Patent Publication Number: US-8991072-B2

Title: Fluid-filled chamber incorporating a flexible plate

Description:
BACKGROUND 
     Articles of footwear generally include two primary elements, an upper and a sole structure. The upper is formed from a variety of material elements (e.g., textiles, foam, leather, and synthetic leather) that are stitched or adhesively bonded together to form a void on the interior of the footwear for comfortably and securely receiving a foot. An ankle opening through the material elements provides access to the void, thereby facilitating entry and removal of the foot from the void. In addition, a lace is utilized to modify the dimensions of the void and secure the foot within the void. 
     The sole structure is located adjacent to a lower portion of the upper and is generally positioned between the foot and the ground. In many articles of footwear, including athletic footwear, the sole structure generally incorporates an insole, a midsole, and an outsole. The insole, which may be located within the void and adjacent to a lower surface of the void, is a thin compressible member that enhances footwear comfort. The midsole, which may be secured to a lower surface of the upper and extends downward from the upper, forms a middle layer of the sole structure. In addition to attenuating ground reaction forces (i.e., providing cushioning for the foot), the midsole may limit foot motions or impart stability, for example. The outsole, which may be secured to a lower surface of the midsole, forms the ground-contacting portion of the footwear and is usually fashioned from a durable and wear-resistant material that includes texturing to improve traction. 
     Generally, the midsole is primarily formed from a foamed polymer material, such as polyurethane or ethylvinylacetate, that extends throughout a length and width of the footwear. In some articles of footwear, the midsole may include a variety of additional footwear elements that enhance the comfort or performance of the footwear, including plates, moderators, fluid-filled chambers, lasting elements, or motion control members. In some configurations, any of these additional footwear elements may be located between the midsole and either of the upper and outsole, embedded within the midsole, or encapsulated by the foamed polymer material of the midsole, for example. Although many midsoles are primarily formed from a foamed polymer material, fluid-filled chambers or other non-foam structures may form a majority of some midsole configurations. 
     Various techniques may be utilized to form fluid-filled chambers for articles of footwear or other products, including a two-film technique, a thermoforming technique, and a blowmolding technique, for example. In the two-film technique, two separate polymer sheets are bonded together at specific locations. The thermoforming technique is similar to the two-film technique in that two polymer sheets are bonded together, but also includes utilizing a heated mold to form or otherwise shape the polymer sheets. In the blow-molding technique, a parison formed from a molten or otherwise softened polymer material is placed within a mold having a cavity with the desired configuration of the chamber. Pressurized air induces the polymer material to conform to surfaces of the cavity. The polymer material then cools and retains the shape of the cavity, thereby forming the chamber. 
     Following each of the techniques discussed above, the chambers are pressurized. That is, a pressurized fluid is injected into the chambers and then sealed within the chambers. One method of pressurization involves forming inflation conduits in residual portions of the polymer sheets or the parison. In order to pressurize the chambers, the fluid is injected through the inflation conduits, which are then sealed. The residual portions of the polymer sheets or the parison, including the inflation conduits, are then trimmed or otherwise removed to substantially complete manufacture of the chambers. 
     SUMMARY 
     Various features of a fluid-filled chamber, which may be incorporated into articles of footwear and other products, are disclosed below. In one configuration, a fluid-filled structure comprises a chamber and a plate. The chamber has an exterior surface and an interior surface located opposite the exterior surface. At least a portion of the interior surface defines a cavity within the chamber that receives a fluid. The plate has a first surface secured to the chamber and a second surface located opposite the first surface. At least the second surface defines a flexion area extending across at least a portion of the plate. The second surface also defines a flexion stop located adjacent to the flexion area. 
     In another configuration, a fluid-filled structure comprises a chamber and a plate. The chamber has an exterior surface and an interior surface located opposite the exterior surface. At least a portion of the interior surface defines a cavity within the chamber that receives a fluid. The plate is located within the cavity. The plate has a first surface secured to the chamber and a second surface located opposite the first surface. The second surface defines an indentation extending into and across at least a portion of the plate. The second surface also defines edge areas located immediately adjacent to the indentation and on opposite sides of the indentation. At least a portion of the edge areas has an interlocking configuration. 
     In a further configuration, an article of footwear has an upper and a sole structure secured to the upper. The sole structure comprises a chamber and a plate. The chamber has an exterior surface and an interior surface located opposite the exterior surface. At least a portion of the interior surface defines a cavity within the chamber that receives a fluid. The plate is located within the cavity. The plate has a first surface secured to the interior of the chamber and a second surface located opposite the first surface. The second surface defines an indentation extending into and across at least a portion of the plate. 
     In yet another configuration, an article of footwear has an upper and a sole structure secured to the upper. The sole structure comprises a chamber and a plate. The chamber defines an interior cavity that receives a pressurized fluid. The chamber extends from a heel region of the footwear to a forefoot region of the footwear and from a lateral side of the footwear to a medial side of the footwear. The chamber has an upper portion, a lower portion, and a sidewall portion. The upper portion forms an upper surface of the chamber and is positioned adjacent to the upper. The lower portion forms an opposite lower surface of the chamber and is positioned adjacent to a ground-contacting surface of the sole structure. The sidewall portion extends between the upper portion and the lower portion to form a sidewall of the chamber. The plate is secured to the upper portion of the chamber. The plate extends from the heel region to the forefoot region and from the lateral side to the medial side. The plate has a first area and a second area located on opposite sides of a flexion line. The first area rotates relative to the second area about the flexion line. 
     The advantages and features of novelty characterizing aspects of the invention are pointed out with particularity in the appended claims. To gain an improved understanding of the advantages and features of novelty, however, reference may be made to the following descriptive matter and accompanying figures that describe and illustrate various configurations and concepts related to the invention. 
    
    
     
       FIGURE DESCRIPTIONS 
       The foregoing Summary and the following Detailed Description will be better understood when read in conjunction with the accompanying figures. 
         FIG. 1  is a lateral side elevational view of an article of footwear. 
         FIG. 2  is a medial side elevational view of the article of footwear. 
         FIG. 3  is a perspective view of a chamber from the article of footwear. 
         FIG. 4  is an exploded perspective view of the chamber. 
         FIG. 5  is a top plan view of the chamber. 
         FIGS. 6A-6D  are cross-sectional views of the chamber, as defined by section lines  6 A- 6 D in  FIG. 5 . 
         FIG. 7  is a lateral side elevational view of the chamber. 
         FIG. 8  is a medial side elevational view of the chamber. 
         FIG. 9  is a perspective view of an upper plate of the chamber. 
         FIG. 10  is a top plan view of the upper plate. 
         FIG. 11  is a lateral side elevational view of the upper plate. 
         FIG. 12  is a medial side elevational view of the upper plate. 
         FIG. 13  is a bottom plan view of the upper plate. 
         FIG. 14  is a cross-sectional view of the upper plate, as defined by section line  14 - 14  in  FIG. 13 . 
         FIG. 15  is a perspective view of a mold for forming the chamber. 
         FIGS. 16A-16D  are side elevational views depicting steps in a process of manufacturing the chamber. 
         FIGS. 17A-17D  are cross-sectional views depicting steps in the process of manufacturing the chamber, as defined by section lines  17 A- 17 A through  17 D- 17 D in  FIGS. 16A-16D . 
         FIGS. 18A-18G  are cross-sectional views corresponding with  FIG. 14  and depicting further configurations of the upper plate. 
         FIGS. 19A-19C  are partial bottom plan views of further configurations of the upper plate. 
         FIG. 20  is a perspective view of a portion of a further configuration of the upper plate. 
         FIGS. 21A-21C  are bottom plan views of further configurations of the upper plate. 
         FIGS. 22A-22C  are bottom plan views of further configurations of the upper plate. 
         FIG. 23  is a bottom plan view of a further configuration of the upper plate. 
         FIGS. 24A-24F  are cross sectional views corresponding with  FIG. 6B  and depicting further configurations of the chamber. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion and accompanying figures disclose various configurations of fluid-filled chambers and methods for manufacturing the chambers. Although the chambers are disclosed with reference to footwear having a configuration that is suitable for running, concepts associated with the chambers may be applied to a wide range of athletic footwear styles, including basketball shoes, cross-training shoes, football shoes, golf shoes, hiking shoes and boots, ski and snowboarding boots, soccer shoes, tennis shoes, and walking shoes, for example. Concepts associated with the chambers may also be utilized with footwear styles that are generally considered to be non-athletic, including dress shoes, loafers, and sandals. In addition to footwear, the chambers may be incorporated into other types of apparel and athletic equipment, including helmets, gloves, and protective padding for sports such as football and hockey. Similar chambers may also be incorporated into cushions and other compressible structures utilized in household goods and industrial products. Accordingly, chambers incorporating the concepts disclosed herein may be utilized with a variety of products. 
     General Footwear Structure 
     An article of footwear  10  is depicted in  FIGS. 1 and 2  as including an upper  20  and a sole structure  30 . For reference purposes, footwear  10  may be divided into three general regions: a forefoot region  11 , a midfoot region  12 , and a heel region  13 , as shown in  FIGS. 1 and 2 . Footwear  10  also includes a lateral side  14  and a medial side  15 . Forefoot region  11  generally includes portions of footwear  10  corresponding with the toes and the joints connecting the metatarsals with the phalanges. Midfoot region  12  generally includes portions of footwear  10  corresponding with the arch area of the foot. Heel region  13  generally includes portions of footwear  10  corresponding with rear portions of the foot, including the calcaneus bone. Lateral side  14  and medial side  15  extend through each of regions  11 - 13  and correspond with opposite sides of footwear  10 . Regions  11 - 13  and sides  14 - 15  are not intended to demarcate precise areas of footwear  10 . Rather, regions  11 - 13  and sides  14 - 15  are intended to represent general areas of footwear  10  to aid in the following discussion. In addition to footwear  10 , regions  11 - 13  and sides  14 - 15  may also be discussed with respect to the individual elements thereof, such as upper  20  and sole structure  30 , and to the foot itself. 
     Upper  20  is depicted as having a substantially conventional configuration incorporating a variety of material elements (e.g., textile, foam, leather, and synthetic leather) that are stitched or adhesively bonded together to form an interior void for securely and comfortably receiving a foot. The material elements may be selected and located with respect to upper  20  in order to selectively impart properties of durability, air-permeability, wear-resistance, flexibility, and comfort, for example. An ankle opening  21  in heel region  13  provides access to the interior void. In addition, upper  20  may include a lace  22  that is utilized in a conventional manner to modify the dimensions of the interior void, thereby securing the foot within the interior void and facilitating entry and removal of the foot from the interior void. Lace  22  may extend through apertures in upper  20 , and a tongue portion of upper  20  may extend between the interior void and lace  22 . Given that various aspects of the present application primarily relate to sole structure  30 , upper  20  may exhibit the general configuration discussed above or the general configuration of practically any other conventional or nonconventional upper. Accordingly, the overall structure of upper  20  may vary significantly. 
     Sole structure  30  is secured to upper  20  and has a configuration that extends between upper  20  and the ground. In effect, therefore, sole structure  30  is located to extend between the foot and the ground. In addition to attenuating ground reaction forces (i.e., providing cushioning for the foot), sole structure  30  may provide traction, impart stability, and limit various foot motions, such as pronation. 
     The primary elements of sole structure  30  are a midsole  31  and an outsole  32 . Midsole  31  may include a fluid-filled chamber  33 . In addition to chamber  33 , midsole  31  may incorporate one or more additional footwear elements that enhance the comfort, performance, or ground reaction force attenuation properties of footwear  10 , including a polymer foam material, such as polyurethane or ethylvinylacetate, plates, moderators, lasting elements, or motion control members. Outsole  32 , which may be absent in some configurations of footwear  10 , is secured to a lower surface of midsole  31  and may be formed from a rubber material that provides a durable and wear-resistant surface for engaging the ground. In addition, outsole  32  may also be textured to enhance the traction (i.e., friction) properties between footwear  10  and the ground. Sole structure  30  may also incorporate an insole or sockliner that is located with in the void in upper  20  and adjacent (i.e., located nearby or close to, although not necessarily in contact with) a plantar surface or lower surface of the foot to enhance the comfort of footwear  10 . 
     In some embodiments of sole structure  30 , the location of fluid-filled chamber  33  may be restricted to one or more particular regions of footwear  10 , such as forefoot region  11  or heel region  13 . In some embodiments of sole structure  30 , midsole  31  may include a fluid-filled chamber  33  but not include a polymer foam material. In such embodiments, an upper portion of fluid-filled chamber  33  may be positioned immediately adjacent to upper  20 , and a lower portion of fluid-filled chamber  33  may be positioned immediately adjacent to outsole  32 . 
     Chamber Configuration 
     Chamber  33  is depicted individually in  FIGS. 3-8  as having a configuration that is suitable for footwear applications. When incorporated into footwear  10 , chamber  33  has a shape that fits within a perimeter of midsole  31  and substantially extends from forefoot region  11  to heel region  13  and also from lateral side  14  to medial side  15 , thereby corresponding with a general outline of the foot. Chamber  33  has an upper portion positioned adjacent to upper  20 , a lower portion positioned adjacent to outsole  32 , and a sidewall portion extending between the upper portion and the lower portion. Although chamber  33  is depicted as forming a sidewall of midsole  31 , a polymer foam material of midsole  31  may form a portion of the sidewall in some configurations of footwear  10 . When the foot is located within upper  20 , chamber  33  extends under substantially all of the foot in order to attenuate ground reaction forces that are generated when sole structure  30  is compressed between the foot and the ground during various ambulatory activities, such as running and walking. In other configurations, chamber  33  may extend under only a portion of the foot. 
     The primary elements of chamber  33  are a barrier  40 , an upper plate  51 , and a supplemental lower plate  52 . Barrier  40  forms an exterior of chamber  33  and (a) defines an interior void that receives a pressurized fluid, upper plate  51 , and lower plate  52 , and (b) provides a durable sealed barrier for retaining the pressurized fluid within chamber  33 . The polymer material of barrier  40  includes an upper barrier portion  41 , an opposite lower barrier portion  42 , and a sidewall barrier portion  43  that extends around a periphery of chamber  33  and between barrier portions  41  and  42 . Upper plate  51  and lower plate  52  are located within the interior void, upper plate  51  being secured to an interior surface of upper barrier portion  41 , and lower plate  52  being secured to an interior surface of lower barrier portion  42  opposite upper plate  51 . Upper plate  51  and lower plate  52  may be secured to barrier  40  by, for example, adhesive bonding or thermobonding. Upper plate  51  has indentations  55 . 
     In manufacturing chamber  33 , a pair of polymer sheets may be molded and bonded during a thermoforming process to define barrier portions  41 - 43 . More particularly, the thermoforming process (a) imparts shape to one of the polymer sheets in order to form upper barrier portion  41  (b) imparts shape to the other of the polymer sheet in order to form lower barrier portion  42  and sidewall portion  43 , and (c) forms a peripheral bond  44  that joins a periphery of each of the polymer sheets and extends around a top edge of sidewall barrier portion  43 . The thermoforming process may also (a) position upper plate  51  and lower plate  52  within chamber  33  and (b) bond upper plate  51  and lower plate  52  to upper barrier portion  41  and lower barrier portion  42 , respectively. Although substantially all of the thermoforming process may be performed with a mold, as described in greater detail below, each of the various parts of the process may be performed separately in forming chamber  33 . 
     In some embodiments, such as embodiments in which chamber  33  may have a low profile, barrier portion  43  may have a less vertical configuration, or a more rounded or tapered configuration. Additionally, in some embodiments, peripheral bond  44  may extend instead around another length of sidewall barrier portion  43 , such as a mid-section of sidewall barrier portion  43 . 
     Following the thermoforming process, a fluid may be injected into the interior void and pressurized. Peripheral bond  44  joins the polymer sheets to form a seal that prevents the fluid from escaping. Accordingly, the pressurized fluid exerts an outward force upon chamber  33 , which tends to separate barrier portions  41  and  42 . In the absence of upper plate  51  and lower plate  52 , barrier portions  41  and  42  may extend, distend, or otherwise bulge outward to impart a rounded or even cylindrical aspect to chamber  33 . Plates  51  and  52 , however, restrict outward expansion of barrier portions  41  and  42 , which imparts a generally planar configuration to upper and lower surfaces of chamber  33 . In some configurations, the combination of upper barrier portion  41  and upper plate  51 , or the combination of lower barrier portion  42  and lower plate  52 , restrict outward expansion of barrier portion  41  or  42  and impart a generally planar configuration to upper or lower surfaces of chamber  33 . More particularly, upper plate  51 , which is bonded to upper barrier portion  41 , restricts the outward expansion (i.e., the upward expansion) of upper barrier portion  41 . Similarly, lower plate  52 , which is bonded to lower barrier portion  42 , restricts the outward expansion (i.e., the downward expansion) of lower barrier portion  42 . As discussed in greater detail below, however, indentations  55  in a lower side of upper plate  51  may allow upper plate  51  to deflect inward and toward a central area of the cavity within chamber  33 . Moreover, indentations  55  have a structure that resists the outward expansion or distension of chamber  33 , while accommodating inward flexing or compression. An advantage of indentations  55  is, therefore, that chamber  33  may deflect inward to enhance the attenuation of ground reaction forces (i.e., impart cushioning) of footwear  10 , while retaining an intended shape of chamber  33 . 
     Chamber  33  is shaped to provide a structure that is suitable for footwear applications. As noted above, chamber  33  has a shape that fits within a perimeter of midsole  31  and extends under substantially all of the foot, thereby corresponding with a general outline of the foot. With reference to  FIGS. 6D-8 , chamber  33  exhibits a tapered configuration between heel region  13  and forefoot region  11 . That is, the portion of chamber  33  in heel region  13  exhibits a greater overall thickness than the portion of chamber  33  in forefoot region  11 . When incorporated into footwear  10 , the tapering of chamber  33  ensures that the heel of the foot is slightly raised in relation to the forefoot. 
     The fluid within chamber  33  may be pressurized between zero and three hundred fifty kilopascals (i.e., approximately fifty-one pounds per square inch) or more. In addition to air and nitrogen, the fluid may include any of the gasses disclosed in U.S. Pat. No. 4,340,626 to Rudy. In some configurations, chamber  33  may incorporate a valve or other structure that permits the individual to adjust the pressure of the fluid. 
     A wide range of polymer materials may be utilized for chamber  33 . In selecting materials for barrier  40 , engineering properties of the material (e.g., tensile strength, stretch properties, fatigue characteristics, dynamic modulus, and loss tangent) as well as the ability of the material to prevent the diffusion of the fluid contained by barrier  40  may be considered. When formed of thermoplastic urethane, for example, barrier  40  may have a thickness of approximately 1.0 millimeter, but the thickness may range from 0.25 to 2.0 millimeters or more, for example. In addition to thermoplastic urethane, examples of polymer materials that may be suitable for chamber  33  include polyurethane, polyester, polyester polyurethane, and polyether polyurethane. Barrier  40  may also be formed from a material that includes alternating layers of thermoplastic polyurethane and ethylene-vinyl alcohol copolymer, as disclosed in U.S. Pat. Nos. 5,713,141 and 5,952,065 to Mitchell, et al. A variation upon this material may also be utilized, wherein a center layer is formed of ethylene-vinyl alcohol copolymer, layers adjacent to the center layer are formed of thermoplastic polyurethane, and outer layers are formed of a regrind material of thermoplastic polyurethane and ethylene-vinyl alcohol copolymer. Another suitable material for barrier  40  is a flexible microlayer membrane that includes alternating layers of a gas barrier material and an elastomeric material, as disclosed in U.S. Pat. Nos. 6,082,025 and 6,127,026 to Bonk, et al. Additional suitable materials are disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Rudy. Further suitable materials include thermoplastic films containing a crystalline material, as disclosed in U.S. Pat. Nos. 4,936,029 and 5,042,176 to Rudy, and polyurethane including a polyester polyol, as disclosed in U.S. Pat. Nos. 6,013,340; 6,203,868; and 6,321,465 to Bonk, et al. 
     In order to facilitate bonding between barrier  40  and plates  51  and  52 , plates  51  and  52  may be formed of, or formed to include, polymer bonding materials (e.g., thermoplastic polymer materials, adhesives, heat-activated bonding agents). When heated, the polymer bonding materials soften, melt, or otherwise begin to change state so that contact with barrier portions  41  and  42  induces material from each of barrier  40  and the polymer bonding materials to intermingle or otherwise join with each other. Upon cooling, therefore, the polymer bonding materials are permanently joined with barrier  40 , thereby joining plates  51  and  52  with barrier  40 . In some configurations, an adhesive may be utilized to secure barrier  40  to plates  51  and  52 . 
     In some embodiments, either upper barrier portion  41  may be co-molded with upper plate  51 , or lower barrier portion  42  may be co-molded with lower plate  52 . In such embodiments, plate  51  or  52  may not be physically separate from barrier portion  41  or  42 , but may instead be physically formed together with or at the same time as barrier portion  41  or  42 . In other words, upper plate  51  may be part of, or may be integrally formed with, upper barrier portion  41 . Similarly, lower plate  52  may be part of, or may be integrally formed with, lower barrier portion  42 . 
     Upper Plate Configuration 
     An initial configuration of upper plate  51 , depicted individually in  FIGS. 9-14 , includes indentations  55 . Upper plate  51  is a generally planar structure being substantially similar in shape to, but slightly smaller than, chamber  33 . As a result, upper plate  51 , like chamber  33 , has a shape corresponding with a general outline of the foot. Indentations  55  are substantially linear and extend across upper plate  51 , between lateral side  14  of upper plate  51  and medial side  15  of upper plate  51 . Indentations  55  are located in forefoot region  11  and midfoot region  12 , on both the forefoot side and the heel side of portions of upper plate  51  corresponding with the joints connecting the metatarsals of the foot with the phalanges of the foot. Indentations  55  are formed on the lower surface or inner side (i.e., the inward-facing side) of upper plate  51 . 
     In the initial configuration, edge areas  56  are substantially linear and are adjacent to indentations  55  on the lower surface or inner side of upper plate  51 . Edge areas are portions of upper plate  51 , including portions of surfaces of upper plate  51 , that are adjacent to and on opposite sides of indentations  55 . 
     Indentations  55  may be flexion areas defined in a surface of upper plate  51 . Upper plate  51  may be significantly more flexible or bendable at flexion areas than away from flexion areas. Additionally, edge areas  56  may be flexion stops defined in a surface of upper plate  51  and located adjacent to indentations  55 . A flexing or bending in one direction of upper plate  51  at a flexion area may be restricted, blocked, or otherwise hindered by a corresponding flexion stop. For example, in the initial configuration, a flexing or bending in one direction of upper plate  51  at indentations  55  may be restricted, blocked, or otherwise hindered by edge areas  56 , whereas a flexing or bending in the opposite direction of upper plate  51  at indentations  55  may be unobstructed. 
     Indentations  55  may be living hinges. Upper plate  51 , or portions of upper plate  51  including indentations  55 , may incorporate a polymer, including thermoplastic polymers such as polyethylene and polypropylene. Upper plate  51 , measured at a living hinge in upper plate  51 , may be thin relative to portions of upper plate  51  apart from a living hinge in upper plate  51 . Upper plate  51  may be compressed at a living hinge, or made thinner than as molded, through a pressuring or coining process. Upper plate  51  may be capable of flexing or bending about a living hinge a great number of times before losing structural integrity. For example, indentations  55  formed to be living hinges may be capable of bending throughout the expected life of footwear  10  without failure or damage. 
     Indentations  55  may be defined in a surface of upper plate  51  to extend into a portion of upper plate  51 . That is, indentations  55  may be cuts, grooves, scores, or other depressions, molded or otherwise formed in upper plate  51 . Edge areas may be defined in a surface of upper plate  51  and may be located immediately adjacent to and on opposite sides of indentations  55 . That is, edge areas  56  may be areas of upper plate  51  adjacent to and on opposite sides of cuts, grooves, scores, or other depressions, molded or otherwise formed in upper plate  51 . 
     A thickness of upper plate  51  in an area spaced from indentations  55  may be greater than a thickness of upper plate  51  within indentations  55 . For example, in the initial configuration, a first thickness of upper plate  51  at an area spaced away from indentations  55  is at least twice as great as a second thickness of upper plate  51  measured within indentations  55 . Referring specifically to  FIG. 14 , for example, a first thickness X is spaced from indentation  55  and is greater than a second thickness Y within indentation  55 . Additionally, in the initial configuration, the thickness of upper plate  51  is substantially uniform outside of indentations  55 , even at areas adjacent to indentations  55 . Accordingly, the thickness of upper plate  51  measured at edge areas  56  is also at least two times the thickness of upper plate  51  measured within indentations  55 . 
     Indentations  55  may be cuts, grooves, scores, or other depressions whose width is narrow relative to other dimensions associated with indentations  55  and edge areas  56 . For example, in the initial configuration, a width of indentations  55  is less than such dimensions as the thickness of upper plate  51  in edge areas  56 , the thickness of upper plate  51  measured at indentations  55 , and the difference between those two thicknesses. 
     Indentations  55  may be flexion lines. Upper plate  51  may flex or bend along a flexion line under application of a flexing force in preference to flexing along another portion of upper plate  51  away from the flexion line. This preferential flexing may be due to structural differences between the flexion line of upper plate  51  and other portions of upper plate  51 , such as differences of size or dimension, or differences of material composition, or differences of treatment or processing of the constituent material or materials. For example, upper plate  51  may have a lesser thickness along a flexion line than another portion of upper plate  51  away from the flexion line. Under an applied flexing force, one area of upper plate  51  may rotate about the flexion line relative to another area of upper plate  51  located on an opposite side of the flexion line. For example, in the initial configuration, a first area  52  of upper plate  51  and a second area  53  of upper plate  52 , located on opposite sides of an indentation  55 , may rotate about the indentation  55  under application of a flexing force between first area  52  and second area  53 . 
     A portion of upper plate  51  immediately adjacent to indentations  55  may obstruct, hinder, or restrict the rotation of one area of upper plate  51  about a flexion line relative to another area of upper plate  51  located on an opposite side of the flexion line. Rotation may be restricted under application of a flexing force in one direction, but not restricted under application of a flexing force in another direction, as depicted in  FIGS. 24E and 24F . For example, in the initial configuration, edge areas  56  restrict the rotation of first area  52  relative to second area  53  about indentation  55  in one direction, but do not restrict the rotation of first area  52  relative to second area  53  about indentation  55  in another direction. More specifically, edge areas  56  restrict rotation about indentations  55  under an upward flexing, but do not restrict rotation about indentations  55  under a downward flexing force. 
     The material of upper plate  51  or lower plate  52  may have a different modulus of elasticity (i.e., stiffness) than the material of barrier portions  41  and  42 . In some configurations, for example, barrier portions  41  and  42  may be formed of a first polymer material having a first stiffness, and plates  51  and  52  may be formed of a second polymer material having a second stiffness, and the first stiffness may be less than the second stiffness. That is, the material of barrier portions  41  and  412  may be less stiff than the material of plates  51  and  52 . 
     Manufacturing Process 
     Although a variety of manufacturing processes may be utilized to form chamber  33 , an example of a suitable thermoforming process will now be discussed. With reference to  FIG. 15 , a mold  60  that may be utilized in the thermoforming process is depicted as including an upper mold portion  61  and a lower mold portion  62 . Mold  60  is utilized to form chamber  33  from a pair of polymer sheets that are molded and bonded to define surfaces  41 - 43 , and the thermoforming process secures upper plate  51  and lower plate  52  to barrier  40 . More particularly, mold  60  (a) imparts shape to one of the polymer sheets in order to form upper barrier portion  41  (b) imparts shape to the other of the polymer sheets in order to form lower barrier portion  42  and sidewall barrier portion  43 , and (c) forms a peripheral bond  44  that joins a periphery of each of the polymer sheets and extends around a top edge of sidewall barrier portion  43 . Mold  60  also respectively bonds upper plate  51  and lower plate  52  to barrier portions  41  and  42 . 
     A securing structure may establish a positional relationship between upper plate  51  and lower plate  52  during various portions of the manufacturing process. In some configurations, one or more upper alignment members  57  of upper plate  51  may be secured to one or more corresponding lower alignment members  58  of lower plate  52 . In some configurations, upper alignment members  57  may be indentations in upper plate  51 , and lower alignment members  58  may be protrusions of lower plate  52 . In other configurations, upper alignment members  57  may protrude from upper plate  51 , and lower alignment members  48  may be indentations within lower plate  52 . Alternatively, each of alignment members  57  and  58  may include any mix or hybrid arrangement of indentations and protrusions capable of fitting against the corresponding alignment member. In other configurations, upper plate  51  and lower plate  52  may be connected by a temporary securing structure, in which one or more upper alignment members  57  of upper plate  51  are bonded to or are otherwise formed to be part of the same structure as one or more lower alignment members  58  of lower plate  52 . The securing of upper alignment members  57  against lower alignment members  58  may operate to establish a positional relationship between upper plate  51  and lower plate  52 . The securing may also operate to maintain that positional relationship for various subsequent portions of the manufacturing process. 
     In manufacturing chamber  33 , one or more of an upper polymer layer  71 , a lower polymer layer  72 , and plates  51  and  52  are heated to a temperature that facilitates bonding between the components. As discussed in greater detail below, polymer layers  71  and  72  respectively become barrier portions  41  and  42  during the manufacturing of chamber  33 . Depending upon the specific materials utilized for plates  51  and  52  and polymer layers  71  and  72 , which form barrier  40 , suitable temperatures may range from 120 to 200 degrees Celsius (248 to 392 degrees Fahrenheit) or more. As an example, a material having alternating layers of thermoplastic polyurethane and ethylene-vinyl alcohol copolymer may be heated to a temperature in a range of 149 to 188 degrees Celsius (300 and 370 degrees Fahrenheit) to facilitate bonding. Various radiant heaters or other devices may be utilized to heat the components of chamber  33 . In some manufacturing processes, mold  60  may be heated such that contact between mold  60  and the components of chamber  33  raises the temperature of the components to a level that facilitates bonding. 
     Following heating, the components of chamber  33  (i.e., polymer layers  71  and  72  and plates  51  and  52 ) are located between mold portions  61  and  62 , as depicted in  FIGS. 16A and 17A . Once positioned, mold portions  61  and  62  translate toward each other and begin to close upon the components such that (a) upper mold portion  61  contacts upper polymer layer  71 , (b) a ridge  64  of lower mold portion  62  contacts lower polymer layer  72 , and (c) polymer layers  71  and  72  begin bending to extend into a cavity within mold  60 , as depicted in  FIGS. 16B and 17B . Accordingly, the components are located relative to mold  60  and initial shaping and positioning has occurred. 
     In alternate embodiments, upper plate  51 , lower plate  52 , or both may be positioned adjacent to an exterior surface of chamber  33 . In some alternate embodiments, upper plate  51  may be placed between mold portion  61  and polymer layer  71 . For example, upper plate  51  may be placed adjacent to a surface of mold portion  61 , and may be positioned with respect to mold portion  61  for the process of forming chamber  33 . In other alternate embodiments, lower plate  52  may be placed between mold portion  62  and polymer layer  72 . For example, lower plate  52  may be placed within mold portion  62 , and may be positioned with respect to mold portion  62  for the process of forming chamber  33 . In such alternate embodiments, polymer layers  71  and  72  may then be drawn into the contours of mold  60  such that at least one of polymer layers  71  and  72  contacts and is bonded to at least one of plates  51  and  52 . A variety of techniques may be utilized to secure plates  51  and  52  to mold portions  61  and  62 , including a vacuum system, various seals, or non-permanent adhesive elements, for example. In addition, plates  51  and  52  may include various tabs that define apertures, and mold portions  61  and  62  may include protrusions that engage the apertures to secure plates  51  and  52  with respect to mold portions  61  and  62 . 
     Subsequently, air may be partially evacuated from the area around polymer layers  71  and  72  through various vacuum ports in mold portions  61  and  62 . The purpose of evacuating the air is to draw polymer layers  71  and  72  into contact with the various contours of mold  60 . This ensures that polymer layers  71  and  72  are properly shaped in accordance with the contours of mold  60 . Note that polymer layers  71  and  72  may stretch in order to extend around plates  51  and  52  and into mold  60 . In comparison with the thickness of barrier  40  in chamber  33 , polymer layers  71  and  72  may exhibit greater thickness. This difference between the original thicknesses of polymer layers  71  and  72  and the resulting thickness of barrier  40  may occur as a result of the stretching that occurs during this stage of the thermoforming process. Moreover, given that lower polymer layer  72  may stretch to a greater degree than upper polymer layer  71  during the manufacturing process, lower polymer layer  72  may have a greater initial thickness than upper polymer layer  71  in order to equalize the resulting thicknesses of barrier portions  41  and  42  in the finished chamber  33 . 
     In order to provide a second means for drawing polymer layers  71  and  72  into contact with the various contours of mold  60 , the area between polymer layers  71  and  72  may be pressurized. During a preparatory stage of this method, an injection needle may be located between polymer layers  71  and  72 , and the injection needle may be located such that upper mold portion  61  and ridge  64  envelop the injection needle when mold  60  closes. A gas may then be ejected from the injection needle such that polymer layers  71  and  72  engage upper mold portion  61  and ridge  64 , thereby forming an inflation conduit  73  (see  FIG. 16C ) between polymer layers  71  and  72 . The gas may then pass through inflation conduit  73 , thereby entering and pressurizing the area between polymer layers  71  and  72 . In combination with the vacuum, the internal pressure ensures that polymer layers  71  and  72  contact the various surfaces of mold  60 . 
     As mold  60  closes further, ridge  64  bonds upper polymer layer  71  to lower polymer layer  72 , as depicted in  FIGS. 16B and 17B , thereby forming peripheral bond  44 . In addition, a movable insert  65  that is supported by various springs  66  (as depicted in  FIG. 17A ) may depress to place a specific degree of pressure upon the components, thereby bonding polymer layers  71  and  72  to upper plate  51  and lower plate  52 . Depressions or channels may be formed near the outside perimeter of movable insert  65  to impart a configuration to sidewall barrier portion  43 . As discussed above, plates  51  and  52  may be formed to include polymer bonding materials in order to facilitate bonding between plates  51  and  52  and barrier  40 . The pressure exerted upon the components by insert  65  ensures that the polymer bonding materials form a bond with polymer layers  71  and  72 . Furthermore, portions of ridge  64  that extend away from plates  51  and  52  form a bond between other areas of polymer layers  71  and  72  to form inflation conduit  73 . 
     When bonding is complete, mold  60  is opened and chamber  33  and polymer layers  71  and  72  are removed and permitted to cool, as depicted in  FIGS. 16C and 17C . A fluid may then be injected from pressure source  80  into chamber  33  through inflation conduit  73 . As the fluid is injected from pressure source  80 , the securing structure comprising upper alignment member  57  and lower alignment member  58  may be separated or broken, and upper plate  51  and lower plate  52  may accordingly separate from each other. By controlling (a) the configuration of mold  60 , (b) an initial positional relationship between upper plate  51  and lower plate  52 , and (c) the bonding of plates  51  and  52  to polymer layers  71  and  72 , the establishment of a positional relationship between upper plate  51  and lower plate  52  within chamber  33  subsequent to pressurization may be facilitated. Subsequently, a sealing process may be utilized to seal inflation conduit  73  adjacent to chamber  33  after pressurization. 
     When pressurization is complete, excess portions of polymer layers  71  and  72  are then removed, thereby completing the manufacture of chamber  33 , as depicted in  FIGS. 16D and 17D . As an alternative, the order of inflation and removal of excess material may be reversed. As a final step in the process, chamber  33  may be tested and then incorporated into midsole  31  of footwear  10 . 
     Based upon the above discussion, mold  60  is utilized to (a) impart shape to upper polymer layer  71  in order to form upper barrier portion  41  (b) impart shape to lower polymer layer  72  in order to form lower barrier portion  42  and sidewall barrier portion  43 , and (c) form peripheral bond  44  between polymer layers  71  and  72 . Mold  60  also (a) bonds upper plate  51  to the portion of upper polymer layer  71  that forms upper barrier portion  41  and (b) bonds lower plate  52  to the portion of lower polymer layer  72  that forms lower barrier portion  42 . 
     The surfaces of mold  60  that shape barrier portions  41  and  42  are depicted as being substantially parallel and planar. Chamber  33 , however, exhibits a tapered configuration between heel region  13  and forefoot region  11 . When chamber  33  is pressurized, tapering may arise due to the configuration of upper mold  61  and lower mold  62 . For example, the cavity of mold  60  used to form chamber  33  may have one height in heel region  13 , and another, lesser height in forefoot region  11 . 
     In some manufacturing processes, chamber  33 , as well as upper plate  51 , lower plate  52 , or both, may incorporate features such as contours, indentations, protrusions, or shaping. For example, chamber  33  or upper plate  51  may incorporate a depression in heel region  13 . Accordingly, the configuration of mold  60  may incorporate corresponding contours, indentations, protrusions, or shaping to facilitate the formation of such features in chamber  33  and to impart such features to chamber  33 . 
     In alternate manufacturing processes, a ridge in upper mold portion  61  corresponding with ridge  64  in lower mold portion  62  may allow upper mold portion  61  to impart a configuration to an upper part of sidewall barrier portion  43 . In such manufacturing processes, peripheral bond  44  may instead extend around a mid-section of sidewall barrier portion  43 . Accordingly, by controlling the ridge in upper mold portion  61  and ridge  64  in lower mold portion  62 , peripheral bond may be located on the same plane as either upper barrier portion  41  or lower barrier portion  42 , or at any mid-section in between. 
     Upper plate  51  and lower plate  52  are depicted in  FIGS. 16A-16D  and in  FIGS. 17A-17D  as having essentially no curvature (i.e., as having a substantially flat shape) throughout the manufacturing process. However, the manufacturing process may change a curvature of or impart a curvature to upper plate  51 , lower plate  52 , or both. For example, upper plate  51  or lower plate  52  may have essentially no curvature before chamber  33  is pressurized, whereas an outward curvature may be imparted to upper plate  51  or lower plate  52 , or portions thereof, after chamber  33  is pressurized. Accordingly, upper plate  51 , lower plate  52 , or both may be formed to have an inward curvature before chamber  33  is pressurized, such that plates  51  and  52  have essentially no curvature after chamber  33  is pressurized, as disclosed in U.S. Pat. No. 7,533,477 to Goodwin et al. 
     Further Configurations 
     Indentations  55  may exhibit further cross-sectional configurations. In the initial configuration of upper plate  51 , as depicted in  FIG. 14 , indentations  55  have a generally rectangular cross-sectional configuration. That is, indentations  55  have sides at substantially right angles with each other, and a depth of indentations  55  is greater than a width of indentations  55 . In contrast, for example, with reference to  FIG. 18A , indentation  55  in cross-section has a generally triangular shape, with two sides of the triangular cross-section imparting a depth greater than a width imparted by a third, inner side of the triangular cross-section. As a further example, with reference to  FIG. 18B , indentation  55  in cross-section has a substantially linear incision portion reaching from a lower exterior area of upper plate  51  to a small, hollow circular portion within upper plate  51 . In another example, with reference to  FIG. 18C , indentation  55  in cross-section is generally water-droplet-shaped, having a hollow portion within upper plate  51  that tapers in a curving manner and ends, with edge areas  56  touching each other, at an exterior of upper plate  51 . Generally, indentations  55  may have any cross-sectional shape, and this shape may result in portions of upper plate  51  immediately adjacent to and on opposite sides of indentations  55  being separated, touching, or exhibiting any mixed configuration incorporating separated portions and touching portions. An advantage to this general configuration is that opposite sides of indentations  55  make contact to prevent outward flexing of upper plate  51 , thereby resisting outward expansion and retaining an intended shape of chamber  33 . 
     Edge areas  56  may also have other cross-sectional configurations. In the initial configuration of upper plate  51 , the thickness of upper plate  51  is substantially uniform outside of indentations  55 , even at areas immediately adjacent to and on opposite sides of indentations  55 . That is, edge areas  56  are substantially flush with other areas of upper plate  51  outside of indentations  55 . In further configurations, edge areas  56  have a greater thickness than other portions of upper plate  51  spaced away from edge areas  56 . In such configurations, the cross-sectional profile from edge areas  56  to portions of upper plate  51  spaced away from edge areas  56  may take any shape. For example, with reference to  FIG. 18D , when tracing the cross-sectional profile from edge areas  56  to portions of upper plate  51  spaced away from edge areas  56 , the thickness of upper plate  51  may decrease substantially linearly. As a further example, with reference to  FIG. 18E , the thickness of upper plate  51  may decrease slowly at first, then decrease rapidly until reaching the thickness of the remainder of upper plate  51 . In another example, with reference to  FIG. 18F , the thickness of upper plate  51  may decrease rapidly at first, then decrease slowly until reaching the thickness of the remainder of upper plate  51 . Generally, edge areas  56 , and the profile of upper plate  51  between indentations  55  and portions spaced away from edge areas  56 , may have any cross-sectional shape. Each of these configurations also have an advantage of resisting outward expansion and retaining an intended shape of chamber  33 . 
     In the initial configuration, a first thickness of upper plate  51  at an area spaced away from indentations  55  is greater than a second thickness of upper plate  51  measured within indentations  55 . In further configurations, a third thickness of upper plate  51  measured at edge areas  56  may be greater than the first thickness of upper plate  51 , where the first thickness is at an area spaced away from both indentations  55  and edge areas  56 . For example, with reference to  FIG. 18D , a first thickness A, a second thickness B, and a third thickness C are defined. Third thickness C of upper plate  51 , which is measured at edge areas  56 , may be two or more times greater than first thickness A of upper plate  51 , which is measured at an area spaced away from indentations  55  and edge areas  56 . Additionally, third thickness C, measured at edge areas  56 , is greater than both first thickness A and second thickness B. The protruding aspect of portions of upper plate  51  having third thickness C (i.e., the areas corresponding with edge areas  56 ) provides an advantage of resisting outward expansion and retaining an intended shape of chamber  33 . 
     In some configurations, edge areas  56  may be outward protrusions in an outer exterior side of upper plate  51 , located on opposite sides of, and immediately adjacent to, indentations  55 . In such configurations, edge areas  56  may serve to restrict the rotation of a first portion of upper plate  51  about indentations  55  with respect to a second portion of upper plate  51 . In other configurations, either or both of edge areas  56  may serve as a hinge stop, obstructing flexing of upper plate  51  in one direction while permitting flexing of upper plate  51  in another direction. A hinge stop may have a first portion located on one side of indentations  55  and a second portion located on an opposite side of indentations  55 . Alternatively, a hinge stop may be one-sided, or may be solely located on one side of indentations  55 . Accordingly, the protruding aspect of portions of upper plate  51  having third thickness C (i.e., the areas corresponding with edge areas  56 ) provides an advantage of resisting outward expansion and retaining an intended shape of chamber  33 . 
     In the initial configuration of upper plate  51 , a thickness at indentations  55  is defined by a lower surface of upper plate  51 . However, in further configurations, a thickness at indentations  55  may be defined by both an upper surface of upper plate  51  and a lower surface of upper plate  51 . For example, with reference to  FIG. 18G , a first thickness D, a second thickness E, and a third thickness F are defined, and second thickness E of upper plate  51  at indentations  55  is defined both by the shape of an upper surface of upper plate  51  and the shape of a lower surface of upper plate  51 . In such configurations, first thickness D may be greater than third thickness F. 
     Generally, the cross-sectional configuration of indentations  55  and edge areas  56  may vary along a length of indentations  55  and edge areas  56 . That is, the cross-sectional configuration of indentations  55  and edge areas  56  may not be uniform along the entire length of indentations  55  and edge areas  56 , but may differ across the entire length, resulting in indentations  55  and edge areas  56  including a variety of cross-sectional configurations. For example, a depth of indentations  55 , or a width of indentations  55 , or both may vary along a length of indentations  55  and edge areas  56 . 
     Edge areas  56  may exhibit further configurations while running generally parallel to corresponding substantially straight indentations  55 , as may be seen from a bottom plan perspective. In the initial configuration, edge areas  56  are formed to be substantially straight, and are parallel to substantially straight indentations  55 . However, in further configurations, edge areas  56  may be otherwise formed. For example, with reference to  FIG. 19A , edge areas  56  may be formed to have a triangular or saw-toothed configuration closely following substantially straight corresponding indentations  55 . As a further example, with reference to  FIG. 19B , edge areas  56  may be formed to have a trapezoidally-shaped configuration closely following substantially straight corresponding indentations  55 . In another example, with reference to  FIG. 19C , edge areas  56  may be formed to have a curving, circular, rippling, or otherwise wavy configuration closely following substantially straight corresponding indentations  55 . Additionally, with reference to  FIGS. 19A-19C , edge areas  56  may have an interlocking configuration, which may restrict the movement of edge areas  56  with respect to each other, or which may establish and maintain an alignment, or a positional relationship, between opposite edge areas  56 . An advantage of maintaining an alignment of opposite edge areas  56  is that a hemorrhaging or distortion of chamber  33  in the area associated with the opposite edge areas  56  may be reduced or prevented. Generally, in closely following corresponding indentations  55 , edge areas  56  may have configurations incorporating any shape or shapes, regular or irregular, periodically instantiated or continuously instantiated, whether under a regular pattern or no pattern. 
     These further cross-sectional configurations of indentations  55 , variations in thickness of edge areas  56 , and configurations of edge areas  56  may be combined. For example, with reference to  FIG. 20 , in some configurations of upper plate  51 , (a) indentations  55  may have a generally water-droplet-shaped cross-section, (b) when tracing the cross-sectional profile from edge areas  56  to portions of upper plate  51  spaced away from edge areas  56 , the thickness of upper plate  51  may decrease rapidly at first, then decrease slowly toward the thickness of the remainder of upper plate  51 , and (c) edge areas  56  may be formed to have a curving, circular, rippling, or otherwise wavy configuration. In other configurations, any cross-sectional configuration of indentations  55  and any cross-sectional configuration of edge areas  56  may be combined, and in such combinations, edge areas  56  may additionally be formed to have any configuration closely following substantially straight corresponding indentations  55 . 
     Chamber  33  may have one or more upper plates  51  or lower plates  52 , which may have various shapes. In the initial configuration, upper plate  51  and lower plate  52  have a shape corresponding with a general outline of the foot. However, upper plate  51  may instead have a shape corresponding with a portion of the foot. Upper plate  51  may extend across at least fifty percent of chamber  33 . For example, with reference to  FIGS. 21A-21C , upper plate  51  in some configurations has a shape corresponding with portions of forefoot region  11  and midfoot region  12  of the foot. Alternatively, with reference to  FIGS. 22A-22C , upper plate  51  in some configurations has a shape corresponding with portions of midfoot region  12  and heel region  13  of the foot. Generally, upper plate  51  or lower plate  52  may have a shape corresponding with any region or regions of the foot. Additionally, more than one upper plate  51  or lower plate  52  may be incorporated into the same chamber  33 , and may be spaced apart from other upper plates  51  or lower plates  52 , respectively, within chamber  33 . 
     Chamber  33  may itself have other configurations. In the initial configuration, chamber  33  has a shape corresponding with a general outline of the foot. In further configurations, however, chamber  33  may have a substantially circular shape, or a filled-in U-shape similar to the shape of a heel, or a shape having a central area from which lobes extend radially outward. In configurations in which upper plate  51  or lower plate  52  correspond in shape with a portion of the foot, chamber  33  may correspond in shape with the same portion of the foot. In general, chamber  33  may have any shape corresponding with one or more portions of the foot. 
     In the initial configuration, indentations  55  are substantially linear and extend across upper plate  51 . However, indentations  55  may generally extend across any portion of upper plate  51  or lower plate  52 . For example, with reference to  FIGS. 21A and 22A , upper plate  51  has a roughly U-shaped configuration, and indentations  55  demarcate and exist between sections of upper plate  51  along the U-shaped configuration. As a further example, with reference  FIGS. 21B and 22B , upper plate  51  has a central portion and separate radial portions, and indentations  55  exist between each radial portion and the central portion. In another example, with reference to  FIGS. 21C and 22C , upper plate  51  has a central portion and a roughly U-shaped portion which includes radial portions, and indentations  55  demarcate and exist both between sections of upper plate  51  along the U-shaped configuration, and between each radial portion and the central portion. In a further example yet, with reference to  FIG. 23 , upper plate  51  has a shape corresponding with a general outline of the foot. In a portion of forefoot region  11  and midfoot region  12  of upper plate  51 , indentations  55  demarcate and exist between sections of upper plate  51 . Similarly, in a portion of heel region  13  and midfoot region  12  of upper plate  51 , indentations  55  demarcate and exist between sections of upper plate  51 . 
     In the initial configuration, indentations  55  have a configuration of single lines. However, in further configurations, indentations  55  may have a configuration of two or more lines intersecting at any angle. For example, with reference to  FIGS. 21C ,  22 C, and  23 , indentations  55  may have a configuration of three lines meeting at one point in a Y-shape or a T-shape. Generally, an indentation  55  may include any plurality of supplemental indentations, which may extend radially outward from indentation  55  in any manner. 
     Indentations  55  may be incorporated in a variety of ways in the various surfaces of upper plates  51  and lower plates  52 . In the initial configuration, indentations  55  are defined in a lower surface of upper plate  51 . In contrast, for example, with reference to  FIG. 24A , indentations  55  are defined in a lower surface of lower plate  52 . As a further example, with reference to  FIG. 24B , indentations  55  are defined both in a lower surface of upper plate  51  and a lower surface of lower plate  52 , and indentations  55  in lower plate  52  are positioned similarly to indentations  55  in upper plate  51 . In another example, with reference to  FIG. 24C , indentations  55  are defined both in a lower surface of upper plate  51  and a lower surface of lower plate  52 , but indentations  55  in lower plate  52  are positioned differently than indentations  55  in upper plate  51 . 
     Additionally, upper plates  51  may be positioned in an upper area of either an interior surface or an exterior surface of chamber  33 , and lower plates  52  may be positioned in a lower area of either an interior surface or an exterior surface of chamber  33 . In the initial configuration, upper plate  51  is secured to an interior surface or lower surface of upper barrier portion  41 , and lower plate  52  is secured to an interior surface or upper surface of lower barrier portion  42 . In contrast, for example, with reference to  FIG. 24D , lower plate  52  is secured to an exterior surface or lower surface of lower barrier portion  42 . 
     In the initial configuration, upper plate  51  and lower plate  52  are substantially planar. However, in further configurations, upper plate  51  or lower plate  51  may not be substantially planar, but may have contours, indentations, protrusions, or shaping. For example, upper plate  51  may be contoured or shaped to better correspond to the shape of the lower surface of a foot, such as by incorporating a depression in heel region  13 . 
     With reference to  FIG. 24E , by being flexible in one direction, an upper plate  51  having a configuration similar to the configuration depicted in  FIG. 20  may allow for compression of chamber  33  in the presence of downward pressure  92 . Meanwhile, with reference to  FIG. 24F , by locking in the reverse direction, upper plate  51  may restrict outward distention of chamber  33  in the presence of outward pressure  91 . Accordingly, in various configurations, the incorporation of upper plate  51  or lower plate  52  into fluid-filled chamber  33  may impart shape-retention properties to chamber  33  in the absence of internal bonds, linkages, or connections between upper barrier portion  41  and lower barrier portion  42 , while retaining properties relating to attenuating ground reaction forces (i.e., cushioning). Additionally, with fewer such internal structures visible, upper plate  51  or lower plate  52  may enhance an overall transparency or see-through quality of chamber  33 . Alternatively, in some configurations, upper plate  51  or lower plate  52  may be incorporated into chambers that also include other internal structures, such as internal bonds, linkages, or connections. In such configurations, the incorporation of upper plate  51  or lower plate  52  into fluid-filled chamber  33  may impart shape-retention properties in some areas of chamber  33  without other internal structures, and may enhance an overall transparency or see-through quality of corresponding portions of chamber  33 . 
     The invention is disclosed above and in the accompanying figures with reference to a variety of configurations. The purpose served by the disclosure, however, is to provide an example of various features and concepts related to the invention, not to limit the scope of the invention. One skilled in the relevant art will recognize that numerous variations and modifications may be made to the configurations described above without departing from the scope of the present invention, as defined by the appended claims.