The most important finding of this study was that females applied a significantly higher peak pressure on the hallux, toes, forefoot, and inside of the foot while both standing and walking than males. Females have a higher pelvic tilt and a center of gravity anterior to males[11], which can result in higher plantar pressure at the toes and forefoot. The movement of knee-in while walking was likely to occur in females because of weaker gluteus medius strength compared to males[12]. Thus, the peak pressure at the inside of the foot may be higher. Putti et al measured plantar pressure using an in-shoe foot pressure measurement and reported no peak pressure differences between females and males[9]. Demirbuken et al measured plantar pressure using the same system and detected higher toe peak pressure in females compared to males and an age-dependent increase of plantar pressure at the female toes and forefoot [10].
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 [13][14]. However, they are relatively heavy (The Pedar system; 400 g, F-scan system; 400 g) and bulky (The Pedar system; 600 ㎤, F-scan system; 300 ㎤). 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 ㎤, 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[15]. It has been suggested that the COP provides greater insight into dynamic foot function compared to pressure at discrete regions[16]. 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[17]. In this study, COP movements were similar across healthy adult subjects 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 diseased, such as flat foot and hallux valgus, are warranted.
Although this study did not include cases of hallux valgus (HV), females had a higher plantar pressure at the hallux than males. Nix reported a meta-analysis that estimated that female HV prevalence (30%) was 2.3 times greater than in males (13%)[18]. 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[19][20], while others have reported an inverse correlation between severity and plantar pressure at the hallux[21]. There are various causes of HV; however, this gender-based difference in pressure can be suggested as one of the causes.
The findings of this study should be considered after taking into consideration three limitations. 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 subjects 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 gender specific design of insole or shoes. Wearing appropriate shoes may help prevent foot and ankle diseases. 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.