progressing position of the piston. The piston is moved relative to the sole plate by dorsiflexion of the sole plate, with the bridge portion in tension, the foot support portion in compression, and the support arm separating the bridge portion and the foot support portion. 
     In some embodiments, the sole plate has a guide track, and the body of the piston has an engagement feature that engages with the guide track, ratcheting the piston incrementally along the guide track with repetitive dorsiflexion of the sole plate. The bending stiffness of the sole structure varies with a position of the piston along the guide track. 
     In some embodiments, the guide track has teeth, and the engagement feature of the piston is at least one tooth that engages with the teeth of the guide track. The guide track may have different segments, and the teeth of the different segments may angle in different directions to guide the piston along a segmented path. For example, in one section, the teeth may angle forward, in the next section, the teeth may angle in a transverse direction, and then in the next section, the teeth may angle rearward. 
     The teeth of the guide track may have a varied spacing. Widely spaced teeth (i.e., teeth with a large pitch) will advance the piston a greater distance along the sole plate with each dorsiflexion than closely spaced teeth (i.e., teeth with a small pitch). The piston may be configured to move along teeth of different spacings. For example, in one embodiment, the piston body includes a rear car and a front car. The teeth of the guide track have a first spacing at a first portion of the guide track. The teeth of the guide track have a second spacing less than the first spacing at second portion of the guide track. The sole plate has an obstruction that blocks ratcheting of the rear car along the guide track at a predetermined position between a start position and a final position of the piston body. The rear car abuts the front car between the start position and the predetermined position such that the front car is moved by the rear car as the rear car is ratcheted along the guide track from the start position to the predetermined position by repetitive dorsiflexion of the sole structure. The front car continues to move relative to the sole plate by repetitive dorsiflexion of the sole structure after the rear car is blocked, by ratcheting along the guide track free of the obstruction from the predetermined position to the final position. 
     In an embodiment, the teeth of the guide track are split in two transversely-spaced sets at the first portion of the guide track. A split tooth of the rear car engages the transversely-spaced set of teeth. A tooth of the front car extends from the front car between the transversely-spaced sets and is not engaged with the guide track when the split-tooth of the rear car progresses along the first portion of the guide track, but engages the teeth of the second portion of the guide track when the front car progresses without the rear car. 
     The guide track may be configured to advance the piston in a linear or nonlinear path relative to the sole plate. For example, the guide track may advance the piston along a curved track, or a track with multiple linear segments. In an embodiment, the guide track is curved toward a lateral side of the sole plate such that bending stiffness of the sole plate under bending in a transverse direction increases as the piston is ratcheted along the guide track. 
     In another embodiment the guide track has different segments that cause the piston to move in different directions relative to the sole plate as the piston progresses along the segments. For example, in an embodiment, the guide track has a first segment with a first series of teeth, and a second segment with a second series of teeth. The second segment is oriented at a first angle with respect to the first segment. A first post extends from the plate between the first segment and the second segment. The first post is positioned on the sole plate so that it contacts the at least one tooth of the piston as the piston is ratcheted along the sole plate. The at least one tooth of the piston is pivotable, and pivots by the first angle when it is in contact with the at least one tooth of the piston, thereby orienting the at least one tooth for subsequent engagement with the second series of teeth. For example, the first series of teeth may progress in a longitudinal direction along the sole plate, and the second series of teeth may progress in a transverse direction along the sole plate. Accordingly, when the at least one tooth is pivoted to engage with the second series of teeth, the piston progresses transversely along the sole plate. The second segment may be relatively short, and a second post may extend from the sole plate between the second segment and a third segment of the guide track that has a third series of teeth. The third segment is oriented at a second angle with respect to the second segment. The second post contacts the at least one tooth of the piston, pivoting the at least one tooth by the second angle after the at least one tooth progresses along the second series of teeth. The at least one tooth is thus oriented to engage with the third series of teeth, which progress in an opposite direction as the first series of teeth so that the piston is ratcheted in the opposite direction along the third series of teeth, having the opposite effect on changing bending stiffness than progression along the first series of teeth. For example, the first series of teeth may progress in a forward direction along the sole plate and the third series of teeth may progress in a rearward direction along the sole plate so that the piston is ratcheted forward along the first series of teeth, with the position of the arm therefore increasing bending stiffness. The piston and is ratcheted rearward along the third series of teeth, with the position of the arm thereby decreasing bending stiffness. 
     In some embodiments, the teeth of the guide track and the at least one tooth of the piston extend transversely relative to the sole plate. For example, each tooth of the guide track extends from a base to a tip in a transverse direction relative to the sole plate, and the at least one tooth of the piston extends from a base to a tip in an opposite transverse direction to engage the teeth of the guide track. 
     The piston and the guide track are not limited to embodiments having teeth that engage with one another. For example, in an embodiment, the guide track includes a first set of directional fibers, and the engagement feature of the piston is a second set of directional fibers that engages with the first set of directional fibers. 
     A sole structure for an article of footwear comprises a sole plate. The sole plate includes a foot-facing surface and a ground-facing surface. The sole plate has a compressive portion above a neutral axis, and a tensile portion below the neutral axis. The sole plate includes a guide track in the foot-facing surface. The guide track includes a series of protrusions. The sole structure includes a piston that has a body disposed above the tensile portion, and a support arm extending from the body, resting on the tensile portion, and disposed below the compressive portion and against the ground-facing surface. The piston includes at least one protrusion engaged with the series of protrusions of the guide track and ratcheting the piston along the guide track as the piston translates relative to the sole plate in response to dorsiflexion of the sole structure. In an embodiment, the sole plate has an opening, the body of the piston extends through the opening, and the support arm extends across the opening. In an embodiment, the bending stiffness of the sole structure varies with a position of the piston along the guide track. 
     In an embodiment, the series of protrusions is a first set of directional fibers, and the at least one protrusion of the piston is a second set of directional fibers engaged with the first set of directional fibers. In another embodiment, the series of protrusions is a set of teeth, and the at least one protrusion of the piston is a tooth that engages with the set of teeth. 
     The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description of the modes for carrying out the present teachings when taken in connection with the accompanying drawings. 
     “A”, “an”, “the”, “at least one”, and “one or more” are used interchangeably to indicate that at least one of the items is present. A plurality of such items may be present unless the context clearly indicates otherwise. All numerical values of parameters (e.g., of quantities or conditions) in this specification, unless otherwise indicated expressly or clearly in view of the context, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. In addition, a disclosure of a range is to be understood as specifically disclosing all values and further divided ranges within the range. All references referred to are incorporated herein in their entirety. 
     The terms “comprising”, “including”, and “having” are inclusive and therefore specify the presence of stated features, steps, operations, elements, or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, or components. Orders of steps, processes, and operations may be altered when possible, and additional or alternative steps may be employed. As used in this specification, the term “or” includes any one and all combinations of the associated listed items. The term “any of” is understood to include any possible combination of referenced items, including “any one of” the referenced items. The term “any of” is understood to include any possible combination of referenced claims of the appended claims, including “any one of” the referenced claims. 
     Those having ordinary skill in the art will recognize that terms such as “above”, “below”, “upward”, “downward”, “top”, “bottom”, etc., may be used descriptively relative to the figures, without representing limitations on the scope of the invention, as defined by the claims. 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the views,  FIG. 1  shows a sole structure  10  for an article of footwear  11  shown in  FIGS. 4B-4C . The sole structure  10  has a resistance to flexion that varies with repeated dorsiflexion of the forefoot region  14  of the sole structure  10  (i.e., flexing of the forefoot region  14  in a longitudinal direction as discussed herein). As further explained herein, due to a piston  28  that moves relative to a sole plate  12  in response to dorsiflexion of the sole structure  10 , the sole structure  10  provides a varying bending stiffness when flexed in a longitudinal direction. More particularly, because the piston  28  has a body  38  supported on a bridge portion  32  of the sole plate  12 , and a support arm  40  extending from the body  38  underneath a ground-facing surface  21  of the sole plate  12 , the sole structure  10  has a bending stiffness that varies with successive dorsiflexion of the sole structure  10 . The bending stiffness is tuned by the selection of various structural parameters discussed herein. As used herein, “bending stiffness” may be used interchangeably with “bend stiffness”. 
     Referring to  FIGS. 1-3 , the sole structure  10  includes the sole plate  12  and a piston  28 , and may include one or more additional plates, layers, or components, as discussed herein. The article of footwear  11  of  FIGS. 4B-4C  includes both the sole structure  10  and an upper  13  (shown in phantom in  FIGS. 4B-4C ). The sole plate  12  is configured to be operatively connected to the upper  13  as discussed herein. The upper  13  may incorporate a plurality of material elements (e.g., textiles, foam, leather, and synthetic leather) that are stitched or adhesively bonded together to form an interior void for securely and comfortably receiving a foot  53  as shown. In addition, the upper  13  may include a lace or other tightening mechanism that is utilized to modify the dimensions of the interior void, thereby securing the foot  53  within the interior void and facilitating entry and removal of the foot  53  from the interior void. Accordingly, the structure of the upper  13  may vary significantly within the scope of the present teachings. 
     The sole structure  10  is secured to the upper  13  and has a configuration that extends between the upper  13  and the ground G (indicated in  FIG. 4B ). The sole plate  12  may or may not be directly secured to the upper  13 . Sole structure  10  may attenuate ground reaction forces (i.e., provide cushioning for the foot  53 ), and may provide traction, impart stability, and limit various foot motions. 
     In the embodiment shown, the sole plate  12  is a full-length, unitary sole plate  12  that has a forefoot region  14 , a midfoot region  16 , and a heel region  18 . In other embodiments, the sole plate  12  may be a partial length plate member. For example, in some cases, the sole plate  12  may include only a forefoot region  14  and may be operatively connected to other components of the article of footwear that comprise a midfoot region and a heel region. The sole plate  12  provides a foot support portion  19  that includes a foot-facing surface  20  (also referred to as a foot-receiving surface). 
     The foot-facing surface  20  extends over the forefoot region  14 , the midfoot region  16 , and the heel region  18 . The foot support portion  19  includes the majority of the sole plate  12  at the foot-facing surface  20 , and supports the foot  53  but is not necessarily directly in contact with the foot  53 . For example, an insole, midsole, strobel, or other layers or components may be positioned between the foot  53  and the foot-facing surface  20 . 
     The sole plate  12  has a medial side  22  and a lateral side  24 . As shown, the sole plate  12  extends from the medial side  22  to the lateral side  24 . As used herein, a lateral side of a component for an article of footwear, including the lateral side  24  of the sole plate  12 , is a side that corresponds with an outside area of the human foot  53  (i.e., the side closer to the fifth toe of the wearer). The fifth toe is commonly referred to as the little toe. A medial side of a component for an article of footwear, including the medial side  22  of the sole plate  12 , is the side that corresponds with an inside area of the human foot  53  (i.e., the side closer to the hallux of the foot of the wearer). The hallux is commonly referred to as the big toe. Both the medial side  22  and the lateral side  24  extend along a periphery of the sole plate  12  from a foremost extent  25  to a rearmost extent  29  of the sole plate  12 . 
     The term “longitudinal”, as used herein, refers to a direction extending along a length of the sole structure  10 , e.g., extending from the forefoot region  14  to the heel region  18  of the sole structure  10 . The term “transverse”, as used herein, refers to a direction extending along the width of the sole structure  10 , e.g., extending from the medial side to the lateral side of the sole structure  10 . The term “forward” is used to refer to the general direction from the heel region  18  toward the forefoot region  14 , and the term “rearward” is used to refer to the opposite direction, i.e., the direction from the forefoot region  14  toward the heel region  18 . The terms “anterior” and “fore” are used to refer to a front or forward component or portion of a component. The term “posterior” and “aft” are used to refer to a rear or rearward component or portion of a component. 
     The heel region  18  generally includes portions of the sole plate  12  corresponding with rear portions of a human foot, including the calcaneus bone, when the human foot is supported on the sole structure  10  and is a size corresponding with the sole structure  10 . The forefoot region  14  generally includes portions of the sole plate  12  corresponding with the toes and the joints connecting the metatarsal bones with the phalange bones of the human foot (interchangeably referred to herein as the “metatarsal-phalangeal joints” or “MPJ” joints). The midfoot region  16  generally includes portions of the sole plate  12  corresponding with an arch area of the human foot, including the navicular joint. Regions  14 ,  16 ,  18  are not intended to demarcate precise areas of the sole structure  10 . Rather, regions  14 ,  16 ,  18  are intended to represent general areas relative to one another, to aid in the following discussion. In addition to the sole structure  10 , the relative positions of the regions  14 ,  16 ,  18 , and medial and lateral sides  22 ,  24  may also be applied to the upper  13 , the article of footwear  11 , and individual components thereof. 
     The sole plate  12  is referred to as a plate, and is generally but not necessarily flat. The sole plate  12  need not be a single component but instead can be multiple interconnected components. For example, both an upward-facing portion of the foot-facing surface  20  and the opposite ground-facing surface  21  may be pre-formed with some amount of curvature and variations in thickness when molded or otherwise formed in order to provide a shaped footbed and/or increased thickness for reinforcement in desired areas. For example, the sole plate  12  could have a curved or contoured geometry that may be similar to the lower contours of the foot  53 . The sole plate  12  may have a contoured periphery (i.e., along the medial side  22  and the lateral side  24 ) that slopes upward toward any overlaying layers, such as a midsole or the upper  13 . 
     The sole plate  12  may be entirely of a single, uniform material, or may have different portions comprising different materials. For example, a first material of the forefoot region  14  can be selected to achieve, in conjunction with the piston  28  and other features and components of the sole structure  10  discussed herein, the desired bending stiffness in the forefoot region  14 , while a second material of the midfoot region  16  and/or the heel region  18  can be a different material that has little effect on the bending stiffness of the forefoot region  14 . By way of non-limiting example, the second portion can be over-molded onto or co-injection molded with the first portion. Example materials for the sole plate  12  include durable, wear resistant materials. For example, a thermoplastic elastomer, such as thermoplastic polyurethane (TPU), a glass composite, a nylon including glass-filled nylons, a spring steel, carbon fiber, ceramic or a foam or rubber material (such as but not limited to a foam or rubber with a Shore A Durometer hardness of about 50-70 (using ASTM D2240-05(2010) standard test method) or an Asker C hardness of 65-85 (using hardness test JIS K6767 (1976))) may be used for the sole plate  12 . 
     In the embodiment shown, the sole plate  12  may be an inner board plate, also referred to as an inner board, an insole board, or a lasting board. The sole plate  12  may instead be an outsole. Still further, the sole plate  12  could be a midsole plate or a unisole plate, or may be any combination of an inner board plate, a midsole plate, or an outsole. For example, in  FIG. 4B , the sole plate  12  is shown with traction elements  69 . The traction elements  69  may be integrally formed as part of the sole plate  12  (e.g., if the sole plate is an outsole or a unisole plate), may be attached to the sole plate  12 , or may be formed with or attached to another plate underlying the sole plate  12 , such as if the sole plate  12  is an inner board plate and the sole structure  10  includes an underlying outsole. For example, the traction elements  69  may be integrally formed cleats. In other embodiments, the traction elements may be, for example, removable spikes. The traction elements  69  protrude below the ground-facing surface  21  of the sole plate  12 . Direct ground reaction forces on the sole plate  12  that could affect operation of the piston  28  are thus minimized. In other embodiments, however, the sole structure  10  may have no traction elements  69 , the ground-facing surface  21  may be the ground-contact surface, or other plates or components may underlie the sole plate  12 . 
     With reference to  FIGS. 1 and 3 , an opening  30  extends through the foot support portion  19  of the sole plate  12  from the foot-facing surface  20  to a ground-facing surface  21  of the sole plate  12  that is best shown in  FIG. 3 . A bridge portion  32  of the sole plate  12  underlies the opening  30  and is secured to (i.e., extends as a unitary part of) the foot support portion  19  fore and aft of the opening  30 . The bridge portion  32  is operatively secured to the foot support portion  19 . As used herein, the bridge portion  32  is “operatively secured” to the foot support portion  19  when it is directly or indirectly attached to the foot support portion  19 . In the embodiment of  FIGS. 1-6D , the bridge portion  32  is a unitary part of and is of the same material as the foot support portion  19 . 
     As best shown in  FIG. 3 , the bridge portion  32  is recessed below the foot support portion  19 . Stated differently, a foot-facing surface  34  of the bridge portion  32  is below the ground-facing surface  21  of the foot support portion  19 , at least when the sole plate  12  is in an unflexed, relaxed state as in  FIGS. 1-3 . The bridge portion  32  is generally the same size and shape as the opening  30 , and both are disposed lengthwise along a longitudinal midline LM of the sole plate  12 . The bridge portion  32  has a thickness T 1 , a width W 1  greater than the thickness T 1 , and a length L 1  greater than the width W 1 . 
     Due to the disposition of the bridge portion  32  below the foot support portion  19 , slots  36  are formed between the ground-facing surface  21  of the foot support portion  19  and the bridge portion  32 . The slots  36  run along the length L 1  of the bridge portion  32  at the medial side  37  and the lateral side  39  of the bridge portion  32 . The lateral slot  36  is visible in  FIGS. 1 and 3 , and the medial slot  36  is indicated in  FIG. 1  between the sole plate  12  and the medial side  27  (shown in hidden lines) of the piston  28 . 
     The piston  28  is shown slightly inverted in  FIG. 1  relative to its assembled and in-use position of  FIGS. 2 and 3  in order to expose the teeth  56 . The piston  28  has an elongated body  38  with a width W 2  slightly less than the width of the opening  30  so that the body  38  can extend through the opening  30 . The piston  28  also has a support arm  40  that extends transversely from the body  38 . The width W 3  of the support arm  40  is greater than the width W 1  of the bridge portion  32  and greater than the width W 2  of the piston body  38  as shown in  FIGS. 2 and 3 . Referring to  FIG. 1 , notches  42  in the foot support portion  19  at the opening  30  create a transverse expanse of the opening  30  that has a width W 4  greater than the width W 3  of the support arm  40 . When the piston  28  is placed above the sole plate  1  with the teeth  56  facing downward, the support arm  40  can be dropped through the opening  30  at the notches  42  so that the bottom surface  46  of the support arm  40  rests on the foot-facing surface  34  of the bridge portion  32 , and the upper surface  47  of the support arm  40  is below the ground-facing surface  21  as shown in  FIG. 3 . In other words, the body  38  extends through the opening  30 , and the support arm  40  is supported on the bridge portion  32 . The foot-facing surface  48  of the piston  28  may rest below or generally level with the foot-facing surface  20  of the foot support portion  19  when the piston  28  is inserted in the opening  30  as described and the sole structure  10  is in an unflexed, generally relaxed state as shown in  FIG. 2 . If the foot-facing surface  48  rests sufficiently below the foot-facing surface  20 , the foot support portion  19  can extend directly over the guide track  50  and the bridge portion  32  so that the foot-facing surface  48  is nested below the foot support portion  19 . 
     With reference to  FIG. 1 , the sole plate  12  includes a guide track  50  slightly recessed at the foot-facing surface  20 . The guide track  50  is shown to have two sections  50 A,  50 B. A forward section  50 A is forward of the bridge portion  32 , and a rear section  50 B is rearward of the bridge portion  32 . In an alternative embodiment, either only the forward section  50 A, or only the rearward section  50 B of the guide track  50  may be provided. The guide track  50  has a series of protrusions  52 . In the embodiment shown, the protrusions  52  are gear teeth and the guide track  50  is a linear gear, also referred to as a rack. The gear teeth  52  have a profile angle that inclines toward tips  54  of the teeth  52  in a forward direction. 
     The piston  28  also has at least one protrusion  56 . In the embodiment shown, the piston  28  has a series of protrusions  56  that are gear teeth. The teeth  56  have a profile angle that inclines toward tips  58  of the teeth  56  in a rearward direction when the piston  28  is in its in-use position of  FIGS. 2 and 3 . The teeth  56  are divided into a forward section  56 A and a rearward section  56 B. 
     It should be appreciated that the overall length L 2  of the piston  28  is less than the length L 3  of the guide track  50  from a front of the forward section  50 A to a rear of the rearward section  50 B. The relative size of the piston  28  and guide track  50  is best shown in  FIG. 2 . The length L 2  is greater than the length L 1 , but less than the length L 3 . The lengths L 2  and L 3  are such that, when the arm  40  is disposed through the notches  42 , the rearward section  50 B engages with the rear section  56 B, and a forward-most tooth  56 C of the piston  28  is engaged with a rearmost tooth  52 C of the forward section  50 A so that teeth  52  forward of the tooth  52 C are not yet engaged with any teeth of the piston  28 . In other embodiments, the tooth  56 C could be engaged with a tooth forward of tooth  52 C, but in all embodiments, when the piston  28  is in a rearmost position, at least some of the teeth  52  of the forward section  50 A are forward of tooth  56 C. This provides room for the piston  28  to progress forward relative to the sole plate  12  during dorsiflexion. In other words, the tooth  56 C is engaged with the tooth  52 C, and ratchets the piston  28  along the guide track  50  as the piston  28  translates relative to the sole plate  12  with repetitive dorsiflexion of the sole structure  10 . 
       FIG. 6A  shows the tooth  52 C relative to tooth  56 C as the piston  28  begins to move during dorsiflexion, and  FIG. 6B  represents a subsequent position of tooth  52 C relative to tooth  56 C when the sole structure  10  flexed at a flex angle A 1  during an initial dorsiflexion with the forefoot region  14  of the sole structure operatively engaged with the ground G (such as through traction elements  69 ). A removable pin (not shown) may extend through the piston  28  and sole plate  12  to temporarily maintain the piston  28  in the initial position until ratcheting of the piston  28  and is desired. For example, the pin may be removed at the beginning of a race. A similar pin may be used in any of the embodiments described herein. During dorsiflexion, and assuming any such pin is removed, the sole plate  12  and the piston  28  will be flexed so that the mating gear tooth faces  52 F,  56 D of teeth  52 C,  56 C, respectively, will be tilted relative to the position shown in  FIG. 6A  to a horizontal disposition or even further, and the forward weight of the foot  53  (arrow A) will urge the piston  28  to move forward relative to the sole plate  12 .  FIGS. 6A and 6B  show the resulting progression of the tooth  56 C up (arrow B) and over (arrow C) the tooth  52 C of the guide track  50 . 
     Following the initial dorsiflexion, as the foot  53  plantar flexes and lifts the forefoot region  14  of the article of footwear  11  out of operative engagement with the ground G, and then the article of footwear  11  comes into contact with the ground G at a point rearward of the forefoot region  14 , such as at the heel region  18  or even a more rearward part of the forefoot region  14  during a sprint, the foot  53  no longer urges the piston  28  forward relative to the sole plate  12 . The foot  53  may urge the piston  28  rearward relative to the sole plate  12 , as indicated by arrow D in  FIG. 6C  showing relative movement of the piston  28  rearward. The faces  55 C,  55 E of the gear teeth  52 C,  56 C opposite to the inclined faces are substantially perpendicular to the foot-facing surface  20  and to the bottom surface  57  of the piston  28 , and prevent further movement of the piston  28  rearward relative to the sole plate  12 . In a subsequent dorsiflexion with the forefoot region  14  in operative engagement with the ground G, the process repeats, and the tooth  56 C progresses up and over the next forward tooth  52 D, as indicated with arrows E and F in  FIG. 6D , with the next rearward tooth  56 E of the piston  28  now encountering the tooth  52 C. In this manner, the tooth  56 C continues to ratchet the piston  28  forward relative to the sole plate  12  tooth by tooth along the series of teeth  52  with repeated dorsiflexion of the sole structure  10  until the tooth  56 C progresses over the forward-most tooth  52 E of the series of teeth  52 , shown in  FIG. 1 . The piston  28  then remains in the forward-most position during any further dorsiflexion as the front wall  61  of the foot support portion  19  forward of the forward section  56 A in combination with the downward force of the wearer prevents forward motion of the piston  28  relative to the sole plate  12 . 
     As will be understood by those skilled in the art, during bending of the sole structure  10  as the foot  53  is dorsiflexed, there is a layer in the sole plate  12  referred to as a neutral plane (although not necessarily planar) or a neutral axis NB above which the sole plate  12  is in compression, and below which the sole plate  12  is in tension. It should be appreciated that the neutral axis NB is not the bend axis about which bending occurs. The bend axis BA is positioned above the foot-facing surface  20 , and represents the axis about which the foot  53  bends. The position of the bend axis BA changes as the foot  53  progresses through dorsiflexion. Those skilled in the art will appreciate that portions of the sole plate  12  (such as portions of the sole plate  12  near the foot-facing surface  20 ) may be placed in compression during dorsiflexion of the sole plate  12 , while other portions of the sole plate  12 , (such as portion of the sole plate  12  near the ground-facing surface  21 ) may be placed in tension during dorsiflexion of the sole plate  12 . The greater the distance from the neutral axis NB that the compressive and tensile forces of the sole plate  12  are applied, the greater the bending stiffness of the sole plate  12 .  FIG. 4B  indicates that the sole plate  12  has a compressive portion CP above the neutral axis NB and a tensile portion TP below the neutral axis NB. The bridge portion  32  is below the neutral axis NB and is thus in tension. The bridge portion  32  is thus also referred to herein as a tensile portion of the sole plate  12 . Generally, greater torque is required to bend material that is further displaced from the neutral bend axis NB, and greater compressive or tensile forces act on the material. Accordingly, increasing the relative distance between the neutral axis NB and the compressive forces and/or the tensile forces increases the bending stiffness of the sole plate  12 , whereas decreasing the relative distance between the neutral axis NB and the compressive forces and/or the tensile forces decreases the bending stiffness of the sole plate  12 . 
     As the piston  28  ratchets along the series of teeth  52 , the bending stiffness of the sole structure  10  varies in accordance with the position along the longitudinal axis of the arm  40  of the piston  28 . The arm  40  interferes with movement of the bridge portion  32  and the foot support portion  19  toward the neutral axis NB.  FIG. 4A  shows the sole plate  12  with the piston  28  removed. During dorsiflexion of the sole plate  12 , the sole plate  12  can relieve bending forces to the extent that the bridge portion  32  can rise up relative to the foot support portion  19  at the lateral and medial sides of the opening  30 . Without the piston  28  in place, the midsection  32 A of the bridge portion  32  is free to flex or bend by rising up toward the foot-facing surface  20 , and the medial section  19 A of the foot support portion  19  and the lateral section  19 B of the foot support portion  19  adjacent the opening  30  are free to bend by moving downward toward the bridge portion  32 . Of course, with the weight of a foot  53  on the sole plate  12 , the midsection  32 A of the bridge portion  32  will not move up further than the foot-facing surface  20 .  FIG. 4A  shows movement of the midsection  32 A beyond the foot-facing surface  20  only because no foot or sole component is shown over the bridge portion  32 . 
     Allowing the midsection  32 A of the bridge portion  32  to move upward and the medial and lateral sections  19 A,  19 B of the foot support portion  19  at the medial and lateral sides of the opening  30  to move downward aligns the midsection  32 A with the medial and lateral sections  19 A,  19 B (assuming a foot  53  or other component is above the bridge portion  32  to prevent its upward movement beyond the foot-facing surface  20 ). This causes the sole plate  12  to behave in bending (i.e., to exhibit a similar bending stiffness) as a single piece of material having an approximate thickness equal to the thickness TS of the sole plate  12  (see  FIG. 3 ) at the bending area. Conversely, if the midsection  32 A cannot rise up (i.e., if no relative movement of the midsection  32 A and the medial and lateral sections  19 A,  19 B is possible), then the sole plate  12  behaves in bending as a piece of material having a thickness D 2  equivalent to the distance from the foot-facing surface  20  to the bottom surface of the bridge portion  32  indicated in  FIGS. 1 and 4C . Bending stiffness can be further varied by providing the bridge portion  32  with a varying thickness in the longitudinal direction. 
     In  FIGS. 4B-4C , the effective thickness discussed with respect to bending stiffness is at the portion of the sole plate  12  below the metatarsal-phalangeal joints. As is understood by those skilled in the art, torque on the sole structure  10  results from a force applied at a distance from a bending axis BA located in the proximity of the metatarsal-phalangeal joints, as occurs when a wearer flexes the sole structure  10 . A flex angle μl is defined as the angle formed at the intersection between a first axis LM 1  and a second axis LM 2 . The first axis LM 1  generally extends along the longitudinal midline LM of the sole plate  12  at the ground-facing surface  21  of the sole plate  12  at a forward part of the bridge portion  32 . The second axis LM 2  generally extends along the longitudinal axis LM of the sole plate  12  at the ground-facing surface  21  of the sole plate  12  at a rearward part of the bridge portion  32 . The sole plate  12  is configured so that the intersection of the first axis LM 1  and the second axis LM 2  is approximately centered both longitudinally and transversely below the metatarsal-phalangeal joints of the foot  53  supported on the foot-facing surface  20  of the sole plate  12 . Changing or repositioning the arm  40  relative to the bridge portion  32  of the sole plate  12  changes the bending stiffness that the sole plate  12  exhibits at similar flex angles A 1 . In other words, the sole plate  12  may exhibit a first bending stiffness at a specific flex angle A 1  with the arm  40  in the first position of  FIG. 4B , and exhibit a second bending stiffness at the same specific flex angle A 1  with the arm  40  in the second position of  FIG. 4C , and other bending stiffness values with the arm  40  at other positions corresponding with different positions of the piston  28  along the guide track  50 . 
     As a wearer&#39;s foot  53  dorsiflexes by lifting the heel region  18  away from the ground G, while maintaining contact with the ground G at the forefoot region  14 , it places torque on the sole structure  10  and causes the sole plate  12  to flex through the forefoot region  14 . Referring to  FIG. 5 , an example plot indicating the bending stiffness (slope of the line) of the sole plate  12  with the arm in the first position is generally shown at  80 . Torque (in Newton-meters) is shown on a vertical axis  82 , and the flex angle (in degrees) is shown on a horizontal axis  84 . As is understood by those skilled in the art, the torque results from a force applied at a distance from a bending axis located in the proximity of the metatarsal-phalangeal joints, as occurs when a wearer flexes the sole structure  10 . The bending stiffness of the sole plate  12  may be constant (thus the plot would have a linear slope) or substantially linear, or may increase gradually (which would show a change in slope with changes in flex angle). As shown in the exemplary plot of  FIG. 5 , the bending stiffness is nonlinear, and increases exponentially and with a positive rate of change of stiffness. Alternatively, the bending stiffness could be nonlinear with a negative rate of change of stiffness with increasing flex angle, or could be linear. 
     The arm  40  of the piston  28  changes the ability of the sole plate  12  and bridge portion  32  to align as described. With reference to  FIG. 4B , when the piston  28  is in the rearmost position in which the arm  40  is directly below the notches  42  and a rear end  60  of the piston  28  (shown in  FIG. 1 ) is adjacent and possibly abutting a rear wall  62  of the foot support portion  19  rearward of the section  50 B, the support arm  40  is trapped below the foot support portion  19  and above the bridge portion  32 . The support arm  40  prevents relative movement of the bridge portion  32  toward the foot support portion  19  at the support arm  40 . Any relative movement of the bridge portion  32  toward the foot support portion  19  can only occur forward of the support arm  40 . With the support arm  40  inserted through the opening  30  as shown, the midsection  32 A of the bridge portion  32  has some movement toward the foot support portion  19 , but cannot raise toward the foot support portion  19  as much as it could when the piston  28  was removed in  FIG. 4A . This causes the sole plate  12  to behave in bending (i.e., to exhibit a similar bending stiffness) as a sole plate having a thickness D 1  equivalent to the distance from the foot-facing surface  20  to the bottom surface  49  of the bridge portion  32 , and bending stiffness is thus higher than in  FIG. 4A . 
     When the piston  28  ratchets as described with respect to  FIGS. 5A-5D , the support arm  40  moves forward with the body  38 , shortening the portion of the bridge portion  32  that is forward of the support arm  40 . The piston  28  is moved relative to the sole plate  12  by dorsiflexion of the sole plate  12 , with the bridge portion  32  in tension, the foot support portion  19  in compression, and the support arm  40  separating the bridge portion  32  and the foot support portion  19 . When the support arm  40  moves forward of the notches  42 , the support arm  40  is trapped below the ground-facing surface  21  by the foot support portion  19 , and extends under the foot support portion  19  at medial and lateral sides  51 A,  51 B of the opening  30 . The upper surface  47  of the support arm  40  will be in contact with the ground-facing surface  21  at least during dorsiflexion. For example, when the support arm  40  is at the position shown in  FIG. 4C , representing the forward-most position in which the forward edge  63  of the piston  28  abuts the front wall  61  of the foot support portion  19  forward of the section  50 A (i.e., slightly more forward than shown in  FIG. 2 ), the arm  40  is in the position shown in  FIG. 4C . In this position, the arm  40  prevents relative movement of the midsection  32 A of the bridge portion  32  toward the medial and lateral sections  19 A,  19 B so the sole structure  10  behaves in bending as a sole plate having the thickness D 2  equivalent to the distance from the foot-facing surface  20  to the bottom surface  49  of the bridge portion  32 . 
     The support arm  40  thus moves with the piston  28  along the longitudinal midline LM of the sole structure  10  to alter or change the bending stiffness of the sole structure  10 . The support arm  40  is at least a semi-rigid material. The substantially semi-rigid material may include any material having a durometer of  50 D or greater. For example, the support arm  40  may be a metal, such as stainless steel or aluminum, or may alternatively include a plastic, such as a nylon material or a thermoplastic polyurethane, although the embodiments are not limited only to those examples listed here, but can also include other similarly and suitably semi-rigid or rigid materials. The support arm  40  extends transversely relative to the longitudinal midline LM and is interlaced with the lateral section  19 B of the foot support portion  19  at the lateral side of the bridge portion  32 , with the bridge portion  32 , and with the medial section  19 A of the foot support portion  19  at the medial side of the bridge portion  32 . 
     The bending stiffness of the sole plate  12  provides the resistance against dorsiflexion of the sole plate  12  in the longitudinal direction along the longitudinal midline LM of the sole plate  12 . In other words, when the arm  40  is moved forward from the first position of  FIG. 4B , the bending stiffness of the sole plate  12  is changed at any specific flex angle when compared to the bending stiffness profile of the sole plate  12  with the arm  40  in the first position at the same flex angle. Accordingly, as shown in  FIG. 5 , the bending stiffness shown by line  80 , with the arm  40  in the first position, is less than the bending stiffness shown by line  86 , with the arm  40  in the second position. 
       FIG. 7  shows another embodiment of a sole structure  110  within the scope of the present teachings. The sole structure  110  is configured with many of the same components that function in the same manner as described with respect to sole structure  10  and are referred to with the same reference numbers. Instead of a guide track with teeth, the sole plate  12  has a guide track  150  that has a first set of directional fibers  152 . The first set of directional fibers  152  is divided into a forward section  152 A forward of the opening  30  and the bridge portion  32 , and a rear section  152 B rearward of the opening  30  and the bridge portion  32 . Instead of a tooth as an engagement feature, the piston  28  has a second set of directional fibers  156  that engages with the first set of directional fibers  152 . The second set of directional fibers  156  has a forward section  156 A and a rearward section  156 B. The forward section  156 A engages with the forward section  152 A, and the rearward section  156 B engages with the rear section  152 B. The directional fibers  152 ,  156  are configured to allow the directional fibers  156  to incrementally ratchet forward over the directional fibers  152  under the force of the foot  53  shown as arrow A and described with respect to  FIG. 6A . The directional fibers  152 ,  156  are arranged as parallel rows of individual fibers  157  laid transverse to the longitudinal midline LM. The fibers  157  protrude from the sole plate  12 , and may be nylon, mohair, or a combination thereof, similar to ski skins on a cross-country ski. A backing of the fibers  152 ,  156  can be adhered to the sole plate  12  and to the piston  28 . Once the directional fibers  156  advance forward on the directional fibers  152 , the protrusions of the fibers  157  are sufficient to prevent rearward movement, as any rearward force of the fibers  156  relative to the fibers  152  is less than the forward force of the fibers  156  against the fibers  152 , represented by arrow A in  FIG. 6A  and experienced during dorsiflexion. 
       FIG. 8  shows another embodiment of a sole structure  210  within the scope of the present teachings. The sole structure  210  is configured with many of the same components that function in the same manner as described with respect to sole structure  10  and are referred to with the same reference numbers. The sole structure  210  has a piston  228 , and is configured with a sole plate  212  that has posts  270 ,  272  and a segmented guide track  250  that enable the piston  228  to move forward, transversely, and rearward relative to the sole plate  212 . More specifically, the guide track  250  has a first segment  250 A with a first series of teeth  252 A, and a second segment  250 B with a second series of teeth  252 B. The second segment  250 B is oriented at a first angle with respect to the first segment  250 A. In the embodiment shown, the first angle is a 90 degree angle. The first series of teeth  252 A progress incline in a forward longitudinal direction, progressing in a forward longitudinal direction along the sole plate  212 . The second series of teeth  252 B progress in a transverse direction along the sole plate  212 , inclining in a direction from the lateral side toward the medial side  22 . Accordingly, the piston  228  is ratcheted along the second series of teeth  252 B in a transverse direction at a 90 degree angle with respect to the direction that it is ratcheted along the first series of teeth  252 A. The guide track  250  also has a third segment  250 C with a third series of teeth  252 C. The third segment  250 C is oriented at a second angle with respect to the second segment  250 B. In the embodiment shown, the second angle is 90 degrees. The third series of teeth  252 C incline in a rear longitudinal direction, thus progressing in an opposite direction as the first series of teeth  252 A so that the piston  228  is ratcheted in the opposite direction along the third series of teeth  252 C. In other embodiments, the first, second, and third segments could be arranged at other angles relative to one another, so that the piston  228  progresses in a different manner. For example, the third segment could be arranged forward of the second segment, so that the third series of teeth progresses in the forward longitudinal direction, just as the first series of teeth. A fourth segment could be arranged between the third segment and the first segment to direct the piston  228  transversely from the third segment back to the first segment, so that the piston  228  loops around the four segments. The segments may correspond to portions of a race in which increasing longitudinal stiffness is first desired (i.e., when the piston  228  moves along the first segment  250 A), followed at some point by decreasing longitudinal stiffness (i.e., when the piston  228  moves along the third segment  250 C). 
     The sole plate  212  has a first post  270  and a second post  272  both of which extend upward at the foot-facing surface  20  of the sole plate. The first post  270  is positioned between the first segment  250 A and the second segment  250 B. The piston  228  has a pivotable tooth  256  that extends downward and interfaces with the teeth  252 A,  252 B,  252 C as described with respect to teeth  56  and teeth  52  in  FIG. 1 . The tooth  256  has a ramped surface  256 D that encounters the inclining faces of the teeth  252 A,  252 B,  252 C as described with respect to face  56 D of tooth  56 C encountering face  52 F of tooth  52 C. In order to encounter the inclining faces which incline in different directions as shown and described, the tooth  256  is pivotable about a center axis  253  extending from the base to the tip of the tooth  256 . The tooth  256  is configured so that it is pivotable upon encountering sufficient force off-centered from its axis  253  so as to cause the tooth to rotate about its axis by 90 degrees in the direction indicated by arrow G. 
     The first post  270  is positioned off center from the tooth  256 , and may have a rounded contact surface  257  that pivots the tooth  256  so that when the first post  270  contacts the tooth  256 , and the dorsiflexion force indicated by arrow A in  FIG. 6A  is applied by the tooth  256  against the first post  270 , the tooth  256  pivots by the first angle (i.e., 90 degrees counter-clockwise in the embodiment shown). The tooth  256  may be held in place with friction between the tooth  256  and the bottom surface of the piston  228 , which friction is overcome by the force of the offset post  270  against the tooth  256 . 
     After the tooth  256  is pivoted, its ramped surface  256 D now faces the ramped surfaces of the teeth  252 B, and further dorsiflexion of the sole structure  210  will cause the piston  228  to ratchet along the second series of teeth  252 B. The second series of teeth  252 B incline in a transverse direction, from the lateral side  24  to the medial side  22  in the embodiment shown. A forward wall  258  at the forward edge of the teeth  252 B prevents the tooth  256  from progressing forward as it moves along the second segment  250 B. The arm  40  does not move forward as the piston progresses along the second series of teeth, so the ability of the bridge portion  32  to flex is unchanged and bending stiffness in dorsiflexion does not vary as the piston  228  progresses over the second series of teeth  252 B. 
     The second post  272  is between the second segment  250 B and the third segment  250 C. and is off-centered from the tooth  256  such that the tooth  256  encounters the second post  272  and is caused to pivot along a rounded surface  259  of the second post  272  to rotate about its axis by 90 degrees in the direction indicated by arrow G. The second post  272  extends upward at a position off-centered from the tooth  256  so that when the second post  272  contacts the tooth  256 , and the dorsiflexion force indicated by arrow A in  FIG. 6A  is applied by the tooth  256  against the second post  272 , the post  272  pivots the tooth  256  by the second angle (i.e., by 90 degrees counter-clockwise in the embodiment shown). After the tooth  256  is pivoted, its ramped surface  256 D now faces the ramped surfaces of the teeth  252 C, and further dorsiflexion of the sole structure  210  will cause the piston  228  to ratchet along the second series of teeth  252 C, progressing rearward. 
     The first series of teeth  252 A progress in a forward direction along the sole plate  212  and the third segment  250 C progress in a rearward direction along the sole plate  212  so that the piston  228  is ratcheted forward along the first series of teeth  252 A, and is ratcheted rearward along the third segment  250 C. Accordingly, the sole structure  210  will have increasing stiffness as the piston  228  progresses along the first series of teeth  252 A, and decreasing stiffness as the piston  228  progresses along the third segment  250 C, in accordance with the location of the arm  40  as described with respect to the embodiment shown in  FIGS. 4B-4C . 
     Alternatively, the tooth  256  may be generally L-shaped, as illustrated by tooth  256 A in  FIG. 9 , in which case the sole plate  312  need only have the first series of teeth  252 A and the third series of teeth  252 C need be provided. Each of the arms  259 A,  259 B has an engaging portion. The engaging portion  261 A of arm  259 A engages with teeth  252 A when the piston  228  is moving forward, and the engaging portion  261 B of arm  259 B engages with teeth  252 C when the piston  228  is moving rearward. As the piston  228  progresses forward along the first series of teeth  252 A, the first arm  259 A of the tooth  256 A interferes with the post  270 , causing the tooth  256 A to pivot 90 degrees clockwise to the position  256 AA shown in  FIG. 9 . Stoppers  271  also extend from the sole plate  212  to limit movement of the tooth  256 A. Once pivoted, the portion of the tooth  256 A on the second arm  259 B engages the third series of teeth  252 B to enable the piston  228  to progress along the third series of teeth  252 C. 
     In still another embodiment, instead of a pivoting tooth, the tooth is non-pivotable, but has two opposing, angled surfaces, one of which engages the first series of teeth when the piston  228  moves forward, and the other of which engages the third series of teeth when the piston  228  moves rearward. No second series of teeth  252 B is needed. In such an embodiment, a foot-facing surface of the piston  228  has an extension extending upward, and a portion of the sole plate  212  directly overlays the piston  228  and has a cam surface along which the extension rides as the piston  228  progresses. The cam surface is configured to guide the extension, thereby guiding the tooth of the piston  228  to engage the first series of teeth  252 A followed by the third series of teeth  252 C. 
       FIG. 10  shows another embodiment of a sole structure  310  within the scope of the present teachings. The sole structure  310  is configured with many of the same components that function in the same manner as described with respect to sole structure  10  and are referred to with the same reference numbers. The sole structure  310  has a piston  328 , and is configured with a sole plate  312  that has a guide track  350  with a forward section  350 A (also referred to as a first section) and a rearward section  350 B (also referred to as a second section). The guide track  350  has a series of teeth  352  rearward of the bridge portion  32  and the opening  30 . The forward section  350 A of the guide track  350  has no teeth. 
     The piston  328  has only a single tooth  356  with a surface  356 D that inclines in a rearward direction from a base to a tip, so that it will interface with the forward-inclining faces  352 D of the teeth  352  to ratchet the piston  328  forward with repetitive dorsiflexion of the sole structure  310  as described with respect to the teeth  52 ,  56  of the sole structure  10  of  FIG. 1 . The recessed area of the foot-facing surface  20  forming the forward section  350 A of the guide track  350  will guide the front of the piston  328 . By locating the interfacing teeth  352 ,  356  only in the rearward section  350 B which is generally in the midfoot region  16 , movement of the tooth  356  over the tooth  352  is not subject to any interference due to the loading of the weight of the wearer, which is borne by the forefoot region  14  during dorsiflexion. 
     The guide track  350  initially curves generally toward the lateral side  24  of the sole plate  312  and then extends generally parallel with the longitudinal midline LM. The arm  40  will thus extend under the foot support portion  19  more on the lateral side  24  than on the medial side  22  as the piston  328  progresses forward. Accordingly, bending that may occur along a transverse axis, such as when running around a curve on a running track, will cause more stiffness at the lateral side  24  of the sole plate  312  than the medial side  22  of the sole plate  312 . After progressing to approximately point  311  to increase the transverse (lateral) bending stiffness when running along a curved portion of the track, the piston  328  then moves generally parallel to the longitudinal midline LM to correspond with a straight portion of the running track, increasing the longitudinal bending stiffness of the sole structure  310 . 
       FIG. 11  shows another embodiment of a sole structure  410  within the scope of the present teachings. The sole structure  410  is configured with many of the same components that function in the same manner as described with respect to sole structure  10  and are referred to with the same reference numbers. 
     The sole structure  410  has a sole plate  412  that has a guide track  450  with a forward section  450 A (also referred to as a first section) and a rearward section  450 B (also referred to as a second section). The guide track  450  has a series of teeth  452  rearward of a bridge portion  432  and the opening  430 . The forward section  450 A of the guide track  450  has no teeth. The teeth  452  of the rearward section  450 B extend from a base to a tip transversely relative to the sole plate within the recessed guide track  450 , instead of vertically from base to tip as the teeth  52  of  FIG. 1 . 
     The sole structure  410  has a piston  428  with a body  429  that is a series of segments  428 A,  428 B,  428 C,  428 D,  428 E,  428 F,  428 G,  428 H, and  428 I, interconnected similarly to links of a chain so that the segments are able to articulate relative to one another. This enables a center longitudinal axis  427  of the piston  428  to change from the straight orientation in  FIG. 11  to a curved orientation. The piston  428  has an engagement feature, which is a protrusion in the form of a single tooth  456  that has a surface  456 D that extends from a base to a tip transversely relative to the sole plate and in an opposite direction than the teeth  452 , and inclines in a rearward direction from a base to a tip. The surface  456 D interfaces with the forward-inclining faces  452 D of the teeth  452  to ratchet the piston  428  forward with repetitive dorsiflexion of the sole structure  410  as described with respect to the teeth  52 ,  56  of the sole structure  10  of  FIG. 1 . The tooth  456  extends from a rearmost one of the segments  428 I. In other embodiments, the piston  428  could have multiple teeth that engage with respective one of the teeth  452 . 
     The sole plate  412  has a bridge portion  432  underlying the foot support portion  419  of the sole plate  412 , and secured to the foot support portion  419  fore and aft of the opening  430 . When the arm  40  of the piston  428  is placed through the notches  42  of the opening  430 , the tooth  456  is engaged with a rearmost one  452 A of the teeth  452  and the body  429  extends through the opening  430 . The support arm  40  is supported on the bridge portion  432  and is trapped below the ground-facing surface of the sole plate  412  by the foot support portion  419 , as described with respect to the piston  28  of  FIG. 1 . 
     The bridge portion  432  and the opening  430  both curve between the longitudinal midline toward the lateral side  24  of the sole plate  412  twice between the rearward section  450 B and the forward section  450 A of the guide track  450 . The curves of the guide track  450  may be configured to correspond with a desired variation in bending stiffness in dorsiflexion and in transverse stiffness for a race having two curved portions, such as a 400 meter track race on an oval track. Repetitive dorsiflexion of the sole structure  410  will cause the piston  428  to ratchet forward along the teeth  452  of the sole plate  412  in a manner similar to that described with respect to teeth  52  and  56  in  FIGS. 6A-6D . Because the piston body  429  is articulated, the orientation of the arm  40  relative to the longitudinal midline LM will vary both in the longitudinal direction and in a transverse direction between the lateral side  24  and the medial side  22  as the piston  428  ratchets forward. For example, the piston  428  will move from a start position with the arm  40  generally below the notches  42  to a position in which the arm  40  corresponds with line  460 . The bridge portion  432  may have a recessed groove running generally along its center. The piston  428  may have a post  435  extending downward from the segment  428 A and engaged in the groove  433 . As the piston body  429  is ratcheted forward by the tooth  456  engaging the teeth  452 , the groove  433  guides the piston  428  via the post  435 . The bending stiffness increases in the longitudinal direction from the start to the position at line  460  due to the effect of the arm  40  on the bridge portion  432  as described with respect to  FIGS. 4B-4C . 
     Further repetitive dorsiflexion of the sole structure  410  causes the piston  428  to progress forward, with the piston body  429  winding along the guide track  450  until the arm  40  is at the position corresponding with line  462 . At this position, the arm  40  will extend under the foot support portion  419  more on the lateral side  24  than on the medial side  22 . Accordingly, bending that may occur along a transverse axis, such as when running around a curve on a curved track, will cause more stiffness at the lateral side  24  of the sole plate  412  than the medial side  22  of the sole plate  412 . 
     Further repetitive dorsiflexion of the sole structure  410  causes the piston  428  to progress forward, with the piston body  429  winding along the guide track  450  until the arm  40  is at the position corresponding with line  464 . At this position, the arm  40  will extend under the foot support portion  419  generally evenly on either side of the longitudinal midline LM. Bending stiffness with dorsiflexion will increase relative to the position at line  462 , and stiffness in bending along a transverse axis will decrease. The position at line  464  may best correlate with running along a straightaway following a curve. 
     Further repetitive dorsiflexion of the sole structure  410  causes the piston  428  to progress forward, with the piston body  429  winding along the guide track  450  until the arm  40  is at the position corresponding with line  466 . At this position, the arm  40  will extend under the foot support portion  419  more on the lateral side  24  than on the medial side  22 . Accordingly, bending that may occur along a transverse axis, such as when running around a curve on a curved track, will cause more stiffness at the lateral side  24  of the sole plate  412  than the medial side  22  of the sole plate  412 . 
     Further repetitive dorsiflexion of the sole structure  410  causes the piston  428  to progress forward, with the tooth  456  engaging with the teeth  452  of the guide track  450  to incrementally ratchet the piston  428  forward, with the piston body  429  winding along the guide track  450  until the arm  40  is at the position corresponding with line  468 . At this position, the arm  40  will extend under the foot support portion  419  generally evenly on either side of the longitudinal midline LM. Bending stiffness with dorsiflexion will increase relative to the position at line  466 , and stiffness in bending along a transverse axis will decrease. The position at line  468  may best correlate with running along a straightaway following a curve, and when relatively high bending stiffness with dorsiflexion is desired. For example, the position at line  468  may correlate with running a straightaway at the end of a 400 meter race. 
       FIGS. 12 and 13  show a sole structure  510  with an alternative embodiment of a piston  528 , a sole plate  512 , and a guide track  550 . The guide track  550  has teeth with a varied spacing. A first series of teeth  552 A at a first portion  582  of the guide track  550  have a relatively large first spacing  580 . A second series of teeth  552 B at a second portion  584  of the guide track are in line with the first series of teeth  552 A and have a second, relatively small spacing  586  (i.e., smaller than the first spacing  580 ). The spacing of the teeth is the distance along the guide track in the forward direction between tips of an adjacent pair of teeth. In the plan view of  FIG. 13 , the tips appear as lines. Only some of the teeth  552 A,  552 B are indicated with reference lines in  FIG. 13 . 
     The piston  528  includes a piston body  529 A,  529 B and the arm  40 . The piston body  529 A,  529 B includes a rear car  529 A and a front car  529 B. The rear car  529 A has an engagement feature that is a tooth  556 A which extends downward at a rear of the rear car  529 A. The tooth  556 A is configured to engage with the first series of teeth  552 A. The front car  529 B has an engagement feature that is a tooth  556 B which extends downward at a rear of the front car  529 B. The tooth  556 B is configured to engage with the second series of teeth  552 B. The sole plate  512  has an obstruction  588  that narrows the guide track  550  at a transition from the first series of teeth  552 A to the second series of teeth  552 B. The obstruction  588  is a pair of transversely-extending arms that extend at the foot-facing surface  20  above the recessed teeth  552 A,  552 B. The obstruction  588  blocks ratcheting of the rear car  529 A along the guide track  550  at a predetermined position between a start position and a final position of the piston body. 
     The rear car  529 A abuts the front car  529 B between the start position (i.e., the position shown in  FIG. 13 ) and a predetermined position such that the front car  529 B is moved by the rear car  529 A as the tooth  556 A of the rear car  529 A engages with the first series of teeth  552 A and is ratcheted along the guide track from the start position to the predetermined position with repetitive dorsiflexion of the sole structure  510 . The predetermined position is the position of the rear car  529 A when the forward ends  590  of the arms  572  abut the obstruction  588 . During this span of ratcheting, the tooth  556 B is too small to engage with the teeth  552 A due to the larger spacing  580  and the greater depth of the teeth  552 A, so it simply sets between adjacent teeth  552 A without necessarily contacting the teeth  552 A. 
     The rear car  529 A is generally U-shaped, with a back  570  and with two arms  572  that extend forward from the back  570 . The front car  529 B has an elongated rectangular forward portion  574  with a neck  576  extending rearward from the forward portion  574 . The neck  576  fits between the two arms  572 . The entire front car  529 B is narrower than the span between the obstructions  588 . 
     During ratcheting, the rear car  529 A abuts the front car  529 B at a rear of the neck  576  and at a rear of the forward portion  574 . The front car  529 B is moved by the rear car  529 A by this abutment as the rear car  529 A is ratcheted along the guide track  550  from the start position to the predetermined position. When the obstruction  588  prevents further forward ratcheting of the rear car  529 A, the front car  529 B has been moved to a position in which the tooth  556 B is engaged with a rearmost one  552 C of the teeth  552 B. Further repetitive dorsiflexion of the sole structure  510  will thus cause the tooth  556 B of the front car  529 B to ratchet the front car  529 B along the second portion  584  of the guide track  550 , free of the obstruction  588 . The front car  529 B will be ratcheted forward in this manner from the predetermined position to a final position in which the tooth  556 B is engaged with a forward-most tooth  552 D of the teeth  552 B. 
     Because the teeth  552 B have closer spacing that the teeth  552 A, the arm  40  will move forward in a direction along the longitudinal axis LM of the sole plate  512  a smaller distance per step between the predetermined position and the final position than the distance per step from the start position to the predetermined position. The larger spacing of teeth  552 A may correspond with an expected relatively large flex angle, such as at the start of a race, and the smaller spacing of the teeth  552 B may correspond with an expected relatively low flex angle, such as shortly after the start. Stiffness of the sole structure  510  is dependent upon the longitudinal position of the arm  40  between the bridge portion  32  and the foot supporting portion, as explained herein. Stiffness will thus vary at larger rate when the rear car  529 A is moving forward than when only the front car  529 B is moving forward. In other embodiments, the rear car  529 A could be any suitable shape to push the front car  529 B. For example, both the rear car and the front car could be rectangular, with the forward edge of the rear car abutting the rear edge of the front car. 
       FIGS. 14-16  show another embodiment of a sole structure  610  with an alternative embodiment of a piston  628 , a sole plate  612 , and a guide track  650 . The guide track  650  has teeth with a varied spacing. A first series of teeth  652 A at a first portion  682  of the guide track  650  have a relatively large first spacing  680 . The first series of teeth  652 A are split into two transversely spaced sets  652 AA,  652 AB, as best shown in  FIG. 16 . A second series of teeth  652 B at a second portion  684  of the guide track are forward of but transversely between the split first series of teeth  552 A and have a second, relatively small spacing  686  (i.e., smaller than the first spacing  580 ). Only some of the teeth  652 A,  652 B are indicated with reference lines in  FIG. 15 . 
     In this embodiment, no obstruction is required to stop ratcheting of the rear car  529 A. Because the teeth  656 B are not in line with the teeth  656 A, the rear car  529 A stops moving forward at the forward-most tooth  656 A, unlike in  FIG. 13  where further dorsiflexion could cause the rear car  529 A to ratchet along the front teeth  556 B if the obstruction  588  was not present. 
     The piston  628  is alike in all aspects as piston  528 , except that the tooth  556 A is replaced with a split tooth (i.e., two transversely-spaced teeth)  656 A,  656 B. Otherwise, like reference numbers are used to reference the features of piston  628  as shown and described with respect to piston  528 . 
     The rear car  529 A abuts the front car  529 B between the start position (i.e., the position shown in  FIG. 15 ) and a predetermined position such that the front car  529 B is moved by the rear car  529 A as the split tooth  656 A,  656 B engages with the two transversely spaced sets  652 AA,  652 AB. respectively, and is ratcheted along the guide track  650  from the start position to the predetermined position with repetitive dorsiflexion of the sole structure  610 . The predetermined position is the position of the rear car  529 A when the split tooth  656 A,  656 B is engaged with a forward-most one  657 A,  657 B of the teeth of the sets  652 AA,  652 BB. During this span of ratcheting, the tooth  556 B has no teeth to engage, and, because it does not extend downward as far as teeth  656 A,  656 B, it is simply carried along with the front car  529 B above the surface of the guide track  650  during ratcheting of the rear car  529 A during repetitive dorsiflexion. 
     When the split tooth  656 A,  656 B is engaged with teeth  657 A,  657 B, the front car  529 B has been moved sufficiently forward that the tooth  556 B is engaged with a rearmost tooth  652 C of the second series of teeth  652 B. Further repetitive dorsiflexion of the sole structure  610  will thus cause the tooth  556 B of the front car  529 B to ratchet the front car  529 B along the second portion  684  of the guide track  650 . The front car  529 B will be ratcheted forward in this manner from the predetermined position to a final position in which the tooth  556 B is engaged with a forward-most tooth  652 D of the teeth  652 B. 
     Because the teeth  652 B have closer spacing that the teeth  652 A, the arm  40  will move forward in a direction along the longitudinal axis LM of the sole plate  12  at a smaller distance per step between the predetermined position and the final position than the distance per step from the start position to the predetermined position. Stiffness of the sole structure  610  is dependent upon the longitudinal position of the arm  40  between the bridge portion  32  and the foot support portion  19 , as explained herein. Stiffness will thus vary at larger rate when the rear car  529 A is moving forward than when only the front car  529 B is moving forward. 
     While several modes for carrying out the many aspects of the present teachings have been described in detail, those familiar with the art to which these teachings relate will recognize various alternative aspects for practicing the present teachings that are within the scope of the appended claims. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not as limiting.