Abstract:
The invention relates to a sliding element for a shoe sole. The sliding element includes an upper sliding surface and a lower sliding surface, wherein the lower sliding surface is arranged below the upper sliding surface so as to be slideable in at least two directions. The upper sliding surface can form a lower side of an upper sliding plate and the lower sliding surface can form an upper side of a lower sliding plate. A relative sliding movement between the upper sliding surface and the lower sliding surface distributes the deceleration of the shoe sole over a greater time period and allows the foot to feel as if it is wearing a conventional shoe that contacts a surface with reduced friction, for example, a soft forest ground. As a result, the force acting on the wearer and the momentum transfer on his or her muscles and bones are reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application incorporates by reference, and claims priority to and the benefit of, German patent application serial number 10244435.8 that was filed on Sep. 24, 2002.  
         TECHNICAL FIELD  
         [0002]    The present invention relates to a sliding element for a shoe sole, in particular a shoe sole with a sliding element that provides cushioning to the shoe in three dimensions.  
         BACKGROUND  
         [0003]    Shoe soles should primarily meet two requirements. First, they should provide good friction with the ground. Second, they should sufficiently cushion the ground reaction forces arising during a step cycle to reduce the strains on the wearer&#39;s muscles and bones. These ground reaction forces can be classified into three mutually orthogonal components, i.e., a component occurring in each of the X-direction, the Y-direction, and the Z-direction. The Z-direction designates a dimension essentially perpendicular (or vertical) to the ground surface. The Y-direction designates a dimension essentially parallel to a longitudinal axis of a foot and essentially horizontal relative to the ground surface. The X-direction designates a dimension essentially perpendicular to the longitudinal axis of the foot and essentially horizontal relative to the ground surface.  
           [0004]    The largest ground reaction force component typically occurs in the Z-direction. Studies have shown that peak forces of approximately 2000 N may occur in the Z-direction during running. This value is about 2.5 to 3 times the body weight of a typical runner. Accordingly, in the past, the greatest attention was directed to the strains of the muscles and the bones caused by this force component and the many different arrangements for optimizing the cushioning properties of a shoe in the Z-direction.  
           [0005]    Ground reaction forces, however, further include noticeable force components in the X-direction and in the Y-direction. Measurements have shown that forces of approximately 50 N in the X-direction and of approximately 250 N in the Y-direction may occur in a heel area during running. During other sports, for example lateral sports such as basketball or tennis, forces of up to 1000 N may occur in a forefoot area in the X-direction during side cuts, impact, and push off.  
           [0006]    The aforementioned horizontal forces in the X- and Y-directions are one reason why running on an asphalt road is considered uncomfortable. When the shoe contacts the ground, its horizontal movement is essentially completely stopped within a fraction of a second. In this situation, the horizontally effective forces, i.e., the horizontal transfer of momentum, are very large. This is in contrast to running on a soft forest ground, where the deceleration is distributed over a longer time period due to the reduced friction of the ground. The high transfer of momentum can cause premature fatigue of the joints and the muscles and may, in the worst case, even be the reason for injuries.  
           [0007]    Further, many runners contact the ground with the heel first. If viewed from the side, the longitudinal axis of the foot is slightly inclined with respect to the ground surface (i.e., dorsal flexion occurs). As a result, a torque, which cannot be sufficiently cushioned by compression of a sole material in the Z-direction alone, is exerted on the foot during first ground contact. This problem becomes worse when the runner runs on a downhill path, since the angle between the shoe sole and the ground increases in such a situation.  
           [0008]    In addition, road surfaces are typically cambered for better water drainage. This leads to a further angle between the sole surface and the ground plane. Additional loads, caused by a torque on the joints and the muscles, are, therefore, created during ground contact with the heel. With respect to this strain, the compression of the sole materials in the Z-direction alone again fails to provide sufficient cushioning. Furthermore, during trail running on soft forest ground, roots or similar bumps in the ground force the foot during ground contact into an anatomically adverse inclined orientation. This situation leads to peak loads on the joints.  
           [0009]    There have been approaches in the field to effectively cushion loads that are not exclusively acting in the Z-direction. For example, International Publication No. WO98/07343, the disclosure of which is hereby incorporated herein by reference in its entirety, discloses 3D-deformation elements that allow for a shift of the overall shoe sole with respect to a ground contacting surface. This is achieved by a shearing motion of an elastic chamber, where the walls are bent to one side in parallel so that the chamber has a parallelogram-like cross-section, instead of its original rectangular cross-section, under a horizontal load.  
           [0010]    A similar approach can be found in U.S. Pat. No. 6,115,943, the disclosure of which is hereby incorporated herein by reference in its entirety. Two plates interconnected by means of a rigid linkage below the heel are shifted with respect to each other. The kinematics are similar to International Publication No. WO98/07343, i.e., the volume defined by the upper and lower plate, which is filled by a cushioning material, has an approximately rectangular cross-section in the starting configuration, but is transformed into an increasingly thin parallelogram under increasing deformation.  
           [0011]    One disadvantage of such constructions is that cushioning is only possible along a single path, as predetermined by the mechanical elements. For example, the heel unit disclosed in U.S. Pat. No. 6,115,943 allows only a deflection in the Y-direction, which is simultaneously coupled to a certain deflection in the Z-direction. With respect to forces acting in the X-direction, the sole is substantially rigid. Another disadvantage of such constructions is that the horizontal cushioning is not decoupled from the cushioning in the Z-direction. Modifications of the material or design parameters for the Z-direction can have side effects on the horizontal directions and vice versa. Accordingly, the complex multi-dimensional loads occurring during the first ground contact with the heel, in particular in the above discussed situations with inclined road surfaces, cannot be sufficiently controlled.  
           [0012]    Further, U.S. Pat. No. 5,224,810, the disclosure of which is also hereby incorporated herein by reference in its entirety, discloses dividing the overall sole of a shoe into two wedge-like halves which are shifted with respect to each other, wherein the movement is limited to the X-direction by means of corresponding ribs. Cushioning for ground reaction forces acting in the longitudinal direction (i.e., the Y-direction) of the shoe is not disclosed. In particular, the system does not provide any cushioning during ground contact with the heel.  
           [0013]    It is, therefore, an object of the present invention to provide a cushioning element for a shoe sole that reduces loads on the muscles and the bones caused by multi-dimensional ground reaction forces, in particular during the first ground contact with the heel, thereby overcoming the above discussed disadvantages of the prior art.  
         SUMMARY OF THE INVENTION  
         [0014]    The invention relates to a sliding element for a shoe sole, in particular a sports shoe with an upper sliding surface and a lower sliding surface, wherein the lower sliding surface is arranged below the upper sliding surface so as to be slideable in at least two directions. A relative movement between the upper sliding surface and the lower sliding surface allows the foot to feel as if it is wearing a conventional shoe that contacts a surface with reduced friction, for example, a soft forest ground. The sliding movement of the surfaces distributes the deceleration of the sole over a greater time period. This, in turn, reduces the amount of force acting on the athlete and the momentum transfer on the muscles and the bones.  
           [0015]    According to the invention, a sliding movement of the upper sliding surface relative to the lower sliding surface may occur in several directions. In contrast to the prior art, strains in the X-direction, as well as in the Y-direction, can therefore be effectively reduced. The two sliding surfaces interact without any side effects on the Z-direction. Thus, proven cushioning systems in the Z-direction can be combined, interference-free, with a sliding element in accordance with the invention.  
           [0016]    Because the horizontal shear-movements can be optimized, the athlete can adjust the orientation of his or her lower extremities in such a way that the ground reaction force, which consists of the three components occurring in the X-, Y- and Z-directions and which is transferred as a load on the joints, is reduced. By reducing the lever arms in the knee joint and the ankle joint, the system can reduce the relevant frontal and transversal moments. Accompanying this reduction is a decrease of the shear-forces in the joints, which is also beneficial to the cartilage of the joints and the bases of the tendons. This is important to runners, because the typical injuries they suffer are degeneration of the cartilage and inflammation of the bases of the tendons.  
           [0017]    In addition, a sliding element in accordance with the invention positively influences the moments and forces arising during running on cambered roads and during downhill running. A comparative study with conventional sole structures has shown that the sliding element allows measurable deflections, which noticeably reduce the loads arising during ground contact.  
           [0018]    In one aspect, the invention relates to a sliding element for a shoe sole. The sliding element includes an upper sliding surface and a lower sliding surface. The lower sliding surface is arranged below the upper sliding surface, such as to be slideable in at least two directions.  
           [0019]    In another aspect, the invention relates to a sole for an article of footwear. The sole includes at least one sliding element, which itself includes an upper sliding surface and a lower sliding surface. The lower sliding surface is arranged below the upper sliding surface, such as to be slideable in at least two directions.  
           [0020]    In yet another aspect, the invention relates to an article of footwear including an upper and a sole. The sole includes at least one sliding element, which itself includes an upper sliding surface and a lower sliding surface. The lower sliding surface is arranged below the upper sliding surface, such as to be slideable in at least two directions.  
           [0021]    In various embodiments of the foregoing aspects of the invention, at least one projection is arranged on one of the two sliding surfaces for engaging a corresponding recess on the other sliding surface to limit the sliding movement of one sliding surface with respect to the other sliding surface. In one embodiment, the lower sliding surface includes the projection for engaging the recess in the upper sliding surface. The projection can have a pin-like shape and the recess can have an elliptical shape. Moreover, the projection can have a starting position arranged at a top end of the elliptically shaped recess and a major axis of the elliptically shaped recess can be inclined with respect to a longitudinal axis of the shoe sole. In a further embodiment, at least one cushioning element is arranged in the recess to cushion the movement of the upper sliding surface with respect to the lower sliding surface.  
           [0022]    In another embodiment, the upper sliding surface forms a lower side of an upper sliding plate and the lower sliding surface forms an upper side of a lower sliding plate. The lower sliding plate and the upper sliding plate can be similarly shaped. Moreover, the upper sliding plate and the lower sliding plate can include corresponding concave or convex shapes and can be slideable relative to one another in at least three directions.  
           [0023]    Furthermore, the sliding element can include a spring element that is deflected by a sliding movement of the upper sliding surface relative to the lower sliding surface. The spring element can form an elastic envelope at least partially encompassing the upper sliding surface and the lower sliding surface. Moreover, the elastic envelope can seal an intermediate space between the upper sliding surface and the lower sliding surface and can include a lower side on which at least one profile element is disposed.  
           [0024]    In still other embodiments, at least one sliding element is arranged in a heel area of the sole, for example on a lateral side of the heel area. In another embodiment, at least one sliding element is arranged in a forefoot area of the sole, for example on a rear section of the forefoot area. In yet another embodiment, the upper sliding surface is attached to a midsole of the sole.  
           [0025]    These and other objects, along with the 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  
       [0026]    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:  
         [0027]    [0027]FIG. 1 is an exploded schematic perspective bottom view of a sliding element in accordance with the invention incorporating a lower sliding plate and an upper sliding plate;  
         [0028]    [0028]FIG. 2 is a schematic perspective bottom view of an embodiment of a spring element for use with the sliding element of FIG. 1;  
         [0029]    [0029]FIG. 3 is an exploded schematic perspective bottom view of an alternative sliding element in accordance with the invention incorporating a lower sliding plate and an upper sliding plate;  
         [0030]    [0030]FIG. 4 is a schematic perspective bottom view of an embodiment of a spring element for use with the sliding element of FIG. 3;  
         [0031]    [0031]FIG. 5 is a schematic plan view of a cushioning element for use with the sliding elements of FIGS. 1 and 3;  
         [0032]    [0032]FIG. 6 is an exploded schematic perspective bottom view of a shoe sole with the sliding elements of FIGS. 1 and 3 and the spring elements of FIGS. 2 and 4;  
         [0033]    [0033]FIG. 7 is a schematic cross-sectional view of the shoe sole of FIG. 6 taken at line  7 - 7 ; and  
         [0034]    [0034]FIG. 8 is a schematic cross-sectional view of the shoe sole of FIG. 6 taken at line  8 - 8 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    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 soles for sports shoes, but rather it is to be understood that the present invention can also be used to produce soles or portions thereof for any article of footwear. Further, only a left or right sole and/or shoe is depicted in any given figure; however, it is to be understood that the left and right soles/shoes are typically mirror images of each other and the description applies to both left and right soles/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.  
         [0036]    [0036]FIG. 1 depicts one embodiment of a sliding element  1  in accordance with the invention. The sliding element  1  includes a lower sliding surface in the form of a lower sliding plate  2  and an upper sliding surface in the form of an upper sliding plate  3 . In FIGS.  1 - 4  and  6 , a bottom view is illustrated. The upper sliding plate  3 ,  43  and the lower sliding plate  2 ,  42 , which are each defined with respect to a shoe in an upright orientation, therefore appear in FIGS. 1, 3, and  6  in an inverted arrangement.  
         [0037]    As shown in FIG. 1, the lower sliding plate  2  and the upper sliding, plate  3  may be slightly curved elements. As such, the sliding element  1  can easily be integrated into the heel area  32  or the forefoot area  34  of a shoe sole  30  (see FIG. 6). In addition, independent cushioning can be added to the heel area  32  to provide cushioning in the Z-direction. In various embodiments, however, the lower sliding plate  2  and the upper sliding plate  3  may be concavely or convexly shaped to permit adaptation to the shoe sole  30  onto which they are arranged, to allow a better adaptation to the gait cycle, and/or to selectively provide a cushioning direction inclined with respect to the X-Y-plane, i.e., in the Z-direction. Alternatively, the sliding plates  2 ,  3  can be generally planar two-dimensional elements. As also shown in FIG. 1, the lower sliding plate  2  and the upper sliding plate  3  may be substantially identical in size and shape; however, the size and shape of the lower sliding plate  2  and upper sliding plate  3  can vary to suit a particular application.  
         [0038]    In one embodiment, to reduce wear on one or both plates  2 ,  3 , the lower sliding plate  2  and the upper sliding plate  3  may be made from materials having good sliding properties. Suitable plastic materials, as well as metals with a suitable coating, such as the Teflon® (polytetrafluoroethylene (PTFE)) brand sold by DuPont or a similar substance, may be used. Besides plastic or polymeric materials and coated metals, it is also possible to coat plastic materials with Teflon® or to compound Teflon® directly into the plastic material. Possible materials and manufacturing techniques are described in greater detail hereinbelow.  
         [0039]    One of the sliding plates  2 ,  3  may include, on the sliding surface directed to the other sliding plate  2 ,  3 , two pin-like projections  4 . As indicated by the dashed lines in FIG. 1, the projections  4  may engage recesses  5  in the corresponding sliding surface  2 ,  3 . In one embodiment, the projections  4  are arranged on the lower sliding plate  2  and the recesses  5  are provided on the upper sliding plate  3 . A reverse arrangement, however, is also possible. Furthermore, it is possible to use only a single projection  4  and a single recess  5 , as well as any other numbers of these elements. As shown in FIG. 1, the projections  4  and corresponding recesses  5  are spaced relatively linearly along a longitudinal axis  9  of the sliding plates  2 ,  3 . Alternatively, the projections  4  and corresponding recesses  5  may be spaced in any arrangement about the sliding plates  2 ,  3 .  
         [0040]    The recess  5  is larger than the projection  4 . The resulting play of the projection  4  within the corresponding recess  5  determines the extent of the relative sliding movement between the lower sliding plate  2  and the upper sliding plate  3 . Excessive shifts of the lower sliding plate  2  relative to the upper sliding plate  3  are avoided, and the stability of the sliding element  1  maintained, through the interaction of the projection  4  and the corresponding recess  5 .  
         [0041]    In general, sliding movements of the lower sliding plate  2  relative to the upper sliding plate  3  are possible in the X-direction as well as in the Y-direction. In the embodiment shown in FIG. 1, the recesses  5  are generally elliptical in shape; however, the shape and size of the recesses  5  can vary to suit a particular application. As shown in FIG. 6, when the sliding element  1  is arranged on a shoe sole  30 , a major axis  7  of the elliptical recess  5  can have an inclined orientation with respect to a longitudinal axis  8  of the shoe sole  30 . Such an arrangement is particularly suitable for cushioning the ground reaction forces occurring in the X- and Y-directions in the heel area  32 , as it allows for maximum deflection of the lower sliding plate  2  along the major axis  7  of the elliptic recess  5 , i.e., in an inclined direction with respect to the longitudinal axis  8  of the shoe sole  30 . Further, as described above, in various embodiments the lower sliding plate  2  and the upper sliding plate  3  may be concavely or convexly shaped. A lower sliding plate  2  and an upper sliding plate  3  having such shapes are particularly useful for cushioning the ground reaction forces occurring in the Z-direction in the heel area  32 , as they allow for a sliding movement of the lower sliding plate  2  relative to the upper sliding plate  3  in an inclined direction with respect to the ground surface (i.e., the X-Y plane).  
         [0042]    [0042]FIG. 2 depicts one embodiment of a spring element  10  in accordance with the invention. In the embodiment shown, the spring element  10  is shaped so that the projection  4  is in the non-deflected, or neutral, position when situated at the front end of the elliptical recess  5 . When the sliding element  1  is positioned in a lateral side  37  of the heel area  32  of the shoe sole  30 , as shown in FIG. 6, maximum deflection of the lower sliding plate  2  relative to the upper sliding plate  3  occurs towards the lateral side  37  and a back end  38  of the shoe sole  30  in a direction inclined relative to the longitudinal axis  8  of the shoe sole  30 . This is one way to compensate for the ground reaction forces that arise during first ground contact. Other movement patterns of the lower sliding plate  2  relative to the upper sliding plate  3  can be easily achieved by modifying the shape of the recess  5 . This may be desirable if the sliding element  1  is to be arranged in a different area of the shoe sole  30  than as shown in FIG. 6.  
         [0043]    In another embodiment, the lower sliding plate  2  or the upper sliding plate  3 , whichever comprises the recesses  5 , is releasably arranged, thereby allowing an athlete to select and mount a differently designed sliding plate  2 ,  3  and to, therefore, easily adapt the sliding element  1  to his or her individual requirements. In yet another embodiment, to allow for multi-level horizontal cushioning, several sliding plates  2 ,  3  may be stacked on top of each other and provided with suitable projections  4  and corresponding recesses  5 .  
         [0044]    Referring again to FIG. 2, the spring element  10  may form an elastic envelope enclosing the lower sliding plate  2  and the upper sliding plate  3 . When the lower sliding plate  2  and the upper sliding plate  3  shift relative to one another, the overall area taken up by the sliding plates  2 ,  3  increases and the spring element  10  is thereby elongated and/or deformed. The spring element  10  provides a restoring force to bring the lower sliding plate  2  and the upper sliding plate  3  back into their neutral or starting positions. The material properties and the wall thickness of the spring element  10  determine the dynamic properties of the sliding element  1 . In other words, the material properties and the wall thickness of the spring element  10  determine the resistance that the spring element  10  will offer against a sliding movement of the lower sliding plate  2  relative to the upper sliding plate  3 .  
         [0045]    Still referring to FIG. 2, the spring element  10  may include on its bottom side a plurality of profile elements  11  in order to provide good friction with the ground. The exact design of the profile elements  11  depends on the intended field of use of the shoe to which the sliding element  1  is arranged. In addition, materials for providing cushioning in the Z-direction (e.g., cushioning elements made from foamed ethylene vinylene acetate (“EVA”)) may be provided on the bottom side of the spring element  10 . In a further embodiment, a thin layer of EVA is arranged between the lower sliding plate  2  and an additional outsole layer  71 . The outsole layer  71  is mounted on the bottom side of the sliding element  1  as a separate component from the spring element  10  (see FIG. 7).  
         [0046]    To ensure that the sliding movement of the lower sliding plate  2  relative to the upper sliding plate  3  is not impaired by the penetration of dirt into an intermediate space between the lower sliding plate  2  and the upper sliding plate  3 , the spring element  10  encompasses the lower sliding plate  2  and the upper sliding plate  3  at least along the sides, thereby enclosing the intermediate space between the plates. Where the sliding element  1  is positioned on the outer surface of the shoe sole  30 , the spring element  10  may enclose the bottom side of the sliding element  1 , which is directed to the ground surface, and profile elements  11  may be arranged on the bottom side of the spring element  10 . The top side of the spring element  10  may be open so that the top side of the upper sliding plate  3  can be directly mounted to the bottom side of a shoe sole  30 .  
         [0047]    Referring again to FIG. 1, cushioning elements  6  may additionally or alternatively be arranged in the recesses  5  to cushion the movements of the projections  4  inside the recesses  5 . These cushioning elements  6  further impact the dynamic properties of the sliding element  1 . In addition, the cushioning elements  6  can provide cushioning to the shoe in the Z-direction.  
         [0048]    [0048]FIG. 5 depicts one embodiment of a cushioning element  6  in accordance with the invention. In the embodiment shown, the outer edge  12  and the inner edge  13  of the cushioning element  6  are generally elliptical in shape; however, the shape of one or both of the outer edge  12  and the inner edge  13  can vary to suit the particular recess  5  in which the cushioning element  6  is disposed. Moreover, any number of projections  14  may be arranged, in any position, on the outer edge  12  of the cushioning element  6  to control the positioning of the cushioning element  6  in the recess  5 . The width of the cushioning element  6 , as measured from the outer edge  12  to the inner edge  13 , can vary to suit the amount of cushioning required by a particular application. In addition, the height of the cushioning element  6  can vary to provide different amounts of cushioning to the shoe in the Z-direction.  
         [0049]    Each projection  4  of one of the sliding plates  2 ,  3  may sit within the aperture defined by the inner edge  13  of one of the cushioning elements  6 . The size and shape of the aperture defined by the inner edge  13  of the cushioning element  6  may determine the extent and direction of the relative sliding movement between the lower sliding plate  2  and the upper sliding plate  3 .  
         [0050]    [0050]FIG. 3 depicts a smaller embodiment of a sliding element  41  in accordance with the invention. The sliding element  41  is similar in structure and operation to sliding element  1 . The sliding element  41  includes a lower sliding surface in the form of a lower sliding plate  42  and an upper sliding surface in the form of an upper sliding plate  43 . One of the sliding plates  42 ,  43  may include, on the sliding surface directed to the other sliding plate  42 ,  43 , pin-like projections  44  for engaging recesses  45  in the corresponding sliding surface  42 ,  43 . Further, cushioning elements  46  may additionally or alternatively be arranged in the recesses  45  to cushion the movements of the projections  44  inside the recesses  45 .  
         [0051]    [0051]FIG. 4 depicts one embodiment of a spring element  50  in accordance with the invention. The spring element  50  is similar in structure and operation to spring element  10 . The spring element  50  may form an elastic envelope enclosing the lower sliding plate  42  and the upper sliding plate  43  and may include on its bottom side a plurality of profile elements  51 .  
         [0052]    The design of this smaller sliding element  41 , which, as shown in FIG. 6, is used in the forefoot area  34  of the shoe sole  30 , differs from the above-described larger embodiment of the sliding element  1 , apart from its smaller dimensions, only by the substantially equal planar shape of the lower sliding plate  42  and the upper sliding plate  43 . This difference in design reflects the different positioning of the two sliding elements  1 ,  41  on the shoe sole  30 , as shown in FIG. 6. Whereas the smaller sliding element  41  is arranged in the almost completely flat rear section  35  of the forefoot area  34 , the larger sliding element  1  is arranged on the lateral side  37  of the back end  38  of the heel area  32 . The larger sliding element  1  facilitates, by its slightly curved configuration, the rolling-off of the shoe.  
         [0053]    The various components of the sliding elements  1 ,  41  can be manufactured by, for example, injection molding or extrusion. 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, for example, the recesses  5 ,  45 . Other manufacturing techniques include melting or bonding additional portions. For example, the projections  4 ,  44  may be adhered to the lower sliding plate  2 ,  42  with a liquid epoxy or a hot melt adhesive, such as ethylene vinyl acetate (EVA). In addition to adhesive bonding, portions can be solvent bonded, which entails using a solvent to facilitate fusing of the portions to be added to the sole  30 . The various components can be separately formed and subsequently attached or the components can be integrally formed by a single step called dual injection, where two or more materials of differing densities are injected simultaneously.  
         [0054]    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 rubber. Other suitable materials will be apparent to those skilled in the art.  
         [0055]    [0055]FIG. 6 depicts one embodiment of the shoe sole  30  for an article of footwear  70  (see FIG. 7) incorporating the above-described sliding elements  1 ,  41  in accordance with the invention. Receiving surfaces  21 , to which the upper sliding plates  3 ,  43  of the respective sliding elements  1 ,  41  may be attached, can be provided on the midsole body  20 . Many different mounting methods, such as gluing or melting, may be used. In another embodiment, the upper sliding surface  3 ,  43  may be directly integrated into the midsole body  20  during its manufacture and corresponding projections  4 ,  44  or recesses  5 ,  45  may be directly arranged in the midsole body  20 .  
         [0056]    The sliding elements  1 ,  41  can be arranged between the midsole  20  and the outsole layer  71 , as shown in the embodiments illustrated in FIGS. 7 and 8. Alternatively, the sliding elements  1 ,  41  may be integrated into the midsole  20  by being arranged between different layers of the midsole  20 . In yet another embodiment, the sliding elements  1 ,  41  may be arranged between the insole  73  (see FIGS. 7 and 8) and the midsole  20 .  
         [0057]    The distribution of the sliding elements  1 ,  41  on the shoe sole  30 , as shown in FIG. 6, is only one possible arrangement. Other arrangements, wherein sliding elements  1  are exclusively arranged in the heel area  32  or sliding elements  41  are exclusively provided in the forefoot area  34 , are also possible. The distribution depends on the preferred field of use for the shoe. With respect to the heel area  32 , for linear sports the sliding element  1  may be arranged on the lateral side  37  and for lateral sports the sliding element  1  may be arranged on the medial side  39 . These are the areas of the sole  30  most affected by the horizontal ground reaction forces during ground contact with the heel. Selectively providing sliding elements  1  in these positions affords maximum cushioning during ground contact with the heel, without substantially influencing the other properties of the sole  30 . With respect to the forefoot area  34 , a sliding element  41  arranged in the rear section  35  of the forefoot area  34  cushions, in particular, the horizontal ground reaction forces occurring during lateral stops and is particularly useful in sports with many changes of direction, such as basketball.  
         [0058]    For a running shoe, sliding elements  1  are particularly useful in the heel area  32 . A basketball shoe may also be equipped with one or more sliding elements  41  in the forefoot area  34 . Thus, in a further embodiment of a basketball shoe (not shown), three decoupled sliding elements  41  are arranged in the forefoot area  34  on the medial side  39  of the shoe sole  30  and two further decoupled sliding elements  1  are arranged in the heel area  32  on the medial side  39  of the shoe sole  30 .  
         [0059]    For reinforcing the attachment of the sliding elements  1 ,  41  to the shoe sole  30  and for a more stable anchoring, the top side of the upper sliding plates  3 ,  43 , which may be directly attached to the shoe sole  30 , may be three-dimensionally shaped to interact with corresponding projections  22  on the receiving surfaces  21 . In one embodiment, the receiving surfaces  21  are part of the midsole body  20 . It is, however, also possible to arrange the sliding elements  1  on suitable areas of the outsole layer  71 .  
         [0060]    In yet another embodiment, the sliding elements  1 ,  41  may be provided as modular components that can be releasably attached to the shoe sole  30 , as required. Such an embodiment is useful for adapting a running shoe to a particular ground surface. For example, one or more sliding elements  1 ,  41  used for running on asphalt may be replaced by lighter common outsole elements for running in the woods, or by other sliding elements  1 ,  41 , which can be optimally adjusted for the respective type of surface.  
         [0061]    [0061]FIG. 7 depicts a cross-sectional view of one embodiment of a shoe sole  30  for an article of footwear  70  in accordance with the invention. The article of footwear  70  can include any type of upper  74 . As shown, the lower sliding plate  2  and the upper sliding plate  3  may be arranged, as described above, between the outsole layer  71  and the midsole  20 . Moreover, as described above, a spring element  10  may form an elastic envelope enclosing the lower sliding plate  2  and the upper sliding plate  3 . Also as shown, the lower sliding plate  2  and the upper sliding plate  3  are at least partially in contact.  
         [0062]    [0062]FIG. 8 also depicts a cross-sectional view of one embodiment of a shoe sole  30  for an article of footwear  70  in accordance with the invention. As shown, the article of footwear  70  can include any type of upper  74 . The lower sliding plate  42  and the upper sliding plate  43  may be arranged, as described above, between an outsole layer  71  and a midsole  20 . Moreover, as described above, a spring element  50  may form an elastic envelope enclosing the lower sliding plate  42  and the upper sliding plate  43 . Also as shown, the lower sliding plate  42  and the upper sliding plate  43  are at least partially in contact.  
         [0063]    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. The described embodiments are to be considered in all respects as only illustrative and not restrictive.