The results of our study show that, with a functional equinus condition, men have significantly greater plantar contact surface areas than women in all phases of the gait for the first, second, and third steps. Although it was not the aim of this research, a complete study of the reliability variables allows it to be inferred that the first step is the most reliable, and comparing the sex differences of the first, second, and third steps, it is appreciated that the phase of minor gender differences in all steps is the takeoff phase.
The surface values in the contact phase increase in both sexes by increasing the number of steps. Therefore, in the third step, the increase is much higher in men than women. There are previous studies that have examined the plantar footprint in healthy people, where the sole was divided for analysis by anatomical regions or studied as a whole [6
]. The number and type of anatomical divisions of plantar areas are not consistent across studies. Putti et al. [4
] found that an in-shoe system showed an increase of the contact area in 10 parts of the sample. These authors considered that the difference in the contact area could be attributed to differences in BMI. The anatomical analysis separates each part of the foot during the entire stance phase. On the other hand, Murhpy et al. [38
] did not find any differences by sex in contact areas in their study of 50 athletes with an in-shoe system. They divided the footprint into four regions. They found a significantly greater value in the area of the rear foot and fore foot in men than women. No significant values were found in the midfoot. The different results between this study and ours could be due to the characteristics of the sample, because athletes participated. Sport activities may change the morphology of the foot [1
]. Wunderlich and Cavanagh [37
] found the same results as the present study for healthy people. They analyzed gender differences in the shape of the foot and found that men have longer and broader feet than women for any given stature. Male feet differed from female feet in a number of shape characteristics, particularly at the arch, the lateral side of the foot, the hallux, and the ball of the foot. Our study is the first to involve a complete biomechanical analysis focused on sex differences in plantar contact surface areas for the first, second, and third steps. The third step has characteristics of a step in the middle of the gait [40
]. We can apply our results to all steps of the gait. Past studies that divided the footprint into anatomical regions took the global dynamic footprint at the end of the step. Then, they separated the heel, midfoot, and forefoot, without taking into consideration the moments where greater forces could be produced. We have chosen the end of each biomechanical phase of the stance phase, because this is where higher velocities of the pressure center occur and the laxity can have a greater influence and generate a greater or equal surface in women than in men. Research has shown greater ligament laxity in women [39
]. During the heel strike phase, there are high impact forces due to vertical ground reaction forces [41
], and during the mid-stance phase, all the body weight falls on the support leg [41
]. Therefore, a greater decrease of the plantar arch and, consequently, a greater surface area, would be expected in both phases. The study of Hills et al. [11
] that compared the third step of the gait in obese adults found a greater surface for women in the mid foot, with anatomical division of the footprint, and found significant increases in pressure under the heel, mid-foot, and metatarsal heads II and IV for men and III and IV for women. Our large sample and biomechanical approach has allowed us to verify that in a functional equinus condition, women present differences during all phases of the stance phase, despite their laxity. These differences remain even in the second and third step, which have more speed [32
] and consequently, display greater forces.
Finding sex differences in the selected population is important for research and clinical practice. A functional equinus condition is initially an asymptomatic pathology that causes biomechanical alterations [19
], changes in the plantar pressures [19
] and tensions and solicitations of the tissues like the Achilles tendon [19
]. The findings imply that footwear in a functional equinus condition must take into account this difference in surface, especially sports shoes, and in plantar foot orthoses. The shoes should be narrower and allow greater application in the midtarsal joint, which suffers from excessive pronation. Further studies should be carried out to check if there are also sex differences in children. In the same way, these differences may increase in an older population [42
] and therefore, further studies are necessary.
The first step is the most reliable. These results can be explained by the biomechanics of the start of the gait. Prior to the heel contact of the first step, a series of neurophysiological mechanisms and movements are produced. These mechanisms includes a series of stereotyped muscle contractions and inhibitions, invariables of a sequential motor program [43
], which is directed at the level of the Central Nervous System, thus unconsciously [43
]. The step phase of minor gender differences in all the steps is the takeoff phase and it is the phase where the values are more constant in the three steps. Additionally, it is necessary to consider that the take-off phase is the less reliable phase. The heel strike and mid-stance phase present a moderate to perfect reliability. All variables increase the reliability by using two different days to collect data. These findings can be very useful and be applied in clinical and studies: the first step is the most reliable step to register, and the take-off phase exhibits the least gender differences for the surface variable.