Plantar pressure sensors indicate women to have a significantly higher peak pressure on the hallux, toes, forefoot, and medial of the foot compared to men

Analysis of the loading condition of the sole during walking and running is vital to improve footwear design [1], sports performance analysis and injury prevention [2], balance control [3], and for diagnosing diseases [4]. Various foot pressure evaluation devices have been developed, and the currently available systems are categorized into the plantar platform system and the in-shoe system.

At present, over 50 different devices are used for foot loading analysis [5]. Some systems require specialized laboratories, whereas others require relatively expensive and delicate insoles with pressure sensors [6]. The in-shoe system is preferable over the plantar platform system because of its portability, simplicity, and applicability on various shoe types [7], and participants can perform natural movement during the experiment without the restriction of space and activity [8]. However, existing in-shoe systems are heavy and bulky, and induce unnatural movement, urging the need of a light and slim in-shoe type sensor.

Skeletal structure and muscular strength differ between women and men. However, the effect of sex on plantar pressure during activities remains undetermined. Putti et al. did not find any sex-based differences in plantar pressure while walking eight steps on the same walkway [9]. Darlene et al. reported that no significant differences were found in normalized plantar pressure values between men and women [10]. On the other hand, in some investigations, there was a significant difference in the peak pressure between men and women. Demirbuken et al. reported that higher toe peak pressure was identified more often in women than in men during early adolescence [11]. Chung et al. indicated that men had higher peak pressure in the medial toe and all forefoot areas than women [12]. McKay et al. reported that there was no significant difference between boys and girls aged 3–9 years, at ages 10–19, men had significantly higher midfoot peak pressure than women and at age 20 and older men had significantly higher rearfoot peak pressures than women [13].

To examine the effect of sex difference on plantar pressure, testing activities should be performed in positions as natural as possible, without any obstruction from the measurement.

Therefore, a slim plantar pressure sensor was newly developed to measure plantar pressure of healthy adults during unrestricted activities. The purpose of this study was to evaluate pressure distribution on the sole during gait and stance using the newly developed in-shoe measurement system and compare the results between sexes.

Women had a significantly higher peak pressure on the hallux, toes, forefoot, and medial aspect of the foot compared to that of men during standing (Table 2. All; p < .05). Similar to the results during standing, a significantly higher peak pressure was observed on the hallux, toes, forefoot, and medial aspect of the foot in women than men during walking (Table 2, All; p < .05). In all cases, greater loading was applied to medial aspect of the foot in the middle of the stance phase than lateral. COP movements were similar across participants such that they translated from the hindfoot through the middle of the midfoot, and finally toward the base of the first toe (Fig. 4). There were no significant differences between sexes (Table 3, All; p > .05).

The most important finding of this study was that women applied a significantly higher peak pressure on the hallux, toes, forefoot, and medial aspect of the foot while both standing and walking than men. Women have a higher pelvic tilt and a center of gravity anterior to men [15], which can result in higher plantar pressure at the toes and forefoot. The movement of knee-in while walking was likely to occur in women because of weaker gluteus medius strength compared to men [16]. Thus, the peak pressure at the medial aspect of the foot may be higher. In previous reports, the results regarding the difference in peak pressure between men and women were varied. Some reports indicated that there was no significant difference between men and women [9, 10], and some reports indicated that men have higher peak pressure in the medial toe and all forefoot areas than women [12]. On the other hand, higher toe peak pressure was identified more often in women than in men during early adolescence [11]. It is also reported that the peak pressure shifts from the hind foot to the forefoot depending on the age [13].

Technical improvement of the measurement system was demonstrated for evaluation of the plantar pressure during natural activities. The Pedar system (Novel gmbh, Munich, Germany) and F-scan system (Tekscan, Inc., Boston, MA) are the main models of the in-shoe plantar pressure measurement system in the previous studies [17, 18]. However, they are relatively heavy (The Pedar system; 400 g, F-scan system; 400 g) and bulky (The Pedar system; 600 cm³, F-scan system; 300 cm³). To evaluate natural walking, the size and weight of a measurement device should be as light and small as possible. The newly developed in-shoe device in the current study weighs 17 g and has a volume of 15 cm³, making it possible to perform standing and walking with less interference than previous products. Additionally, conventional products have 50 Hz of sampling rate, whereas this device has 200 Hz of sampling rate. Therefore, the new system could have less chance to miss some instant and important changes of the plantar pressure during activities. The newly introduced device can be applied to faster movements, such as running and sports activities.

The COP is defined as the centroid of the total number of active sensors, which suggests the spatial distribution of pressure over time [19]. It has been suggested that the COP provides greater insight into dynamic foot function compared to pressure at discrete regions [20]. Buldt et al. reported a difference between the planus and a normal foot in relation to the medial shift of the lateral-medial force index during terminal stance [21]. In this study, COP movements were similar across healthy adult participants such that they translated from the hindfoot through the middle of the midfoot, and finally toward the base of the first toe. Future studies comparing healthy feet with pathological conditions, such as flat foot and hallux valgus, are warranted.

Although this study did not include cases of hallux valgus (HV), women had a higher plantar pressure at the hallux than men. Nix reported a meta-analysis that estimated that female HV prevalence (30%) was 2.3 times greater than in men (13%) [22]. Studies have reported plantar pressure in HV in the past; however, their results have been inconsistent. Some have reported a high plantar pressure on the hallux [23, 24], while others have reported an inverse correlation between severity and plantar pressure at the hallux [25]. There are various causes of HV; however, this sex-based difference in pressure can be suggested as one of the causes.

The findings of this study should be considered after taking into consideration five limitations. We have not recorded the foot posture of the participant in this study, however, according to a systematic review of foot posture by Buldt et al., there was a significant difference in peak pressure between the pes cavus, the pes planus, and the normal foot [6]. Since we selected healthy foot participants, it was considered that foot posture had little effect on peak pressure. We have not recorded the foot morphology of the participant in this study. Mahshid et al. reported that Japanese women had narrower feet in the heel and forefoot, and their instep, first and fifth toes and navicular height were also lower than men’s [26]. Zhao et al. demonstrated that Japanese men generally had longer, larger and higher arched feet than women [27]. Since we adjusted the shoe size to 25.5 cm and compared, it was considered that foot morphology had little effect on peak pressure. Plantar pressure measurement systems are limited in that they only measure force perpendicular to the sensor surface. Therefore, other relevant forces including shear force cannot be measured. However, the current study examined the force during standing and walking when a vast majority of force is applied perpendicular to the foot. The influence of other related forces might be considered when more active sporting activities are involved, i.e. turning, stop-and-go motions. Footwear characteristics such as sole bending stiffness are likely to influence the parameters. To avoid this impact, the same shoes of different sizes were used in the current study; however, care should be taken when using different shoes in future studies. Lastly, although statistical significance was achieved in the comparisons, the sample size might be insufficient to apply this study result to the general population of wider age and/or other cultural backgrounds. It should be noted that the current study results stem from participants who were relatively younger than most foot and ankle patients, but older than most active sport athletes.

The current study result could be used to develop sex specific design of insole or shoes. Wearing appropriate shoes may help prevent foot and ankle pathological conditions. In addition, checking the COP movement may help improve gait balance and gait posture. In order to invent therapeutic ones, further detailed evaluations of plantar pressure in pathological feet are needed, and the new measurement tool might be applicable.

Women tend to put more pressure on the front and medial side of the foot during natural standing and walking than men.