Patent Publication Number: US-2021177087-A1

Title: Midsole for a shoe, in particular a running shoe

Description:
The invention concerns a midsole having an arch support, in particular a midsole for running shoes. One type of running shoes of the state of the art has in common the concept of protection of the foot. More precisely, the shoe is considered a sheltering instrument for the foot. This protection concept has led to relatively heavy running shoes, which often have a sole or insole with a high degree of cushioning in order to mitigate the force reactions stemming from the heel strike and acting on the ankle joint and the leg. Another type of running shoes are ultra lightweight shoes which often are below 300 grams. This type is minimalist having thin soles and thin uppers. When designing shoes, the shoe industry has for a long period had the natural moving foot as the ideal state of motion, e.g. barefoot running on grass, where the foot unconstrained by a shoe is allowed to perform its natural motion. However, once the shoe is on the foot, natural motion of the foot is impeded. As an example, the angle of the metatarsal phalangeal joint is reduced considerably when wearing shoes. The metatarsal joint angle is the angle between the ground and the metatarsal phalanges. If measured at the instant just before pushing off from the ground, this angle is in barefoot running close to 60 degrees and in so called technical or athletic running, where running shoes are used, reduced to only 35 degrees. Impediment of the natural motion of the foot means among other things that the muscles of the leg and foot which are active during barefoot running are also constrained. These muscles are not allowed to act with their full strength, and thus the shoe, if wrongly designed, will limit the ability of the runner to move efficiently. His performance is lowered as compared to barefoot running. Some of the key muscles during walking and running are musculus flexor hallucis longus and musculus extensor hallucis longus. The importance of these strong muscles when considering barefoot running in relation to running with shoes has already been acknowledged in U.S. Pat. No. 5,384,973, which is incorporated herein by reference. More specifically U.S. Pat. No. 5,384,973 describes a midsole for a running shoe which sole has a multiple of flex joints or grooves in longitudinal and transversal direction. A number of discrete outsole elements are connected to the midsole. 
     This structure allows the toes of the foot to act independently and to increase the stability of the shoe. In particular, the flex joints have created an isolated sole area for the hallux, hereby allowing flexor hallucis and extensor hallucis longus to play a greater role during running. U.S. Pat. No. 5,384,973 describes the relatively thick midsole of current running shoes as a reason for instability leading to risk of injuries. In order to reduce this risk, U.S. Pat. No. 5,384,973 provides as already described a solution with flex joint grooves in the sole and particularly along the hallux between the first and second toe. This prior art solution is an improvement over earlier prior art, in that injuries from running can be lowered. 
     Other measures can be taken in order to lower the risk of injury. J P 2001-0291 10 teaches a basketball shoe with asymmetric support in the midfoot area. The midsole is extended upwardly on the lateral side, and upwardly on the medial side, but the lateral side is higher than the medial side. This asymmetry is caused by the frequent side wards movements in basketball. Also U.S. Pat. No. 6,108,943 describes a sports shoe which is asymmetric and has a midsole with distinctly performing lateral and medial portions. The attention is particularly directed to the stability of the lateral side due to the frequent side wards movements in tennis. However, running places other demands on the midsole design. Further, the prior art midsole of U.S. Pat. No. 6,108,943 is made of a soft foam material with high cushioning characteristics in order to cushion the impact forces. While this solution may work well in some sports as tennis, cushioning is not an optimum way to reduce the risk of injury during running, because cushioning absorbs too much energy from the runner. 
     In the light of the foregoing, the object of the present invention is to reduce further the risk of injury during running while at the same time reducing the loss of energy experienced by a runner. 
     This is achieved with a midsole according to claim  1 . 
     The invention has its starting point in the basic assumption that natural running is the ideal situation, and that a midsole should be designed in a way that brings running as close to the ideal situation as possible. Instead of extensive cushioning in running shoes, or extreme reduction of the weight, a concept of supporting the foot in its natural motion during running has been developed. The present invention is characterized in that the medial arch support structure of the midsole is covering an area larger than the lateral support structure. Realizing that the foot during running especially needs support on the medial side has led to this design where the midsole has a medial arch support structure which extends upwardly to support the medial upper arch. Further, a lateral support structure is extending upwardly to support the lateral side of the midfoot. As the medial side needs more support than the lateral side, the medial arch support structure covers an area larger than the lateral support structure. The medial upper arch support structure has the advantage that it offers an elastic adjustable support and allows the foot to move naturally. The invention is further characterized in that the medial support structure is connected to an upper heel portion of the midsole which portion essentially covers the tuberosity of the calcaneus of a wearer, and that a toe end of the midsole is extended upwardly. Extending the upper heel portion to vertically cover the tuberosity of the human calcaneus, and having the area of midsole material supporting the heel on the medial side of the upper heel side larger than the supporting area of the midsole material on the lateral side has the advantage, that the midsole firmly supports the heel. This extended midsole heel so to speak grabs around the human heel and follows its motions intimately. Due to the larger material surface on the medial side of the heel, support is given already at heel strike when the foot moves from typically the lateral side towards the medial side into pronation. As the midsole is made from a material with a higher stiffness than textile, the material around the tuberosity will structurally and mechanically support the foot. The toe end of the midsole is extended upwardly and finishes the stabilizing embracement of the foot made by the inventive midsole. The raised toe end, which is an integrated part of the midsole, provides protection and stabilization at the same time. It improves fixation of the foot inside the shoe by limiting longitudinal movement of the foot during running without the need for a discrete toe cap to be applied during manufacturing. In total, these supporting structures reduce the risk of injuries due to the mechanical stabilization they provide, and the integration of these structures into the midsole enables the omission of extra support materials, e.g. for cushioning, that would add to the weight of the shoe. 
     Preferably, the medial support structure extends vertically to at least the start of the navicular bone of the foot. This vertical extension of the structure ensures a sufficient support in the situation after heel strike where the foot typically tends to pronate. The medial arch support structure is as mentioned intended to reduce the effects of such pronation. 
     Advantageously, the medial support structure contains openings devoid of midsole material. This enables a further reduction of the weight of the midsole. 
     On the lateral side of the foot, a bone known as tuberositas ossis creates an a protrusion. This bone, if encapsulated by a relatively stiff sole material, will be subjected to friction between head and sole material, and will reduce the flexibility of the shoe. In order to avoid this friction and to allow the bone and the corresponding joint free movement, an opening is made in the lateral support structure. 
     The lateral support structure and the medial arch support structure are manufactured with a certain mechanical tension, in that they are moulded with an inclination to follow the shape of the foot and are extending towards the lacing area. Thus, these support structures will support the foot not only during running, but also contribute to keep the shape of the shoe over time. 
     Preferably, not only the medial arch support structure but also the lateral support structure is connected to the upper heel portion which surrounds and covers the tuberosity of the calcaneus of a wearer. Via a vertically extending medial heel portion and a vertically extending lateral heel portion the upper heel portion is materially connected to the supporting structures. This connection creates on the medial side a supporting wall which extends longitudinally approximately to the proximal end of the metatarsal phalanges. 
     The supporting structures on the medial and the lateral side can advantageously have a mesh-like architecture with supporting arms creating reinforcing cross sections. This mesh-like structure allows reduction of weight due to openings in the structure, and the reinforcing cross sections ensure that sufficient mechanical supporting force is left. 
     If the height of the stabilizing midsole and the outsole is too high, the risk of injury is increased. By keeping the heel spring of the midsole between 8 and 12 millimetres this risk is reduced. 
     In order to support the concept of getting close to natural running, extensive data had to be collected and turned into practical measures. The last used for the inventive midsole is a so called anatomical last which means that is has a higher degree of similarity to the foot compared to a normal foot shaped last. In other words, the anatomical last is in shape very close to the human foot. The high degree of similarity has been achieved by measuring 2200 feet. By examination of the many data from the feet we have created so to speak “an average human foot” and put this shape into the last. During manufacturing of the shoe, the sole material, which is injected, will follow the shape of the anatomical last and hereby take the shape of the average human foot. The foot sole will rest comfortably on the manufactured sole, because the sole is a mirror of the foot sole. 
    
    
     
       The invention is now described in detail by way of the drawings in which 
         FIG. 1 a    is a split view of the sole with an inventive midsole and a shank 
         FIG. 1 b    is a cut away view of the sole of  FIG. 1 a    along an axis A-A 
         FIG. 2 a    is a split view of another sole with an inventive midsole and a shank 
         FIG. 2 b    is a cut away view of the sole of  FIG. 2 a    along an axis A-A 
         FIG. 3 a    shows the shank used in a perspective view 
         FIG. 3 b    shows the shank of  FIG. 3 a    in a side view 
         FIG. 3 c    shows the shank of  FIG. 3 a    in a rear view 
         FIG. 4  is a view of a first embodiment of the bottom of the inventive midsole 
         FIG. 5  is a drawing showing the bones of the medial side of the foot 
         FIG. 6  shows the right human foot as seen from below 
         FIG. 7  is a second embodiment of the bottom of the inventive midsole with an outsole 
         FIG. 8  is a third embodiment of the bottom of the inventive midsole with an outsole 
         FIG. 9  is a fourth embodiment of the bottom of the inventive midsole with an outsole 
         FIG. 10  is a view of the inventive midsole from the lateral side 
         FIG. 11  is a view of the inventive midsole from the medial side 
         FIG. 12  is a view of an alternative inventive midsole from the medial side 
         FIG. 13  is a view of an alternative inventive midsole from the lateral side 
         FIG. 14  is a view of a first heel embodiment of the inventive midsole 
         FIG. 15  is a view of a second heel embodiment of the inventive midsole 
     
    
    
       FIG. 1 a    is a perspective view of the sole  7 . In a preferred embodiment, the sole consists of three layers, namely as first layer a midsole  1 , a second intermediate layer  2 , and a third layer  3  constituting the outsole. A shank  4  is placed on top of the midsole.  FIG. 1 b    shows the sole in a longitudinal cut along the axis A-A of  FIG. 1 a   . For reasons of clarity, the medial support structure has been cut away in the view of  FIG. 1 a    but it can be seen as reference numeral  158  in  FIG. 11 . 
     Midsole  1  is in the preferred embodiment made of light polyurethane (PU) material, also called PU light. This material is a known special variant of PU which has a low density (0.35 g/cm 3 ), i.e. is a lightweight material. A further characteristic is a good return of energy absorbed from the runner, which characteristic is of importance for long distance running. Shore A hardness is between 38 and 40. Alternatively, also ethylene vinyl acetate (EVA) can be used as midsole because it has a lower specific gravity than PU light resulting in a lighter sole. However, EVA tends to quick ageing under frequent force influence from the foot. This ageing is seen as wrinkles in the material. It is not form stable, and after a while it is compressed and does not return to its original shape. 
     Midsole  1  is in this preferred embodiment covered with the second intermediate layer  2  which has the same profile as the midsole.  FIG. 1 b    shows this profile and the second layer  2  is so to speak a replica of the bottom of the midsole  1 . Layer  2  has the function of a protective layer, consists of thermoplastic polyurethane (TPU), and is an intermediate layer which is thin, typically 0.5-2 millimetres. It has a shore A value of 65 plus/minus 3. 
     The third layer  3  is the outsole, which consists of a number of discrete outsole elements (e.g. reference numbers  120 - 123  in  FIG. 8 ), which together add up to be the outsole. Under the term “discrete outsole element” is understood a piece of outsole that is not cast or moulded in the same process as the midsole or the intermediate layer  2 , but is added or bonded to e.g. layer  2  later. Further, a discrete outsole element is not connected to the other outsole elements. In more detail, the outsole  3  consists of a plurality of outsole elements which can be perceived as islands that are not interconnected, separated by one or more grooves in the midsole. The elements are preferably made of rubber. Instead of rubber, TPU can be used as material for the discrete outsole elements, but the gripping characteristics of TPU are inferior compared to rubber. The rubber used is a conventional Nitril Butadine Rubber (NBR), which is preferred for running shoes because of its relative low weight. It has a shoe A value of 55 plus/minus 3. For other types of shoes, latex (comprised of a mixture of natural and synthetic rubber) can be used. The outsole elements are spaced apart with grooves  5 ,  6  in the intermediate TPU layer  2  and in the midsole  1 , and are placed on protrusions or pads  10 ,  11 ,  12 ,  13  ( FIG. 1 b   ) made in the intermediate TPU layer. The pads and grooves of the intermediate layer mate with the corresponding pads and grooves of the midsole. 
       FIG. 2 a    shows another sole, which has an inventive midsole  1  with lateral and medial support structures, and a shank  4  amended as compared to the shank in  FIGS. 1 a  and 1 b   .  FIG. 2 b    shows the sole of  FIG. 2 a    in a cut away view. The reference numerals of  FIGS. 1 a  and 1 b    are the same in  FIGS. 2 a    and  2   b.    
     Manufacturing of the sole  7  consisting of the sole parts  1 ,  2  and  3 . is made in the following way. In a first step, the TPU intermediate layer  2  and the outsole elements  3  are produced in a separate manufacturing process to become an integrated entity. In a second step, the midsole  1  is connected to the integrated entity consisting of layer  2  and outsole  3 . Step one and step two will now be described.
         In step one, the TPU intermediate layer  2  and the discrete outsole elements  3  are manufactured to become an integrated entity. First the discrete outsole elements are manufactured in a rubber vulcanisation process. Then the outsole elements are placed in a mould, where TPU is inserted above the elements. The mould is closed, and under application of heat and pressure the TPU is shaped into the desired shape. After a curing time, the integrated entity of outsole elements and TPU intermediate layer is finished. Although the TPU layer is manufactured in a casting process, alternative manufacturing processes are available for producing the second layer  2 . Thus, the TPU can be injection moulded in a known manner, or the TPU can be a foil-like raw material like a sheet placed above the outsole elements  3  before joining these elements and the TPU using heat and pressure.       

     Bonding between the TPU intermediate layer  2  and the outsole elements  3  are made with glue which is activated by the heat during moulding the TPU onto the outsole elements. A simple adhesion without glue between TPU and rubber during the moulding process proved not durable. Before adding glue between TPU intermediate layer  2  and outsole elements  3 , the rubber surface of the outsole elements  3  must be halogenated in a process which removes fat from the rubber and thus enhances the adhesion. 
     In step two of the manufacturing of sole  7 , the midsole  1  is unified with the integrated entity consisting of layer  2  and outsole elements  3  from step one, as well as with a shoe upper. More specifically, the TPU intermediate layer  2  with the outsole elements  3  is placed in an injection mould together with the shoe upper, after which PU is injected into the mould and bonds to the shoe upper and the integrated entity consisting of layer  2  and outsole elements  3 . The PU thus bonds to the side of the TPU intermediate layer  2  which is closest to the human foot. After this second step, sole elements  1 ,  2  and  3  have become integrated into one entity. Preferably, shank  4  is only partly embedded in PU during the injection process. 
     The TPU intermediate layer  2  has a double function in that it lowers the breakability of the midsole and reduces the cycle time on the PU injection machinery. This will be detailed in the following. 
     In principle, the TPU intermediate layer can be omitted, and the isolated outsole elements placed directly in the mould by the human operator before PU injection. This would however cost processing time on the PU injection machine, because placement of the many discrete outsole elements takes time. Instead, by manufacturing the TPU intermediate layer  2  and outsole elements  3  in a separate process as described above, the PU injection machine is free to manufacture midsoles most of the time. Machine waiting time is reduced. However, the use of the TPU intermediate layer has a further advantage, namely reducing a tendency of the PU light midsole to break. If the discrete outsole elements  3  are placed directly against the PU light midsole without any intermediate layer  2 , the midsole tends to break in durability tests. Such breakage will allow water to enter the shoe during wear. The reason is that when injecting PU into the mould during manufacturing, air bubbles tend to occur in the midsole. The bubbles occur because the PU is not able to press out air around sharp edges in the channels of the mould. This is probably due to the low specific gravity of the PU. The result is that air bubbles are contained in the midsole, thus making the sole liable to penetration of water when the midsole breaks or experiences cracks. TPU has a larger specific gravity, and does not cause problems with trapped air bubbles during manufacture. In other words, the midsole  1  is not liable to water penetration caused by air bubbles and breakage due to protection by the intermediate layer  2 , which contributes to keeping the interior of the shoe dry. 
     As material for midsole  1  PU has been chosen over TPU. In principle, the whole midsole could be made of TPU, but PU light has a lower specific gravity thus lowering the weight of the shoe. Further, PU has good shock absorbing characteristic which is important especially for running shoes. 
     Between the midsole  1  and an insole (not shown on the figures) is the shank  4  ( FIGS. 3 a  to 3 c   ), which consists of a mixture of thermoplastic polyethylene (TPE) and nylon and is partly flexible. It extends from the heel portion to the toes, and has in the heel portion preferably an opening  8 , where the polyurethane used for the midsole  1  enters during the injection process. This feature improves the shock absorption in the heel. In the front end, the shank has two curved fingers  15  and  16  extending under a curvature in the longitudinal direction, and a small finger  14  in the middle. These fingers support in particular the first, fourth and fifth metatarsal phalanges. It has been found that two to three fingers suffice instead of having one supporting finger for each ray in the foot. The shank is designed to be “anatomical”, i.e. it follows the average foot more closely than conventional shanks. The shank is manufactured in an injection process, and is made bendable in the transversal direction just where the fingers of the shank starts, corresponding to the proximal end of the first, fourth and fifth metatarsal phalanges, see the line indicated by reference number  18  in  FIGS. 1 a , 2 a  and 3 a   . Thus, the shank is bendable in a direction orthogonal to the longitudinal axis of the sole. The bend ability is achieved in a process during manufacturing of the shank, where thermoplastic polyethylene is injected from the heel end and nylon from the toe end. The two compositions meet at the bending line and the sole is bendable from this line  18  because polyester is soft compared to hard glass fibre. As a further measure, the shank is also flexible in its longitudinal direction along a line  19  ( FIGS. 1 a  and 2 a   ), because the shank should preferably be more flexible on the lateral side than on the medial side. With this measure, the torsional stiffness in the longitudinal direction is adjustable.  FIG. 3 a    shows the small finger  14 . Tests have shown that pushing off in the forefoot during running is improved by increasing the stiffness in this area of the foot. 
     Preferably, the shank  4  is placed on top of the midsole. Alternatively it could have been placed between the midsole  1  and the intermediate layer  2 , but this placement would lead to friction problems between the human heel and the heel of the midsole. During running, the midsole would compress and decompress in the heel area, each compression allowing the human heel a movement downwards, and each decompression allowing the human heel to move upwards. Repeated movements downwards and upwards against the heel creates friction and discomfort for the runner. Instead, by placing the shank on top of the midsole, friction is lowered because the shank as an early stiffening layer reduces the length of downwards and upwards movements. 
     In one embodiment, the shank is integrated in the strobel sole, which is a flexible sole connected and typically sewn to the upper (not shown in the figures). The strobel sole is often a textile. The integration of the shank into the strobel sole gives a harder sole because the strobel sole contributes to the hardness. This embodiment has the advantage of an easier manufacturing, because the shank is sewn into the strobel sole and does not have to be placed in the mould before PU injection as described above. In the preferred embodiment however, the shank is glued to the strobel sole, which together with the upper is mounted on the last. The last is placed in the mould which is closed, after which PU is injected into the mould. 
     The shank  4  has an offset heel area  25  as shown in  FIG. 3 a   . This offset heel area defines a cavity  17  for receipt of PU or other material. The offset heel area functions as a platform for the PU entering the essentially elliptically shaped opening  8 . The cavity is made by a rim in the shank, which rim follows around the opening  8 . The rim is sloping inwardly towards the centre of the opening, hereby defining the cavity  17 . In one embodiment of the invention, the PU fully fills the cavity, which, when taken at the centre of the opening, gives the following layering in the heel area from top to outsole: strobel sole, PU, TPU intermediate layer  2  and outsole  3 . In the arch area of the sole however, the order of the layers is: strobel sole, PU, shank  4 , PU, and TPU intermediate layer  2 . As there is no shank material in the opening  8  of the heel, this area is more flexible. 
     In order to lower the hardness in the heel area even further, a comfort element  9  ( FIGS. 2 a  and 2 b   ) can be placed in the cavity. In this embodiment, the PU only fills the opening  8  of the shank. Such comfort elements are well known and commercially available. The comfort element is 9 millimetres in height, the PU midsole below is 8 millimetres, the TPU intermediate layer 1 millimetre and the discrete rubber outsole  3  is 2 millimetres. The ratio between the height of the comfort element and the PU midsole below can be varied in a wide range, but should not exceed 1,5:1. Otherwise, the design would approach the conventional cushioning techniques, which as already described has drawbacks. Advantageously, the PU bonds to the comfort element, hereby ensuring a fixation of the material without any further manufacturing steps. 
     Referring to  FIG. 3 b   , a transition zone  39  in the shank between the arch area and the heel area should preferably not make an angle β of more than 50 degrees with the horizontal plane of the offset heel area. A larger angle provides discomfort to the runner due to a sharp edge. Advantageous angles are around 30 degrees.  FIG. 3 c    shows the shank in a rear view. The transition zone  39  not only slopes from the arch area towards the heel area, but also from the medial side of the shank to the lateral side. In this way the shank is raised to give support to the arch of the foot. 
     The shank  4  is in both embodiments (i.e. cavity fully or partly filled with PU) fully or partly embedded in the PU midsole. In the forefoot and in the arch area, the shank is placed close to the strobel sole, either with or without PU in between strobel sole and shank. In the offset heel area the shank is placed close to the outsole. 
     Thus, by offsetting the longitudinally extending shank in the heel area of the sole, a cavity in the heel zone is created. This offset heel area has a platform on which the PU from the midsole is embedded during the injection process. The PU enters the cavity through a hole made in the platform, or, more precisely, through an opening made in the offset heel area of the shank. The heel area is offset towards the outsole to a second horizontal plane different from a first horizontal plane of the arch area of the shank. Our tests have shown, that this design gives a better running experience because the heel area of the sole has become softer. 
     A special insole has been provided. The insole consists of two layers. The upper layer is a polyester material, which is lightweight, and breathable. The bottom layer is made in two versions. For class A runners the bottom layer consists of EVA, which advantageously has a low weight, and for class B runners the bottom layer is made of PU foam. This is a more expensive solution, but gives a better insole. The bottom layer has through-going holes for breathing. In the heel portion of the insole an area with shock absorbing material is placed, and in the forefoot area of the insole an energy return material is placed which during push off releases most of the energy received during heel strike and full foot contact. Instead of placing the shock absorbing material in the insole it can also be embedded during the injection process in the heel of the midsole  1 . 
     The inventive midsole  1  is shown in  FIG. 4  with a direct view from the bottom. The midsole has a forefoot portion  23 , a top end  22 , a lower heel portion  20 , an arch portion  21  and a lateral side portion  24 . Four flex grooves  27 ,  29 ,  31  and  34  traverse the forefoot  23 . The grooves have a depth of approximately 50-60% of the thickness of the forefoot midsole, in this example 3-4 millimetres. A curved flex groove  63  extends from the medial side  49  of the arch portion  21  and continues along portions  48 ,  32 ,  59 ,  60  and  61 . The flex grooves create protrusions or pads  26 ,  28 ,  30 ,  33 ,  35 ,  38 ,  40 ,  46 ,  50 ,  52 ,  54 ,  56 ,  62  which in shape correspond to the shape of the discrete outsole elements  3  but have a larger area. Thus, the pads are closer to each other than the discrete outsole elements mounted on the TPU intermediate layer  2 . As will be described later, this has shown to have a positive effect on slip resistance. Pads  33  and  35  are extended in the lateral horizontal direction to become the most extreme points on the lateral side of the sole. When outsole elements are placed on the pads, this extension will contribute to stabilizing especially when the foot supinates. A reinforcement bar  47  runs slanted from the medial side to the lateral side. The reinforcement bar is part of the midsole and made during the injection process. It is thicker than the midsole on the lateral portion  37  and on the medial portion  49 , and adds stiffness to the midsole. It runs parallel with the shank  4  (not visible on  FIG. 4 ) which is placed on the other side of the midsole, i.e. the side facing the foot. 
     The curved flex groove is substantially wider than the other flex grooves. In one embodiment it is six millimetres wide, the flex groove  34  three millimetres and the flex groove  31  four millimetres. As a rule, the curved flex groove is between 1.5 and 3 times wider than the other flex grooves. The width of the curved flex groove can be varied, but it has preferably a width corresponding to 1-2 times the distance between the third and fourth metatarsal phalanges. However, the distance may not be too wide because this would cause too much flexibility. Further, the flex groove has essentially a constant width along its curve in the forefoot. 
     The curved flex groove  63  intersects the transverse flex grooves  29 ,  31  and  34 . The curved flex groove thus runs in longitudinal direction from the medial side of the arch to an apex point  59  in the metatarsal zone of the foot. From this apex point the groove continues in the opposite direction along path  60  and crossing flex grooves  57  and  55 . It ends approximately under the ball of the big toe in flex groove  61 . The curvature of the groove in essence gives the sequence of midsole pads a spiral shaped character: Thus, starting in an origo point ◯ in pad  62 , a curve  64  can be drawn which describes a somewhat compressed or eccentric spiral graph. When mounted later in the manufacturing process, the discrete outsole elements  3  will describe the same curve. 
     The function of the curved flex groove  63  is to enable natural running by giving the midsole a bending line in longitudinal direction between the third and the fourth metatarsal phalanges and hereby giving the characteristic “2-3 split” of the rays of the foot attention. This will be detailed in the following.  FIG. 5  shows the bones of a right foot from the medial side with first metatarsal phalange  85 , calcaneus  69 , the tuberosity  68  and the superior tuberosity  67 .  FIG. 6  shows a right human foot from below. Reference number  70  describes the talus,  71  the navicular bone, and  72 ,  73  and  74  the three cuneiform bones, i.e. the medial, the intermediate and the lateral cuneiform bone respectively. Line  89  represents a folding line in the human foot between cuboid bone  87  on the one hand, and the lateral cuneiform bone  74  and the navicular bone  71  on the other. The foot is flexible and bendable along this folding line meaning that if bending is made along a longitudinal axis running between the fourth metatarsal phalanges  82  and the third metatarsal phalanges  83 , the three most medial phalanges ( 83 ,  84 ,  85 ) will bend to one side, and the two most lateral phalanges ( 81 ,  82 ) will bend to the other side. Recognizing this bending line by allowing the sole to be bent along this axis enable the supinating and pronating muscles to compensate faster after heel strike in the situation where the foot either pronates or supinates. Thus, in the case of a too large pronation, i.e. the case where the arch of the foot is moved to the medial side, the supinating muscle flexor hallucis longus will counteract by a plantar flexing reaction on the medial side of the foot. Counteraction will be faster with a sole having a curved flex groove, because musculus flexor hallucis does not have to “lift” the whole sole, but only a part of it, namely the part on the medial side of the curved flex groove, i.e. the part which comprises the first, second and third metatarsal phalanges. This supinating counteraction happens in order to get the ankle into neutral position where ideally no supination or pronation exists. 
     The outline of the curved flex groove  63  is shown with the line  90  in  FIG. 6 . This line shows where the curved flex groove is placed in the midsole  1 . Note that the flex groove  63  is placed on the side of the midsole facing the outsole. Curved flex groove  63 , represented by line  90  in  FIG. 6 , emanates from the medial side of the arch and starts under the navicular bone  71 . Alternatively the medial cuneiform bone  72 . It crosses the lateral cuneiform bone  74  and continues between the third and fourth metatarsal phalanges up to the beginning of the joints between the metatarsal and proximal phalanges ( 75 ,  76 ,  77 ,  78 ,  79 ). These joints are shown by line  92 , which also represents flex groove  31  in  FIG. 4 . The curvature of line  90  (i.e. groove  63 ) in the region of the cuneiform bones can be changed. Also the starting point of the curve on the medial side can be raised towards the toe end or lowered towards the heel. 
     Turning back to  FIG. 4 , an ideal landing point A is shown in the lower heel portion. This point is the optimum point of landing for a runner, and it is placed just below the calcaneus, offset to the lateral side. Real life test shows however that in practice this optimum landing point cannot be reached. Typically, real life runners touch ground somewhere along the line marked B, reference number  41 . The point of landing is dependent on the speed of running, and may even be different from right foot to left foot. However, moving the point closer to A results in improved force and energy consumption, and tests have shown that the point of landing with the sole can be moved to approximately C shown in  FIG. 4 . The basic idea with moving the point of landing as close to A as possible is the recognition that the muscles in the leg responsible for propulsion can be activated at an earlier time to become mechanically active—they are earlier in tension and able to create forward propulsion. In order to move this landing point as close to A as possible, two measures have been taken in the design. First, the height of the heel has been lowered or more specifically, the height of the lower heel portion  20  has been lowered in order to get the human foot as close as possible to the ground. Compared to state of the art running shoes, this height can be reduced, because the inventive design does not make use of extra cushioning materials in the sole. Cushioning is an inherent characteristic of the PU midsole material used. In general, cushioning should not be avoided but kept to a minimum because it absorbs energy without returning it to the foot. In the preferred embodiment the maximum height or thickness of the midsole in lower heel portion  20  is between eight and twelve millimetres, preferably eight millimetres. This is the heel spring of the midsole and corresponds to the thickness of the heel in point A of  FIG. 4 . The second measure taken in order to move the point of landing closer to A is by designing the lower heel portion  20  of the midsole  1  with a double tapering.  FIG. 14  shows the rear of the foot  150  wearing a shoe with the inventive midsole  1  and discrete outsole element  124 . The midsole in the rear foot area is asymmetrical around a vertical line B-B dividing the midsole into two halves. In the optimum upright standing position, the vertical axis B-B would go through the ankle joint and the tibia. The midsole is split into a medial heel portion  143  and a lateral heel portion  151 . Further, a horizontal line C-C divides the midsole in the rear foot area into the lower heel portion  20  and an upper heel portion  142 . The lines B-B and C-C together divides the heel of the midsole into four sections: I, II, III and IV. It is clear from the drawing that none of the four sections I-IV are identical. The tapering  141  enables the foot to touch down in point C ( FIG. 4 ). As seen in  FIG. 14 , the tapering is not only in section III, but also partly in section IV. In section IV, i.e. on the medial side of lower heel portion  20 , the tapering stops, and becomes aligned with a geometric plane corresponding to the geometric plane of surface  149  ( FIG. 10 ).  FIG. 10  shows the tapering in more detail, and it will be understood that the tapering not only runs from the centre of the lower heel portion  20  towards the lateral side as depicted in  FIG. 14 , but also from the centre towards the heel end.  FIG. 11  shows with reference number  153  that on this point of the medial inner side of the heel, the lower heel portion has full contact with the ground via an outsole element. Supports  147  are an integral part of the midsole. 
     On heel strike, the midsole and outsole is designed to allow so called horizontal flexing. This is achieved with the curved heel flex groove  45  of  FIG. 4 , which groove is deeper and wider than the transverse flex grooves in the forefoot, and has the function of decoupling the heel of the sole from the forefoot sole in order to allow “horizontal flex”, i.e. in order to allow horizontal movement of the heel portion especially during heel strike. This functionality can be compared to the human fat padding in the heel area which also allows a small horizontal movement back and forth. A second curved heel flex groove  42  is decoupling the pad  40  from the pad  38  at heel strike. Preferably, one discrete outsole element is applied to pad  40  and another element to pad  38 . Pad  38  and pad  46  are fully horizontal, i.e. when the discrete outsole elements have been applied, these elements have full ground contact and are not curved as pad  40 . The full ground contact of pad  46  is important to reduce the effects of overpronation, i.e. the situation where the foot continues pronating during the mid-stance. The double tapering of pad  40 , as already described, is delimited by the second curved heel flex groove  42  from where the tapering starts. Also in points  43  and  44  pad  40  is tapered. 
     In  FIG. 15  a second embodiment  168  of the heel of the midsole is shown. The lower heel portion  20  is provided with steps  169 ,  170  and  171 . These steps are staggered in relation to each other and made as part of the midsole in PU. The staggered steps  170  and  171  are made in order to stiffen the lower heel portion. Such stiffening effect is provided by direct injected PU in edge zones. Step  169  which is also shown in  FIG. 14 , clearly extends longer to the lateral side than the rest of the midsole in the heel portion, e.g. as compared to support arm  145 , and is provided to achieve enhanced stability. It will be noted from  FIGS. 14 and 15  that the medial heel portion  143  essentially can be aligned with a vertical line D, whereas the lateral heel portion  151  is aligned with a slanted line E. 
     Comparative tests between the inventive running shoe and a state of the art running shoe have been made.  12  male test persons were using the inventive shoes and the state of the art shoes. Using a goniometer placed on the heel of the persons, foot switches for detecting ground contact and an accelerometer mounted on the tibia muscle, different parameters as angles, velocities and accelerations have been measured. Table 1 shows the comparative test results. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparative test 
               
            
           
           
               
               
               
            
               
                   
                 Inventive 
                 State of the art 
               
               
                   
                 shoe 
                 running shoe 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 Rear foot angle at touchdown 
                 −3.4° 
                 −2.8° 
               
               
                 (negative angle = inversion) 
               
               
                 Maximal rear foot angle 
                 10.2° 
                 10.1° 
               
               
                 (positive angle = eversion) 
               
               
                 Rear foot angle velocity at touchdown 
                 175°/s 
                 340°/s 
               
               
                 Maximal rear foot angle velocity 
                 390°/s 
                 480°/s 
               
               
                 Mean rear foot angle velocity 
                 200°/s 
                 290°/s 
               
               
                   
               
            
           
         
       
     
     The rear foot angle at touchdown was a bit larger than in the state of the art shoe. Thus the heel as a mean value was turned 3.4° to the lateral side measured in relation to the ideal zero degree situation. The maximal eversion angle on the other hand was found to be 10.2° as compared to 10.1° of the state of the art shoe. The maximal eversion angle is the angle measured when the heel of the foot turns to the medial side. Of particular interest are the velocity dynamics during touchdown, where the maximal rear foot angle velocity is 390°/s (degrees per second) as compared to 480°/s on the state of the art shoe and the mean rear foot angle velocity 200°/s as compared to 290°/s. In the eyes of the applicant this is a significant difference, because the lower mean and maximum velocity results in a more stable shoe. This means that from the instant the heel hits the ground until eversion is finished, the inventive shoe is significantly slower and thus more stable. The result is a reduced risk of injuries in the ankle. The low mean rear foot angle velocity is partly due to the fact that the shoe has a low heel which advantageously brings the foot very close to the ground. 
       FIG. 7  shows a second embodiment of a midsole  118  slightly modified in comparison to midsole  1  of  FIG. 4 . Apart from the modified midsole,  FIG. 7  departs from  FIG. 4  in that the midsole  118  has discrete circular outsole elements ( 101 ,  102 ,  104 ,  105 ,  106 ,  108 ,  110 ,  111 ,  112 ,  114 ,  115 ) mounted on the midsole. Further,  FIG. 7  shows the curved flex groove as reference number  103  following a path  119  up to transversal flex line  113  and  107 . This flex line corresponds to line  92  in  FIG. 6 . Also in the embodiment of  FIG. 7 , an imaginary eccentric spiral curve can be drawn starting with an origo ◯ (curve not shown) in outsole element  105  and continuing via  104 ,  106 ,  108 ,  110 ,  111 ,  112 ,  114  and ending at  115 , hereby curving around the curved flex groove  103 . Also here, the outsole elements are discrete. Thus, elements  104 ,  105  and  106  although bridged by connection  109 , can be made as isolated outsole elements. Element pair  108 ,  110  is another discrete outsole element.  FIG. 7  shows that the curved flex groove  103  can stop at the level of flex line  113 . This sole design will also contribute to increased flexibility of the foot and faster reaction to excessive supination or pronation. In the heel portion a tapered area  117  enables moving the point of landing closer to the centre of the heel sole. An outsole element  100  is spaced apart from a reinforcement bar  99  by a heel flex groove  116 . 
     Improvements can be reached by further continuing the curved flex groove. Turning back to  FIG. 6 , the curved line  90  continues as curved forefoot line  91  across the third and second proximal phalanges and makes a U-turn in the direction of the heel. The curve  91  now runs in an opposite direction between the first and second metatarsal phalanges. This trajectory is also the one shown in the midsole of  FIG. 4  and corresponds to the one seen in  FIG. 8 . 
     In more detail,  FIG. 8  shows a third embodiment of the inventive midsole, which in the figure has a TPU intermediate layer  2  and discrete outsole elements ( 120 ,  121 ,  122 ,  124 ,  125 ) fixated. The discrete outsole elements function as the tread of the shoe. Due to the flex grooves between the discrete outsole elements, the total outsole area is small compared to conventional outsoles. This has an effect on the slip resistance. The outsole area, which can also be perceived as a contact area between outsole and ground, has been further minimised by removing material from the central portion of the outsole elements. More specifically, the contact area of an outsole element in the elements of  FIG. 8  is the area close to the edge of the element, whereas the centre of the outsole element is either devoid of material or only having a small contact area. Removing material from the outsole elements has the advantage of reducing the weight of the shoe, which is of particular interest in running shoes. Despite this reduction and the small surface area, a surprising effect has been seen regarding icy surfaces, because the grip of the sole has been improved compared to conventional soles. This is partly due to the material of the sole which is as mentioned rubber, and partly due to the “islandic” structure of the sole. As an example, the discrete outsole element  125  of  FIG. 8  has a first plane surface  126  and a second plane surface  127 . The second surface is lowered in relation to the first surface and a third surface  128  is in the same plane as the first. A fourth plane surface  133  constitutes the surface of the TPU intermediate layer  2 , and is lower than plane surfaces  126  and  127 . The surface area  133  essentially corresponds to the surface area of a pad of the midsole (see pad  35  in  FIG. 4 ), albeit a bit larger due to the TPU intermediate layer which is covering the pad. As can be seen on  FIG. 8 , the discrete outsole element  125  covers a smaller area than the corresponding pad in the midsole. This means that neighbouring discrete outsole elements have a larger distance to each other than the pads in the midsole as can be seen by comparing the distance between outsole elements  125  and  123  of  FIG. 8 . In the current embodiment, the distance between outsole elements  123  and  125  is five millimetres, and the distance between element  122  and  125  ten millimetres. The relatively large distance between the discrete outsole elements increases the flexibility of the sole, and has, as already described, led to good characteristics on slip resistance. Further, by making the area of an outsole element smaller than the corresponding area of TPU intermediate layer and pad, peeling effects on the outsole elements can be avoided. They will be less inclined to loosen as the bonding between TPU and rubber is made on a plane surface away from edges of the surface  133 . 
     The discrete outsole element  125  has sharp edges in an angle of about 90 degrees. When walking on an icy surface, the sharp edges penetrate the ice which creates a better grip. The total length of the sharp edges amounts to the sum of the circumference of the discrete outsole elements. The longer, the better grip one gets. However, with the invention, the grip has been even further improved. Without being bound by the following theory, it is believed that the flexible discrete outsole elements allow the foot to react in a natural way in the case of an icy surface. If you slip on one part of the foot base, the human brain will via a muscle action instruct another part of the same foot base to instantly and automatically compensate and try to get a grip on the ground. Conventional outsoles prevent this compensation because the compensational muscle reaction is constrained by the normal sole. A discrete outsole as in the invention, however, having flexible outsole islands, allows the discrete action of one or more of the  32  muscles in the foot. The improved gripping characteristic of the inventive sole was confirmed in laboratory tests in comparison with state of the art running shoes. Slip resistance showed to be improved both in relation to a wet surface and in relation to an icy surface. An improvement in slip resistance of the embodiment of  FIG. 8  can be made by building channels  129  into the first surface  126 . On wet surfaces, aqua planning can arise because water is trapped in the groove of the lower second surface  127 . Channels  129  will allow the water to escape, hereby lowering the risk of aqua planning and increasing the slip resistance even further. 
       FIG. 9  shows a fourth embodiment of an inventive midsole  135 , which midsole has a TPU intermediate layer  2  and an alternative tread. Discrete outsole element  130  exhibits undulating channels  131 , which acts as grooves transporting the water away. Typically, grooves of one millimetre are used. The embodiment in  FIG. 9  shows the use of a mixture of the outsole elements of  FIGS. 8 and 9 . The discrete outsole element  132  in the lower heel portion exhibits undulating channels in a direction slanted to the longitudinal direction of the sole. 
       FIG. 10  shows in a lateral side view an embodiment of the inventive midsole  135  with discrete outsole elements  139  and a TPU intermediate layer  134 . The heel end  137  extends vertically to a top point  152  on the medial side of the midsole and to a lower point  140  at the centre of the heel end  137 . The top or apex of the upper heel portion is approximately at the same level as the instep of the shoe upper, see  FIG. 12 . The upper heel portion thus extends to the location where the Achilles&#39; tendon is fixed to the calcaneus, and the upper heel portion essentially covers the tuberosity of the calcaneus on the medial and the lateral side. An opening  144  is made on the lateral side in order to increase flexibility by lowering the structural support given in this area. However, in principle the whole calcaneus can be supported by the vertically extended midsole material. The heel is extended vertically to a point essentially corresponding to the superior tuberosity of the calcaneus, see reference number  67  in  FIG. 5 . A support arm  145  connects the heel end  137  with the lateral heel portion  151 , and ensures stability. By extending the heel of the midsole into an upper heel portion which forms an integrated entity with the midsole (preferably as described injection moulded), the heel cap of traditional shoes can be omitted, hereby simplifying the shoe and reducing weight and cost. In an exemplary embodiment the vertical height measured from the geometric plane corresponding to surface  149  to lower top point  140  is 61 millimetres. With TPU intermediate layer  2  and discrete outsole elements mounted the height becomes 65 millimetres. 
     On the lateral side of the midsole  135 , a measure is taken to compensate for the proximal head of the fifth metatarsal phalanges which causes a protrusion or a local extremity of the foot, also known as tuberositas ossis, see reference number  86  in  FIG. 6 . This head, if encapsulated by a relatively stiff sole material, will be subjected to friction between head and sole material, and will reduce the flexibility of the shoe. In order to avoid this friction and to allow the head and the joint free movement, an opening or window  148  as shown in  FIG. 10  is created in the midsole material. Thus, in this area of the midsole, the midsole is devoid of sole material. 
       FIG. 11  shows midsole  135  from the medial side with the large support area of the medial heel portion  143 . As described, top point  152  is in the area of the superior tuberosity of the calcaneus. From this point, the edge of the midsole of the medial heel portion degrades in a direction towards the toe end along a curve  154  via supporting arm  155  to the forefoot. A corresponding support arm is found on the lateral side, reference number  156  ( FIG. 10 ). Thus the midsole  1  is raised vertically on the lateral side and on the medial side with the idea of supporting the foot by using support structures  157  and  158  respectively. These structures give the medial upper arch an elastic and adjustable support. Thus, support structure  158  adds support shortly after heel strike e.g. in a case where the foot tends to pronate. The support is achieved because the PU material of the midsole has sufficient mechanical strength to exert a stabilizing force. In principle the support structure  158  could be made without window  159 , but the supporting arm  155  has proved to give sufficient support. Additionally, structural element  160  has been added for further reinforcement. The vertical height of support structure  158  extends up to or above the upper half of the navicular bone  71  and medial cuneiform bone  72 , and support structure  158  extends in longitudinal direction to approximately the start of the first metatarsal phalanges. 
     Preferably, the support structures  158  and  157  are inclined inwardly to follow the shape of the foot. As the support structures are an integrated part of the midsole and thus made of polyurethane in the preferred embodiment, the support structures have the same material characteristics as PU and are thus able to keep the inclination during use and to exert a pressure against the upper  166  and the arch. The lateral and medial support structures are bonded to the upper in a polyurethane injection process. 
     Toe end  36  ( FIGS. 1 a , 1 b , 2 a , 2 b   ,  10 ,  11 ,  12  and  13 ) is likewise bonded to the upper in the injection process, and forms an integrated part of the midsole. The toe end is materially connected with the support structures  163  and  162  through a rim in the forefoot area, and is extended vertically from the base of the midsole  1  and curved inwardly and pointing towards the heel. The design of this integrated toe cap follows the general inventive concept, namely to increase the supporting material surface on the medial side as compared to the lateral side. Thus, as shown on  FIG. 1   1 , toe end  36  covers on its medial side an area larger than on the lateral side as shown in  FIG. 10 . The extended toe end  36  is offset from a longitudinal centre line through the midsole to the medial side, and stabilizes the foot during running and protects the toes and the upper. 
       FIGS. 12 and 13  show an even further embodiment of an inventive midsole  161  provided with an upper  166 . Support structures  162  and  163  are in this embodiment made as a supporting mesh with openings  164  and  165 . Looking at the medial side in  FIG. 12 , sufficient structural support is ensured by support arms  172  extending upwards to the lacing area  173  and creating crossing sections  167 ,  172 . The support structure  163  describes a structural mechanical stabilizing connection between the medial heel end and the medial forefoot, which ends in the upwardly extending toe end  36 . 
     The described embodiments can be combined in different ways.