Plantar Pressure Characteristics with Foot Postures and Balance Abilities in Indigenous Taiwanese: A Preliminary Exploration

Introduction

There are approximately 640 000 Indigenous Taiwanese, accounting for 2.2% of the total population in Taiwan. They are divided into 16 ethnic groups and are widely spread along Taiwan’s East coast (Amis, Puyuma, and Yami) as well as the central mountain ranges (Atayal, Bunun, Paiwan, Rukai, Saisiyat, Seediq, Taroko, Thao, and Tsou), and are recently recognized as the Hla’alua, Kanakanavu, Kavalan, and Sakizaya [1]. Indigenous Taiwanese are known for their excellent physical fitness and athletic performance, which has been documented in numerous case studies [2–4]. The study by He et al mentioned that Indigenous students had significantly better static balance than non-Indigenous students, mainly because they were more physically active, which may explain why they perform better in athletics than non-Indigenous students [4]. Chang et al also argued that Indigenous youth baseball players had better dynamic balance ability in a conventional baseball training program compared to non-Indigenous youth [5]. While many studies have shown that Indigenous people have better athletic performance and balance abilities than non-Indigenous people, comparative studies have also shown that Indigenous populations have a higher prevalence of chronic musculoskeletal problems than non-Indigenous populations [6]. Peng et al found that Indigenous people of New Zealand (Māori) had higher rates of sports injuries than non-Māori [7]. This phenomenon is similar to that of Indigenous Taiwanese. The proportion of foot and back injuries among Taiwan Indigenous students is higher than that of non-Indigenous students [7]. Foot complications and musculoskeletal injuries are caused by multiple factors, one of which is directly related to increased plantar pressure [8].

Currently, there has been little progress in research exploring plantar pressure and foot postural characteristics of Indigenous peoples around the world. A recent study suggested that Aboriginal Australians are more susceptible to musculoskeletal injuries, mainly because their midfoot has higher peak pressures (PP) and/or higher pressure time integrals (PTI) [8]. The study further noted that knee and back injuries were associated with higher PP on the midfoot, but were primarily directly related to PTI [8]. In addition, Charles found that Aboriginal Australians generally have higher arches, and examined the presence of higher PP around the first to third metatarsal heads and the big toe in Aboriginal Australians, while the pressure in the midfoot or rearfoot is lower [8]. The characteristics of high arches and reduced ankle dorsiflexion (ankle equinus) are also common among modern Aboriginal Australians [9,10]. The finding is consistent with similar evidence that ancient Aboriginal Australians had high arches [11]. Such foot shapes tend to increase the duration and degree of forefoot loading. As a result of overloading the forefoot, it reduces the range of motion at the ankle and can contribute to plantar fasciopathy, Achilles tendonitis, midfoot arthritis, and knee pathology [9,10]. However, Gurney et al reported that healthy Māori had significantly higher static and dynamic arch index (AI) and sub-arch angle values than healthy New Zealand White people [12]. Such results appear to support earlier survey findings by Stout, Stout, and Duncan, who documented that most Māori soldiers in World War II had flat feet, but that this foot shape did not cause them to have functional disabilities [13]. Nonetheless, this suggests that Māori have congenital abnormalities in midfoot morphology and are prone to developing flat feet [12]. Puti et al also found that the PP of heel, the first to third and fifth metatarsal heads, and the PTI under the first and fifth metatarsal heads of Indians were lower than those of White people [14].

Based on the research mentioned above, since Indigenous people in various countries around the world have different foot shapes, plantar pressure distributions (PPDs), and foot postural characteristics, there has been little progress in related research, especially in Asia. There are very few case studies about foot characteristics of Indigenous population groups, particularly those that focus on the correlation between foot characteristics and balance ability performance. Review of the literature shows the balance abilities and musculoskeletal impairments of Indigenous people are caused by multiple factors, one of which may be related to their unique foot characteristics. The present study aimed to expand the exploration of the distinctive features of plantar pressure and foot posture among Indigenous Taiwanese based on previous research context and methodologies [15–18], with the aim of exploring the AI, PPDs, the centers of gravity balance, and rearfoot posture alignment of Indigenous Taiwanese. Our hypothesis was that among Indigenous Taiwanese, the feet will develop a unique arch type with PPD concentrated primarily in the forefoot and midfoot. As a result of maintaining good physical fitness and inborn athletic abilities, they may also have balanced centers of gravity and normal rearfoot postural alignment.

Material and Methods

PARTICIPANTS:

This study was a cross-sectional survey of college-level Taiwan Indigenous students during their education from June 2016 to March 2019, with subsequent data analysis and supplementation. We recruited a total of 348 eligible participants and categorized them as follows: 165 Indigenous students (referred to as the Indigenous group, 83 males, 82 females) and 183 non-Indigenous students (referred to as the control group, 95 males, 88females). Participants in the Indigenous group were Indigenous students from Taipei City, New Taipei City, Hualien, Taitung, Kaohsiung, and Pingtung areas in Taiwan. The inclusion criteria were: (1) general college and university students aged 19–21 years; (2) body mass index (BMI) in the healthy physical range of 18.5–23.9 as defined by the World Health Organization (WHO) [19]; (3) not elite athletes or receiving professional sports training; (4) the parents are both Indigenous Taiwanese; (5) no dysfunctions of the lower extremities; (6) have not received medication or physical therapy in the past week; and (7) never wore foot orthotics or orthotic insoles. These students were selected from among 192 Indigenous students enrolled at St. John’s University, National Dong Hwa University, National Taitung University, R.O.C. Military Academy, and National Pingtung University. Approximately 14% of eligible Indigenous students were excluded during the recruitment process for the following reasons: (1) their absence rate; (2) previously treated for fractures or surgery; (3) neuromuscular diseases affecting the lower limb function, such as cerebral palsy or poliomyelitis; and (4) acquired foot conditions such as plantar fasciitis, calcaneal osteitis, metatarsitis, osteoarthritis, corns, and calluses. For the control group, we recruited 183 eligible participants from 195 age-matched healthy college students and university students, whose inclusion criteria were the same as for the Indigenous group except that point 4 was not applicable. The elimination rate during the entire recruitment process was about 6%, mainly due to: (1) the absence rate; (2) BMI outside the healthy physical range; (3) professional training in sports; and (4) treated for fractures and had surgery, or musculoskeletal disorders such as calcaneal spurs, rheumatoid arthritis, or neuropathies within the last 6 months.

To ensure the validity of the research sample, participants in both groups came from a relatively homogeneous population. They had similar ages, body types, learning levels of subject knowledge and skills, and the only difference lies in their ethnic origin. All participants were informed in detail about the purpose, process, requirements, and methods of the experiments before conducting the study, and then signed a written informed consent indicating their full participation. Their age, height, weight, and BMI were recorded at the beginning of the study. The anthropometric characteristics of the eligible subjects were collected after the experiment and summarized in Table 1. The study was approved by the National Taiwan University Research Ethics Committee (NTU-REC No.: 201506 ES016). All experiments were conducted according to ethical approval guidelines and following the Helsinki Declaration.

INSTRUMENTATION:

The JC Mat optical plantar pressure analyzer combined with the built-in FPDS-Pro-V2 software (View Grand International Co., Ltd, New Taipei City, Taiwan, sampling frequency of 15 Hz) was applied to measure the AI, PPDs, and balance of the foot centers of gravity parameters of the study participants [20]. The present study inherits the previous research context and methodologies since the device has been proven to be repeatable and reproducible in many previous studies [15–18,21]. Based on the same principle and technology as the Harris footprint measurement, the system has been shown to have high reliability and good validity, and the main characteristics are as follows: (1) there are 25 sensors on each side of the JC Mat (32×17 cm), which can be used to measure plantar pressure precisely; (2) the sensitive pressure sensing system is equipped with a large working area that can display and mark the delicate pressure image of the plantar foot with fine dots; (3) the system is equipped with FPDS-Pro software so users can synchronously analyze certain parameters, such as AI values, PPD values, the center of gravity balance, and toe angle; (4) convenient and immediate correction to make PPDs and footprint images consistent with the results of weight calibration experiments; (5) a portable type provides the convenience and efficiency of collecting large amounts of data on subjects; (6) easy and effective identification of foot characteristics with color foot pressure images and real barefoot images; (7) the pressure distribution of colored footprints and real barefoot images can be captured synchronously.

EXPERIMENTAL PROCEDURES:

To ensure the consistency and reliability of the experiment during the study, the experiments were scheduled every Monday and Thursday from 9 AM to 12 noon. The experiments were conducted in the classrooms of each college or university. Before the experiment began, the basic physiological characteristics of all participants were recorded using anthropometric measurements. Each participant was then instructed to perform the following steps in a brief trial of static upright standing to obtain preliminary results of static footprint:

FOOT DATA ANALYSIS:

The values of AI, PPD, and center of gravity balance of both feet from footprints were estimated using a built-in FPDS-Pro program on the JC Mat. Once the plantar pressure detection procedure has been completed, the researcher can then use the software to create a straight line (a perpendicular line) from the base of the second toe to the heel center from the image of the subject’s footprint. Meanwhile, the software automatically constructs 4 equidistant parallel lines from the base of the toes to the heel perpendicular to the straight line, and further divided the footprint into 3 equal regions (regions A, B, and C) and 6 subregions (subregions 1–6) [18]. Three regions of A, B, and C from the front to the rear direction of the footprint excluding the toes were sequentially defined as the forefoot, midfoot, and rearfoot regions. From these 3 regions, 2 regions of D and E were equally divided by left and right directions and were referred to as lateral and medial foot, respectively. In addition, segmentation further identified the 6 distinct subregions (subregions 1, 2, 3, 4, 5, and 6) from the 3 regions: (1) the lateral metatarsal bone (LM), (2) the lateral longitudinal arch (LLA), (3) the lateral heel (LH), (4) the medial metatarsal bone (MM), (5) the medial longitudinal arch (MLA), and (6) the medial heel (MH). The PPD values of each region was calculated as a percentage of the relative loads. The individual center of gravity balance was estimated based on comparing the difference between percentage of the relative loads of both feet.

Furthermore, the calculation of AI value was based on a previous study by Cavanagh and Rodgers, who suggested that AI is calculated as the ratio of the middle third of the footprint to the entire footprint excluding the toes, AI=B/(A+B+C) [22]. The Cavanagh and Rodgers definition of AI indicates that an AI below 0.21 indicates a high arch, an AI between 0.21 and 0.26 indicates a normal arch, and an AI greater than 0.26 indicates a flat arch.

REARFOOT POSTURAL ALIGNMENT ANALYSIS:

To determine rearfoot pronation/supination behavior, the rearfoot postural alignment assessment was conducted following the analysis of foot pressure profiles. In the procedure, we first instructed each participant to stand upright statically on a 30-cm high platform with their feet naturally spaced apart (around 12–15 cm). Once the participant stands still and stable on the platform, we made sure that the rearfoot of both feet are standing on the same horizontal line. At this time, the researcher used a fixed-point digital camera to capture an image of each participant’s rearfoot posture alignment from the posterior perspective. The researcher then used the Biomech 2019-postural analysis software (Loran Engineering SrL, EmiliaRomagna, Italy, sampling frequency of 100 Hz) to calculate the rearfoot static angle by connecting the 3 anatomical points of the digital photos [23]. The 3 anatomical points of the lower limb, in order from bottom to top, were positioned as follows: (1) the posterior calcaneal tuberosity, (2) the second point above the center of the calcaneus, and (3) the lower third of the leg. Once the points of the 3 anatomical positions have been marked, the software then automatically generates the first standard straight line (a solid line) of the lower extremity, which connects the lower third of the leg to the center of the calcaneus. Located in the lower extremity, a second flip angle line (dotted line) is also created simultaneously, which extends from the posterior tubercle of the calcaneus to the center of the calcaneus. Upon intersecting these 2 lines, the flip angle was created in the middle, and the angles of the rearfoot were thus determined as normal type (0° to 5°), varus type (<0°) or valgus type (>5°) [15–18,24].

SAMPLE SIZE ESTIMATION:

The sample size estimation was performed using statistical power analysis software (G*Power version 3.1.9.7, Universität Düsseldorf, Düsseldorf, Germany). The effect size Cohen’s d was calculated through the mean and standard deviation (SD) of the AI values of the groups (ie, control group: 0.23±0.07; Indigenous group: 0.27±0.09), and the effect size obtained was 0.496. Under the condition that the alpha error probability (significance level) for the independent sample t test was set to 0.05, and to achieve a statistical power (1−β) of 0.95, the minimum number of samples required for each group was 113 for the controls and 101 for the Indigenous subjects.

STATISTICAL ANALYSIS:

Statistical analysis of the data was conducted using SPSS 23.0 software (IBM Corp., Somers, NY, USA). The Kolmogorov-Smirnov test was used to examine whether the numerical variables conformed to a normal distribution, where P>0.05 indicates normally distributed data. The Mann-Whitney U test was further applied in the comparison of 2 independent groups that were not normally distributed. Normally distributed data are expressed as mean±SD and determined using independent sample t tests. Statistical significance was determined by P<0.05 and P<0.01 for all tests.

Results

ANTHROPOMETRIC CHARACTERISTICS OF SUBJECTS:

The anthropometric characteristics of the participants in the groups were similar in terms of average age and height, but there were significant differences in average weight and BMI values. In particular, the BMI values of the male Indigenous group and the weight of the female Indigenous group were significantly lower than those of the control group (Table 1).

BIPEDAL ARCH INDEX:

As a result of the static standing of bipedal AI analysis, the average value of the control group falls within the normal range (ie, defined as 0.20–0.26), while the average value of the Indigenous group was significantly higher than that of the control group (P<0.01). The same results were also found in the male and female Indigenous groups, especially for the males (P<0.01), suggesting the arch height was generally lower in the Indigenous group (Table 2).

BIPEDAL PPDS OF THE 5 REGIONS:

Based on the PPDs for the 5 regions, the Indigenous group’s relative loads were predominantly distributed on the forefoot, midfoot, and medial foot regions of both feet, while relative loads on the rearfoot and lateral foot regions of both feet were relatively lower (P<0.01). Similar trends were found among Indigenous males and females (Table 3).

BIPEDAL PPDS OF THE 6 SUBREGIONS:

The PPDs results for the 6 subregions were extended from those of the 5 regions. As compared with the control group, the Indigenous group experienced greater average relative loads at the medial and lateral longitudinal arches as well as the medial metatarsals of both feet (P<0.01), while being relatively lower at the medial heels of both feet and the lateral metatarsal bone of the right foot (P<0.05). This situation of PPDs also occurred in the male Indigenous group, but in the female Indigenous group, the relative loads were mainly distributed at the medial and lateral longitudinal arches of both feet, and the medial metatarsal bone of the right foot (P<0.01) (Table 4).

BALANCE OF THE CENTERS OF GRAVITY:

The centers of gravity are presented as a percentage of gravity. As compared with the control group, the static centers of gravity of both feet were relatively symmetrical (left foot: 49.53±4.38%; right foot: 50.47±4.38%; P<0.01), particularly in the male Indigenous group (left foot: 49.89±3.81%; right foot: 50.11±3.81%; P<0.01). Additionally, no significant differences were observed between both feet within the respective groups (Figure 1).

BIPEDAL REARFOOT POSTURAL ALIGNMENT:

Static rearfoot postural alignment of both feet was measured in angles and expressed in degrees (deg). The results showed that the static rearfoot angles of participants in both groups were within the normal range, but compared with the control group, the values in the Indigenous group were significantly lower, especially in males (P<0.05) (Table 5).

CHARACTERISTICS OF THE FOOTPRINT IMAGES:

Based on plantar pressure analysis, each homogenized representative subject’s footprint image was derived by averaging the results within the respective group. The recognizable diagram of the foot anatomy corresponding to the subject’s colored footprint image is shown in Figure 2. In the male Indigenous group, the plantar pressure was greater at the medial and lateral longitudinal arches, while it was less at the heels. In addition, the footprint characteristics of the female Indigenous group also showed higher plantar pressure at the bipedal medial and lateral longitudinal arches, and lower heel pressure (Figure 3).

Discussion

Based on preliminary results, we found that participants in the Indigenous group had different BMI values than the non-Indigenous group, particularly the males. Among the age-matched healthy participants in the study, the results suggest that Indigenous participants appeared to have relatively lower BMI than non-Indigenous participants. According to the results for both feet of the groups, the bipedal AI values were considerably symmetrical to each other within the respective groups. All participants in the control group had a normal AI, whereas the values for the Indigenous group were significantly higher, especially for the male participants. Although the values of the female Indigenous group were categorized into the normal range, they were also significantly higher than those of the control group. This result means that the foot arch type of the Indigenous Taiwanese was relatively lower than that of the general population. It seems this situation echoes those studies conducted by Charles, who suggested that midfoot pressure distribution is related to the prevalence of musculoskeletal injuries among Australian Aboriginals [8,10]. These studies found that knee and back injuries were associated with midfoot peak pressure, but mainly with PTI, which is the amount of time spent on the midfoot during gait. The findings indicated that the Aboriginal people paused the midstance phase of the gait, thus prolonging the pressure beneath the midfoot region, resulting in knee and back injuries [8,10]. Igbigbi et al also mentioned that healthy Indigenous Kenyans and Tanzanians without foot pain had a higher AI [25]. They found that the males had a significantly higher AI than the females in both groups, and Kenyans had a prevalence of flat feet of 43.2%, which was twice as high as Tanzanians, with 20.3% [25]. They also found in an earlier study that Malawians had a significantly higher AI than American Whites and Europeans, in whom the incidence of pes planus was 24.3% [26]. Although these studies did not clearly indicate the details of the PPDs, and the ethnic groups are not geographically and historically related, it confirmed that the Indigenous Taiwanese and the Aboriginal populations of various ethnic groups abroad had low arches.

As for the bipedal pressure distributions for the 5 foot regions, the results showed that the relative loads were mainly located in the forefoot, midfoot, and medial foot regions, but lower in the rearfoot and lateral foot regions than in the control group. Such results differ in some ways from those of Charles’s studies, which found that Australian Aboriginals tended to have a high prevalence of high-arched foot [8–10,27], with PP around the hallux and 1st to 3rd metatarsal heads of the forefoot, but less in the midfoot or rearfoot [8]. Charles also mentioned in an earlier study that Aboriginal Australians have a high incidence of ankle equinus [9]. Therefore, Aboriginal Australians may experience elevated forefoot and midfoot plantar loads as a result of gait compensation due to ankle equinus, increasing the risk of pressure-related foot complications [9]. Equinus is characterized by excessive forefoot pressure, first metatarsal elevation, and midfoot collapse during gait [10]. Based on these findings, the study further explored the detailed distribution of plantar pressure in the 6 subregions. Our results indicated that the relative loads of the bipedal 6 subregional PPDs were largely concentrated at the medial and lateral longitudinal arches of the midfoot as well as the medial metatarsals of the forefoot, whereas the medial heels of the rearfoot and the lateral metatarsals of the right foot were lower, particularly for male Indigenous individuals. In analyzing footprint images of homogenized representative subjects with an average of plantar pressure, it was found that the Indigenous group had a higher loading rate in the midfoot, especially among males, who had lower arches with navicular and cuboid collapse footprints. Such characteristics of the foot were similar to that of the equinous foot pressure distribution [10]. It is possible for elevated first metatarsals and collapsed midfoot to negatively impact the windlass mechanism during normal gait. An abnormal windlass mechanism of the foot may contribute to increased foot pressure on the hallux and 1st to 5th metatarsal heads [28,29]. Therefore, the tendency to increase the duration and degree of forefoot loading also reflects what Charles suggested may contribute to Aboriginal Australians’ athleticism [8,27,30]. A review of the PPDs between Indigenous Taiwanese and Australian aborigines showed there were still some different features, but both groups had higher midfoot plantar loads. However, the arch type of Indigenous Taiwanese in the present study was classified as low-arched feet, whose characteristics are significantly different from those of high-arched feet in Australian Aborigines.

Based on the analysis of centers of gravity, the Indigenous group’ static centers of gravity for both feet were unexpectedly equal and symmetrical compared to the control group. Such well-balanced control of the centers of gravity was similar to observed in earlier studies by He et al showing that Taiwan Indigenous students had significantly lower values for anteroposterior sway, mediolateral sway, total length track, area of the centers of pressure, and movement speed than non-Indigenous students when they were standing in the following postures: left-single-leg stand with open eyes, double-leg in tandem with open and/or close eyes [4]. The study also indicated that Taiwan Indigenous students had significantly lower total length track and area of the centers of pressure than non-Indigenous students in the following standing postures: right-single-leg stand with open eyes, double-leg stand with open and/or close eyes, indicating that Indigenous students have better balance ability in standing postures [4]. In addition, comparing the study results with those of our previous research on healthy students and special athletes [17,18], it may be a preliminary and important indication that Indigenous people have greater control over their center of gravity balances than the general population. Athletes who are able to control their centers of gravity and body posture will be able to exert skeletomuscular strength effectively during exercise, leading to smoother movements and better sports performance [4]. As previously discussed, however, this may also be one of the reasons why Indigenous people had excellent performance in sports and exercise training. Furthermore, based on such indications, we refer to the results of the rearfoot postural alignment assessment, indicating that Indigenous participants displayed a normal range of rearfoot postural angles. Such results were compared to past analyses of the rearfoot postural angles in both ordinary people and special athletes [15–18], the preliminary conclusion is that Indigenous participants in the study had normal centers of gravity balance and rearfoot postural alignment angles, suggesting that Indigenous people may have better musculoskeletal alignment of the lower extremities than the general population. Regarding the association between equinous foot type and musculoskeletal pain of the lower extremities in Aboriginal Australians as discussed in previous studies, our study did not further explore this issue, and the relationships among the walking PPDs, foot shape characteristics, possible musculoskeletal painful areas of the lower limbs, and the specific sports performances need to be further explored.

A limitation of this study was that it did not consider to which of 16 Indigenous ethnic groups in Taiwan the parents of the participants belonged. It is possible that both parents of the participants come from different ethnic groups. Nonetheless, it was confirmed that the participants recruited in this study were qualified Indigenous people, and that both parents were Indigenous as well. The study participants in the Indigenous group came from 11 Indigenous ethnic groups. In addition, most Indigenous students in school had previous experiences in sports or exercise training. To trace the features of the foot in the Indigenous person who is generally without training experiences in sports, recruitment and selection of the participants for the study must be cautious and take a long time. Although the cross-sectional nature of these studies required a long time to recruit subjects of interest for analysis, the findings are not generalizable to all Taiwan Indigenous populations. However, even with the limited ability to generalize our results, this study recruited a substantial number of Indigenous participants and differentiated those eligible for the study by their age, sex, and BMI, as well as sports backgrounds. The results of the plantar pressure profiles associated with the centers of gravity balance and rearfoot posture of Indigenous Taiwanese may provide insight into how they perform well in sports and exercise training. In future studies, it would be worthwhile to investigate the relationships among the dynamic foot characteristics, possible lower-limb pains, and excellent athletic performance to further reveal the foot characteristics of Indigenous Taiwanese. These findings will not only improve uneven plantar pressure distribution and alleviate possible musculoskeletal pain in the lower limbs through the use of insoles or orthopedic appliances, but will also enhance athletic performance and thus prolong the careers of athletes.

Conclusions

We found that Indigenous Taiwanese had lower arches compared with controls, which were characterized by higher plantar loads at the medial and lateral longitudinal arches of the midfoot and the medial metatarsals of the forefoot. The footprint images further illustrated pronounced cuboid and navicular collapse appearances. Nevertheless, the performances of centers of gravity balance and rearfoot postural angles unexpectedly were within the normal range and outperformed the control group. The results may facilitate further exploration of the correlation between foot characteristics, athletic potential, and musculoskeletal injuries among Indigenous Taiwanese.