Abstract:
A shoe having a toe region, a middle region, a heel region, and a multi-layer, multi-density midsole; the midsole being comprised of at least a shank and a lower layer; the bottom surface of the shank having at least one longitudinal concavity and at least one longitudinal convexity, the longitudinal concavity typically occupying a substantial portion of the heel region and the longitudinal convexity typically occupying a portion of the middle region. Collectively, these elements contribute to making the shoe appropriate for both walking and higher impact activities such as running, and simulating the effect, and imparting the fitness benefits, of use on a sandy beach or on a giving or uneven surface regardless of the actual hardness of the surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of patent application Ser. No. 12/776,253 filed on May 7, 2010 which is a continuation in part of patent application Ser. No. 12/557,276 filed on Sep. 10, 2009 which claims the benefit of priority based on U.S. Provisional Application No. 61/122,911 filed Dec. 16, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to footwear and, in particular, to a shoe with fitness benefits which can be used during high impact activities such as running. The fitness benefits are imparted by a unique running or walking motion which is induced primarily by the shoe&#39;s midsole. The midsole has multiple layers and multiple densities. One of the layers of the midsole is a shank that allows the shoe to be lighter and to have a lower-profile which results in the user&#39;s foot being positioned closer to the ground; the shank also provides increased heel and midfoot support. As a result of these qualities/characteristics, the shoe can be worn during high impact activities such as running. The motion induced by the shoe mimics the effect of running or walking on a sandy beach or on a giving or uneven surface. 
         [0004]    2. Description of the Related Art 
         [0005]    Shoes are designed for many purposes—from protection on the job, to performance during athletic activity, to everyday use. Shoes have also been used to promote physical health and activity. Increasingly, shoes have been designed to increase the fitness benefits that users get from everyday uses such as walking. However, there continues to be a need for such shoes that increase the fitness benefits to users yet are comfortable, easy to use, and able to be used for high impact activities such as running. 
         [0006]    Walking and running are the easiest and most beneficial forms of exercise. When done properly and with the appropriate footwear, they strengthen the heart, improve cardiovascular health, increase one&#39;s stamina and improve posture. Walking and running also help to strengthen and tone one&#39;s muscles and maintain joint flexibility. 
         [0007]    Prior art shoes have attempted to improve the user&#39;s fitness by mimicking walking barefoot. See, for example, U.S. Pat. No. 6,341,432 to Müller. Such shoes can include an abrupt, discrete pivot point provided by a hard inclusion. Consequently, in every step taken during normal walking while wearing such shoes, the user is forced to overcome this abrupt, discrete pivot point. This can result in significant pain and discomfort. 
         [0008]    Prior art shoes that have attempted to mimic walking barefoot have been rather large and clunky. They also have not been suitable for running or other high impact activities due to their relatively significant weight, high midsole profile, and low level of heel and midfoot support. In order for a shoe to be optimum for running and other high impact activities, it must have a relatively low profile which allows the foot to be positioned closer to the ground. In addition, the shoe must be light weight and provide sufficient support to the user&#39;s foot. 
         [0009]    The present invention aims to provide a way of mimicking running or walking on a sandy beach or on a giving or uneven surface, while not inducing any pain or discomfort from doing so. By mimicking running or walking on a sandy beach and/or on an uneven surface, the present invention aims to significantly increase the fitness and health benefits of everyday running or walking by requiring the user to exert additional effort and energy and to use muscles that the user otherwise would not use if wearing ordinary footwear, again all without inducing any pain or discomfort. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of the present invention to provide a shoe that can be used during high impact activities such as running and which provides certain fitness benefits not imparted by ordinary shoes. It does this by mimicking the effects of running or walking on a sandy beach or on a giving or uneven surface without inducing any pain or discomfort from doing so. 
         [0011]    The present invention is a shoe comprising an upper, an outsole, and a midsole, each having a medial side and a lateral side. In a preferred embodiment, the midsole is affixed to the upper and the outsole is affixed to the midsole. The upper, midsole, and outsole each has a frontmost point and a rearmost point substantially opposite the frontmost point. As the terms imply, each frontmost point is closer to the user&#39;s toes than each rearmost point while at the same time each rearmost point is closer to the user&#39;s heel than each frontmost is point. 
         [0012]    The midsole is unique in that it comprises a plurality of layers. In a preferred embodiment, the midsole comprises an upper layer, a shank and a lower layer. In a preferred embodiment, the upper layer has a first density and the lower layer has a second density. The second density of the lower layer is less than the first density of the upper layer. 
         [0013]    Throughout the midsole, the thickness of the upper layer and lower layer may vary. In some instances, the lower layer is thicker than the upper layer or vice versa. In the regions in which the less dense lower layer is thicker, such as the heel, the midsole is less stable. Therefore, it provides the effect of walking or running on sand or an uneven surface. However, in regions in which the less dense lower layer is thicker, the relatively denser upper layer and shank provide some compensating stability to the user&#39;s foot. The benefits of the different densities and thicknesses will be further discussed herein below. 
         [0014]    The shank is positioned in between the upper layer and the lower layer. The addition of the shank provides at least two groups of benefits. The first group of benefits is that the shank allows the midsole to be constructed with a relatively thinner upper layer. Because the midsole is made thinner due to the shank, the users&#39; foot is placed closer to the ground and therefore provides better footing for high impact activities such as running. Furthermore, the thinner upper layer not only is more aesthetically pleasing, but since there is less material, the midsole is lighter than a midsole with a relatively thick upper layer, thereby making the entire shoe lighter. The second group of benefits is that the shank provides enhanced support to the user&#39;s foot and thus allows the user to engage in faster paced activities such as running. The shank also disperses the force and pressure from the foot strike more evenly throughout the shoe. 
         [0015]    The shoe has a front tip that is located at the farthest forward point of the shoe when moving from the rear portion to the front portion. The shoe has a rear tip that is located at the farthest rearward point of the shoe when moving from the front portion to the rear portion. In a preferred embodiment, the front tip coincides with the frontmost point of the upper, the frontmost point of the midsole, or the frontmost point of the outsole while the rear tip coincides with the rearmost point of the upper, the rearmost point of the midsole, or the rearmost point of the outsole. In a preferred embodiment, the frontmost point of the upper, the frontmost point of the midsole, and the frontmost point of the outsole are all located relatively close to one another while the rearmost point of the upper, the rearmost point of the midsole, and the rearmost point of the outsole are all located relatively close to one another. 
         [0016]    The upper, midsole, and outsole each has a toe region. The toe region includes the region that extends substantially from the medial side to the lateral side at a location that begins in the vicinity of the front tip of the shoe and extends from there to a location that is approximately one third of the distance toward the rear tip of the shoe. 
         [0017]    The upper, midsole, and outsole each has a heel region. The heel region includes the region that extends substantially from the medial side to the lateral side at a location that begins in the vicinity of the rear tip of the shoe and extends from there to a location that is approximately one third of the distance toward the front tip of the shoe. 
         [0018]    The upper, midsole, and outsole each has a middle region. The middle region includes the region that extends substantially from the medial side to the lateral side at a location that extends approximately between the toe region and the heel region. 
         [0019]    In a preferred embodiment, the midsole further comprises an upper layer, shank and a lower layer, the upper layer having a first density and the lower layer having a second density different from the first density. In between the upper layer and lower layer, there is a shank that extends longitudinally from the heel region to the toe region. The upper layer, the shank and the lower layer each has a top surface and a bottom surface. 
         [0020]    In a preferred embodiment, the bottom surface of the upper layer rests on the top surface of the shank, and the bottom surface of the shank rests on the top surface of the lower layer. 
         [0021]    In a preferred embodiment, the shank extends from the heel region to the toe region and extends longitudinally along the entire midsole. However, without deviating from the scope of the invention, the shank may extend from the heel region to the middle region or part of the toe region without extending the entire length of the shoe. 
         [0022]    In a preferred embodiment, the bottom surface of the upper to layer is in substantially continuous contact with, and substantially conforms to, the top surface of the shank. Likewise, the bottom surface of the shank is in substantially continuous contact with, and substantially conforms to, the top surface of the lower layer. 
         [0023]    In a preferred embodiment, the shank is comprised of two portions, a top portion and a bottom portion. The top portion and the bottom portion of the shank can be separate pieces which are affixed together or alternatively they can comprise one unitary structure. 
         [0024]    In a preferred embodiment, as the shank longitudinally extends along the midsole from the heel region to the toe region, the bottom surface of the shank forms a single longitudinal concavity (as defined below) that occupies a substantial portion of the heel region and terminates at a point in the middle region. Upon termination of the longitudinal concavity, the bottom surface of the shank forms a longitudinal convexity (as defined below) that occupies a portion of the middle region. The longitudinal convexity then terminates. Upon termination of the longitudinal convexity, a second longitudinal concavity begins on the bottom surface of the shank. The second longitudinal concavity on the bottom surface of the shank occupies a portion of the middle and/or toe regions of the midsole. 
         [0025]    In a preferred embodiment, due to the shape of the top portion and bottom portion of the shank, a cavity is formed within the shank. For reference, the cavity begins at a point longitudinally closer to the heel region and that point is referred to as the start of the cavity. The cavity terminates at a point longitudinally closer to the middle region and that point is referred to as the end of the cavity. The cavity is completely open from the lateral to medial side of the shoe. The cavity causes the shank to provide better support to the heel and midfoot areas of the foot and disperses the force and pressure of the foot strike more evenly throughout the shoe. 
         [0026]    In a preferred embodiment, the invention includes an outsole that, when no load is applied, gently curves continuously upward in a direction toward the upper beginning at a location near the middle region of the outsole and ending at a location near the rearmost point of the upper. 
         [0027]    In this preferred embodiment, the upper layer, shank and the lower layer of the midsole each extend from at least the vicinity of the front tip of the shoe to at least the vicinity of the rear tip of the shoe. 
         [0028]    In this preferred embodiment, the upper layer is made from a material having a first density sufficiently dense to provide some support and stabilization of the user&#39;s foot. Typically, in this preferred embodiment, the upper layer has a durometer hardness between about 45 and about 65 on the Asker C scale. The upper layer typically has a relatively low compressibility so that it compresses a relatively low, or small, amount under a given load. 
         [0029]    The lower layer, which may or may not be made of the same material as the upper layer, has a second density that is different from the first density and is sufficiently low in density and high in compressibility so as to allow the lower layer to compress and deform a higher, or greater, amount under a given weight than the upper layer would compress and deform under that same weight. Typically, the lower layer has a durometer hardness between about 20 and about 45 on the Asker C scale. The density of the lower layer is sufficiently low and the compressibility of the lower layer is sufficiently high so that under normal running or walking conditions, the user&#39;s foot, first in the heel region, then in the middle region, and then finally in the toe region, sinks toward the ground as the lower layer compresses and deforms during use. 
         [0030]    In this preferred embodiment, the shank is made from a material having a third density sufficiently dense to provide the primary support and stability to the user&#39;s foot. Typically, the shank has a durometer hardness between about 50 and about 70 on the Shore D scale. The shank in the area of the heel region and the middle region is relatively thick and rigid and thereby provides support and stability to the user&#39;s foot in those areas. In contrast, the shank in the toe area is relatively thin and may even have a fork-like structure or be completely absent, thus allowing the toe region to flex during use. 
         [0031]    Due to the hardness and rigidity of the shank, the upper layer of the midsole may be relatively thin or completely absent. 
         [0032]    During walking or running while wearing a preferred embodiment of the instant invention, when the curved heel region of the outsole strikes the ground, the heel region of the lower layer, which is less dense and more easily compressed than the upper layer, deforms to a relatively large degree compared to the upper layer and the shank. After each such initial heel region contact with the ground, the user&#39;s heel sinks or moves toward the ground more than it would sink or move in a conventional shoe. This sinking or downward movement is due primarily to deflection of the heel region of the outsole and compression of the heel region of the midsole as they each respond to the increasing weight being transmitted through the user&#39;s heel as the step progresses and the user&#39;s heel continues to bear an increasing amount of the user&#39;s weight until it reaches a maximum. The impact is akin to a heel striking a sandy beach or a giving or uneven surface. Then, as the user&#39;s weight begins to shift toward the middle region of the shoe, the shoe rolls forward in a smooth motion, without the user having to overcome any abrupt or discrete pivot points. Then the lower layer of the midsole in the middle region and then the toe region compresses and deforms under the increasing weight of the user&#39;s foot in those regions as the step progresses. This compression and deformation allows the user&#39;s foot to sink further toward the ground than would be the case with a conventional shoe. The user then completes the step by pushing off with the forefoot ball area of the user&#39;s foot. This push-off further compresses and deforms the lower layer in the toe region. 
         [0033]    As used herein, “longitudinal convexities” and “longitudinal concavities” mean, refer to, and are defined as, respectively, convexities and concavities that lie only in vertical, longitudinal planes that extend from any local frontmost point of the shoe to a corresponding local rearmost point of the shoe when the shoe is in its normal, upright position. As used herein, “transverse convexities” and “transverse concavities” mean, refer to, and are defined as, respectively, convexities and concavities that lie only in vertical, transverse planes that extend from any local medialmost point of the shoe to a corresponding local lateralmost point of the shoe when the shoe is in its normal, upright position. 
         [0034]    All convexities and concavities in the instant invention, both longitudinal and transverse, are all identified herein as being on, and being a part of, the bottom surface of the shank. Under this convention, each longitudinal convexity and each transverse convexity identified herein is, to some degree, an outward bulge of the bottom surface of the shank and each longitudinal concavity and each transverse concavity identified herein is, to some degree, an inward depression in the bottom surface of the shank. The inward depression of each longitudinal concavity and of each transverse concavity means that the lower layer is relatively thick wherever the bottom surface of the shank has a longitudinal or to transverse concavity. Similarly, the outward bulge of each longitudinal convexity and of each transverse convexity means that the lower layer is relatively thin wherever the shank has a longitudinal or transverse convexity. 
         [0035]    Each concavity and convexity, as described above, has at least five primary variables that control the effect of each such concavity and each such is convexity. These primary variables are (1) the location where each concavity and each convexity is located from a point where it begins to a point where it ends, (2) the sharpness or shallowness of each such concavity or convexity, i.e., its radius of curvature or radii of curvature, (3) the length or wavelength of each such concavity or convexity as measured from a point where it begins to a point where it ends, (4) the amplitude, i.e., the greatest height of each such concavity or the greatest depth of each such convexity, and (5) the firmness or compressibility of the upper layer material with which each such concavity or convexity is formed. These variables are some of the primary means by which the effects of the shoe on the user are controlled. These effects comprise primarily (1) the degree of softness or hardness felt by the user&#39;s foot throughout each step while wearing the shoe, (2) the amount of energy and effort needed for the user to complete each step, and (3) the amount of muscle use, control and coordination necessary for the user to maintain the user&#39;s balance throughout each step. 
         [0036]    The degree of softness or hardness felt by the user&#39;s foot immediately after the heel strike is controlled primarily by a longitudinal concavity in the bottom surface of the shank located in the heel region of the lower layer of the midsole. This longitudinal concavity is typically relatively large, i.e., it typically has a long length, a large radius of curvature or radii of curvature, and a large amplitude. This relatively large longitudinal concavity allows a relatively thick lower layer to be used in the heel region that can absorb and soften the initial heel strike of each step. Whereas each longitudinal concavity and each transverse concavity imparts a relatively soft feel to the user&#39;s foot while walking, each longitudinal convexity and each transverse convexity imparts a relatively hard feel to the user&#39;s foot while walking. This relative hardness is due to the decreased thickness of the soft, highly compressible lower layer at each location where a longitudinal or transverse convexity occurs. 
         [0037]    The shank allows the midsole to be thinner because it provides a further hardness and rigidity in addition to or in place of the upper layer. Due to the inclusion of the harder and more rigid shank, the lower layer can compress and, at the same time, guide the user&#39;s motion without compromising support and stability. Due to the hardness and rigidity of the shank, as the lower layer sinks toward the ground due to the compressibility of the lower layer, the user&#39;s foot is still supported and prevented from excessive lateral movement in the midfoot and heel areas during use. 
         [0038]    The amount of energy and effort required by the user in each step is related to the degree of softness or hardness felt by the user as discussed in the preceding paragraph insofar as each longitudinal or transverse concavity corresponds to a softer feel which, in turn, requires more energy and effort to overcome in each step. 
         [0039]    The amount of muscle use, control and coordination necessary for the user to maintain the user&#39;s balance throughout each step increases in direct proportion to each one of the following: (1) increased size, primarily in wavelength and amplitude, of the longitudinal concavity and/or transverse concavity and (2) increased compressibility of the lower layer. Increased longitudinal and/or transverse concavity size in the form of greater amplitude corresponds to a thicker lower layer. The compressibility of the lower layer is a physical property inherent in the material out of which the lower layer is made. It is a measure of the readiness with which the lower layer compresses under a given load. A high compressibility means that the lower layer is highly compressible and can be compressed a high amount with relative ease. As the compressibility increases, the user must use more muscle control and coordination to maintain the user&#39;s balance during each step as the weight of the user compresses the lower layer. This compression is accompanied by a downward movement of the user&#39;s foot as it compresses the lower layer during each step. This downward compression movement requires balancing by the user to accommodate inherent instability that accompanies the compression. This inherent instability is also affected by the thickness of the lower layer. This thickness, as mentioned above, increases as longitudinal and/or transverse concavity size of the bottom surface of the shank increases. As the thickness of the lower layer increases, the inherent instability increases. Thus, longitudinal and/or transverse concavities on the bottom surface of the shank contribute to a less stable walking/running nature of the shoe. The relative opposite effect is achieved with a longitudinal and/or transverse convexity on the bottom surface of the shank. 
         [0040]    As mentioned above, the instability results in the user having to exert more effort and energy while running or walking than they would if they had been wearing conventional footwear. This, in turn, imparts various fitness benefits to the user such as increased muscle toning, better posture and greater burning of calories. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0041]    By way of example only, selected embodiments and aspects of the present invention are described below. Each such description refers to a particular figure (“FIG.”) which shows the described matter. All such figures are shown in drawings that accompany this specification. Each such figure includes one or more reference numbers that identify one or more part(s) or element(s) of the invention. 
           [0042]      FIG. 1  is an exploded perspective view of an embodiment of the midsole and outsole of the shoe. 
           [0043]      FIG. 2  is a side elevation view of an embodiment of the midsole and outsole of the shoe. 
           [0044]      FIG. 2A  is an exploded side elevation view of an embodiment of the midsole and outsole of the shoe. 
           [0045]      FIG. 3  is a side elevation view of an embodiment of the shank. 
           [0046]      FIG. 3A  is a front elevation view in cross section of an embodiment of the shank along line  3 A in the direction of the appended arrows. 
           [0047]      FIG. 3B  is a front elevation view in cross section of an alternative embodiment of the shank along line  3 A in the direction of the appended arrows. 
           [0048]      FIG. 3C  is a front elevation view in cross section of another alternative embodiment of the shank along line  3 A in the direction of the appended arrows. 
           [0049]      FIG. 4  is a perspective view of an embodiment of the shank. 
           [0050]      FIG. 5A  is a side elevation view of a representative shoe that embodies the instant invention without any load. 
           [0051]      FIG. 5B  is a side elevation view of the shoe of  FIG. 5A  showing the heel region bearing the load of a user. 
           [0052]      FIG. 5C  is a side elevation view of the shoe of  FIG. 5A  showing the middle region bearing the load of a user. 
           [0053]      FIG. 5D  is a side elevation view of the shoe of  FIG. 5A  showing the toe region bearing the load of a user. 
           [0054]      FIG. 6  is an exploded elevation view of  FIG. 2  that includes view plane lines. 
           [0055]      FIG. 6A  is a top plan view of the top surface of the upper layer of the midsole along line  6 A- 6 A in the direction of the appended arrows. 
           [0056]      FIG. 6B  is a bottom plan view of the bottom surface of the upper layer of the midsole along line  6 B- 6 B in the direction of the appended arrows. 
           [0057]      FIG. 6C  is a top plan view of the top surface of the shank along line  6 C- 6 C in the direction of the appended arrows. 
           [0058]      FIG. 6D  is a bottom plan view of the bottom surface of the shank along line  6 D- 6 D in the direction of the appended arrows. 
           [0059]      FIG. 6E  is a top plan view of the top surface of the lower layer of the midsole along line  6 E- 6 E in the direction of the appended arrows. 
           [0060]      FIG. 6F  is a bottom plan view of the bottom surface of the lower layer of the midsole along line  6 F- 6 F in the direction of the appended arrows. 
           [0061]      FIG. 7  is an exploded perspective view of an alternative embodiment of the midsole and outsole of the shoe. 
           [0062]      FIG. 8  is a side elevation view of an alternative embodiment of the midsole and outsole of the shoe. 
           [0063]      FIG. 8A  is an exploded side elevation view of an alternative embodiment of the midsole and outsole of the shoe. 
           [0064]      FIG. 9A  is a top plan view of the bottom surface of an alternative embodiment of the shank along line  6 C- 6 C in the direction of the appended arrows. 
           [0065]      FIG. 9B  is a top plan view of the bottom surface of an alternative embodiment of the shank along line  6 C- 6 C in the direction of the appended arrows. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0066]    The invention will now be described with reference to the preferred embodiment shown in  FIG. 1 .  FIG. 1  is an exploded perspective view of a preferred embodiment of a midsole  103  and an outsole  105  of the shoe. The outsole  105  is not part of the midsole  103 . As shown in  FIGS. 1 ,  2  and  2 A, the outsole  105  is below the midsole  103  when the shoe is in its normal, upright position. This normal, upright position is shown with respect to the ground  100  in  FIGS. 5A-5D . As used herein, “above” and “below” refer to relative locations of identified elements when the shoe is in this normal, upright position as shown in  FIGS. 5A-5D . The midsole  103  is located between the shoe upper  106  and the outsole  105 . 
         [0067]    The midsole  103 , as shown in  FIGS. 1 ,  2  and  2 A, comprises an upper layer  107 , a shank  111 , and a lower layer  109 . The upper layer  107  and/or the lower layer  109  may each comprise two or more sub-layers. As described more fully hereinafter in an alternative embodiment, the upper layer  107  may also be eliminated completely. 
         [0068]    In the preferred embodiment shown in  FIGS. 1 ,  2  and  2 A, upper layer  107  has a top surface  113  substantially opposite a bottom surface  115 . Top surface  113  is shown in  FIG. 6A . Bottom surface  115  is shown in  FIG. 6B . The shank  111  has a top surface  181  substantially opposite a bottom surface  183 . Top surface  181  is shown in  FIG. 6C  and bottom surface  183  is shown in  FIG. 6D . The shank has a top portion  186  and a bottom portion  187 . Top portion  186  and bottom portion  187  are shown in  FIG. 3 . The lower layer  109  has a top surface  117  substantially opposite a bottom surface  121 . Top surface  117  is shown in  FIG. 6E . Bottom surface  121  is shown in  FIG. 6F . The outsole  105  has a top surface  119  substantially opposite a bottom surface  123 . As shown in  FIG. 1 , when the shoe is in its normal, upright position, the shank  111  is below the upper layer  107 . The lower layer  109  is below the shank  111 , and the outsole  105  is below the lower layer  109 . 
         [0069]      FIG. 2  is a side elevation view of an embodiment of the midsole and outsole of the shoe. The shoe has a front tip  140  located at the farthest point toward the front of the shoe and a rear tip  142  located at the farthest point toward the rear of the shoe. The upper layer  107  includes a toe region  151  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the front tip  140  and extends from there to a location that is approximately one third of the distance toward the rear tip  142 . The shank  111  includes a toe region  251  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the front tip  140  and extends from there to a location that is approximately one third of the distance toward the rear tip  142 . The lower layer  109  includes a toe region  161  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the front tip  140  and extends from there to a location that is approximately one third of the distance toward the rear tip  142 . The outsole  105  includes a toe region  171  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the front tip  140  and extends from there to a location that is approximately one third of the distance toward the rear tip  142 . 
         [0070]    The upper layer  107  includes a heel region  153  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the rear tip  142  and extends from there to a location that is approximately one third of the distance toward the front tip  140 . The shank  111  includes a heel region  253  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the rear tip  142  and extends from there to a location that is approximately one third of the distance toward the front tip  140 . The lower layer  109  includes a heel region  163  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the rear tip  142  and extends from there to a location that is approximately one third of the distance toward the front tip  140 . The outsole  105  includes a heel region  173  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the rear tip  142  and extends from there to a location that is approximately one third of the distance toward the front tip  140 . 
         [0071]    The upper layer  107  includes a middle region  152  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that extends approximately between the toe region  151  and the heel region  153 . The shank  111  includes a middle region  262  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that extends approximately between the toe region  251  and the heel region  253 . The lower layer  109  includes a middle region  162  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that extends approximately between the toe region  161  and the heel region  163 . The outsole  105  includes a middle region  172  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that extends approximately between the toe region  171  and the heel region  173 . 
         [0072]    Typically, the lower layer  109  of the midsole  103  is on average thicker in the heel region  163  than it is in the toe region  161 . The upper layer  107  has a first density. The lower layer  109  has a second density different from the first density and is typically less dense than the first density. The upper layer  107  has a first compressibility and the lower layer  109  has a second compressibility that is different from the first compressibility. The compressibility of the lower layer  109  is typically relatively high. Due to this relatively high compressibility, the lower layer  109  undergoes a relatively high amount of deformation when subjected to a given load. The upper layer  107  is typically made from polyurethane, polyvinyl chloride, rubber or thermal plastic rubber. However, the upper layer  107  can be made from any other material without departing from the scope of the present invention. Typically the upper layer  107  will have a durometer hardness between about 45 and about 65 on the Asker C scale. 
         [0073]      FIG. 2A  is an exploded side elevation view of  FIG. 2 . The lower layer  109  is made of a compressible and deformable yet resilient material which may or may not be the same material of which the upper layer  107  is made. Typically the lower layer  109  will have a durometer hardness between about 20 and about 45 on the Asker C scale. The top surface  113  of the upper layer  107  is typically positioned below an insole board (not shown) which is typically positioned below a sockliner (not shown). As shown in  FIGS. 2 and 2A , the bottom surface  115  of the upper layer  107  is in substantially continuous contact with the top surface  181  of the shank  111 . Due to this substantially continuous contact between the bottom surface  115  of the upper layer  107  and top surface  181  of the shank  111  in this embodiment, bottom surface  115  of the upper layer  107  substantially conforms to top surface  181  of the shank  111 . In other embodiments, such substantially continuous contact between bottom surface  115  of the upper layer  107  and top surface  181  of the shank  111  may not be present. The upper layer  107  has a bottom surface  115  that may be connected to the top surface  181  of the shank  111  by either friction and/or an adhesive and/or other similar means. Alternatively, substantially the entire bottom surface  115  of the upper layer  107  may be molded to substantially the entire top surface  181  of the shank  111 . Alternatively, the upper layer may be eliminated in alternative embodiments. 
         [0074]    The shank  111  has a frontmost point  250  and a rearmost point  255 . The shank  111  can be made from polyurethane, polyvinyl chloride, rubber, thermal plastic rubber, carbon fiber or carbon fiber reinforced plastic. However, the shank  111  can be made from any other material without departing from the scope of the present invention. Typically the shank  111  will have a durometer hardness between about 50 and about 70 on the Shore D scale. 
         [0075]    The outsole  105  typically curves upwardly in the heel region. The outsole  105  has a frontmost point  170  and a rearmost point  174 . When the shoe is in its typical upright, unloaded state, the frontmost point  170  and the rearmost point  174  are both relatively high above the ground  100 . From a point at or near the vicinity of the frontmost point  170 , the outsole  105  has a gradual downward curve  195  that continues through at least a portion of the toe region  171  of the outsole  105 . Starting in the middle region  172 , the outsole  105  has a gradual, upward curve  196  that continues to curve upward through at least a portion of the heel region  173  of the outsole  105 . This gradual upward curve  196  typically continues until the outsole  105  approaches the vicinity of the rear tip  142  of the shoe. This upward curve  196  is typically sharper than downward curve  195  in the toe region  171 . Upward curve  196  may be substantially sharper than shown in  FIG. 2A  or substantially shallower than shown in  FIG. 2A . The outsole  105  has a bottom surface  123  that typically contains grooves and/or patterns for optimal traction and wear. 
         [0076]      FIG. 3  is a side elevation view of a preferred embodiment of the shank  111 . In the preferred embodiment, the shank  111  comprises a top portion  186  and a bottom portion  187 . The shank  111  has a top surface  181  and a bottom surface  183 . The bottom surface  183  of the shank  111  has a longitudinal concavity  303 , a longitudinal convexity  305  and another longitudinal concavity  307 . 
         [0077]    The bottom surface  183  of the shank  111  has a longitudinal concavity  303  that comprises at least a downward curve  190  located in at least a portion of the heel region  253 . “Downward curve,” as used here and throughout this specification, unless otherwise noted, refers to a direction that moves toward the ground  100  from any specified location on the shoe when the shoe is oriented in its typical upright position in which the bottom surface  123  of the outsole  105  is in unloaded contact with the ground  100 . 
         [0078]    The shank  111  has a frontmost point  250  and a rearmost point  255 . Downward curve  190  of the longitudinal concavity  303  begins at or near the vicinity of, the rearmost point  255  of the shank  111  and gradually and continuously descends downwardly from there through a point at or near the vicinity of the middle region  262 . The portion of the shank  111  indicated by lines extending from, and associated with, reference numeral  303  indicates the approximate range wherein longitudinal concavity  303  is typically primarily located. Longitudinal concavity  303  may, or may not, be entirely located within the range indicated by the lines extending from, and associated with, reference numeral  303 . Longitudinal concavity  303 , as shown in  FIG. 2A , is relatively shallow due to its large radius of curvature or radii of curvature. Longitudinal concavity  303  may comprise a curve or curves in addition to downward curve  190 . The radius of curvature throughout longitudinal concavity  303  may be completely constant, may have one or more constant portions mixed with one or more non-constant portions, or may be completely non-constant. Downward curve  190 , as well as any other curve or curves that are part of longitudinal concavity  303 , may, at any point on any of those curves, have a slope that is gradual, moderate or steep. Although downward curve  190  of longitudinal concavity  303  is shown in  FIG. 2A  as beginning near the rearmost point  255 , downward curve  190  of longitudinal concavity  303  may instead begin at some other location on the bottom surface  183  of the shank  111 . Although longitudinal concavity  303  is shown in  FIG. 2A  as ending at a location in the middle region  262  or the location where the heel region  253  transitions into the middle region  262 , longitudinal concavity  303  may end at some other location on the bottom surface  183  of the shank  111 . 
         [0079]    The bottom surface  183  of the shank  111 , as shown in  FIG. 2A , to has a longitudinal concavity  307  that comprises at least an upward curve  192  located in at least a portion of the middle region  262 . “Upward curve,” as used here and throughout this specification, unless otherwise noted, refers to a direction that moves away from the ground  100  from any specified location on the shoe when the shoe is oriented in its typical upright position in which the bottom surface  123  of the outsole  105  is in unloaded contact with the ground  100 . 
         [0080]    Upward curve  192  of longitudinal concavity  307  begins at, or near the vicinity of the middle region  262  of the bottom surface  183  and gradually and continuously ascends upwardly from there through at least a portion of the toe region  251 . The portion of the bottom surface  183  indicated by lines extending from, and associated with reference numeral  307  indicates the approximate range wherein longitudinal concavity  307  is typically primarily located. Longitudinal concavity  307  may, or may not, be entirely located within the range indicated by the lines extending from, and associated with, reference numeral  307 . Longitudinal concavity  307 , as shown in  FIG. 2A , is relatively shallow due to its large radius of curvature or radii of curvature. Longitudinal concavity  307  may comprise a curve or curves in addition to upward curve  192 . The radius of curvature throughout longitudinal concavity  307  may be completely constant, may have one or more constant portions mixed with one or more non-constant portions, or may be completely non-constant. Upward curve  192 , as well as any other curve or curves that are part of longitudinal concavity  307 , may, at any point on any of those curves, have a slope that is gradual, moderate or steep. Although upward curve  192  of longitudinal concavity  307  is shown in  FIG. 2A  as beginning near the middle region  262 , upward curve  192  of longitudinal concavity  307  may instead begin at some other location on the bottom surface  183 . Although longitudinal concavity  307  is shown in  FIG. 2A  as ending at a location in the toe region  251 , longitudinal concavity  307  may end at some other location on the bottom surface  183  of the shank  111 . 
         [0081]    The bottom surface  183  of the shank  111 , as shown in  FIG. 2A , has a longitudinal convexity  305  that is defined by downward curve  190  and upward curve  192  and that is typically located in at least a portion of the middle region  262 . 
         [0082]    Longitudinal convexity  305  may, or may not, be entirely located within the range indicated by the lines extending from, and associated with, reference numeral  305 . Longitudinal convexity  305 , as shown in  FIG. 2A , is relatively shallow due to its large radius of curvature or radii of curvature. Longitudinal convexity  305  may comprise a curve or curves in addition to upward curve  192  and downward curve  190 . The radius of curvature throughout longitudinal convexity  305  may be completely constant, may have one or more constant portions mixed with one or more non-constant portions, or may be completely non-constant. Downward curve  190  and upward curve  192 , as well as any other curve or curves that are part of longitudinal convexity  305 , may, at any point on any of those curves, have a slope that is gradual, moderate or steep. Although longitudinal convexity  305  is shown in  FIG. 2A  as ending at a location where the middle region  162  transitions into the toe region  161 , longitudinal convexity  305  may end at some other location on the bottom surface  183  of the shank  111 . 
         [0083]    The shank  111 , has a cavity  309  which is formed by the top portion  186  and bottom portion  187 . The cavity has a beginning point  311  and an end point  313 . The cavity  309  begins at the beginning point  311  longitudinally closer to the heel region. The cavity  309  terminates at end point  313  closer to the middle region. The shank  111  has a bottom surface  183  that may be connected to the top surface  117  of the bottom layer  109  by either friction and/or an adhesive and/or other similar means. Alternatively, substantially the entire bottom surface  183  of the shank  111  may be molded to substantially the entire top surface of the bottom layer  109 . As shown in  FIGS. 2 and 2A , the top surface  117  of the lower layer  109  is in substantially continuous contact with the bottom surface  183  of the shank  111 . Due to this substantially continuous contact between the top surface  117  of the lower layer  109  and bottom surface  183  of the shank  111  in this embodiment, top surface  117  of the lower layer  109  substantially conforms to bottom surface  183  of the shank  111 . In other embodiments, such substantially continuous contact between top surface  117  of the lower layer  109  and bottom surface  183  of the shank  111  may not be present. 
         [0084]      FIG. 3A  is a front elevation view in cross section of an embodiment of the shank  111  along line  3 A- 3 A in the direction of the appended arrows. As shown, the bottom surface  183  of the shank  111  along line  3 A- 3 A is straight. 
         [0085]      FIG. 3B  is a front elevation view in cross section of an alternative embodiment of the shank  111  along line  3 A- 3 A in the direction of the appended arrows. As shown, the bottom surface  183  of the shank  111  along line  3 A- 3 A contains a transverse concavity. 
         [0086]      FIG. 3C  is a front elevation view in cross section of another alternative embodiment of the shank  111  along line  3 A- 3 A in the direction of the appended arrows. As shown, the bottom surface  183  of the shank  111  along line  3 A- 3 A contains a transverse convexity. 
         [0087]      FIG. 4  is a perspective view of a preferred embodiment of the shank  111  as seen in  FIGS. 1 ,  2 ,  2 A and  3 .  FIG. 4  illustrates the cavity  309  being open from the lateral to medial side of the shoe. 
         [0088]    In normal use of the shoe, each forward step taken by the user begins when the heel region  173  of the outsole  105  begins to make contact with the ground  100 . The lower layer  109  of the midsole  103  in the heel region  163  that is made of less dense and more readily compressible material then begins to compress and deform, allowing the heel of the user&#39;s foot to sink toward the ground  100  to a greater extent than it would sink while wearing a conventional shoe. Due to longitudinal concavity  303 , the lower layer  109  is relatively thick in the heel region  163 . Since this relatively thick heel region  163  of the lower layer  109  is also relatively soft and highly compressible, it mimics the effect of walking or running on a sandy beach, thereby requiring the user to exert more energy while walking or running than would be required when walking or running while wearing conventional shoes. Additionally, since the heel region  163  of the lower layer  109  is relatively thick and highly compressible, it has a degree of inherent longitudinal and transverse instability that is not present in conventional shoes. This inherent instability forces the user to engage in a balancing effort and use muscles and muscle control and coordination to maintain a normal walking gait that would not be required with conventional shoes. However, while also maintaining an inherent instability due to the lower layer  109  as discussed above, the shank  111 , due to its rigidity and structure is able to provide proper support to the user&#39;s heel so that although the heel region  163  compresses and provides instability, the shank  111  provides stability and does not compress. 
         [0089]    As the step continues, the user&#39;s weight shifts to the middle regions  152 ,  162 ,  262 , and  172  and the shoe rolls forward in a smooth motion without the user having to overcome any abrupt pivot point. The lower layer  109  of the midsole  103  in the middle region  162  then compresses and deforms, allowing the user&#39;s foot in that region to sink toward the ground  100  more than it would sink if the user were wearing conventional shoes, due to the inherent instability due to the lower layer  109  as discussed above. As with the above, the shank  111 , due to its rigidity and structure is able to provide proper support to the user&#39;s midfoot area. The cavity  309  in the shank  111 , may cause the bottom portion  187  of the shank  111  to compress a small amount in the area directly below the cavity  309 . This compression provides cushioning and imparts some instability, but the shank  111  still maintains adequate support to the user&#39;s foot. 
         [0090]    As the step continues, the user&#39;s weight then shifts to the toe regions  151 ,  161 ,  251 , and  171 . The lower layer  109  of the midsole  103  in the toe region  161  then compresses and deforms, allowing the user&#39;s foot in that region to sink toward the ground  100  more than it would sink if the user were wearing conventional shoes. As shown in  FIG. 2A , the thickness of the lower layer  109  in the toe region  161  is typically not as great as it is in the heel region  163 . This decrease in thickness of the lower layer  109  results in relatively more stability in the toe region  161 . This allows the user, when completing his/her step more control when pushing off with the forefoot ball of the user&#39;s foot. 
         [0091]    All of this simulates the effect, and imparts the fitness benefits, of running or walking on a sandy beach or on a giving or uneven soft surface regardless of the actual hardness of the surface. 
         [0092]      FIGS. 5A-5D  show a side elevation exterior view of a representative shoe that embodies the instant invention.  FIG. 5A  shows this representative shoe in a fully unloaded state.  FIGS. 5B ,  5 C, and  5 D show this representative shoe undergoing normal loading that occurs when a user walks or runs while wearing the shoe. In  FIGS. 5A-5D , the shank  111  does not undergo a significant amount of compression aside from the area occupied by cavity  309 . Thus the compression of the shank is not shown aside from the area occupied by cavity  309 . 
         [0093]    In  FIGS. 5A-5D , the straight lines identified by, respectively, reference numerals  501 A- 501 D,  502 A- 502 D, and  503 A- 503 D each represent the thickness of the upper layer  107  at the location where each such straight line  501 A- 501 D,  502 A- 502 D, and  503 A- 503 D appears. The straight lines identified by, respectively, reference numerals  504 A- 504 D,  505 A- 505 D, and  506 A- 506 D each represent the thickness of the lower layer  109  at the location where each such straight line  504 A- 504 D,  505 A- 505 D, and  506 A- 506 D appears. The straight lines identified by, respectively, reference numerals  509 A- 509 D each represent the area occupied by the cavity  309 . A decrease in the area represented by numeral  509 A- 509 D represents a compression in the cavity  309  of shank  111 . 
         [0094]    As shown in the unloaded state in  FIG. 5A , the upper layer  107  and lower layer  109  are not undergoing any compression. As also shown in  FIG. 5A , the outsole  105  is not undergoing any deflection or deformation. In this fully uncompressed state, the thickness of the upper layer  107  and the thickness of the lower layer  109  are each at their respective maximum thickness. This maximum thickness is indicated by, and corresponds to, the length of each straight line  501 A- 506 A, each one of which is at its maximum length as shown in  FIG. 5A . Furthermore, the area occupied by the cavity is at its maximum. This maximum area is indicated by and corresponds to the length of the straight line  509 A. 
         [0095]      FIG. 5B  shows the representative shoe in an orientation where the user&#39;s heel (not shown) is imparting a load in the heel regions  153 ,  163 ,  253 , and  173 , shown in  FIGS. 1 and 2 . In normal use of the shoe, each forward step taken by the user begins when the heel region  173  of the outsole  105  begins to make contact with the ground  100 . The lower layer  109  of the midsole  103  in the heel region  163  that is made of less dense and more readily compressible material then begins to compress and deform, allowing the heel of the user&#39;s foot to sink toward the ground  100  to a greater extent than it would sink while wearing a conventional shoe. Due to longitudinal concavity  303 , the lower layer  109  is relatively thick in the heel region  163 . Since this relatively thick heel region  163  of the lower layer  109  is also relatively soft and highly compressible, it mimics the effect of walking or running on a sandy beach, thereby requiring the user to exert more energy during use than would be required with conventional shoes. Additionally, since the heel region  163  of the lower layer  109  is relatively thick and highly compressible, it has a degree of inherent longitudinal and transverse instability that is not present in conventional shoes. This inherent instability forces the user to engage in a balancing effort and use muscles and muscle control and coordination to maintain a normal gait that would not be required with conventional shoes. However, while also maintaining an inherent instability due to the lower layer  109  as discussed above, the shank  111 , due to its rigidity and structure is able to provide proper support to the user&#39;s heel so that although the heel region  163  compresses and provides instability, the shank  111  provides stability and does not compress. Under this loading condition, the heel region  153  of the upper layer  107  is undergoing a relatively small amount of compression. This relatively small amount of compression results in a relatively small decrease in the thickness of the heel region  153  of the upper layer  107 . This relatively small decrease in thickness is indicated by  501 B. Under this same loading, the heel region  163  of the lower layer  109  is undergoing a relatively large amount of compression. This relatively large amount of compression results in a relatively large decrease in the thickness of the heel region  163  of the lower layer  109 . This relatively large decrease in thickness is indicated by  504 B. Under this same loading, the heel region  173  of the outsole  105  is undergoing a relatively large amount of deflection. This relatively large amount of deflection in the heel region  173  of the outsole  105  is caused by the heel region  173  conforming to the ground  100  as it bears the load of the user. This deflection and conformity of the heel region  173  of the outsole  105  is indicated by the straight portion of the outsole  105  where it contacts the ground  100  as shown in  FIG. 5B . 
         [0096]      FIG. 5C  shows the representative shoe in an orientation where the user&#39;s foot (not shown) is imparting a load in the middle regions  152 ,  162 ,  262 , and  172 , shown in  FIGS. 1 and 2 . As the step continues, the user&#39;s weight shifts to the middle regions  152 ,  162 ,  262 , and  172  and the shoe rolls forward in a smooth motion without the user having to overcome any abrupt pivot point. The lower layer  109  of the midsole  103  in the middle region  162  then compresses and deforms, allowing the user&#39;s foot in that region to sink toward the ground  100  more than it would sink if the user were wearing conventional shoes, due to the inherent instability due to the lower layer  109  as discussed above. As with the above, the shank  111 , due to its rigidity and structure is able to provide proper support to the user&#39;s midfoot region. The cavity  309  in the shank  111 , may cause the bottom portion  187  of the shank  111  to compress a small amount in the area directly below the cavity  309 . That compression provides cushioning and imparts some instability, but the shank  111  still maintains adequate support to the user&#39;s foot. Under this loading condition, the middle region  152  of the upper layer  107  is undergoing a relatively small amount of compression. This relatively small amount of compression results in a relatively small decrease in the thickness of the middle region  152  of the upper layer  107 . This relatively small decrease in thickness is indicated by  502 C. Under this same loading, the middle region  162  of the lower layer  109  is undergoing a relatively large amount of compression. This relatively large amount of compression results in a relatively large decrease in the thickness of the middle region  162  of the lower layer  109 . This relatively large decrease in thickness is indicated by  505 C. Under this same loading, the middle region  172  of the outsole  105  is undergoing a relatively large amount of deflection. This relatively large amount of deflection in the middle region  172  of the outsole  105  is caused by the middle region  172  conforming to the ground  100  as it bears the load of the user. This deflection and conformity of the middle region  172  of the outsole  105  is indicated by the straight portion of the outsole  105  where it contacts the ground  100  as shown in  FIG. 5C . Furthermore, the area occupied by the cavity  309  is decreased due to the weight of the user&#39;s foot with respect to the ground. The decrease in area of cavity  309  is shown in line  509 C. 
         [0097]      FIG. 5D  shows the representative shoe in an orientation where the user&#39;s foot (not shown) is imparting a load in the toe regions  151 ,  161 ,  251 , and  171 , shown in  FIGS. 1 and 2 . As the step continues, the user&#39;s weight then shifts to the toe regions  151 ,  161 ,  251 , and  171 . The lower layer  109  of the midsole  103  in the toe region  161  then compresses and deforms, allowing the user&#39;s foot in that region to sink toward the ground  100  more than it would sink if the user were wearing conventional shoes. As shown in  FIG. 2A , the thickness of the lower layer  109  in the toe region  161  is typically not as great as it is in the heel region  163 . This decrease in thickness of the lower layer  109  results in relatively more stability in the toe region  161 . This allows the user, when completing his/her step more control when pushing off with the forefoot ball of the user&#39;s foot. Under this loading condition, the toe region  151  of the upper layer  107  is undergoing a relatively small amount of compression. This relatively small amount of compression results in a relatively small decrease in the thickness of the toe region  151  of the upper layer  107 . This relatively small decrease in thickness is indicated by  503 D. Under this same loading, the toe region  161  of the lower layer  109  is undergoing a relatively large amount of compression. This relatively large amount of compression results in a relatively large decrease in the thickness of the toe region  161  of the lower layer  109 . This relatively large decrease in thickness is indicated by  506 D. Under this same loading, the toe region  171  of the outsole  105  is undergoing a relatively large amount of deflection. This relatively large amount of deflection in the toe region  171  of the outsole  105  is caused by the toe region  171  conforming to the ground  100  as it bears the load of the user. This deflection and conformity of the toe region  171  of the outsole  105  is indicated by the straight portion of the outsole  105  where it contacts the ground  100  as shown in  FIG. 5D . The area in the cavity  309  is now returned to its original state as shown in line  509 D, which is equal to line  509 A. 
         [0098]      FIGS. 7 ,  8  and  8 A show another embodiment of the invention. The midsole  703  in this alternative embodiment does not have an upper layer but rather is comprised of a shank  711  and a lower layer  709 . The lower layer  709  can be comprised of two or more sub-layers. 
         [0099]    In this alternative embodiment, lower layer  709  has a top surface  717  substantially opposite a bottom surface  721 . The shank  711  has a top surface  781  substantially opposite a bottom surface  783 . The shank has a top portion  786  and a bottom portion  787  similar to the embodiment of shank  111  shown in  FIG. 3 . The outsole  705 , which is not part of the midsole  703 , has a top surface  719  substantially opposite a bottom surface  723 . As shown in  FIG. 7 , when the shoe is in its normal, upright position, the lower layer  709  is below the shank  711  and the outsole  705  is below the lower layer  709 . 
         [0100]      FIG. 8  is a side elevation view of the alternative embodiment. The shoe has a front tip  740  located at the farthest point toward the front of the shoe and a rear tip  742  located at the farthest point toward the rear of the shoe. The shank  711  includes a toe region  851  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the front tip  740  and extends from there to a location that is approximately one third of the distance toward the rear tip  742 . The lower layer  709  includes a toe region  761  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the front tip  740  and extends from there to a location that is approximately one third of the distance toward the rear tip  742 . The outsole  705  includes a toe region  771  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the front tip  740  and extends from there to a location that is approximately one third of the distance toward the rear tip  742 . 
         [0101]    The shank  711  includes a heel region  853  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the rear tip  742  and extends from there to a location that is approximately one third of the distance toward the front tip  740 . The lower layer  709  includes a heel region  763  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the rear tip  742  and extends from there to a location that is approximately one third of the distance toward the front tip  740 . The outsole  705  includes a heel region  773  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that begins in the vicinity of the rear tip  742  and extends from there to a location that is approximately one third of the distance toward the front tip  740 . 
         [0102]    The shank  711  includes a middle region  862  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that extends approximately between the toe region  851  and the heel region  853 . The lower layer  709  includes a middle region  762  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that extends approximately between the toe region  761  and the heel region  763 . The outsole  705  includes a middle region  772  that extends substantially from the medial side of the shoe to the lateral side of the shoe at a location that extends approximately between the toe region  771  and the heel region  773 . 
         [0103]      FIG. 8A  is an exploded side elevation view of  FIG. 8 . The lower layer  709  is made of a compressible and deformable yet resilient material. Typically the lower layer  709  will have a durometer hardness between about 20 and about 45 on the Asker C scale. The top surface  781  of the shank  711  is typically positioned below an insole board (not shown) which is typically positioned below a sockliner (not shown). As shown in  FIGS. 8 and 8A , top surface  717  of the lower layer  709  is in substantially continuous contact with, and substantially conforms to, the bottom surface  783  of the shank  711 . In other embodiments, such substantially continuous contact between top surface  717  and bottom surface  783  may not be present. 
         [0104]    The bottom surface  783  of the shank  711 , as shown in  FIG. 8A , has a longitudinal concavity  782  that comprises at least a downward curve  790  located in at least a portion of the heel region  853 . 
         [0105]    The shank  711  has a frontmost point  750  and a rearmost point  755 . Downward curve  790  of longitudinal concavity  782  begins at, or near the vicinity of, the rearmost point  755  of the shank  711  and gradually and continuously descends downwardly from there through a point at or near the vicinity of the middle region  862 . The portion of the bottom surface  783  of the shank  711  indicated by lines extending from, and associated with, reference numeral  782  indicates the approximate range wherein longitudinal concavity  782  is typically primarily located. Longitudinal concavity  782  may, or may not, be entirely located within the range indicated by the lines extending from, and associated with, reference numeral  782 . Longitudinal concavity  782 , as shown in  FIG. 8A , is relatively shallow due to its large radius of curvature or radii of curvature. Longitudinal concavity  782  may comprise a curve or curves in addition to downward curve  790 . The radius of curvature throughout longitudinal concavity  782  may be completely constant, may have one or more constant portions mixed with one or more non-constant portions, or may be completely non-constant. Downward curve  790 , as well as any other curve or curves that are part of longitudinal concavity  782 , may, at any point on any of those curves, have a slope that is gradual, moderate or steep. Although downward curve  790  of longitudinal concavity  782  is shown in  FIG. 8A  as beginning near the rearmost point  774 , downward curve  790  of longitudinal concavity  782  may instead begin at some other location on the shank  711 . Although longitudinal concavity  782  is shown in  FIG. 8A  as ending at a location in the middle region  862  or the location where the heel region  853  transitions into the middle region  862 , longitudinal concavity  782  may end at some other location on the bottom surface  783  of the shank  711 . 
         [0106]    The bottom surface  783  of the shank  711 , as shown in  FIG. 8A , has a longitudinal concavity  785  that comprises at least an upward curve  792  located in at least a portion of the middle region  862 . Upward curve  792  of longitudinal concavity  785  begins at, or near the vicinity of, the middle region  862  of the lower layer  709  and gradually and continuously ascends upwardly from there through at least a portion of the toe region  851 . The portion of the bottom surface  783  of the shank  711  indicated by lines extending from, and associated with, reference numeral  785  indicates the approximate range wherein longitudinal concavity  785  is typically primarily located. Longitudinal concavity  785  may, or may not, be entirely located within the range indicated by the lines extending from, and associated with, reference numeral  785 . Longitudinal concavity  785 , as shown in  FIG. 8A , is relatively shallow due to its large radius of curvature or radii of curvature. Longitudinal concavity  785  may comprise a curve or curves in addition to upward curve  792 . The radius of curvature throughout longitudinal concavity  785  may be completely constant, may have one or more constant portions mixed with one or more non-constant portions, or may be completely non-constant. Upward curve  792 , as well as any other curve or curves that are part of longitudinal concavity  785 , may, at any point on any of those curves, have a slope that is gradual, moderate or steep. Although upward curve  792  of longitudinal concavity  785  is shown in  FIG. 8A  as beginning near the middle region  762 , upward curve  792  of longitudinal concavity  785  may instead begin at some other location on the bottom surface  783  of the shank  711 . Although longitudinal concavity  785  is shown in  FIG. 8A  as ending at a location in the toe region  851 , longitudinal concavity  785  may end at some other location on the bottom surface  783  of the shank  711 . 
         [0107]    The bottom surface  783  of the shank  711 , as shown in  FIG. 8A , has a longitudinal convexity  789  that comprises the downward curve  790  and upward curve  792  and that is typically located in at least a portion of the middle region  862 . Longitudinal convexity  789  may, or may not, be entirely located within the range indicated by the lines extending from, and associated with, reference numeral  789 . Longitudinal convexity  789 , as shown in  FIG. 8A , is relatively shallow due to its large radius of curvature or radii of curvature. Longitudinal convexity  789  may comprise a curve or curves in addition to upward curve  792  and downward curve  790 . The radius of curvature throughout longitudinal convexity  789  may be completely constant, may have one or more constant portions mixed with one or more non-constant portions, or may be completely non-constant. Downward curve  790  and upward curve  792 , as well as any other curve or curves that are part of longitudinal convexity  789 , may, at any point on any of those curves, have a slope that is gradual, moderate or steep. Although longitudinal convexity  789  is shown in  FIG. 8A  as ending at a location where the middle region  762  transitions into the toe region  761 , longitudinal convexity  789  may end at some other location on the bottom surface  783  of the shank  711 . 
         [0108]    As shown in  FIGS. 8 and 8A , the outsole  705  typically curves upwardly in the heel region. The outsole  705  has a frontmost point  770  and a rearmost point  774 . When the shoe is in its typical upright, unloaded state, the frontmost point  770  and the rearmost point  774  are both relatively high above the ground  100 . From a point at or near the vicinity of the frontmost point  770 , the outsole  705  has a gradual downward curve  795  that continues through at least a portion of the toe region  771  of the outsole  705 . Starting in the middle region  772 , the outsole  705  has a gradual, upward curve  796  that continues to curve upward through at least a portion of the heel region  773  of the outsole  705 . This gradual upward curve  796  typically continues until the outsole  705  approaches the vicinity of the rear tip  742  of the shoe. This upward curve  796  is typically sharper than downward curve  795  in the toe region  771 . Upward curve  796  may be substantially sharper than shown in  FIG. 8A  or substantially shallower than shown in  FIG. 8A . 
         [0109]      FIG. 9A  depicts a top plan view of the top surface of an alternative embodiment of a shank  901  along line  6 C- 6 C in the direction of the appended arrows. As shown, the shank  901  shown in  FIG. 9A  differs from the shank  111  shown in  FIG. 6C . The shank  901 , instead of having a fork-like structure as shown in  6 C, does not have any open areas and occupies substantially all of the area from the medial to the lateral side of the shoe between the rear tip  142  and the front tip  140 . 
         [0110]      FIG. 9B  depicts a top plan view of the top surface of another alternative embodiment of a shank  903  along line  6 C- 6 C in the direction of the appended arrows. As shown, the shank  903  shown in  FIG. 9B  differs from the shank  111  shown in  FIG. 6C . The shank  903 , instead of extending from the rear tip  142  to the front tip  140 , extends only from the rear tip  142  to an area close to the middle region  262  and does not extend to the front tip  140 . 
         [0111]    While the foregoing detailed description sets forth selected embodiments of a shoe in accordance with the present invention, the above description is illustrative only and not limiting of the disclosed invention. The claims that follow herein collectively cover the foregoing embodiments. The following claims further encompass additional embodiments that are within the scope and spirit of the present invention.