Abstract:
The invention is directed to intelligent systems for articles of footwear that adjust automatically in response to a measured performance characteristic. The intelligent systems include one or more adjustable elements coupled to a mechanism that actuates the adjustable elements in response to a signal from a sensor to modify the performance characteristic of the article of footwear. The intelligent system adjusts the performance characteristics of the article of footwear without human intervention.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and incorporates herein by reference in its entirety, U.S. patent application Ser. No. 11/522,727, filed on Sep. 18, 2006 now U.S. Pat. No. 7,506,460, which is a continuation of U.S. patent application Ser. No. 10/385,300, filed on Mar. 10, 2003, now U.S. Pat. No. 7,188,439. 
    
    
     TECHNICAL FIELD 
     The invention generally relates to intelligent systems for articles of footwear. In particular, the invention relates to automatic, self-adjusting systems that modify a performance characteristic of the article of footwear. 
     BACKGROUND INFORMATION 
     Conventional athletic shoes include an upper and a sole. The sole is usually manufactured of a material chosen to attempt to optimize a particular performance characteristic of the shoe, for example, stability or stiffness. Typically, the sole includes a midsole and an outsole, either of which can include, a resilient material to protect a wearer&#39;s foot and leg. One drawback with conventional shoes is that performance characteristics, such as cushioning and stiffness, are not adjustable. The wearer must, therefore, select a specific shoe for a specific activity. For example, for activities requiring greater cushioning, such as running, the wearer must select one type of shoe and for activities requiring greater stiffness for support during lateral movement, such as basketball, the wearer must select a different type of shoe. 
     Some shoes have been designed to allow for adjustment in the degree of cushioning or stiffness provided by the sole. Many of these shoes employ a fluid bladder that can be inflated or deflated as desired. A disadvantage presented by these shoes is that one or more of the bladders can fail, rendering the cushioning system effectively useless. Moreover, many of the shoes employing fluid bladders do not allow for small-scale changes to the degree of cushioning provided by the sole. Often, the change to the degree of cushioning provided by the sole in pressurizing or depressurizing, or in partially pressurizing or partially depressurizing, a bladder will typically be larger than that desired by the wearer. In other words, bladders are typically not capable of fine adjustments. 
     A further disadvantage of many of the shoes designed to allow for adjustment in the degree of cushioning or stiffness provided by the sole is that they are only manually adjustable. Accordingly, in order to adjust such shoes the wearer is required to interrupt the specific activity in which he/she is engaged. With some shoes, the wearer may also be required to partially disassemble the shoe, re-assemble the shoe, and even exchange shoe parts. Moreover, the wearer, to his or her dissatisfaction, may be limited in the amount of adjustment that can be made. 
     Some shoes have been designed to automatically adjust the degree of cushioning or stiffness provided by the sole. These shoes measure the amount of force or pressure exerted on the sole by the wearer&#39;s foot when the wearer&#39;s foot strikes the ground. Through analysis and investigation, it has been discovered that the mere measurement of force or pressure alone, however, is too limited, as it provides no information relating to the performance of the shoe. For example, measuring force provides no indication as to whether the sole has either over-compressed or under-compressed for that particular wearer without prior investigation into the normal forces exerted by the wearer during the activity. If the sole is either over-compressed or under-compressed, the shoe is poorly matched to the wearer&#39;s activity and needs. In essence, the wearer&#39;s body has to adapt to the shoe. The biomechanical needs of the wearer are poorly met, if at all. 
     In sum, shoes that have been designed to allow for some adjustment in the degree of cushioning or stiffness provided by the sole still fall short of accommodating the wearer&#39;s needs. Specifically, they are not fully adjustable throughout the range of the biomechanical needs of the particular wearer or lack the ability to sense the true needs of the wearer. As a result, the wearer must still, in some way, adapt his or her body to the environment presented by the shoe. 
     There is, therefore, a need for a shoe that senses the biomechanical needs of the wearer, automatically adjusts a performance characteristic of the shoe to accommodate the biomechanical needs of the wearers for example the degree of cushioning or stiffness provided by the sole, and avoids the drawbacks of bladder cushioning or manually adjustable shoes. 
     SUMMARY OF THE INVENTION 
     The invention is directed to intelligent systems for articles of footwear that adjust a feature of the footwear in response to the footwear&#39;s environment, without human interaction. In other words, the footwear is adaptive. For example, the intelligent system can continuously sense the biomechanical needs of the wearer and concomitantly modify the footwear to an optimal configuration. The intelligent system includes a sensing system, a control system, and an actuation system. 
     The sensing system measures a performance characteristic of the article of footwear and sends a signal to the control system. The signal is representative of the measured performance characteristic. The control system processes the signal to determine if, for example, the performance characteristic deviates from an acceptable range or exceeds a predetermined threshold. The control system sends a signal to the actuation system relative to the deviation. The actuation system modifies a feature of the footwear in order to obtain an optimal performance characteristic. 
     In one aspect, the invention relates to an intelligent system for an article of footwear. The system includes a control system, a power source electrically coupled to the control system, an adjustable element, and a driver coupled to the adjustable element. The driver adjusts the adjustable element in response to a signal from the control system. 
     In another aspect, the invention relates to an article of footwear including an upper coupled to a sole and an intelligent system at least partially disposed in the sole. The system includes a control system, a power source electrically coupled to the control system, an adjustable element, and a driver coupled to the adjustable element. The driver adjusts the adjustable element in response to a signal from the control system. 
     In various embodiments of the foregoing aspects, the system modifies a performance characteristic of the article of footwear, such as compressibility, resiliency, compliancy, elasticity, damping, energy storage, cushioning, stability, comfort, velocity, acceleration, jerk, stiffness, or combinations thereof. In one embodiment, the adjustable element is adjusted by at least one of translation, rotation, reorientation, modification of a range of motion, or combinations thereof. The system may include a limiter for limiting a range of motion of the adjustable element. The control system includes a sensor and electrical circuitry. The sensor may be a pressure sensor, a force transducer, a hall effect sensor, a strain gauge, a piezoelectric element, a load cell, a proximity sensor, an optical sensor, an accelerometer, a hall element or sensor, a capacitance sensor, an inductance sensor, an ultrasonic transducer and receiver, a radio frequency emitter and receiver, a magneto-resistive element, or a giant magneto-resistive element. In various embodiments, the driver may be a worm drive, a lead screw, a rotary actuator, a linear actuator, a gear train, a linkage, or combinations thereof. 
     In still other embodiments, the adjustable element may be at least partially disposed in at least one of a forefoot portion, a midfoot portion, and a rearfoot portion of the article of footwear. In one embodiment, the article of footwear has a sole including an outsole and a midsole and the adjustable element is disposed at least partially in the midsole. In various embodiments, the adjustable element may be generally longitudinally disposed within the article of footwear, or the adjustable element may be generally laterally disposed within the article of footwear, or both. For example, the adjustable element may extend from a heel region to an arch region of the article of footwear or from an arch region to a forefoot region of the article of footwear or from a forefoot region to a heel region of the article of footwear. Furthermore, the adjustable element may be at least partially disposed in a lateral side, or a medial side, or both of the article of footwear. 
     In another aspect, the invention relates to a method of modifying a performance characteristic of an article of footwear during use. The method includes the steps of monitoring the performance characteristic of the article of footwear, generating a corrective driver signal, and adjusting an adjustable element based on the driver signal to modify the performance characteristic of the article of footwear. In one embodiment, the steps are repeated until a threshold value of the performance characteristic is obtained. 
     In various embodiments of the foregoing aspect, the generating step includes the substeps of comparing the monitored performance characteristic to a desired performance characteristic to generate a deviation and outputting a corrective driver signal magnitude based on the deviation. In one embodiment, the corrective driver signal has a predetermined magnitude. Further, the monitoring step may include the substeps of measuring a magnetic field of a magnet with a proximity sensor, wherein at least one of the magnet and the sensor are at least partially disposed within the sole and are vertically spaced apart in an unloaded state, and comparing the magnetic field measurement during compression to a threshold value. In one embodiment, the monitoring step involves taking multiple measurements of the magnetic field during compression and comparing an average magnetic field measurement to the threshold value. 
     In additional embodiments, the method may include the step of limiting a range of motion of the adjustable element with a limiter and the adjusting step may include adjusting the limiter a predetermined distance. The adjustment step may be performed when the article of footwear is in an unloaded state. In one embodiment, the adjustment step is terminated when a threshold value of the performance characteristic is reached. 
     In various embodiments of all of the foregoing aspects of the invention, the adjustable element may be an expansion element, a multiple density foam, a skeletal element, a multidensity plate, or combinations thereof. The adjustable element may exhibit an anisotropic property. In one embodiment, the adjustable element may be a generally elliptically-shaped expansion element. Further, the system may include a manual adjustment for altering or biasing the performance characteristic of the adjustable element, or an indicator, or both. The manual adjustment may also alter a threshold value of the performance characteristic. The indicator may be audible, visual, or both. For example, the indicator may be a series of light-emitting diodes. 
     In another aspect, the invention relates to a system for measuring compression within an article of footwear. The system includes a sensor at least partially disposed within a sole of the article of footwear and a magnet generally aligned with and spaced from the sensor. The sensor may be a hall effect sensor, a proximity sensor, a hall element or sensor, a capacitance sensor, an inductance sensor, an ultrasonic transducer and receiver, a radio frequency emitter and receiver, a magneto-resistive element, or a giant magneto-resistive element. The system may include a processor. In one embodiment, the sensor measures a magnetic field generated by the magnet and the processor converts the magnetic field measurement into a distance measurement representing an amount of compression of the sole in correlation with respective time measurements. The processor may convert the distance measurements into a jerk value. 
     In various embodiments of the foregoing aspect, the system further includes a driver coupled to the sensor and an adjustable element coupled to the driver. The system may include a limiter for limiting a range of motion of the adjustable element. In one embodiment, a performance characteristic of the article of footwear is modified in response to a signal from the sensor. In one embodiment, the signal corresponds to an amount of compression of the sole. 
     In another aspect, the invention relates to a method of providing comfort in an article of footwear. The method includes the steps of providing an adjustable article of footwear and determining a jerk value. The method may further include the step of modifying a performance characteristic of the adjustable article of footwear based on the jerk value. 
     These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIG. 1  is a partially exploded schematic perspective view of an article of footwear including an intelligent system in accordance with one embodiment of the invention; 
         FIG. 2A  is an exploded schematic perspective view of the sole of the article of footwear of  FIG. 1 ; 
         FIG. 2B  is an enlarged schematic side view of the intelligent system of  FIG. 2A  illustrating the operation of the adjustable element; 
         FIG. 3  is a schematic perspective view of an alternative embodiment of an adjustable element in accordance with the invention; 
         FIGS. 4A-4E  are schematic side views of alternative embodiments of an adjustable element in accordance with the invention; 
         FIG. 5A  is a schematic side view of the article of footwear of  FIG. 1  showing select internal components; 
         FIG. 5B  is an enlarged schematic view of a portion of the article of footwear of  FIG. 5A ; 
         FIG. 6  is a schematic top view of a portion of the sole of  FIG. 2A  with a portion of the sole removed to illustrate the layout of select internal components of the intelligent system; 
         FIG. 7  is a block diagram of an intelligent system in accordance with the invention; 
         FIG. 8  is a flow chart depicting one mode of operation of the intelligent system of  FIG. 1 ; 
         FIG. 9  is a circuit diagram of one embodiment of the intelligent system of  FIG. 1 ; 
         FIG. 10  is a circuit diagram of the voltage regulator system of  FIG. 9 ; 
         FIG. 11  is a circuit diagram of the sensing system of  FIG. 9 ; 
         FIG. 12  is a circuit diagram of the control system of  FIG. 9 ; 
         FIG. 13  is a circuit diagram of the actuation system of  FIG. 9 ; 
         FIG. 14  is a circuit diagram of another embodiment of the intelligent system of  FIG. 1 ; 
         FIG. 15A  is a schematic side view of an article of footwear including an alternative embodiment of an intelligent system in accordance with the invention; 
         FIG. 15B  is a schematic perspective view of a portion of the intelligent system of  FIG. 15A ; 
         FIG. 16A  is a schematic side view of an article of footwear including yet another alternative embodiment of an intelligent system in accordance with the invention; 
         FIGS. 16B-16D  are schematic side views of the intelligent system of  FIG. 16A  in various orientations; 
         FIG. 17A  is a schematic side view of an article of footwear including yet another alternative embodiment of an intelligent system in accordance with the invention; 
         FIG. 17B  is a schematic side view of the intelligent system of  FIG. 17A  throughout a range of adjustment; 
         FIG. 18  is a graph depicting a performance characteristic of a specific embodiment of an adjustable element; 
         FIG. 19  is a flow chart depicting one embodiment of a method of modifying a performance characteristic of an article of footwear during use; 
         FIGS. 20A and 20B  are flow charts depicting additional embodiments of the method of  FIG. 19 ; and 
         FIG. 21  is a flow chart depicting one embodiment of a method of providing comfort in an article of footwear. 
     
    
    
     DESCRIPTION 
     Embodiments of the present invention are described below. It is, however, expressly noted that the present invention is not limited to these embodiments, but rather the intention is that modifications that are apparent to the person skilled in the art are also included. In particular, the present invention is not intended to be limited to any particular performance characteristic or sensor type or arrangement. Further, only a left or right shoe is depicted in any given figure; however, it is to be understood that the left and right shoes are typically mirror images of each other and the description applies to both left and right shoes. In certain activities that require different left and right shoe configurations or performance characteristics, the shoes need not be mirror images of each other. 
       FIG. 1  depicts an article of footwear  100  including an upper  102 , a sole  104 , and an intelligent system  106 . The intelligent system  106  is laterally disposed in a rearfoot portion  108  of the article of footwear  100 . The intelligent system  106  could be disposed anywhere along the length of the sole  104  and in essentially any orientation. In one embodiment, the intelligent system  106  is used to modify the compressibility of a heel area of the article of footwear  100 . In another embodiment, the intelligent system  106  can be located in a forefoot portion  109  and can be moved into and out of alignment with a flex line or otherwise configured to vary a push-off characteristic of the footwear  100 . In yet another embodiment, the footwear  100  could include multiple intelligent systems  106  disposed in multiple areas of the footwear  100 . The intelligent system  106  is a self-adjusting system that modifies one or more performance characteristics of the article of footwear  100 . The operation of the intelligent system  106  is described in detail hereinbelow. 
       FIG. 2A  depicts an exploded view of a portion of the sole  104  of  FIG. 1 . The sole  104  includes a midsole  110 , an outsole  112   a ,  112   b , an optional lower support plate  114 , an optional upper support plate  116 , and the intelligent system  106 . The upper and lower support plates may, among other purposes, be included to help constrain the intelligent system  106  in a particular orientation. The intelligent system  106  is disposed within a cavity  118  formed in the midsole  110 . In one embodiment, the midsole  110  is a modified conventional midsole and has a thickness of about 10 mm to about 30 mm, preferably about 20 mm in the heel portion. The intelligent system  106  includes a control system  120  and an actuation system  130  in electrical communication therewith, both of which are described in greater detail hereinbelow. The actuation system  130  includes a driver  131  and an adjustable element  124 . The control system  120  includes a sensor  122 , for example a proximity sensor, a magnet  123 , and electrical circuitry (see  FIGS. 9-14 ). In the embodiment shown, the sensor  122  is disposed below the adjustable element  124  and the magnet  123  is vertically spaced from the sensor  122 . In this particular embodiment, the magnet  123  is disposed above the adjustable element  124  and is a Neodymium Iron Bore type magnet. The actual position and spacing of the sensor  122  and magnet  123  will vary to suit a particular application, for example, measuring and modifying the compressibility of the sole. In this particular embodiment, the sensor  122  and magnet  123  are located in a spot that corresponds generally to where maximum compression occurs in the rearfoot portion  108  of the footwear  100 . Typically, the spot is under the wearer&#39;s calcaneous. In such an embodiment, the sensor  122  and magnet  123  are generally centered between a lateral side and a medial side of the sole  104  and are between about 25 mm and about 45 mm forward of a posterior aspect of the wearer&#39;s foot. 
       FIG. 2B  depicts a portion of the intelligent system  106 , in particular the actuation system  130 , in greater detail. The intelligent system  106  is preferably encased in a sealed, waterproof enclosure. The actuation system  130  generally includes a driver  131 , which includes a motor  132  and a transmission element  134 , and an adjustable element  124 , which includes a limiter  128 , an expansion element  126 , and a stop  136 . The embodiment of the particular driver  131  shown is a lead screw drive, made up of a bi-directional electric motor  132  and a threaded rod that forms the transmission element  134 . In one embodiment, the motor  132  can be a radio-controlled servomotor of the type used in model airplanes. The threaded rod could be made of steel, stainless steel, or other suitable material. 
     The motor  132  is mechanically coupled to the transmission element  134  and drives the element  134  in either a clockwise or counter-clockwise direction as indicated by arrow  138 . The transmission element  134  threadedly engages the limiter  128  and transversely positions the limiter  128  relative to the expansion element  126 , as shown generally by arrow  140 . Because the limiter  128  is threadedly engaged with the transmission element  134  and prevented from rotation relative to the motor  132  and the footwear  100 , no power is required to maintain the limiter&#39;s position. There is sufficient friction in the actuation system  130  and a sufficiently fine thread on the transmission element  134  to prevent inadvertent rotation of the element  134  during a heel strike. In one example, the limiter  128  advances toward the expansion element  126  when the motor  132  drives the transmission element  134  in the clockwise direction and the limiter  128  moves away from the expansion element  126  when the motor  132  drives the transmission element  134  in the counter-clockwise direction. Alternatively, other types of drivers are possible. For example, the driver  131  could be essentially any type of rotary or linear actuator, a gear train, a linkage, or combinations thereof. 
     The expansion element  126  is generally cylindrical, with an elongated circular or elongated generally elliptically-shaped cross-section. The arcuate ends of the expansion elements are not necessarily semi-circular in shape. The radius of the arcuate ends will vary to suit a particular application and can be varied to control the amount of longitudinal expansion of the expansion element  126  when under compressive loading vertically. In general, the larger the radius of the arcuate end, the greater longitudinal expansion is possible under vertical compression loading. The expansion element  126  has a solid outer wall  142  and a optional compressible core  144  of foam or other resilient material. The size, shape, and materials used in the expansion element  126  will be selected to suit a particular application. In the embodiment shown, the transmission element  134  extends through the expansion element  126  and connects to a stop  136 . The stop  136  prevents movement of the expansion element  126  in a direction away from the limiter  128 . Alternatively, the stop  136  could be a rear wall of the cavity  118 . 
     The general operation of the adjustable element  124  is described with respect to an application where the intelligent system  106  is used to modify cushioning in the article of footwear  100  in response to a measured parameter, for example compression of the midsole  110 . The expansion element  126  is allowed to compress when acted on by a vertical force, depicted generally by arrows  146 . The expansion element  126  expands in the horizontal direction (arrow  148 ) when compressed. The limiter  128  is used to control this movement. As the horizontal movement is limited, the vertical movement is limited as well. The expansion element  126  has a bi-modal compression response, which is discussed in greater detail below with respect to  FIG. 18 . 
     The intelligent system  106  can control the amount of compression a user creates in the article of footwear  100 . As an example, when a user wearing the article of footwear  100  engages a ground surface during a stride, the vertical force  146  is applied to the expansion element  126  via the sole  104 . The force  146  causes the expansion element  126  to expand during ground contact until it contacts the limiter  128 , thereby controlling the compression of the sole  104 . 
     During compression, the sensing portion of the control system  120  measures field strength of the magnet  123 . In the embodiment shown, the sensor  122  is disposed proximate the bottom of the midsole  110  and the magnet  123  is disposed proximate the top of the midsole  110 . The magnetic field strength detected by the sensor  122  changes as the magnet  123  moves closer to the sensor  122 , as the midsole  110  is compressed. The system can be calibrated, such that this magnetic field strength can be converted to a distance. It is the change in distance that indicates how much the midsole  110  has been compressed. The control system  120  outputs a signal to the actuation system  130  based on the change in distance or compression measurement. 
     The actuation system  130  then modifies the hardness or compressibility of the midsole  110  based on the signal received from the control system  120 . The actuation system  130  utilizes the transmission element  134  as the main moving component. The operation of the intelligent system  106  is described in greater detail below, with respect to the algorithm depicted in  FIG. 8 . 
       FIG. 3  depicts a portion of an alternative embodiment of an intelligent system  306  in accordance with the invention, in particular the actuation system  330 . The actuation system  330  includes a driver  331  and an adjustable element  324 . The adjustable element  324  includes an expansion element  326  and limiter  328  similar to that described with respect to  FIG. 2B . The driver  331  includes a motor  332  and a transmission element  334 , in this embodiment a hollow lead screw  325  through which a cable  327  passes. The cable  327  runs through the expansion element  326  and has a stop  336  crimped to one end. The limiter  328  is a generally cylindrically-shaped element that is slidably disposed about the cable  327  and acts as a bearing surface between the screw  325  and the expansion element  326 , in particular a bearing arm  339  coupled to the expansion element  326 . A similar bearing arm is disposed proximate the stop  336 , to distribute loads along the depth of the expansion element  326 . In one embodiment, the motor  332  is a 6 mm pager motor with a 300:1 gear reduction. The cable  327 , screw  325 , limiter  328 , and bearing arm  339  may be made of a polymer, steel, stainless steel, or other suitable material. In one embodiment, the cable  327  is made from stainless steel coated with a friction-reducing material such as that sold by DuPont under the trademark Teflon®. 
     In operation, the cable  327  is fixedly attached to the driver  331  and has a fixed length. The cable  327  runs through the screw  325 , which determines the amount of longitudinal travel of the expansion element  326  that is possible. For example, as a vertical force is applied to the expansion element  326 , the element  326  expands longitudinally along the cable  327  until it hits the limiter  328 , which is disposed between the expansion element  326  and the end of the screw  325 . The motor  332  rotates the screw  325  to vary the length of the cable  327  that the limiter  328  can slide along before contacting the screw  325  and expansion element  326 . The screw  325  moves a predetermined distance either towards or away from the element  326  in response to the signal from the control system. In one embodiment, the screw  325  may travel between about 0 mm to about 20 mm, preferably about 0 mm to about 10 mm. 
     In an alternative embodiment, the adjustable element  324  includes two motors  332  and cables  327  oriented substantially parallel to one another. Two cables  327  aid in holding the expansion element  326  square relative to a longitudinal axis  360  of the adjustable element  324  depicted in  FIG. 3 . In addition, other types of expansion element/limiter arrangements are possible. For example, a circumferential or belly band type limiter may be used instead of a diametral or longitudinal type limiter. In operation, the driver  331  varies the circumference of the belly band to vary the range of expansion of the element  326 , the larger the circumference, the larger the range of expansion. Other possible arrangements include shape memory alloys and magnetic rheological fluid. 
       FIGS. 4A-4E  depict alternative adjustable elements, with each shown in an unloaded state. In particular,  FIGS. 4A-4D  depict certain different possible shapes for the expansion element. In  FIG. 4A , the expansion element  426  includes two cylinders  428  having generally elliptically-shaped cross-sections and formed as a single element. Alternatively, the cylinder cross-sectional shape could be any combination of linear and arcuate shapes, for example, hexagonal or semi-circular. The cylinders  428  include a wall  432  and a pair of cores  434  that may be hollow or filled with a foam or other material.  FIG. 4B  depicts an expansion element  446  having two separate cylinders  448  having generally circular cross-sections and coupled together. The cylinders  448  each have a wall  452  and a core  454 .  FIG. 4C  depicts an expansion element  466  including two cylinders  448  as previously described. In  FIG. 4C , the expansion element  466  includes a foam block  468  surrounding the cylinders  448 . The foam block  468  may replace the core or be additional to the core.  FIG. 4D  depicts yet another embodiment of an expansion element  486 . The expansion element  486  includes a cylinder  488  having an elongate sector cross-sectional shape. The cylinder includes a wall  492  and a core  494 . The cylinder  488  includes a first arcuate end  496  and a second arcuate end  498 . The first arcuate end  496  has a substantially larger radius than the second arcuate end  498 , thereby resulting in greater horizontal displacement at the first arcuate end when under load. Additionally, the wall thickness of any cylinder can be varied and/or the cylinder could be tapered along its length. In embodiments of the expansion element  126  that use a foam core, it is undesirable to bond the foam core to the walls of the expansion element  126 . Bonding the foam to the walls may inhibit horizontal expansion. 
       FIG. 4E  depicts an alternative type of adjustable element  410 . The adjustable element  410  includes a relatively flexible structural cylinder  412  and piston  414  arrangement. The internal volume  416  of the cylinder  412  varies as the piston  414  moves into and out of the cylinder  412 , shown generally by arrow  418 . The piston  414  is moved linearly by the driver  131  in response to the signal from the control system  120 . By varying the volume  416 , the compressibility of the cylinder  412  is varied. For example, when the piston  414  is moved into the cylinder  412 , the volume is reduced and the pressure within the cylinder is increased; the greater the pressure, the harder the cylinder. While this system may appear similar to that of an inflatable bladder, there are differences. For example, in this system, the amount of fluid, e.g., air, stays constant, while the volume  416  is adjusted. Further, bladders primarily react based on the pressure within the bladder, whereas the element  410  depicted in  FIG. 4E  uses the structure of the cylinder in combination with the internal pressure. The two are fundamentally different in operation. For example, the inflatable bladder, like a balloon, merely holds the air in and provides no structural support, while the cylinder, like a tire, uses the air to hold up the structure (e.g. the tire sidewalls). In addition, the piston  414  and driver  131  arrangement allows for fine adjustment of the pressure and compressibility of the adjustable element  410 . 
       FIG. 5A  depicts a side view of the article of footwear  100  of  FIG. 1 . The intelligent system  106  is disposed generally in the rearfoot portion  108  of the article of footwear  100 . As shown in  FIG. 5A , the intelligent system  106  includes the adjustable element  124  with the limiter  128  and the driver  131 . Also shown is a user-input module  500  ( FIG. 5B ) including user-input buttons  502 ,  504  and an indicator  506 . The user can set the compression range or other performance characteristic target value of the article of footwear  100 , by pushing input button  502  to increase the target value or pushing input button  504  to decrease the target value or range. In an alternative embodiment, the user-input module  500  can be remotely located from the shoe. For example, a wristwatch, personal digital assistant (PDA), or other external processor could be used alone or in combination with the user-input module  500  disposed on the article of footwear, to allow the user to customize characteristics of the intelligent system  106 . For example, the user may press buttons on the wristwatch to adjust different characteristics of the system  106 . In addition, the system  106  may include an on and off switch. 
     The user-input module  506  is shown in greater detail in  FIG. 5B . The indicator(s)  506  may be one or more light emitting diodes (LEDs) or organic light emitting diodes (OLE&#39;s), for example. In the embodiment shown, the indicator  506  is a series of LEDs printed on a flex-circuit that glow to indicate the range of compression selected; however, the indicators could also indicate the level of hardness of the midsole or some other information related to a performance characteristic of the footwear  100 . Alternatively or additionally, the indicator may be audible. 
       FIG. 6  depicts a top view of one possible arrangement of select components of the intelligent system of  FIG. 1 . The adjustable element  124  is disposed in the rearfoot portion  108  of the midsole  110  with the expansion element  126  laterally disposed within the cavity  118 . The driver  131  is disposed adjacent to the expansion element  126 . Adjacent to the driver  131  is the control system  120 . The control system  120  includes a control board  152  that holds two micro-controllers, one for controlling the driver  131  and one for processing the algorithm. Further, the system  106  includes a power source  150 , for example a 3.6V ½ AA battery. The power source  150  supplies power to the driver  131  and the control system  120  via wires  162  or other electrical connection, such as a flexcircuit. 
     The system  106  further includes the magnet  123  and the aligned sensor  122  (not shown), which is located under the expansion element  126  and is electrically coupled to the control system  120 . The magnet  123  is located above the expansion element  126 , but below an insole and/or sock liner. Further, the entire intelligent system  106  can be built into a plastic casing to make the system  106  waterproof. In addition, the system  106  can be built as a single module to facilitate fabrication of the sole  104  and may be pre-assembled to the lower support plate  114  (not shown in  FIG. 6 ). In one embodiment, the system  106  is removable, thereby making the system  106  replaceable. For example, the outsole  112   a ,  112   b  may be configured (e.g., hinged) to allow the system to be removed from the cavity  118  of the midsole  110 . 
     The system  106  may also include an interface port  160  that can be used to download data from the intelligent system  106 , for example to a PDA or other external processor. The port  106  can be used to monitor shoe performance. In an alternative embodiment, the data can be transmitted (e.g., via radio waves) to a device with a display panel located with the user. For example, the data can be transmitted to a wristwatch or other device being worn the user. In response to the data, the user may adjust certain characteristics of the shoe by pressing buttons on the wristwatch, as described above. These adjustments are transmitted back the system  106  where the adjustments are implemented. 
     A block diagram of one embodiment of an intelligent system  106  is shown in  FIG. 7 . The intelligent system  706  includes a power source  750  electrically coupled to a control system  720  and an actuation system  730 . The control system  720  includes a controller  752 , for example one or more micro-processors, and a sensor  722 . The sensor may be a proximity-type sensor and magnet arrangement. In one embodiment, the controller  152  is a microcontroller such as the PICMicro® manufactured by Microchip Technology Incorporated. In another embodiment, the controller  152  is a microcontroller manufactured by Cypress Semiconductor Corporation. The actuation system  730  includes a driver  731 , including a motor  732  and a transmission element  734 , and an adjustable element  724 . The driver  731  and control system  720  are in electrical communication. The adjustable element  724  is coupled to the driver  731 . 
     Optionally, the actuation system  730  could include a feedback system  754  coupled to or as part of the control system  720 . The feedback system  754  may indicate the position of the adjustable element  724 . For example, the feedback system  754  can count the number of turns of the motor  732  or the position of the limiter  728  (not shown). The feedback system  734  could be, for example, a linear potentiometer, an inductor, a linear transducer, or an infrared diode pair. 
       FIG. 8  depicts one possible algorithm for use with the intelligent system  106 . The intelligent system  106  measures a performance characteristic of a shoe during a walk/run cycle. Before the system  106  begins to operate, the system  106  may run a calibration procedure after first being energized or after first contacting the ground surface. For example, the system  106  may actuate the adjustable element  124  to determine the position of the limiter  128  and/or to verify the range of the limiter  128 , i.e., fully open or fully closed. During operation, the system  106  measures a performance characteristic of the shoe (step  802 ). In one embodiment, the measurement rate is about 300 Hz to about 60 KHz. The control system  120  determines if the performance characteristic has been measured at least three times (step  804 ) or some other predetermined number. If not, the system  106  repeats step  802  by taking additional measurements of the performance characteristic until step  804  is satisfied. After three measurements have been taken, the system  106  averages the last three performance characteristic measurements (step  806 ). The system  106  then compares the average performance characteristic measurement to a threshold value (step  808 ). At step  810 , the system  106  determines if the average performance characteristic measurement is substantially equal to the threshold value. If the average performance characteristic measurement is substantially equal to the threshold value, the system  106  returns to step  802  to take another performance characteristic measurement. If the average performance characteristic measurement is not substantially equal to the threshold value, the system  106  sends a corrective driver signal to the adjustable element  124  to modify the performance characteristic of the shoe. The intelligent system  106  then repeats the entire operation until the threshold value is reached and for as long as the wearer continues to use the shoes. In one embodiment, the system  106  only makes incremental changes to the performance characteristic so that the wearer does not sense the gradual adjustment of the shoe and does not have to adapt to the changing performance characteristic. In other words, the system  106  adapts the shoe to the wearer, and does not require the wearer to adapt to the shoe. 
     Generally, in a particular application, the system  106  utilizes an optimal midsole compression threshold (target zone) that has been defined through testing for a preferred cushioning level. The system  106  measures the compression of the midsole  110  on every step, averaging the most recent three steps. If the average is larger than the threshold then the midsole  110  has over-compressed. In this situation, the system  106  signals the driver  131  to adjust the adjustable element  124  in a hardness direction. If the average is smaller than the threshold, then the midsole  110  has under-compressed. In this situation, the system  106  signals the driver  131  to adjust the adjustable element in a softness direction. This process continues until the measurements are within the target threshold of the system. This target threshold can be modified by the user to be harder or softer. This change in threshold is an offset from the preset settings. All of the above algorithm is computed by the control system  120 . 
     In this particular application, the overall height of the midsole  110  and adjustable element  124  is about 20 mm. During testing, it has been determined that an optimal range of compression of the midsole  110  is about 9 mm to about 12 mm, regardless of the hardness of the midsole  110 . In one embodiment, the limiter  128  has an adjustment range that corresponds to about 10 mm of vertical compression. The limiter  128 , in one embodiment, has a resolution of less than or equal to about 0.5 mm. In an embodiment of the system  106  with user inputs, the wearer may vary the compression range to be, for example, about 8 mm to about 11 mm or about 10 mm to about 13 mm. Naturally, ranges of greater than 3 mm and lower or higher range limits are contemplated. 
     During running, the wearer&#39;s foot goes through a stride cycle that includes a flight phase (foot in the air) and a stance phase (foot in contact with the ground). In a typical stride cycle, the flight phase accounts for about ⅔ of the stride cycle. During the stance phase, the wearer&#39;s body is normally adapting to the ground contact. In a particular embodiment of the invention, all measurements are taken during the stance phase and all adjustments are made during the flight phase. Adjustments are made during the flight phase, because the shoe and, therefore, the adjustable element are in an unloaded state, thereby requiring significantly less power to adjust than when in a loaded state. In most embodiments, the shoe is configured such that the motor does not move the adjustable element, therefore lower motor loads are required to set the range of the adjustable element. In the embodiments depicted in  FIGS. 15 ,  16 , and  17 , however, the adjustable element does move, as described in greater detail hereinbelow. 
     During operation, the system  106  senses that the shoe has made contact with the ground. As the shoe engages the ground, the sole  104  compresses and the sensor  122  senses a change in the magnetic field of the magnet  123 . The system  106  determines that the shoe is in contact with the ground when the system  106  senses a change in the magnetic field equal to about 2 mm in compression. It is also at this time that the system  106  turns off the power to the actuation system  130  to conserve power. During the stance phase, the system  106  senses a maximum change in the magnetic field and converts that measurement into a maximum amount of compression. In alternative embodiments, the system  106  may also measure the length of the stance phase to determine other performance characteristics of the shoe, for example velocity, acceleration, and jerk. 
     If the maximum amount of compression is greater than 12 mm, then the sole  104  has over-compressed, and if the maximum amount of compression is less than 9 mm, then the sole  104  has under-compressed. For example, if the maximum compression is 16 mm, then the sole  104  has over-compressed and the control system  120  sends a signal to the actuation system  130  to make the adjustable element  124  firmer. The actuation system  130  operates when the shoe is in the flight phase, i.e., less than 2 mm of compression. Once the system  106  senses that the compression is within the threshold range, the system  106  continues to monitor the performance characteristic of the shoe, but does not further operate the actuation system  130  and the adjustable element  124 . In this way, power is conserved. 
     In alternative embodiments, the intelligent system  106  can use additional performance characteristics alone or in combination with the optimal midsole compression characteristic described above. For example, the system  106  can measure, in addition to compression, time to peak compression, time to recovery, and the time of the flight phase. These variables can be used to determine an optimum setting for the user, while accounting for external elements such as ground hardness, incline, and speed. Time to peak compression is described as the amount of time that it takes from heel strike to the maximum compression of the sole while accounting for surface changes. It may be advantageous to use the area under a time versus compression curve to determine the optimum compression setting. This is in effect a measure of the energy absorbed by the shoe. In addition, the time of the flight phase (described above) can contribute to the determination of the optimum setting. The stride frequency of the user can be calculated from this variable. In turn, stride frequency can be used to determine changes in speed and to differentiate between uphill and downhill motion. 
       FIG. 9  illustrates one embodiment of an electrical circuit  900  suitable for implementing an intelligent system  106  in accordance with the invention. The electrical circuit  900  includes a sensing system  1100  ( FIG. 11 ), a control system  1200  ( FIG. 12 ), and an actuation system  1300  ( FIG. 13 ). The control system  1200  further includes a voltage regulator system  1000  ( FIG. 10 ). 
     The voltage regulator system  1000  is a step-up DC/DC voltage regulator system; however, other types of voltage regulator systems are possible, including no voltage regulator system. Referring to  FIG. 10 , the input voltage of a power supply  1004  is stepped up to a higher voltage at the output  1008  of the voltage regulator system  1000 . In the embodiment shown, the voltage regulator system  1000  includes the power supply  1004 , a switch  1012 , an input bypass capacitor  1016 , and a TC125 PFM step-up DC/DC regulator  1020  coupled to an external resistor  1024 , an inductor  1028 , an output diode  1032 , an output capacitor  1036 , and three connection points (output  1008 , ground  1040 , and output  1014 ). The power supply  1004  is a 3.6 volt DC battery and the stepped-up voltage is 5 volts DC at the output  1008  of the voltage regulator system  1000 . The switch  1012  acts as a basic on/off switch for the electrical circuit  900  (not shown). With the switch  1012  closed, the input bypass capacitor  1016  is connected in parallel with the power supply  1004 . The ground  1040  of the power supply  1004  is connected to the ground terminal pin  1048  of the TC125 regulator  1020  and to a pin  1052  of the TC125 regulator  1020 . The output  1008  of the voltage regulator system  1000  is connected to the power and voltage sense input pin  1056  of the TC125 regulator  1020 . The output  1008  of the voltage regulator system  1000  provides both internal chip power and feedback voltage sensing for closed-loop regulation to the TC125 regulator  1020 . The external inductor  1028  is connected, when switch  1012  is closed, between the positive terminal  1044  of the power supply  1004  and the inductor switch output pin  1060  of the TC125 regulator  1020 . The output diode  1032 , which, in the embodiment shown, is a Schottky diode, is connected between the inductor switch output pin  1060  and the power and voltage sense input pin  1056  of the TC125 regulator  1020 . The output capacitor  1036  is connected between the ground  1040  of the power supply  1004  and the power and voltage sense input pin  1056  of the TC125 regulator  1020 . The resistor  1024  is connected between the shutdown input pin  1066  of the TC125 regulator  1020  and the ground  1040  of the power supply  1004 . 
     Referring to  FIG. 11 , the sensing system  1100  includes a hall element  1104 , an operational amplifier (“op amp”)  1108 , and resistors  1112 ,  1116 ,  1120 , and  1124 . In an alternate embodiment, the hall element  1104  and the op amp  1108  may be replaced with a hall sensor that provides the equivalent functionality in a single package. The op amp  1108 , as described below, produces a pulsed output signal. Alternatively, the output  1008  of the voltage regulator system  1000  is connected to a terminal  1128  of the sensor  1104  and provides power to the sensor  1104 . A micro-controller  1204  of the control system  1200  is connected to a terminal  1132  of the sensor  1104  via connection point  1248 . See  FIG. 12 . The micro-controller  1204  alternately pulses the sensor  1104  on, by sending it a ground signal, and then off. The sensor  1104  is pulsed on to measure magnetic field strength, as described above, and then off to conserve power. When pulsed on, the sensor  1104  outputs voltages at its terminals  1136  and  1140 . The resistor  1112  is connected between the terminal  1140  of the sensor  1104  and the inverting input  1144  of the op amp  1108 . The resistor  1120  is connected between the terminal  1136  of the sensor  1104  and the non-inverting input  1148  of the op amp  1108 . The resistor  1116  is connected between the inverting input  1144  of the op amp  1108  and the micro-controller  1204  of the control system  1200  via connection point  1160 . The resistor  1124  is connected between the non-inverting input  1148  of the op-amp  1108  and the ground  1040  of the power supply  1004  of the voltage regulator system  1000 . The positive supply voltage terminal  1152  of the op amp  1108  is connected to the output  1008  of the voltage regulator system  1000  and the negative supply voltage terminal  1156  of the op amp  1108  is connected to the ground  1040  of the power supply  1004  of the voltage regulator system  1000 . In the embodiment shown, the op amp  1108  amplifies the difference in voltage between the terminal  1136  and the terminal  1140  of the hall effect sensor  1104 . With a 1 kΩ resistor  1112 , a 200 kΩ resistor  1116 , a 1 kΩ resistor  1120 , and a 200 kΩ resistor  1124 , the voltage at the op amp output  1160  is the difference in voltage between the terminal  1136  and the terminal  1140  of the sensor  1104  amplified by a factor of 200 (i.e., V1160=200 [V1136−V1140]). Other amplification factors may be achieved by choosing appropriate resistances for resistors  1112 ,  1116 ,  1120 , and  1124 . The op amp output  1160  is connected to micro-controller  1204  of the control system  1200 . The pulsed output signal of the op amp  1108  is thus inputted to the micro-controller  1204 . 
     Referring to  FIG. 12 , the control system  1200  includes the voltage regulator system  1000 , a micro-controller  1204 , light emitting diodes (“LEDs”)  1208 , two switches  1212 ,  1216 , and an external RC oscillator including a resistor  1240  and a capacitor  1244 . The output  1008  of the voltage regulator system  1000  is connected to the micro-controller  1204  in order to power the micro-controller  1204 . The output  1008  of the voltage regulator system  1000  is further connected through a resistor  1220  to a different pin of the micro-controller  1204  to allow for active low reset of the micro-controller  1204 . The ground  1040  of the power supply  1004  is connected to the micro-controller  1204  to provide it with a ground reference. The LEDs  1208  provide a visual output to the user of, for example, the current softness/hardness setting of the midsole. The cathodes  1224  of the LEDs  1208  are connected to the micro-controller  1204  and the anodes  1228  of the LEDs  1208  are connected through resistors  1232  to the output  1008  of the voltage regulator system  1000 . The micro-controller  1204  turns on or off one or several of the LEDs  1208 . Switches  1212  and  1216  are connected between the ground  1040  of the power supply  1004  and, through resistors  1236 , the positive terminal  1044  of the power supply  1004  when the switch  1012  is closed. Switches  1212  and  1216 , when closed, connect various pins of the micro-controller  1204  to the ground  1040  of the power supply  1004 . The user may adjust, for example, the midsole compression threshold by closing either switch  1212  or  1216  of the control system  1200 . The user does so by actuating push buttons, located on the outside of the shoe, which control switches  1212  and  1216  of the control system  1200 . The resistor  1240  of the external RC oscillator is connected between the output  1008  of the voltage regulator system  1000  and an input pin to the timing circuitry of the micro-controller  1204 . The capacitor  1244  of the external RC oscillator is connected between the ground  1040  of the power supply  1004  and the same input pin to the timing circuitry of the micro-controller  1204 . 
     Referring to  FIG. 13 , the actuation system  1300  includes transistor bridges  1304  and  1308 , a motor  1312  connected in parallel with a capacitor  1316 , and a potentiometer  1320 . In the embodiment shown, the transistor bridge  1304  includes an n-Channel MOSFET  1324  and a p-Channel MOSFET  1328  and the transistor bridge  1308  includes an n-Channel MOSFET  1332  and a p-Channel MOSFET  1336 . The source  1340  of MOSFET  1324  and the source  1344  of MOSFET  1332  are connected to the ground  1040  of the power supply  1004 . The source  1348  of MOSFET  1328  and the source  1352  of MOSFET  1336  are connected to the output  1014  of the power supply  1004  when the switch  1012  is closed. The gate  1356  of MOSFET  1324  and the gate  1360  of MOSFET  1336  are connected to the X pin of the micro-controller  1204  via connection point  1252 . The gate  1364  of MOSFET  1328  and the gate  1368  of MOSFET  1332  are connected to the Y pin of micro-controller  1204  via connection point  1256 . The drain  1372  of MOSFET  1324  and the drain  1376  of MOSFET  1336  are connected to a terminal  1380  of the motor  1312 . The drain  1384  of MOSFET  1328  and the drain  1388  of MOSFET  1332  are connected to a terminal  1392  of the motor  1312 . In order to drive the motor  1312  in one direction, the micro-controller  1204  turns on MOSFETs  1324  and  1328  while MOSFETs  1332  and  1336  are turned off. In order to drive the motor  1312  in the opposite direction, the micro-controller  1204  turns on MOSFETs  1332  and  1336  while MOSFETs  1324  and  1328  are turned off. A terminal  1396  of the potentiometer  1320  is connected to the output  1008  of the voltage regulator system  1000  and a terminal  1394  of the potentiometer  1320  is connected to the ground  1040  of the power supply  1004 . The voltage at the wiper terminal  1398  of the potentiometer  1320  is measured by the micro-controller  1204  via connection point  1260 . Depending on the direction that the motor  1312  is made to turn by the micro-controller  1204 , the wiper terminal  1398  of the potentiometer  1320  is moved in one direction or the other. The voltage at the wiper terminal  1398  of the potentiometer  1320  therefore indicates the position of the limiter  128  based on the number of turns that the motor  1312  has made. 
       FIG. 14  illustrates an alternative embodiment of an electrical circuit  900 ′ suitable for implementing an intelligent cushioning system in accordance with the invention. As in the first embodiment, the electrical circuit  900 ′ includes a sensing system  1100 ′, a control system  1200 ′, and an actuation system  1300 ′. Again, as in the first embodiment, the control system  1200 ′ includes a voltage regulator system  1000 ′. 
     As opposed to the first embodiment, the voltage regulator system  1000 ′ is a step-down DC/DC voltage regulator system. The input voltage of a power supply  1004 ′ is stepped down to a lower voltage at the output  1008 ′ of the voltage regulator system  1000 ′. In the embodiment shown, the voltage regulator system  1000 ′ includes the power supply  1004 ′, a switch  1012 ′, an input capacitor  1404 , an LTC3405A step-down DC/DC regulator  1408 , an inductor  1412 , an output capacitor  1416 , resistors  1420  and  1424 , and a capacitor  1428 . The power supply  1004 ′ is a 3.6 volt DC battery and the stepped-down voltage at the output  1008 ′ of the voltage regulator system  1000 ′ may be chosen by selecting appropriate resistances for the resistors  1420  and  1424 . The switch  1012 ′ acts as a basic on/off switch for the electrical circuit  900 ′. When the switch  1012 ′ is closed, the input capacitor  1404  is connected in parallel with the power supply  1004 ′. Moreover, when the switch  1012 ′ is closed, the positive terminal  1044 ′ of the power supply  1004 ′ is connected to the run control input pin  1432  and the main supply pin  1436  of the LTC3405A regulator  1408 . The ground  1040 ′ of the power supply  1004 ′ is connected to the ground pin  1440  and the mode select input pin  1444  of the LTC3405A regulator  1408 . The inductor  1412  is connected between the switch node connection to inductor pin  1448  of the LTC3405A regulator  1408  and the output  1008 ′ of the voltage regulator system  1000 ′. The output capacitor  1416  is connected between the output  1008 ′ of the voltage regulator system  1000 ′ and the ground  1040 ′ of the power supply  1004 ′. The resistor  1420  is connected between the feedback pin  1452  of the LTC3405A regulator  1408  and the ground  1040 ′ of the power supply  1004 ′. The capacitor  1428  is connected in parallel with the resistor  1424 . Both the resistor  1424  and the capacitor  1428  are connected between the feedback pin  1452  of the LTC3405A regulator  1408  and the output  1008 ′ of the voltage regulator system  1000 ′. 
     The sensing system  1100 ′, including a hall element type sensor  1104 ′ and an op amp  1108 ′, is similar to the sensing system  1100  of the electrical circuit  900 . In an alternate embodiment, the hall element  1104 ′ and the op amp  1108 ′ may be replaced with a hall sensor that provides the equivalent functionality in a single package. The op amp  1108 ′ produces the same output signal at its output  1160 ′ as does the op amp  1108  at its output  1160 ; however, the sensing system  1100 ′ is different in several respects. First, rather than being connected to the output of a step-up DC/DC voltage regulator system, the terminal  1128 ′ of the sensor  1104 ′ and the positive supply voltage terminal  1152 ′ of the op amp  1108 ′ are instead connected, when the switch  1012 ′ is closed, to the positive terminal  1044 ′ of the power supply  1004 ′. Second, the micro-controller  1204 ′, in addition to being connected to a terminal  1132 ′ of the sensor  1104 ′, is also connected to the negative supply voltage terminal  1156 ′ of the op amp  1108 ′. The micro-controller  1204 ′, therefore, alternately pulses the op amp  1108 ′ on and then off, to conserve power, in tandem with the sensor  1104 ′. Finally, the resistor  11124 ′, rather than being connected to the ground  1040 ′ of the power supply  1004 ′, is instead connected to the pin of the micro-controller  1204 ′ that is used to alternately pulse the sensor  1104 ′ and the op amp  1108 ′ on and then off. Nevertheless, when the micro-controller  1204 ′ pulses a ground signal to the terminal  1132 ′ and the negative supply voltage terminal  1156 ′ to turn on the sensor  1104 ′ and the op amp  1108 ′, respectively, the resistor  1124 ′ is effectively connected to the pulsed ground signal. 
     The control system  1200 ′ is similar to the control system  1200  of the electrical circuit  900 ; however, the control system  1200 ′ is different in several respects. First, rather than being connected to and powered by the output of a step-up DC/DC voltage regulator system, the micro-controller  1204 ′ is, when the switch  1012 ′ is closed, directly connected to the positive terminal  1044 ′ of the power supply  1004 ′ and therefore directly powered by the power supply  1004 ′. Second, rather than being connected to the output of a step-up DC/DC voltage regulator system, the resistors  1220 ′,  1232 ′, and  1240 ′ are connected, when the switch  1012 ′ is closed, to the positive terminal  1044 ′ of the power supply  1004 ′. 
     The actuation system  1300 ′ is similar to the actuation system  1300  of the electrical circuit  900 ; however, the actuation system  1300 ′ is different in several respects. First, rather than being connected to the positive terminal  1044 ′ of the power supply  1004 ′ when the switch  1012 ′ is closed, the source  1348 ′ of MOSFET  1328 ′ and the source  1352 ′ of MOSFET  1336 ′ are instead connected to the output  1008 ′ of the voltage regulator system  1000 ′. Second, rather than being connected to the output of a step-up DC/DC voltage regulator system, the terminal  1396 ′ of the potentiometer  1320 ′ is instead connected, when the switch  1012 ′ is closed, to the positive terminal  1044 ′ of the power supply  1004 ′. Finally, rather than being connected to the ground  1040 ′ of the power supply  1004 ′, the terminal  1394 ′ of the potentiometer  1320 ′ is instead connected to the pin of the micro-controller  1204 ′ that is used to alternately pulse the sensor  1104 ′ and the op amp  1108 ′ on and then off. Nevertheless, when the micro-controller  1204 ′ pulses a ground signal to the terminal  1132 ′ and the negative supply voltage terminal  1156 ′ to turn on the sensor  1104 ′ and the op amp  1108 ′, respectively, the terminal  1394 ′ of the potentiometer  1320 ′ is effectively connected to the pulsed ground signal. 
       FIGS. 15A and 15B  depict an article of footwear  1500  including an alternative intelligent system  1506 . The article of footwear  1500  includes an upper  1502 , a sole  1504 , and the intelligent system  1506 . The intelligent system  1506  is disposed in the rearfoot portion  1508  of the sole  1504 . The intelligent system  1506  includes a driver  1531  and an adjustable element  1524  of one or more similar components. The adjustable element  1524  is shown in greater detail in  FIG. 15B  and includes two dual density tuning rods  1525  that are rotated in response to a corrective driver signal to modify a performance characteristic of the footwear  1500 . The dual density rods  1525  have an anisotropic property and are described in detail in pending U.S. patent application Ser. No. 10/144,440, the entire disclosure of which is hereby incorporated herein by reference. The dual density rods  1525  are rotated by the motor  1532  and the transmission element  1534  to make the sole  1504  harder or softer. The transmission element  1534  is coupled to the dual density rods  1525  at about a lateral midpoint of the rods  1525 , for example by a rack and pinion or worm and wheel arrangement. 
       FIG. 16A  depicts an article of footwear  1600  including an alternative intelligent system  1606 .  FIGS. 16B-16D  depict the adjustable element  1624  in various states of operation. The article of footwear  1600  includes an upper  1602 , a sole  1604 , and the intelligent system  1606 . The intelligent system  1606  includes a driver  1631  and an adjustable element  1624 . The adjustable element  1624  includes two multi-density plates  1625 ,  1627 . One of the plates, in this embodiment lower plate  1627 , is slid relative to the other plate, in this embodiment upper plate  1625 , by the driver  1631 , in response to the corrective driver signal to modify the performance characteristic of the shoe (arrow  1680 ). 
     The plates  1625 ,  1627  are made of alternating density materials. In particular, the plates  1625 ,  1627  are made up of alternating strips of a relatively soft material  1671  and a relatively hard material  1673 . The alignment of the different density portions of the plates  1625 ,  1627  determines the performance characteristic of the shoe. In  FIG. 16B , the relatively hard materials  1673  are substantially aligned, thereby resulting in a relatively hard adjustable element  1624 . In  FIG. 16C , the different density materials  1671 ,  1673  are only partially aligned, thereby resulting in a softer adjustable element  1624 . In  FIG. 16D , the relatively hard materials  1673  and the relative soft materials  1671  are substantially aligned, thereby resulting in the softest possible adjustable element  1624 . 
       FIGS. 17A and 17B  depict an article of footwear  1700  including an alternative intelligent system  1706 . The article of footwear  1700  includes an upper  1702 , a sole  1704 , and the intelligent system  1706 . The intelligent system  1706  is disposed in the rearfoot portion  1708  of the sole  1704 . The intelligent system  1706  includes a driver  1731  (not shown, but similar to those described hereinabove) and an adjustable element  1724 . The adjustable element  1724  is a multi-density heel portion  1726  that swivels relative to the sole  1704  (see arrow  1750  in  FIG. 17B ). Swiveling the heel portion  1726  modifies the mechanical properties of the footwear  1700  at a heel strike zone  1782 . The heel portion  1726  swivels about a pivot point  1784  in response to a force from the driver  1731 . 
     The various components of the adjustable elements described herein can be manufactured by, for example, injection molding or extrusion and optionally a combination of subsequent machining operations. Extrusion processes may be used to provide a uniform shape, such as a single monolithic frame. Insert molding can then be used to provide the desired geometry of the open spaces, or the open spaces could be created in the desired locations by a subsequent machining operation. Other manufacturing techniques include melting or bonding additional elements. For example, the cylinders  448  may be joined with a liquid epoxy or a hot melt adhesive, such as ethylene vinyl acetate (EVA). In addition to adhesive bonding, components can be solvent bonded, which entails using a solvent to facilitate fusing of various components or fused together during a foaming process. 
     The various components can be manufactured from any suitable polymeric material or combination of polymeric materials, either with or without reinforcement. Suitable materials include: polyurethanes, such as a thermoplastic polyurethane (TPU); EVA; thermoplastic polyether block amides, such as the Pebax® brand sold by Elf Atochem; thermoplastic polyester elastomers, such as the Hytrel® brand sold by DuPont; thermoplastic elastomers, such as the Santoprene® brand sold by Advanced Elastomer Systems, L.P.; thermoplastic olefin; nylons, such as nylon 12, which may include 10 to 30 percent or more glass fiber reinforcement; silicones; polyethylenes; acetal; and equivalent materials. Reinforcement, if used, may be by inclusion of glass or carbon graphite fibers or para-aramid fibers, such as the Kevlar® brand sold by DuPont, or other similar method. Also, the polymeric materials may be used in combination with other materials, for example natural or synthetic rubber. Other suitable materials will be apparent to those skilled in the art. 
     In a particular embodiment, the expansion element  126  can be made of one or more various density foams, non-foamed polymer materials, and/or skeletal elements. For example, the cylinder could be made of Hytrel® 4069 or 5050 with a 45 Asker C foamed EVA core. In another embodiment, the cylinder is made of Hytrel® 5556 without an inner core foam. The expansion element  126  can have a hardness in the range of about 40 to about 70 Asker C, preferably between about 45 and about 65 Asker C, and more preferably about 55 Asker C. In an alternative embodiment, the tuning rods  1525 , the multiple density plates  1625 ,  1627 , or the upper and lower support plates  114 ,  116  may be coated with an anti-friction coating, such as a paint including Teflon® material sold by DuPont or a similar substance. The various components can be color coded to indicate to a wearer the specific performance characteristics of the system and clear windows can be provided along the edge of the sole. The size and shape of the various components can vary to suit a particular application. In one embodiment, the expansion element  126  can be about 10 mm to about 40 mm in diameter, preferably about 20 mm to about 30 mm, and more preferably about 25 mm. The length of the expansion element  126  can be about 50 mm to about 100 mm, preferably about 75 mm to about 90 mm, and more preferably 85 mm. 
     In addition, the expansion element  126  can be integrally formed by a process called reverse injection, in which the cylinder  142  itself forms the mold for the foam core  144 . Such a process can be more economical than conventional manufacturing methods, because a separate core mold is not required. The expansion element  126  can also be formed in a single step called dual injection, where two or more materials of differing densities are injected simultaneously to create integrally the cylinder  142  and the core  144 . 
       FIG. 18  is a graph depicting a performance characteristic of an adjustable element at two different settings (curves A and B). The graph depicts the amount of deformation of the adjustable element in a loaded condition, i.e., under compression. As can be seen, each curve A, B has two distinct slopes  1802 ,  1804 ,  1806 ,  1808 . The first slope  1802 ,  1806  of each curve generally represents the adjustable element from first contact until the adjustable element contacts the limiter. During this phase, the resistance to compression comes from the combined effect of the structural wall and core of the adjustable element, which compress when loaded. The second slope  1804 ,  1808  of each curve represents the adjustable element under compression while in contact with the limiter. During this phase, very little additional deformation of the adjustable element is possible and the additional force attempts to bend or buckle the structural wall. 
     At setting A, which is a relatively hard setting, the adjustable element deforms about 6.5 mm when a force of 800 N is applied to the adjustable element, as represented by slope  1802 . At this point, the adjustable element has contacted the limiter and very little additional deformation is possible. As slope  1804  represents, the additional deformation of the adjustable element is only about 2 mm after an additional force of 800 N is applied to the adjustable element. At setting B, which is a relatively soft setting, the adjustable element deforms about 8.5 mm when a force of 800 N is applied to the adjustable element, as represented by slope  1806 . At this point, the adjustable element has contacted the limiter and very little additional deformation is possible. As slope  1808  represents, the additional deformation of the adjustable element is only about 2.5 mm after an additional force of 800 N is applied to the adjustable element. 
       FIG. 19  depicts a flow chart representing a method of modifying a performance characteristic of an article of footwear during use. The method includes monitoring the performance characteristic of the article of footwear (step  1910 ), generating a corrective driver signal based on the monitored performance characteristic (step  1920 ), and adjusting an adjustable element based on the driver signal to modify the performance characteristic of the article of footwear (step  1930 ). In a particular embodiment, the steps are repeated until a threshold value of the performance characteristic is obtained (step  1940 ). 
     One possible embodiment of the monitoring step  1910  is expanded in  FIG. 20A . As shown, monitoring the performance characteristic involves measuring a magnetic field of a magnet with a proximity-type sensor (substep  2010 ) and comparing the magnetic field measurement to a threshold value (substep  2020 ). Optionally, monitoring the performance characteristic may include taking, multiple measurements of the magnetic field and taking an average of some number of measurements. The system then compares the average magnetic field measurement to the threshold value (optional substep  2030 ). The system could repeat these steps as necessary (optional substep  2040 ) until the magnetic field measurement is substantially equal to the threshold value, or within a predetermined value range. 
     One possible embodiment of the generating step  1920  is expanded in  FIG. 20B . As shown, generating the corrective driver signal involves comparing the monitored performance characteristic to a desired performance characteristic (substep  2050 ), generating a deviation (substep  2060 ), and outputting a corrective driver signal magnitude based on the deviation (substep  2070 ). In one embodiment, the corrective driver signal has a predetermined magnitude, such that a predetermined amount of correction is made to the performance characteristic. In this way, the system makes incremental changes to the performance characteristic that are relatively imperceptible to the wearer, thereby eliminating the need for the wearer to adapt to the changing performance characteristic. 
       FIG. 21  depicts a flow chart representing a method of providing comfort in an article of footwear. The method includes providing an adjustable article of footwear (step  2110 ) and determining a jerk value (step  2120 ). Jerk is represented as a change of acceleration over a change in time (Δa/Δt). The jerk value can be derived from the distance measurement, based on the changing magnetic field, over a known time period. A control system records the change in the magnetic field over time and is able to process these measurements to arrive at the jerk value. The method may further include modifying a performance characteristic of the adjustable article of footwear based on the jerk value (optional step  2130 ), for example, to keep the jerk value below a predetermined maximum value. 
     Having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.