Patent Publication Number: US-2021169172-A1

Title: Shoe bottoms and shoes

Description:
TECHNICAL FIELD 
     The present invention relates to shoe bottoms of shoes. 
     BACKGROUND ART 
     In the related art, attempts have been made for providing various functions to shoes by devising soles of shoe bottoms (e.g., see Patent Document 1). 
     [Patent Document 1] International Publication No. 2017/046959 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     When doing a front bridge motion or the like during core training, a wearer of shoes (hereinafter simply referred to as a wearer) may take a standing on tiptoe posture. The expression “standing on tiptoe posture” in the present specification means a posture in which at least a rearfoot portion of the sole is lifted from the ground under the condition that a forefoot portion of the sole described later is in contact with the ground. There is no particular limitation on the angle that the sole makes with the ground at parts other than the part in contact with the ground. 
     If the muscular strength of the wearer&#39;s legs is weak, it tends to be difficult to stably maintain the standing on tiptoe posture. From the viewpoint of supporting the exercise of the wearer, it is desirable to propose shoe bottoms that allow for good stability while taking a standing on tiptoe posture. As a result of study based on such a viewpoint, the inventors of the present invention have come to realize that there is room for improvement in the shoe bottoms described in Patent Document 1, as described later in detail. 
     One embodiment of the present invention has been made in view of such problems, and one of the purposes of the invention is to provide shoe bottoms that are capable of improving the stability of a standing on tiptoe posture. 
     Means to Solve the Problem 
     One embodiment of the present invention relates to a shoe bottom that is a shoe bottom comprising a sole, wherein when a midfoot portion of the sole is divided by a predetermined sole center line into a medial midfoot region and a lateral midfoot region one on each side in the foot width direction, the shoe bottom has a rigidity lowering portion that is provided in the medial midfoot region, and wherein, in such a manner that the bending rigidity of the medial midfoot region around a foot width direction axis becomes smaller than that of the lateral midfoot region, the rigidity lowering portion in the medial midfoot region reduces the bending rigidity of the medial midfoot region due to another factor other than the shape of the medial edge and the shape of the lateral edge of the sole in a planar view. 
     Advantage of the Invention 
     According to the present invention, shoe bottoms that are capable of improving the stability of a standing on tiptoe posture can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a sole that serves as one invention example; 
         FIG. 2  is a plan view showing the skeleton of a foot of a human body; 
         FIG. 3  are diagrams in which the skeleton of the right foot of a wearer is viewed from the front side in the foot longitudinal direction;  FIG. 3A  shows a positional relationship when the toes and heel of the wearer are in contact with the ground;  FIG. 3B  shows a state where a crossing angle is larger compared to the positional relationship of  FIG. 3A ; 
         FIG. 4  are diagrams showing the calcaneocuboid joint surface and talonavicular joint surface of the right foot of the wearer; 
         FIG. 4A  shows a positional relationship when the toes and heel of the wearer are in contact with the ground;  FIG. 4B  shows a state where a crossing angle is larger compared to the positional relationship of  FIG. 4A ; 
         FIG. 5  are diagrams for explaining the axis of motion of a Chopart joint;  FIG. 5A  is a plan view of the skeleton of the right foot;  FIG. 5B  is a view of the skeleton thereof viewed from the medial side in the foot width direction; 
         FIG. 6  is a bottom view showing another invention example of a sole provided with rigidity lowering portions; 
         FIG. 7  is a bottom view of a sole that serves as still another invention example; 
         FIG. 8  is a perspective view schematically showing a model simulating a sole used for analysis; 
         FIG. 9  is a diagram showing the result of the analysis; 
         FIG. 10  is a diagram for explaining an external torsional resistance expected region; 
         FIG. 11  is a diagram showing a sole according to a reference example used for the analysis; 
         FIG. 12  is a graph showing torsional frequencies obtained by the analysis; 
         FIG. 13A  is a diagram showing the measurement result for torsion angles obtained by an experiment;  FIG. 13B  is a diagram showing the measurement result for the amount of the ankle unstableness; 
         FIG. 14  is a bottom view of a sole according to a first exemplary variation; 
         FIG. 15  is a side view of a shoe using a shoe bottom according to a first embodiment as viewed from the medial side in the foot width direction; 
         FIG. 16  is a bottom view of a sole according to the first embodiment; 
         FIG. 17  is a bottom view of a sole according to a second embodiment; 
         FIG. 18A  is a side view of the sole according to the second embodiment as viewed from the medial side in the foot width direction;  FIG. 18B  is a side view of the sole viewed from the lateral side in the foot width direction; 
         FIG. 19A  is a bottom view of a sole according to a second exemplary variation;  FIG. 19B  is a bottom view of a sole according to a third exemplary variation;  FIG. 19C  is a bottom view of a sole according to a fourth exemplary variation; 
         FIG. 20  is a side view of a shoe bottom according to a third embodiment as viewed from the same viewpoint as that of  FIG. 15 ; and 
         FIG. 21  is a side view of a shoe bottom according to a fourth embodiment as viewed from the same viewpoint as that of  FIG. 15 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Terms used in this specification will be explained.  FIG. 1  is a plan view showing a sole  10 , which serves as one invention example. A “foot longitudinal direction Lx” in the present specification means a direction along a straight line connecting a tip  10   a  on the toe side and an end  10   b  on the heel side of the sole  10 . The toe side in the foot longitudinal direction Lx is also referred to as the front side, and the heel side is also referred to as the back side. A “foot width direction Y” refers to a horizontal direction orthogonal to the foot longitudinal direction Lx. A first toe side of the foot of a wearer supported by the sole  10  is referred to as the medial side, and a fifth toe side is referred to as the lateral side. A “full length La” in the foot longitudinal direction Lx is the longest length in the foot longitudinal direction Lx, and a “full width Lb” in a foot width direction Ly is the longest length in the foot width direction Ly. 
       FIG. 2  is a plan view showing the skeleton of a foot of a human body. The foot of a human body is mainly composed of cuneiform bones Ba, a cuboid bone Bb, a navicular bone Bc, a talus Bd, a calcaneus Be, metatarsal bones Bf, and phalanges Bg. The joint of the foot includes an MP joint Ja, a Lisfranc joint Jb, and a Chopart joint Jc. The Chopart joint Jc includes a calcaneocuboid joint Jc 1  formed by the cuboid bone Bb and the calcaneus Be and a talonavicular joint Jc 2  formed by the navicular bone Bc and the talus Bd. A “midfoot portion” of a wearer (hereinafter, simply referred to as a human midfoot portion) in the present specification means the portion from the MP joint Ja to the Chopart joint Jc. 
       FIG. 1  is referred back. A straight line along the foot width direction Y, which is assumed to pass the heel side end of the MP joint Ja of the wearer, is defined as a line p. A straight line along the foot width direction Y, which is assumed to pass the toe side end of the Chopard joint Jc of the wearer, is defined as a line q. The line p and the line q are, for example, straight lines along the foot width direction Y that divide the full length La of the sole  10  in the foot longitudinal direction Lx into 1.5:1.0:1.1 from the toe side to the heel side. A “forefoot portion  12 ” of the sole  10  in the present specification means a region on the toe side of the line p, a “midfoot portion  14 ” (hereinafter simply referred to as a sole midfoot portion  14 ) of the sole  10  means a region from the line p to the line q, and a “rearfoot portion  16 ” of the sole  10  means a region on the heel side of the line q. The sole midfoot portion  14  can be also considered to be a region that is assumed to overlap with the range from the heel side end of the MP joint Ja to the toe side end of the Chopard joint Jc of the wearer, that is, a range that is assumed to overlap with the human midfoot portion. 
     The background for how the shoe bottom according to the present embodiment has been conceived of will be explained. As described above, if the muscular strength of the wearer&#39;s legs is weak, it tends to be difficult to stably maintain the standing on tiptoe posture. Further, if the muscular strength of the wearer&#39;s legs is weak, a decrease in propulsive force in a pushing-off motion during the terminal stance of a gait cycle is known as a factor for falling down. This wearer is, for example, a woman, an elderly person, or the like. 
     From the viewpoint of solving these problems, the inventors of the present invention found out, based on the anatomical viewpoint of the foot of the human body, that it was effective to induce a bony locking mechanism in the human midfoot portion of the wearer. 
       FIGS. 3 and 4  are diagrams in which the skeleton of a right foot of the wearer is viewed from the front side in the foot longitudinal direction Lx.  FIG. 3  are external views of the skeleton, and  FIG. 4  are diagrams showing a calcaneocuboid joint surface Sja and a talonavicular joint surface Sjb of the right foot.  FIGS. 3A and 4A  show a positional relationship when the toes and heel of the wearer are in contact with the ground, and  FIGS. 3B and 4B  show a state where a crossing angle θc described later is larger compared to the positional relationship of  FIGS. 3A and 4A , respectively. The crossing angle between a joint axis Aj 1  of the calcaneocuboid joint Jc 1  of the Chopart joint Jc viewed from the front in the foot longitudinal direction Lx and a joint axis Aj 2  of the talonavicular joint Jc 2  is defined to be θc. 
       FIG. 5  area diagram for explaining the axis of motion of the Chopart joint Jc.  FIG. 5A  is a plan view of the skeleton of the right foot, and  FIG. 5B  is a view of the skeleton thereof viewed from the medial side in the foot width direction. The Chopart joint Jc has a longitudinal axis and a clinoaxis as two axes of motion, of which the longitudinal axis serves as the joint axis Aj 1  of the calcaneocuboid joint Jc 1  and the clinoaxis serves as the joint axis Aj 2  of the talonavicular joint Jc 2 . Although there are individual differences in terms of a skeleton, in general, the calcaneocuboid joint Jc 1  is an axis obtained by tilting the toe side inward by 9 degrees in the foot width direction with respect to the horizontal plane and tilting the toe side upward by 15 degrees with respect to the sagittal plane, based on the state where the toe and the heel are in contact with the ground. Normally, the talonavicular joint Jc 2  is an axis obtained by tilting the toe side inward by 57 degrees in the foot width direction with respect to the horizontal plane and tilting the toe side upward by 52 degrees with respect to the sagittal plane, based on the state where the toe and the heel are in contact with the ground. 
     As shown in  FIG. 3B , the bony locking mechanism is realized when the crossing angle θc is increased to a certain extent compared to that obtained when the wearer&#39;s toe and heel are in contact with the ground. Due to an increase in this crossing angle θc, the mobility of the Chopart joint Jc is reduced compared with a case where the crossing angle θc is small, and the Chopart joint Jc can be turned into a rigid body. This allows unstable between a plurality of bones constituting the Chopart joint Jc to be prevented when taking a standing on tiptoe posture, and the stability of the standing on tiptoe posture can be improved. Further, as a result of turning the Chopart joint Jc into a rigid body, the propulsive force transmission between the plurality of bones constituting the Chopart joint Jc becomes smooth, and the propulsive force in the pushing-off motion can be improved. 
     The crossing angle θc of the plurality of joint axes forming the Chopart joint Jc described above is known to increase as the amount of external torsion at the human midfoot portion increases. Therefore, in inducing the bony locking mechanism, it is necessary to increase the amount of external torsion at the human midfoot portion. The external torsion means that the heel is twisted in the supination direction with respect to the toes based on the positional relationship obtained when the toes and heel of the human body are in contact with the ground. The present inventors have found that it is preferable to satisfy the following condition in order to achieve such an increase in the amount of external torsion at the human midfoot portion. 
     When the human midfoot portion is attempted to be twisted outward (external torsion), the sole  10  is also attempted to be twisted outward in a range including the sole midfoot portion  14  following the deformation of the human midfoot portion. Therefore, in order to increase the amount of external torsion at the human midfoot portion, it is desirable to reduce the external torsional resistance of the sole  10  within the range including the sole midfoot portion  14 . 
     In order to respond to such a demand, the inventors of the present invention found out that it is effective to provide a rigidity lowering portion  32  for lowering the bending rigidity around a foot width direction axis (hereinafter, simply referred to as “bending rigidity”) in a medial midfoot region  20  of the sole midfoot portion  14 , as shown in  FIG. 1 . The medial midfoot region  20  means a region located on the medial side when the sole midfoot portion  14  is divided into two regions one on each side in the foot width direction by a predetermined sole center line s. Of these two regions, the region located on the lateral side is referred to as a lateral midfoot region  22 . 
     This sole center line s is defined as a line passing through the center part of the sole  10  in the foot width direction Y. In this example, a straight line along the foot longitudinal direction X that divides the full width Lb of the sole  10  into 1.2:1.0 from the medial side to the lateral side in the foot width direction is defined as the sole center line s. The sole center line s in this example is also a part on which a foot width direction center part of the foot of the wearer is assumed to be located. The foot width direction center part is assumed to be a part located on a straight line passing through a third metatarsal bone Bf 3  and a medial process of calcaneal tuberosity Bel of the calcaneus Be of a human body.  FIG. 1  shows a range in which the medial process of calcaneal tuberosity Bel is assumed to be located. 
     Rigidity lowering portions  32  of the medial midfoot region  20  reduce the bending rigidity of the medial midfoot region  20  such that the bending rigidity of the medial midfoot region  20  becomes smaller than that of the lateral midfoot region  22 . The expression “the bending rigidity of the medial midfoot region  20  becomes smaller than that of the lateral midfoot region  22 ” includes the following two cases. The first case is a case where, in the lateral midfoot region  22  and the medial midfoot region  20 , only the bending rigidity of the medial midfoot region  20  is reduced. The second case is a case where, when reducing the bending rigidity of both the lateral midfoot region  22  and the medial midfoot region  20 , the amount of decrease in the bending rigidity in the medial midfoot region  20  is set to be larger than that in the lateral midfoot region  22 . 
     The rigidity lowering portions  32  in the medial midfoot region  20  reduce the bending rigidity of the medial midfoot region  20  due to another factor other than the shape of a medial edge  10   c  and the shape of a lateral edge  10   d  of the sole  10  in a planar view. This “another factor” is, for example, any one or a combination of two of recessed portions that are open on the ground contact surface of the sole  10  and the elongation characteristic of the material constituting the sole  10  such as those explained in the following. 
     The expression “recessed portions that are open on the ground contact surface of the sole  10 ” means those that are recessed upward from the ground contact surface of the sole  10 , which comes into contact with the road surface. The recessed portions may be groove portions continuous in the in-plane direction of a ground contact surface  10   e  of the sole  10  or may not be continuous in the in-plane direction thereof.  FIG. 6  is a bottom view showing another invention example of the sole  10  provided with rigidity lowering portions  32 . When the recessed portions constituting the rigidity lowering portion  32  are not continuous in the in-plane direction, the recessed portions may be provided intermittently so as to be aligned on a virtual line such as a straight line, a curved line or the like. As the recessed portions,  FIG. 1  shows medial transverse groove portions  34  extending from the medial edge  10   c  of the sole  10  in the foot width direction Y. When such recessed portions are provided in the medial midfoot region  20 , the bending rigidity of the medial midfoot region  20  can be lowered compared to that in a case where such recessed portions do not exist. The bending rigidity of the medial midfoot region  20  being reduced due to the recessed portions means such a situation. When the recessed portions are the medial transverse groove portions  34 , the bending rigidity can be effectively reduced. 
     Further, the expression “the elongation characteristic of the material constituting the sole  10 ” means, specifically, the Young&#39;s modulus [N/mm 2 ] in the foot longitudinal direction X of the material constituting the sole  10 . The rigidity lowering portions  32  are formed using a second material having a smaller Young&#39;s modulus in the foot longitudinal direction X than that of a first material constituting portions adjacent to the rigidity lowering portions  32  of the sole  10 . This allows the bending rigidity of the medial midfoot region  20  to be lowered compared to a case where the rigidity lowering portions  32  are formed using the first material. The bending rigidity of the medial midfoot region  20  being reduced due to the elongation characteristic of the material constituting the sole  10  means such a situation. 
     On the medial edge  10   c  of the sole midfoot portion  14 , a curved-in part  10   f  recessed outward in the foot width direction X is formed. The bending rigidity of the medial midfoot region  20  of the sole  10  is often smaller than the bending rigidity of lateral midfoot region  22  due to the influence of the curved-in part  10   f . In order to exclude the influence of this curved-in part  10   f , the shape of the medial edge  10   c  and the lateral edge  10   d  of the sole  10  in a planar view is excluded from the above-mentioned factors, which cause a decrease in bending rigidity. 
     By providing such a rigidity lowering portion  32  in the medial midfoot region  20 , it is easier to lower the bending rigidity of the medial midfoot region  20  than that of the lateral midfoot region  22  as compared with the case where the rigidity lowering portion  32  is not provided. As the bending rigidity of the medial midfoot region  20  is lowered compared to that of the lateral midfoot region  22 , the elongation amount of the medial midfoot region  20  at the ground contact surface can be increased compared to that of the lateral midfoot region  22  when the sole  10  is bendingly deformed around the foot width direction axis. This means that when the wearer is taking a standing on tiptoe posture, the medial midfoot region  20  can be more easily deformed in an extended manner in the foot longitudinal direction X than the lateral midfoot region  22 , that is, the medial midfoot region  20  tends to be easily twisted outward. In other words, it means that external torsional resistance at the sole midfoot portion  14  can be lowered compared with the case where a medial midfoot region  20  is not provided with a rigidity lowering portion  32 . Therefore, by providing a rigidity lowering portion  32  in the medial midfoot region  20 , when the wearer attempts to twist his/her human midfoot portion while taking a standing on tiptoe posture, the amount of external torsion can be increased compared to the case where a rigidity lowering portion  32  is not provided. As a result, it is possible to induce the bony locking mechanism and thus improve the stability of the standing on tiptoe posture and improve the propulsive force in the pushing-off motion. 
     The medial midfoot region  20  is formed such that the bending rigidity thereof is smaller than that of the lateral midfoot region  22 . This is realized by providing a rigidity lowering portion  32  in the medial midfoot region  20  or due to the shape of the medial edge  10   c  or the lateral edge  10   d  of the sole  10  in a planar view. These bending rigidities may be evaluated based on the strain amount of the ground contact surface in the foot longitudinal direction obtained when a bending moment of a predetermined size around the foot width direction axis is applied toward the upper surface of the sole at a toe side end portion and a heel side end portion of the midfoot region being mentioned. It means that the bending rigidity becomes small as this strain amount increases. The bending rigidity of the medial midfoot region  20  being smaller than that of the lateral midfoot region  22  means that the strain amount in the medial midfoot region  20  is larger than the strain amount in the lateral midfoot region  22 . The strain amount may be acquired by actually cutting out the midfoot region being mentioned from the sole  10  and measuring the strain amount using the piece that has been cut out. 
     Further, as described above, the line q indicates a portion where the Chopard joint of the foot of the wearer is assumed to be located. As a rigidity lowering portion  32  is located closer to this line q, the sole midfoot portion  14  becomes more likely to be twisted outward at a location closer to the Chopart joint Jc, and the bony locking mechanism is more likely to be induced accordingly. Therefore, the rigidity lowering portion  32  is preferably provided in a region on the heel side of a straight line y along the foot width direction Y, which bisects the full length of the sole midfoot portion  14  in the foot longitudinal direction, in the medial midfoot region  20  of the sole midfoot portion  14 . 
     During the standing on tiptoe posture, a load for twisting the sole  10  outward is applied to the sole  10  via the upper of the shoe in a state where the forefoot portion  12  of the sole  10  is restrained. At this time, the toe side end portion of the sole midfoot portion  14  is fixed, and an external torsion load is applied to the heel side end portion thereof. At this time, the most deformed portion of the sole midfoot portion  14  is a region on the toe side of the sole midfoot portion  14  close to the forefoot portion  12  restrained in the sole  10 . In this region on the toe side of the sole midfoot portion  14 , it is possible to effectively twist the sole midfoot portion  14  outward by having different bending rigidity on the medial side and the lateral side in the foot width direction of the sole midfoot portion  14 . Therefore, the rigidity lowering portion  32  is also preferably provided in the region on the toe side of the straight line y bisecting the sole midfoot portion  14  in the foot width direction. 
     Next, another condition will be described that is preferably satisfied in order to increase the amount of external torsion at the human midfoot portion. A case is taken into consideration where an external torsional load for twisting the sole  10  outward is applied to the sole  10  via the upper of the shoe when the wearer is taking a standing on tiptoe posture. A case is taken into consideration where a transverse groove portion extending from the medial edge  10   c  to the lateral edge  10   d  is formed in the rearfoot portion  16  of the sole  10 . In this case, even when the external torsional load described above is applied to the sole  10 , the bending deformation at the transverse groove portion of the rearfoot portion  16  thereof becomes dominant, and the amount of external torsion at the sole midfoot portion  14  becomes small. As a result, when the wearer attempts to twist his/her human midfoot portion outward while taking a standing on tiptoe posture, it is difficult to increase the amount of external torsion at the human midfoot portion due to resistance from parts other than the sole midfoot portion  14 . 
       FIG. 7  is a bottom view showing a sole  10 , which serves as another invention example. In order to solve the above problems, as another condition, it is defined that a continuous surface  16   c  continuous in the foot longitudinal direction from a toe side end portion  16   a  to a heel side end portion  16   b  of a rearfoot portion  16  of the sole  10  is formed on the ground contact surface of the sole  10 . In this figure, the range in which the continuous surface  16   c  is formed is indicated by hatching with two-dot chain lines. This means that a transverse groove portion extending from a medial edge  10   c  to a lateral edge  10   d  is not formed in the rearfoot portion  16  of the sole  10 . In the illustrated example, this continuous surface  16   c  is formed in the entire range in the foot width direction Y; however, the continuous surface  16   c  may be formed in at least a part of the range in the foot width direction Y. 
     Thereby, when the wearer attempts to twist the midfoot portion outward while taking a standing on tiptoe posture, the bending deformation of the sole  10  at the rearfoot portion  16  can be suppressed by the continuous surface  16   c , and a situation can be prevented where the amount of external torsion at the sole midfoot portion  14  becomes small in accordance with the bending deformation. Accordingly, by satisfying the above-mentioned conditions, it becomes easier to obtain the effect of reducing the external torsional resistance at the sole midfoot portion  14 , and it thus becomes easier to increase the amount of external torsion at the human midfoot portion. 
     When a reinforcing member such as a shank is attached to the sole midfoot portion  14 , the bending rigidity of the shoe bottom becomes excessively increased, and the external torsional resistance of the sole midfoot portion  14  thus becomes excessively increased. Therefore, in the shoe bottom according to the present embodiment, a reinforcing member such as a shank is preferably not attached to the sole midfoot portion  14 . This prevents an excessive increase in the bending rigidity of the sole midfoot portion  14  and allows the external torsional resistance of the sole midfoot portion  14  to be easily reduced. 
     The reinforcing member used in this case is those other than a midsole  56  and an outer sole  58  of the sole  10  described later. This reinforcing member is used, for example, for enhancing bending rigidity around the foot width direction axis of the shoe bottom just like a shank or the like and is formed using a material whose hardness is larger than the maximum hardness of the sole  10 . This material is, for example, various metals or synthetic resins having a JIS A hardness of 80 degrees or more. The JIS A hardness is a value obtained by measurement using an A type hardness meter in compliance with JIS K 6301. The hardness of the midsole  56  is, for example, 35 to 75 degrees in terms of JIS C hardness, and the hardness of the outer sole  58  is, for example, 50 to 75 degrees in terms of JIS A hardness. The JIS C hardness is a value obtained by measurement using a C type hardness meter in compliance with JIS K 6301. 
     Even when the reinforcing member is not attached to the sole midfoot portion  14 , a reinforcing member may be attached to the sole forefoot portion  12  and the sole rearfoot portion  16 . Even under this configuration, the external torsional resistance of the sole midfoot portion  14  can be easily reduced. 
     Next, an analysis performed for coming up with the shoe bottom according to the embodiment will be explained.  FIG. 8  is a perspective view schematically showing a model simulating a sole  10  used for the analysis. In this analysis, a sole having the same size as that of the sole  10  shown in  FIG. 7  was used. The sole  10  had a full length La of 280 mm, a full width Lb of 200 mm, and a uniform thickness of 20 mm. The physical conditions of the sole  10  were set to have a Young&#39;s modulus of 6 [N/mm 2 ], a Poisson&#39;s ratio of 0.25 [-], and a density of 3×10 2  [kg/m 3 ]. It is assumed that this analysis reproduce the deformed state of the sole  10  during a front bridge motion. For this reason, a region Sa in which the ball of foot of the toes of the wearer were assumed to hit was completely restrained, and an upward load Fz was applied to the rearfoot portion  16  of the sole  10 . In order to apply an external torsional load to the sole  10 , a load Fy directed outward in the foot width direction Y was applied to the rearfoot portion  16  of the sole  10 . 
       FIG. 9  is a diagram showing the result of this analysis. In this figure, the distribution of the maximum principal stress at the bottom surface of the sole  10  obtained under the above-described conditions is shown. The higher the density of dots, the greater the stress. It can be confirmed that when the external torsional load is applied to the sole  10 , the stress becomes larger in a region  24 , which includes the medial midfoot region  20  and the peripheral region of the sole  10 , than those in other regions. This means that this region  24  is strongly resisting the external torsion of the sole midfoot portion  14 . Therefore, it is considered that the external torsion at the sole midfoot portion  14  can be effectively reduced by providing a rigidity lowering portion  32  in the foregoing region  24  (hereinafter referred to as an external torsional resistance expected region  24 ), which is assumed to be resisting the external torsion of the sole midfoot portion  14 . Therefore, as a region in which a rigidity lowering portion is preferably provided, the external torsional resistance expected region  24  obtained by this analysis is used. 
       FIG. 10  is a diagram for explaining the external torsional resistance expected region  24 . The external torsional resistance expected region  24  is geometrically specified in relation to the shape of the sole  10 . Hereinafter, an explanation will be given with reference to the positional relationship of the sole  10  in the planar view. 
     The definition of a line s, a line p, and a line q is the same as the definition described above. A straight line along the foot width direction Y that divides a region on the heel side of the line q of the sole  10  into 0.2:0.9 is defined as a line r. Being viewed from a point o 1 , which is the intersection point of the line p and the line s, a straight line obtained by rotating the line p by 13 degrees around the point o 1  in an outward direction Pa, which rotates the toe side outward in the foot width direction, is defined as a line t. Being viewed from the point o 1 , which is the intersection point of the line s and the line p, a straight line obtained by rotating the toe side of the line s by 8 degrees around the point o 1  in the aforementioned outward direction Pa is defined as a line u. Being viewed from a point o 2 , which is the intersection point of the line u and the line q, a straight line obtained by rotating the line q by 5 degrees around the point o 2  in the outward direction Pa is defined as a line v. Being viewed from a point P, which is the intersection point of the line r and the line u, a straight line obtained by rotating the liner by 4 degrees around the point P in the outward direction Pa is defined as a line w. A straight line connecting a point o 5 , which is the intersection point of the medial edge  10   c  of the sole  10  and the line w, and the point o 2  is defined as a line x. 
     At this time, the external torsional resistance expected region  24  is defined to be formed of a first region  26  surrounded by the line t, the line u, the line v, and the medial edge  10   c  of the sole  10 . This external torsional resistance expected region  24  is provided on the ground contact surface of the sole  10  in the planar view of the sole  10 . A rigidity lowering portion  32  is preferably provided in the foregoing external torsional resistance expected region  24 . It is considered that by providing the rigidity lowering portion  32  in this external torsional resistance expected region  24 , the external torsional resistance of the sole midfoot portion  14  can be effectively reduced. 
     The rigidity lowering portion  32  is preferably provided in apart (the part indicated by a range S 1 ) that belongs to the external torsional resistance expected region  24  outside the medial midfoot region  20 , other than the part that belongs to the external torsional resistance expected region  24  in the medial midfoot region  20 . The rigidity lowering portion  32  provided in the part belonging to the external torsional resistance expected region  24  outside the range of the medial midfoot region  20  also lowers the bending rigidity of the part due to a recessed portion that is open on the ground contact surface of the sole  10 , the elongation characteristic of the material constituting the sole  10 , and the like. 
     Referring to the analysis result of  FIG. 9 , the first region  26  defined as the external torsional resistance expected region  24  of the sole  10  mainly spreads largely in a direction Lb heading toward the heel side of the foot longitudinal direction Lx. Further, the first region  26  also spreads somewhat in a direction Lc heading toward the outside in the foot width direction Y. This analysis is intended for a front bridge motion. It is expected that a larger external torsional load will be applied to the sole  10  in other motions such as running or the like. If a large load is applied to the sole  10 , the external torsional resistance expected region  24  is considered to first spread in the direction Lb heading toward the heel side in the foot longitudinal direction Lx. Further, the external torsional resistance expected region  24  is considered to spread in the direction Lc heading toward the outside in the foot width direction Y in an extent smaller than how the external torsional resistance expected region  24  spreads toward the heel side of the foot longitudinal direction Lx. 
     Therefore, as shown in  FIG. 10 , the external torsional resistance expected region  24  may be defined to be formed of the first region  26  and a second region  28  surrounded by the line v, the line x, and the medial edge  10   c  of the sole  10  in planar view. It is considered that by providing the rigidity lowering portion  32  in the foregoing external torsional resistance expected region  24 , the external torsional resistance of the sole midfoot portion  14  can be effectively reduced when a large external torsional load is applied to the sole  10 . 
     This rigidity lowering portion  32  is also preferably provided in parts (the range S 1  and the part indicated by a range S 2 ) that belong to the external torsional resistance expected region  24  outside the medial midfoot region  20 , other than the part that belongs to the external torsional resistance expected region  24  in the medial midfoot region  20 . 
     Further, the external torsional resistance expected region  24  may be defined to be formed of the first region  26 , the second region  28 , and a third region  30  surrounded by the line s, the line u, the line x, and the line w in planar view. It is considered that by providing the rigidity lowering portion  32  in the foregoing external torsional resistance expected region  24 , the external torsional resistance of the sole midfoot portion  14  can be effectively reduced when a larger external torsional load is applied to the sole  10 . 
     This rigidity lowering portion  32  is also preferably provided in parts (the range S 1 , the range S 2 , and the part indicated by a range S 3 ) that belong to the external torsional resistance expected region  24  outside the medial midfoot region  20 , other than the part that belongs to the external torsional resistance expected region  24  in the medial midfoot region  20 . 
     Next, the effects of the invention based on the presence or absence of the above-described conditions will be explained using analysis.  FIG. 11  shows a sole  100  of a reference example used for the analysis. The sole  10  according to an exemplary embodiment is shown in  FIG. 7 . The dimensional conditions and physical conditions of the soles  10  and  100  were set to be the same as those in the analysis of  FIG. 8 . 
     In each of the sole  100  according to the reference example and the sole  10  according to the exemplary embodiment, a transverse groove portion  40  is provided at a part corresponding to the MP joint in the forefoot portion  12  of the sole such that the sole is bent at the forefoot portion  12  of the sole around the foot width direction axis during a standing on tiptoe posture. In the sole  10  according to the exemplary embodiment, two medial transverse groove portions  34  are provided as rigidity lowering portions  32  that lower the bending rigidity of the medial midfoot region  20 . Further, in the sole  10  according to the exemplary embodiment, one more medial transverse groove portion  34  is provided as a rigidity lowering portion  32  that lowers the bending rigidity of the external torsional resistance expected region  24  in the part S 1  located outside the range of the medial midfoot region  20 . The three medial transverse groove portions  34  extend in the foot width direction Y from the medial edge  10   c  of the sole  10  and are provided at intervals in the foot longitudinal direction Lx. No similar rigidity lowering portion  32  is provided in the sole midfoot portion  14  according to the reference example. 
     The respective deformation characteristics of the soles  10  and  100  with respect to external torsion were evaluated by eigenvalue analysis. More specifically, the respective torsional frequencies, which were the respective natural frequencies occurring when the characteristic vibration mode of the soles  10  and  100  was torsional vibration, were obtained by the eigenvalue analysis, and the deformation characteristics of the soles  10  and  100  were evaluated using the respective torsional frequencies. It means that the smaller the torsional frequencies become, the smaller the respective external torsional resistances of the soles  10  and  100  become. 
       FIG. 12  is a graph showing the torsional frequencies obtained by this analysis. As shown in this figure, the torsional frequency of the sole  10  according to the exemplary embodiment was smaller than that of the sole  100  according to the reference example. This indicates that the sole  10  according to the exemplary embodiment had smaller external torsional resistance than the sole  100  according to the reference example. 
     Next, the effects of the invention based on the presence or absence of the above-described conditions will be explained using an experiment example. In this experiment, a sole whose size and physical properties as the same as those of the two types of soles shown in  FIGS. 7 and 11  was used. In this experiment, shoes using these soles were worn. Using these shoes, the wearer kept a posture for 40 seconds where the wearer focused on keeping his/her body torso lifted from the ground contact surface while having his/her elbows touching the ground and having the respective forefoot portions  12  of the soles touching the ground such that the body parts from the head to the heel became straight. 
     The result of this experiment was evaluated using the torsion angle of the sole midfoot portion  14  and the amount of the ankle unstableness of the wearer. Using a motion capture system, this torsion angle was measured by acquiring three-dimensional positional information of markers attached to a plurality of parts of the sole  10 . This torsion angle is defined as the angle formed by the ground contact surface of the sole midfoot portion with respect to the ground contact surface of the sole rearfoot portion. In the same way as in the torsion angle, the amount of the ankle unstableness of the wearer was also measured by acquiring three-dimensional positional information of markers attached to the ankle. 
       FIG. 13A  shows the measurement result for torsion angles obtained by the experiment, and  FIG. 13B  shows the measurement result for the amount of the ankle unstableness. As compared with the sole  100  according to the reference example, it can be confirmed that the sole  10  according to the exemplary embodiment had a larger torsion angle of the sole midfoot portion  14 . Based on this, it can be confirmed that the sole  10  according to the exemplary embodiment had smaller external torsional resistance than the sole  100  according to the reference example. Further, it can be confirmed that the sole  10  according to the exemplary embodiment had a smaller amount of the ankle unstableness than the sole  100  according to the reference example. Based on this, it can be confirmed that good stability can be obtained during a take standing on tiptoe posture by the shoe using the sole  10  according to the example. This is considered to be due to the bony locking mechanism being able to be induced in accordance with an increase in the torsion angle of the sole midfoot portion  14 , as described above. 
       FIG. 14  is a bottom view showing a sole  10  according to a first exemplary variation. Lateral transverse groove portions  44 , which are open on the ground contact surface  10   e  of the sole  10  and extend from the lateral edge  10   d  of the sole  10  in the foot width direction Y, are formed in the sole midfoot portion  14  and the rearfoot portion  16  of the sole  10  according to the first exemplary variation. In this case, the bending rigidity decreases in a range including a lateral midfoot region  22  of the sole  10 . In accordance with this, it is difficult to provide a sufficient difference in bending rigidity between the medial midfoot region  20  and a lateral midfoot region  22  of the sole  10 . In order to sufficiently obtain the effect of reducing the external torsional resistance at the sole midfoot portion  14 , this difference in the bending rigidity is preferably as large as possible. 
     Therefore, the lateral transverse groove portions  44  extending from the lateral edge  10   d  of the sole  10  in the foot width direction Y are preferably not formed in a partial range Sb of the sole  10  in the foot longitudinal direction X. The partial range Sb includes a range Sb 1  in the foot longitudinal direction X where all rigidity lowering portions  32  are provided and all of a range Sb 2  on the heel side of the range Sb 1 . This allows a sufficient difference in bending rigidity to be provided between the medial midfoot region  20  and the lateral midfoot region  22  of the sole  10 , and the effect of reducing the external torsional resistance at the sole midfoot portion  14  can thus be more easily obtained. From the same point of view, it can be considered that the lateral transverse groove portions  44  are preferably not formed in a range Sc, which is located on the heel side of the line y described above. 
     First Embodiment 
       FIG. 15  is a side view of a shoe  52  using a shoe bottom  50  according to a first embodiment as viewed from the medial side in the foot width direction. The shoe  52  is used, for example, for exercise in a room such as a gym; however, the usage thereof is not particularly limited. The shoe  52  includes a shoe bottom  50 , which supports the wearer&#39;s foot, and an upper  54 , which covers the wearer&#39;s foot. 
     The shoe bottom  50  includes a sole  10 . The sole  10  according to the present embodiment includes a midsole  56 . The sole  10  has a ground contact surface  10   e , which comes into contact with the road surface. The ground contact surface  10   e  according to the present embodiment is formed by the lower surface of the midsole  56 . The midsole  56  mainly has a role of alleviating the impact of landing. The midsole  56  is formed using, for example, a foam or non-foam resin, or the like. 
       FIG. 16  is a bottom view of the sole  10 . A plurality of medial transverse groove portions  34  are formed on the sole  10 . The plurality of medial transverse groove portions  34  are formed so as to be open on the ground contact surface  10   e  of the sole  10  and to extend in the in-plane direction of the ground contact surface  10   e . The plurality of medial transverse groove portions  34  extend in the foot width direction Y from the medial edge  10   c  of the sole  10  toward the lateral edge  10   d . The plurality of medial transverse groove portions  34  are provided at intervals in the leg longitudinal direction Lx. The respective end portions of the plurality of medial transverse groove portions  34  on the lateral side in the foot width direction Y are provided at intermediate positions in the foot width direction Y of the sole  10 . 
     The extending direction of the medial transverse groove portions  34  is set to be a direction oblique to the foot width direction axis. More specifically, the extending direction is set to be a direction that is the same as the direction along a line t in the planar view. As shown in  FIG. 2 , this direction along the line t is a direction that is the same as the direction along a straight line Ld connecting the rear end portion of the first metatarsal bone Bf 1  to the rear end portion of the fifth metatarsal bone Bf 5  constituting the Lisfranc joint Jb. The expression “the same” includes not only the case where the directions are the same as interpreted literally but also the case where the directions are almost the same. When this condition is satisfied, the heel side end portion of the sole  10  easily turns in the supination direction, and as a result, the sole midfoot portion  14  can be easily twisted outward. 
     The depth of the medial transverse groove portions  34  from the ground contact surface  10   e  is preferably as deep as possible from the viewpoint of effectively reducing the bending rigidity of the medial midfoot region  20  of the sole  10 . From this viewpoint, the depth of the medial transverse groove portions  34  is preferably 1% or more, more preferably 5% or more, and particularly preferably 10% or more, with respect to the average thickness of the entire sole  10 . 
     The groove width of the medial transverse groove portions  34  is preferably 1 mm or more. The groove width means the opening width of the medial transverse groove portions  34  at the ground contact surface  10   e  of the sole  10 . The groove width is set to 1 mm or more in order to effectively reduce the bending rigidity of the medial midfoot region  20  of the sole  10 . Although the upper limit value of the groove width is not particularly limited, the groove width is preferably, for example, 20 mm or less. 
     An example is shown in which the shape of the medial transverse groove portions  34  is a straight line shape extending in the in-plane direction; however, the shape is not limited thereto. For example, a curved shape extending in the in-plane direction or a shape such as a combination of a straight line and a curved line may be employed. 
     Each of the plurality of medial transverse groove portions  34  constitutes a rigidity lowering portion  32 , which lowers the bending rigidity of the medial midfoot region  20 . A plurality of rigidity lowering portions  32  are thus provided. One medial transverse groove portion  34 , which is a part of the plurality of medial transverse groove portions  34 , is formed so as to extend from the medial midfoot region  20  to the lateral midfoot region  22 . As described above, the rigidity lowering portions  32  are assumed to be provided in the medial midfoot region  20 ; however, the rigidity lowering portions  32  may be provided such that a portion of the rigidity lowering portions  32  extends over the lateral midfoot region  22 . Further, the plurality of rigidity lowering portions  32  are formed so as to be located on the first region  26 , the second region  28 , and the third region  30  of the external torsional resistance expected region  24 , respectively. Even when the rigidity lowering portions  32  are provided in the external torsional resistance expected region  24  as described above, the rigidity lowering portions  32  may be provided so as to extend outside the external torsional resistance expected region  24 . 
     Second Embodiment 
       FIG. 17  is a bottom view showing a sole  10  according to a second embodiment.  FIG. 18A  is a side view of the sole  10  viewed from the medial side in the foot width direction, and  FIG. 18B  is a side view of the sole  10  viewed from the lateral side in the foot width direction. 
     The sole  10  according to the second embodiment has a longitudinal groove portion  36  extending in the foot longitudinal direction X in addition to a plurality of medial transverse groove portions  34 . The longitudinal groove portion  36  is open on the ground contact surface  10   e  of the sole  10 . The longitudinal groove portion  36  is connected to the end portion on the lateral side in the foot width direction of each of the plurality of medial transverse groove portions  34 . The longitudinal groove portion  36  according to the present embodiment is provided so as to fit in a medial midfoot region  20  and is not provided in a lateral midfoot region  22 . 
     The longitudinal groove portion  36  according to the present embodiment has a heel side portion  36   b  provided on the heel side of an intermediate portion  36   a  in the foot longitudinal direction X thereof and a toe side portion  36   c  provided on the toe side of the intermediate portion  36   b . The intermediate portion  36   a  of the longitudinal groove portion  36  according to the present embodiment is provided so as to form a convex shape toward the lateral side in the foot width direction. The heel side portion  36   b  is provided being inclined with respect to the foot longitudinal axis so as to become closer to the medial edge  10   c  of the sole  10  toward the heel side of the sole  10 . The distal end portion of the heel side portion  36   b  connects with the medial edge  10   c  of the sole  10 . The toe side portion  36   c  is provided being inclined with respect to the foot longitudinal axis so as to become closer to the medial edge  10   c  of the sole  10  toward the toe side of the sole  10 . The distal end portion of the toe side portion  36   c  connects with the medial edge  10   c  of the sole  10 . The longitudinal groove portion  36  is provided such that a part of the range thereof that extends from the intermediate portion  36   a  to the heel side overlaps with a line u. 
     A plurality of island-like regions  38  surrounded by the plurality of medial transverse groove portions  34 , the longitudinal groove portion  36 , and the medial edge  10   c  of the sole  10  are formed in the medial midfoot region  20  of the sole  10 . The island-like regions  38  are separated from the other region including the lateral midfoot region  22  of the sole  10  by a groove portion including the longitudinal groove portion  36 . The “groove portion including the longitudinal groove portion  36 ” in the present embodiment refers to only the longitudinal groove portion  36 . If the longitudinal groove portion  36  does not connect with the medial edge  10   c  of the sole  10 , the medial transverse groove portion  34  closest to the toes or the heel is also included. It can be considered that the island-like regions  38  are separated from the region including the lateral midfoot region  22  by the groove portion including the longitudinal groove portion described above. 
     Thereby, when the medial midfoot region  20  is attempted to be bent and deformed at a part where the plurality of medial transverse groove portions  34  are formed, the deformation of the plurality of medial transverse groove portions  34  is prevented from influencing the lateral midfoot region  22  side of the longitudinal groove portion  36 . Therefore, it becomes easier to design such that the bending rigidity of the medial midfoot region  20  and the bending rigidity of the lateral midfoot region  22  are different from each other. 
     The groove width of the longitudinal groove portion  36  is set to be larger than the respective groove widths of the medial transverse groove portions  34 . The medial transverse groove portion  34  that connects with the end portion of the longitudinal groove portion  36  and is the closest to the toes is also set to be larger than the respective groove widths of the other medial transverse groove portions  34 . 
     A plurality of second transverse groove portions  42  are formed in a forefoot portion  12  and a midfoot portion  14  of the sole  10  according to the second embodiment. The plurality of second transverse groove portions  42  are provided at intervals in the leg longitudinal direction Lx. Some second transverse groove portions  42  of the plurality of second transverse groove portions  42  are provided so as to reach the medial edge  10   c  from the lateral edge  10   d  of the sole  10 . The other second transverse groove portions  42  of the plurality of second transverse groove portions  42  are provided so as to extend toward the medial edge  10   c  from the lateral edge  10   d  of the sole  10 . The respective end portions of the other second transverse groove portions  42  are provided at intermediate positions in the foot width direction of the sole  10 . Any of the second transverse groove portions  42  is provided on the toe side of the above-described line y. 
       FIG. 19A  is a bottom view of a sole  10  according to a second exemplary variation.  FIG. 19B  is a bottom view of a sole  10  according to a third exemplary variation.  FIG. 19C  is a bottom view of a sole  10  according to a fourth exemplary variation. Regarding the longitudinal groove portion  36  in the example of  FIG. 17 , an example has been explained where both end portions of the longitudinal groove portion  36  connect with the medial edge  10   c  of the sole  10 . Both end portions of the longitudinal groove portion  36  according to the present example are provided at locations away from the medial edge  10   c  of the sole  10  in the foot width direction. In the longitudinal groove portion  36  according to the present example, the end portions of the medial transverse groove portions  34  and the end portions of the longitudinal groove portion  36  are connected so as to form corner portions with the medial transverse groove portions  34 . In addition to this, the end portions of the longitudinal groove portion  36  may be provided so as to end without connecting with other groove portions. 
       FIG. 19A  shows an example where a single longitudinal groove portion  36  is provided, and  FIG. 19B  shows an example where a plurality of longitudinal groove portions  36 -A and  36 -B (hereinafter, generically referred to as longitudinal groove portions  36 ). The plurality of longitudinal groove portions  36 -A and  36 -B include a first longitudinal groove portion  36 -A on the lateral side in the foot width direction and a second longitudinal groove portion  36 -B on the medial side in the foot width direction. The first longitudinal groove portion  36 -A is provided so as to connect with end portions of the plurality of medial transverse groove portions  34 . The second longitudinal groove portion  36 -B is provided so as to connect with intermediate portions of the plurality of medial transverse groove portions  34  such that the second longitudinal groove portion  36 -B intersect with the intermediate portions in a T-shape or X-shape. 
       FIGS. 19A and 19B  show examples where the respective longitudinal groove portions  36  are provided so as to extend in a linear manner along the respective lines s, and in  FIG. 19C  shows an example where the longitudinal groove portion  36  is provided so as to extend in a linear manner along the line u. The expression “linear” means a shape looking like a straight line and does not mean a strictly linear shape in a geometrical manner. An extending direction Pb in which the linear longitudinal groove portion  36  extends from the toe side to the heel side as described above is set to have, for example, an angle formed by the direction axis thereof with respect to the line s of from 0 to 15 degrees. 
     Third Embodiment 
       FIG. 20  is a side view of a shoe bottom  50  according to a third embodiment as viewed from the same viewpoint as that of  FIG. 15 . In the above-described embodiment, an example has been explained where a sole  10  has only a midsole  56 ; however, the sole  10  may have an outer sole  58  as well. 
     The outer sole  58  is disposed below the midsole  56  and is attached to the lower surface of the midsole  56  by adhesion or the like. The ground contact surface  10   e  of the sole  10  is formed by the lower surface of the outer sole  58 . The outer sole  58  mainly has a role of securing grip performance against the road surface. The outer sole  58  is formed using, for example, a non-foam or foam rubber, or the like. The midsole  56  is formed to be thicker than the outer sole  58  from the viewpoint of playing the role of alleviating the impact of the landing. Further, since the outer sole  58  plays a role of securing the grip performance, the outer sole  58  may have hardness that is larger than that of the midsole  56 . The medial transverse groove portions  34  according to the present embodiment are formed within a range that does not reach the midsole  56  from the ground contact surface  10   e  of the outer sole  58 . 
     Fourth Embodiment 
       FIG. 21  is a side view of a shoe bottom  50  according to a fourth embodiment as viewed from the same viewpoint as that of  FIG. 15 . Different from the example of  FIG. 20 , medial transverse groove portions  34  according to the present example are formed within a range that reaches a midsole  56  from the ground contact surface  10   e  of an outer sole  58 . 
     As described, a sole  10  may have either one or both of the midsole  56  and the outer sole  58 . For example, although not shown in the figure, the sole  10  may have only the outer sole  58 . 
     Described above is a detailed explanation of the embodiments of the present invention. All of the above-described embodiments merely show specific examples for carrying out the present invention. The details of the embodiments do not limit the technical scope of the present invention, and many design changes such as change, addition, deletion, etc., of constituent elements may be made without departing from the spirit of the invention as defined by the claims. In the above-described embodiments, such details that are changeable in a design manner are explained with notations of “according to the embodiment”, “in the embodiment”, etc.; however, it does not mean that design changes are not allowed for features without such notations. Further, hatching applied to cross sections of the drawings does not limit the material of an object with the hatching. 
     The expression “foot longitudinal direction Lx” may be defined as a direction along a straight line connecting the toe side end portion of the second toe to the rearmost end portion (calcaneus tuberosity) of the calcaneus of the wearer&#39;s foot, which is assumed to be on the sole  10  by design. 
     As the sole center line s, a straight line extending along the foot longitudinal direction Y, which divides the full width Lb of the sole  10  into 1:1, may be used. From another viewpoint, a straight line along the foot longitudinal direction Y may be used by which the full width Lb of the sole  10  is divided from 1:1 to 3.7:3.2 from the medial side in the foot width direction to the lateral side in the foot width direction. 
     For example, the midsole  56  may be formed by stacking two or more parts having different material properties in the vertical direction or arranging the parts in the foot longitudinal direction. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
       10  sole,  10   c  medial edge,  10   d  lateral edge,  10   e  ground contact surface,  14  midfoot portion,  16  rearfoot portion,  16   a  toe side end portion,  16   b  heel side end portion,  16   c  continuous surface,  20  medial midfoot region,  22  lateral midfoot region,  24  external torsional resistance expected region,  26  first region, 28 second region,  30  third region,  32  rigidity lowering portion,  34  medial transverse groove portion,  36  longitudinal groove portion,  44  lateral transverse groove portion,  50  shoe bottom,  52  shoe,  56  midsole,  58  outer sole 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to shoe bottoms of shoes.