1. Introduction
Mountaineering is a popular recreational activity that requires carrying heavy backpacks during long-term uphill walking [
1]. The added external load and uphill gradient can increase the stress on the bones, ligaments, and muscles [
2]. Therefore, this type of activity has always been highly strenuous and fatiguing for mountain climbers. It also induces biomechanical changes, including increased-propulsion vertical and anterior ground reaction force [
1], increased frontal ankle range of motion [
1], increased plantar pressure [
3,
4], and increased low-limb muscle activity [
2]. These biomechanical changes cause a potential risk for metatarsal stress fractures [
4], blister development [
1,
5], ankle sprain [
1], and muscle fatigue [
2]. These risks of injury suggest that neuromuscular and skeletal systems are unable to accommodate the demands of load carriage during uphill walking.
Injuries to the foot are commonly prevented using prefabricated orthopedic insoles, which are inexpensive and convenient [
5,
6]. Prefabricated orthopedic insoles are generally designed based on features and characteristics including arch support, heel cups, and cushion properties [
7,
8]. Two studies have shown a decrease in plantar pressure and the risk of foot injury using these devices while carrying a heavy backpack during level walking [
5,
9]. Peduzzi de Castro, Abreu, Pinto, Santos, Machado, Vaz, and Vilas-Boas [
9] designed a plantar pressure relief insole and found that it was effective in reducing foot plantar pressure in the little toes (second to fifth toe), medial midfoot, lateral midfoot, and forefoot. Melia, Siegkas, Levick, and Apps [
5] reported that a soft orthopedic insole attenuated plantar pressure of the lateral and medial forefoot and increased the contact area over the whole foot. However, to date, there appears to be a gap in the literature regarding the effect of prefabricated orthopedic insoles on plantar pressure during uphill walking while carrying a heavy backpack.
A previous study found that the muscle activity of the erector spinae, vastus medialis, soleus, and gastrocnemius increased when carrying a load during uphill walking compared with unloading during level walking [
2]. The author suggests that muscles require more effort to overcome external loads and uphill gradients, resulting in muscle overexertion, muscle fatigue, and injury. In addition, muscle fatigue may increase the risk of falls and endanger the lives of mountain climbers. Therefore, resisting muscle fatigue and assisting in improving the biomechanical efficiency of walking are critical issues. Previous studies have found that a custom-made orthopedic insole could decrease muscle fatigue during level walking [
10], which is associated with improving the mechanical energy that is stored and returned at each step [
11]. However, it is unclear whether prefabricated orthopedic insoles help with resistance muscle fatigue while carrying a load during uphill walking.
Prefabricated orthopedic insoles are made of varying materials which induce different biomechanical effects. The stiff orthopedic insole slightly deforms when bearing weight, which results in less impact attenuation and greater plantar pressure; however, a method to control abnormal foot motion is better, such as overpronation of the foot [
12,
13,
14]. Conversely, the soft orthopedic insole can fit the geometrical shape of the plantar because of the characteristics of the material, which is likely to contribute to attenuating impact and foot pressure [
5,
12,
15]. This hypothesis was confirmed by Melia, Siegkas, Levick, and Apps [
5], who found that low-density shoe insoles (soft) were better at decreasing plantar pressure than high-density shoe insoles (hard), as they produced more contact area under the whole foot. However, overdeformation of the insole occurs if the material of the insole is too soft, which negatively influences the impact absorption and plantar pressure attenuation [
5]. Therefore, the effect of carrying a heavy backpack during uphill walking while wearing prefabricated orthopedic insoles with different hardness values should be considered, because this issue is critical to developing an appropriate prefabricated orthopedic insole for mountaineering activity.
This study aimed to investigate the effect of varying the hardness of a prefabricated orthopedic insole on plantar pressure and muscle fatigue during uphill walking while carrying a heavily loaded backpack. It was hypothesized that a soft prefabricated orthopedic insole may decrease plantar pressure and muscle fatigue.
4. Discussion
The major findings of the current study were that both MI and SI, with lower hardness values in the fore- and rear-foot regions, significantly reduced the peak plantar pressure in the fifth metatarsal and lateral heel, and MI significantly reduced the pressure–time integral in the lateral heel compared with FI. The HI, with the hardest hardness in the fore-, mid-, and rear-foot regions, significantly increased the peak plantar pressure and pressure–time integral in the toes, but decreased the contact area in the metatarsal compared with the other insoles with lower hardness values. The MI, HI, and SI, which were prefabricated orthopedic insoles with foot arch support and heel cups, significantly decreased the peak plantar pressure in the lateral heel compared with the FI. No difference was found among all insoles in terms of reducing muscle fatigue. Thus, these results partially support our hypothesis that the soft prefabricated orthopedic insole decreases plantar pressure, but cannot reduce muscle fatigue.
Previous studies have indicated that soft orthopedic insoles could contribute to reducing plantar pressure during loading gait and level walking [
5]. The results of this study further confirmed that soft, prefabricated orthopedic insoles can reduce plantar pressure, specifically in the fore- and rear-foot regions, during loading gait and uphill walking. Based on the obtained plantar pressure data, Peduzzi de Castro, Abreu, Pinto, Santos, Machado, Vaz, and Vilas-Boas [
9] used finite element analysis to develop insoles for obese and weight-bearing individuals, and found that insoles configured with soft cork gel in the forefoot and heel can redistribute the plantar pressure in these areas. For example, the pressure of the rearfoot of the weight-bearing person is transferred from the lateral to the medial heel, and the pressure of the forefoot and the lateral mid-foot of an obese person is decreased. The relatively reduced plantar pressures observed in the toes and forefoot during weight-bearing walking may decrease the risk of foot blisters [
25,
45], metatarsalgia, and stress fractures in these foot regions [
46], as well as plantar fasciitis [
47]. Nevertheless, softer is not always better, as the MI significantly reduced the pressure–time integral in the lateral heel, but the SI did not. The MI showed greater hardness in the fore- and midfoot regions than the SI did. This could partly contribute to the stiff arch support of the midfoot MI.
Wearing MI, HI, and SI was shown to decrease peak plantar pressure in the lateral heel compared with FI during weight-bearing uphill walking in this study. The prefabricated orthopedic insoles used herein featured arch support and deep heel cups. Previous research has shown that a flat foot wearing a custom-made arch support insole can reduce the average pressure and peak pressure in the rearfoot by 9.6% and 17%, respectively [
48], which is similar to the results of a study that achieved reduced peak pressure in the rear foot. The outcome of the current study was not in accordance with previous studies reporting that orthopedic insoles cannot decrease plantar pressure in the rearfoot during weight-bearing level walking [
5,
9]. This could be attributed mainly to the different features, such as the presence of arch support and deep or shallow heel cups of the orthopedic insoles, rather than the hardness of the insoles.
This study demonstrated that the SI showed a greater contact area in the metatarsal compared with the HI during weight-bearing uphill walking. Previous research has indicated that insoles with high hardness values are mainly used for foot realignment, while insoles with low hardness values are mainly used for foot shock absorption [
49]. Low-density materials can easily reduce peak plantar pressure, and their soft and compliant properties allow the foot to better adapt to the load on the plantar geometry and produce a more uniform pressure distribution [
50]. The use of insoles can increase the contact area and reduce the peak pressure on the foot [
51]. The SI, with soft material in the forefoot, was considered to fit the geometry of the metatarsal, causing uniform pressure distribution. Moreover, the increased contact area was subjectively considered to increase the comfort of the insoles [
5].
This study found that tibialis anterior fatigue significantly increased after weight-bearing uphill walking in all insoles. This result is consistent with those of previous studies [
39,
52]. Previous studies have indicated that the tibialis anterior can ensure adequate space between the foot and ground during the swing phase; correctly align ankle and calcaneus at initial heel contact; and eccentrically drop the mid- and forefoot to the ground during the stance phase, thus avoiding tumbling and falling during weight-bearing uphill walking [
39]. Moreover, there was a trend toward decreased tibialis anterior fatigue in the MI (1.87%) after weight-bearing uphill walking compared with the HI, SI, and FI (2.58%, 4.12%, and 6.99%, respectively). The slight decrease in tibialis anterior fatigue while wearing the MI may imply that there is a decreased demand for absorption of impact forces of the muscles of the hip and knee [
39].
The current study had some limitations. This study was a short-term test and was only performed on a treadmill indoors, which may not be sufficiently similar to the outdoor environment. A pack frame with only one loading weight (25 kg) was used, and the results may not be applicable to other weights. Nonetheless, all participants’ perceptions of effort in carrying out the tests using a 25 kg load reached the hard/heavy level on the Borg rating of perceived exertion scale (15.4 ± 0.6). The subjects were healthy male recreational athletes; thus, the results may not apply to other populations.