Comprehensive biomechanical characterization of feet in USMA cadets: Comparison across race, gender, arch flexibility, and foot types

Abstract

Lower extremity musculoskeletal injuries are common, complex, and costly problems. Literature supports associations between static foot structure and dynamic foot function, as well as between overuse injury and demographic characteristics. Previous studies failed to provide a comprehensive biomechanical foot characteristics of at-risk military personnel. In this study, foot structure, function, and arch height flexibility (AHF) were objectively measured in 1090 incoming cadets (16.3% female, mean age of 18.5 years and BMI of 24.5 kg/m²) of the United States Military Academy at the start of their training. A Generalized Linear Model with an identity link function was used to examine the effects of race, gender, foot types, and AHF while accounting for potential dependence in bilateral data. Planus and flexible feet independently demonstrated over-pronation, as measured by reduced Center of Pressure Excursion Index (CPEI). When comparing across race, Black participants showed a significantly lower arch height index (AHI), a larger malleolar valgus index (MVI), and a higher prevalence of pes planus (91.7% versus 73.3% overall). However, Asian participants with flexible arches, rather than Black with low arch, displayed over-pronation in gait. Females showed no significant difference in standing AHI and MVI but demonstrated a significantly greater AHF and a reduced CPEI than male participants. This was the first large scale investigation that comprehensively characterized biomechanical foot in a cohort of young at-risk individuals with lower limb musculoskeletal injuries. Long-term goal is to examine the relationship between these biomechanical features and injuries, ultimately to develop effective preventive measures.

1. Introduction

Musculoskeletal (MSK) lower extremity disorders are common and costly problems [1]. More than two-thirds of unintentional injuries in the US annually are in the MSK system with total direct costs of $176.1 billion between 2009 and 2011 [1]. In military personnel, between 15% to 35% of men and 38% to 67% of women sustain at least one injury during Basic Combat Training, with 77% located in the pelvis and lower extremity [2]. MSK injuries are also the leading reason for medical care during active tour of duty. More than 75% of non-battle medical evacuations from Iraq or Afghanistan were for MSK conditions [3]. About 10% of active duty soldiers are non-deployable due to temporary or permanent MSK pathologies. Lower extremity pain and pes planus represented approximately 11% of all discharges [4]. Incidence of osteoarthritis is significantly higher in service personnel, compared to age-matched general population [5]. These acute and chronic conditions associated with MSK injuries can pose life-long challenges in maintenance of a healthy lifestyle and well-being. As stated by Teyhen et al., “Correctly identifying people with foot types susceptible to severe lower extremity MSK injuries could help inform clinical decision making, reduce recurrent injuries and injury-related costs^” [6].

While ankle injuries are most prevalent, lower limb stress fractures can impede readiness of combat unit as the affected individuals are not able to engage in weight-bearing activities for 6–8 weeks. A study of 3025 US Marine Recruits during 12-week basic training showed a 2.45 relative risk of stress fractures or stress reactions in White recruits [7]. Another study, which included all US service members from 2009 to 2012, similarly showed higher unadjusted incidence rate for lower extremity stress fractures in White (6.08 per 1000 person-years), compared to African-American (5.21) and subjects of other race categories (4.41) [8]. A prospective study of 449 Navy Sea, Air, and Land (SEAL) trainees reported that the risk for developing stress fractures in either low or high arch was twice that of normal arch feet [9].

A number of investigations focused on cadets of United States Military Academy (USMA) – an ideal setting for a number of reasons, including high prevalence of overuse injuries, standardized rigorous training, and enclosed health care system with integrated injury tracking and treatment. Cosman et al. reported at least one stress fracture (58% metatarsal and 29% tibial) in 5.7% of male and 19% of female USMA cadets primarily during the first 3 months, which encompasses basic training [10]. In males, the risk of stress fracture was higher for those who exercised less than 7 h per week during the prior year, with smaller tibial cortical area, lower bone density, and smaller femoral neck diameter. In females, the risk of stress fracture was higher for those who had a shorter time since menarche and smaller femoral neck diameter. These factors accounted for less than 10% of model variance; the authors suggested considering biomechanical factors such as foot type, leg length discrepancy, or external hip rotation. Levy et al. showed a correlation between severity of flatfoot and the incidences of lower extremity injuries at USMA cadets. However, female cadets experienced a higher incidence of lower extremity injuries than males in the absence of severe flatfoot [11].

Studies suggest potential importance of arch flexibility to foot function and MSK injuries. Three-dimensional motion analysis of the navicular displacement in the pediatric feet showed uncoupling of the navicular motion, which suggest impaired midfoot function in flatfoot [12]. Subjects with medial tibial stress syndrome demonstrated increased navicular drop in quiet standing and increased arch deformation in gait than healthy control [13]. Subjects with patellofemoral pain displayed a more medially oriented plantar loading during drop jump, and those who experienced an immediate decrease in peak plantar pressure with the use custom foot orthoses were more likely to report improvements after 12 weeks of use [14,15]. While females demonstrated greater arch motion or flexibility than males [16,17], significance of arch flexibility on foot function and lower limb MSK injuries remained largely unknown.

A number of large-scale studies have been conducted to assess the effcacy of shoe or foot orthoses on MSK injuries with inconsistent findings. Selection of running shoes based on visual inspection of plantar shape in 722 Marine Corps in basic training had little influence even after considering other injury risk factors [18]. Cochrane review of 16 trials concluded that the use of cushioning shoe insert probably reduced lower limb MSK injuries [19]. Authors also concluded that there is insuffcient evidence to determine the best insert design and recommended consideration of comfort and tolerance.

There is a significant gap in literature. Specifically, no investigation to date provided comprehensive measures of foot biomechanics in a young active cohort. A comprehensive foot assessment is needed as a baseline description of study cohort to elucidate relationship between foot structure, function, and flexibility; to eventually identify risk factors associated with lower limb MSK injuries, and to develop effective interventions. The purpose of this study was to employ a comprehensive set of foot measurements (1) to describe biomechanical characteristics of feet using a large cohort of healthy young participants at risk for lower limb MSK injuries and (2) to compare differences in foot structure, function, and flexibility across race and gender.

2. Participants and methods

2.1. Participants

The study protocol was approved by the Institutional Review Board of the United States Military Academy (USMA). Of the 1173 incoming cadets at the USMA, 1124 (95.8%) subjects volunteered to participate in the study and provided informed consent. Data were collected within 3 days of the 6-week basic training in the summer of 2013, while study participants were fitted for their boots. To accommodate their rigorous training schedule, some participants were instructed to bypass measurement stations, resulting in varying sample sizes for different parameters.

3. Methods

Overall foot geometry and arch flexibility were assessed using the Arch Height Index Measurement System (JakTool LLC, Cranbury) [20]. This device provides a quick and reliable means to measure foot length, truncated foot length (distance from heel to the first metatarsophalangeal joint), and arch height in sitting and standing positions. Arch Height Index (AHI) is a ratio of the dorsal arch height measured at half the foot length in sitting, normalized by the truncated foot length. AHI was calculated for sitting and standing postures. Each foot was categorized as planus, rectus, or cavus foot type based on previously published standing AHI criteria [21]. Arch drop is the change in arch height from sitting to standing. Foot elongation (ΔFL) is the change in foot length from sitting to standing. Arch height flexibility (AHF, mm/kn) is defined as the arch drop, normalized to change in load, estimated to be 40% of the body weight [21]. AHF was stratified into 3 categories: largest quintile as flexible, middle 3 quintiles as referent, and smallest quintile as stiff.

Twelve stations were set up to measure AHI of each participant, with two testers at each station. Rater measured the foot dimensions while the recorder entered the data into a customized Filemaker Pro app running on an iPad. The two-person team allowed for efficient data acquisition (about 90 s per participant) and a means to check for accuracy of data entry. Resulting foot dimensions were also used to facilitate boot fitting. Podiatric medical students were trained to measure AHI. To ensure consistency of measurements, 12 randomly selected teams of trained students measured AHI on 12 volunteers twice in non-consecutive order and yielded an intra-rater reliability (ICC (2,1)) of 0.88 and an inter-rater reliability (ICC (3, 1)) of 0.84 [22,23].

Standing hindfoot alignment was assessed using the Malleolar Valgus Index (MVI). Instead of using a computer flatbed scanner as originally described [24], one station was set up with a Plexiglas plat- form, a mirror, and a digital camera to expedite data collection. While a subject stood comfortably in his/her base and angle of stance, the sole of each foot was photographed using a mirror position at 45°. The camera (Nikon D5200 with a 35 mm lens) was physically fixed relative to the mirror to minimize potential parallax. A 5.08 cm² area calibration square was photographed simultaneously with the foot, to provide a conversion factor needed to calculate the MVI. Custom developed software was used to compute MVI (%) – larger MVI is associated with greater hindfoot valgus or pronatory foot posture. Consistency of photo-based MVI was compared to the original scanner-based MVI (non-published). Two independent measurements of 14 healthy participants (28 feet) by 2 raters using both methods showed no significant difference between two methods (p = 0.587, mean difference = 0.24%). Intra-rater reliability (ICC 2,k) was 0.941 and inter-rater reliability (ICC 3,k) was 0.917 [22,23].

Each participant’s dynamic plantar pressure distribution was captured with a second-step barefoot protocol at comfortable walking speed using emed-X (novel gmbH, Munich) with a resolution of 4 sensors per cm² and sampling rate of 100 Hz. Five stations were set up to collect dynamic plantar pressures on both feet, 5 trials per foot. Peak pressure (PP, N/cm²) and the Center of Pressure Excursion Index (CPEI, %) were calculated as the mean of 5 trials per foot using novel scientific software (version 24) [25].

3.1. Statistical analysis

Data were compiled and reviewed prior to analysis. A total of 34 subjects were excluded from all analysis: 4 participants had duplicated subject numbers, 28 had missing AHI data, and 2 had incomplete AHI data on one foot. In addition, 15 participants were missing datum on race, 9 on gender, and 9 on body weight. Consequently, these participants were excluded from comparison across race, gender, and AHF, respectively. A complete set of AHI and AHF data were available on 2180 feet (1090 participants). MVI data included 1760 feet (891 participants) while plantar pressure included 1972 feet (987 participants). Descriptive statistics and normality testing (Shapiro-Wilk) were performed using SPSS software version 22 (IBM, Chicago, IL, USA). Few variables (arch drop, foot elongation, AHF, and peak pressure) were not normally distributed. Qualitative review showed that distribution of these variables closely resembled that of a normal distribution; statistical analysis was performed using an assumption of normal distribution. The lower limb was used as the unit of observation. A Generalized linear model with an identity link function was used to examine the effects of race, gender, AHF, and foot type while accounting for potential dependence in bilateral data. The Wald Chi-square was calculated for each dependent variable with significance set at p < 0.05. Post hoc pairwise comparisons for all pairs were performed using the Generalized Chi-Square test at p < 0.05. Pearson correlation analysis was performed between static posture, flexibility, and functional parameters.

4. Results

Participants were 18.5 ± 1.1 years old with mean body mass index of 24.5 ± 3.0 kg/m². Measures of foot structure, function, and flexibility are summarized by race/ethnicity in Table 1. Six Native Americans (0.6%) and 21 of other race/ethnicity categories (2%) were excluded from further analysis due to small sample size. Statistically significant differences were noted across groups. Black participants showed significantly lower standing AHI and sitting AHI as well as greater MVI, compared to the other categories. Asian participants exhibited significantly greater AHF (p < 0.001) and a significantly smaller CPEI (p = 0.003) than the other 3 categories. White participants exhibited a significantly stiffer AHF than Hispanic and Asian participants and greater CPEI than Asian participants. Black participants exhibited a significantly greater prevalence of pes planus (91.7% versus 73.4% overall).

Table 1:

Measures of Foot Structure, Flexibility, and Function for Total Sample and Comparisons of Measures across Race.

Variable	All (n=2,096 feet)		White (N=1,516 feet; 72.3%)		Black (N=192 feet; 8.9%)		Hispanic (N=234 feet; 10.8%)		Asian (N=154 feet; 7.1%)		x2	p- value	Post hoc comparisons
(unit)	Mean	SE	Mean	SE	Mean	SE	Mean	SE	Mean	SE			W v B	W v H	W v A	B v H	B v A	H v A
AHI, sitting	0.349	0.001	0.352	0.001	0.333	0.003	0.354	0.003	0.357	0.003	49.648	0.000	0.000	0.572	0.166	0.000	0.000	0.467
AHI, standing	0.324	0.001	0.329	0.001	0.310	0.003	0.328	0.003	0.327	0.003	46.351	0.000	0.000	0.473	0.455	0.000	0.000	0.899
Arch Drop (cm)	0.49	0.01	0.44	0.01	0.46	0.02	0.49	0.02	0.55	0.02	41.23	0.000	0.095	0.001	0.000	0.202	0.000	0.018
ΔFL (cm)	0.40	0.01	0.37	0.01	0.36	0.02	0.40	0.02	0.45	0.03	13.67	0.003	0.479	0.099	0.001	0.075	0.001	0.086
AHF (mm/kN)	16.98	0.30	14.97	0.23	15.31	0.55	17.81	0.68	19.83	0.76	49.46	0.000	0.571	0.000	0.000	0.005	0.000	0.047
MVI (%)	10.67	0.24	10.63	0.18	12.01	0.50	10.27	0.51	9.77	0.60	10.10	0.018	0.009	0.510	0.171	0.015	0.004	0.523
CPEI (%)	21.80	0.24	21.97	0.20	22.02	0.59	22.93	0.47	20.27	0.55	13.74	0.003	0.941	0.063	0.004	0.227	0.030	0.000
PP (N/cm2)	59.19	0.77	58.33	0.56	61.24	1.70	59.09	1.63	58.11	1.89	2.80	0.423	--	--	--	--	--	--

Mean and Standard Error of means are listed. A generalized linear model with an identity link function was used to examine the effects of race while accounting for potential dependence in bilateral data. The Wald Chi-square was calculated for each dependent variable with significance set at p < 0.05. Post hoc pairwise comparisons were performed using Generalized Chi Square test at p < 0.05.

Black participants demonstrated the smallest standing AHI and the largest MVI – features commonly associated with planus foot structure. However, the greatest over-pronation (i.e., smallest CPEI) in gait was observed in Asian participants with the largest AHF.

Females comprised 176 of the 1081 (16.3%) participants, which is consistent with female enrollment in each incoming USMA class. Female participants were significantly younger (18.1 years versus 18.5 years, p < 0.001) and lighter in body weight (63.1 kg versus 78.7 kg, P < 0.001; BMI 22.8 kg/m² versus 24.9 kg/m², p < 0.001) than the male participants. No significant difference was noted in weight-bearing foot posture (standing AHI and MVI) between genders, see Table 2. Female participants exhibited significantly greater AHF, larger arch drop, and lower CPEI but no significant difference in foot elongation than male participants.

Table 2:

Measures of Foot Structure, Flexibility, and Function for Total Sample and Comparisons of Measures across Gender.

Variable	All (N=2,162 feet)		Male (N=1,810 feet; 83.7%)		Female (N=352 feet; 16.3%)		x2	p-value
(unit)	Mean	SE	Mean	SE	Mean	SE
AHI, sitting	0.353	0.001	0.350	0.001	0.355	0.002	4.34	0.037
AHI, standing	0.327	0.001	0.328	0.001	0.326	0.002	0.45	0.504
Arch Drop (cm)	0.475	0.008	0.445	0.006	0.505	0.014	14.79	0.000
ΔFL (cm)	0.376	0.008	0.384	0.006	0.368	0.014	1.00	0.317
AHF (mm/kn)	17.790	0.348	14.698	0.190	20.882	0.669	79.17	0.000
MVI (%)	10.63	0.222	10.65	0.167	10.62	0.411	0.01	0.942
CPEI (%)	20.89	0.233	22.48	0.179	19.30	0.430	46.83	0.000
PP (N/cm2)	57.90	0.647	59.08	0.533	56.72	1.179	3.32	0.068

Females comprised of 16.3% of the study participants. No significant difference in static foot posture was noted between gender as measured by standing AHI and MVI. Still, female participants demonstrated significantly greater AHF and smaller CPEI (over-pronation in gait). Given significantly greater arch drop, commensurate foot elongation was expected in female participants. However, female participants showed no significant difference in foot elongation (ΔFL) when compared to male participants.

Comparisons across 3 AHF categories are summarized in Table 3. Flexible feet displayed a significantly greater pronatory foot posture (lower standing AHI, greater MVI); greater arch drop and foot elongation; and reduced CPEI than the other two groups − all consistent with flexible pronatory foot structure and function. In contrast, feet with stiff arch demonstrated the largest standing AHI, the smallest arch drop, the smallest foot elongation, and the largest CPEI.

Table 3:

Measures of Foot Structure, Flexibility, and Function for Total Sample and Comparisons of Measures across AHF Categories.

Variable	All (N=2,162)		Flexible (N=436 feet)		Referent (N=1,308 feet)		Stiff (N=418 feet)		X2	p-value	Post hoc
(unit)	Mean	SE	Mean	SE	Mean	SE	Mean	SE			F v R	F v S	R v S
AHI, sitting	0.351	0.001	0.358	0.001	0.351	0.001	0.345	0.001	60.127	0.000	0.000	0.000	0.000
AHI, standing	0.327	0.001	0.320	0.001	0.327	0.001	0.334	0.001	90.618	0.000	0.000	0.000	0.000
Arch Drop (cm)	0.46	0.00	0.72	0.01	0.45	0.00	0.21	0.01	2187.19	0.000	0.000	0.000	0.000
ΔFL (cm)	0.39	0.01	0.46	0.01	0.37	0.01	0.33	0.01	60.94	0.000	0.000	0.000	0.001
AHF (mm/kn)	16.05	0.14	26.70	0.33	15.00	0.09	6.45	0.22	2661.69	0.000	0.000	0.000	0.000
MVI (%)	10.80	0.18	11.76	0.32	10.41	0.20	10.23	0.36	14.74	0.001	0.000	0.002	0.667
CPEI (%)	22.01	0.19	21.18	0.35	21.93	0.19	22.91	0.32	13.81	0.001	0.041	0.000	0.005
PP (N/cm2)	58.82	0.52	57.69	0.81	58.55	0.54	60.22	0.88	5.40	0.067

Feet are categorized based on AHF. Extreme quintiles are categorized as either flexible or stiff while the middle 3 quintiles are categorized as referent. Those feet with flexible arch showed the lowest standing AHI and the largest MVI (pronatory foot structure), the largest foot elongation (flexibility), and the smallest CPEI (pronatory foot function), follow by referent, and those feet with stiff AHF.

Measures of foot structure, function, and flexibility of the study participants are summarized across 3 foot types in Table 4, Fig. 1, Fig. 2. Pes planus demonstrated the most pronatory foot posture (reduced AHI and greater MVI), the largest AHF, and lower CPEI compared to rectus and cavus foot types. Cavus foot type showed the largest sitting AHI, smallest MVI, smallest arch drop, and stiffer AHF than the other groups. No significant differences were noted in CPEI (p = 0.051) and foot elongation (p = 0.117) between rectus and cavus foot types.

Table 4:

Measures of Foot Structure, Flexibility, and Function for Total Sample and Comparisons of Measures across 3 foot types.

Variable	All (N=2,180)		Planus (N=1,601 feet; 73.4%)		Rectus (N=443 feet; 20.3%)		Cavus (N=113 feet; 6.2%)		X2	P-Value	Post hoc
(unit)	Mean	SE	Mean	SE	Mean	SE	Mean	SE			P v R	P v C	R v C
AHI, sitting	0.369	0.001	0.342	0.001	0.370	0.001	0.394	0.002	1113.32	0.000	0.000	0.000	0.000
AHI, standing	0.347	0.001	0.317	0.001	0.349	0.001	0.375	0.001	2409.050	0.000	0.000	0.000	0.000
Arch Drop (cm)	0.41	0.01	0.48	0.01	0.41	0.01	0.36	0.21	54.12	0.000	0.000	0.000	0.017
ΔFL (cm)	0.41	0.01	0.37	0.01	0.41	0.01	0.45	0.03	17.93	0.000	0.000	0.001	0.117
AHF (mm/kn)	14.27	0.29	16.47	0.23	13.97	0.37	12.38	0.66	55.05	0.000	0.000	0.000	0.019
MVI (%)	9.66	0.24	11.10	0.18	9.74	0.34	8.13	0.61	31.89	0.000	0.000	0.000	0.021
CPEI (%)	22.74	0.23	21.59	0.19	22.78	0.31	23.86	0.51	23.68	0.000	0.000	0.000	0.051
PP (N/cm2)	57.99	0.60	59.13	0.53	57.38	0.77	57.47	1.19	4.96	0.084

Feet are categorized into planus, rectus, and cavus foot types based on mean standing AHI obtained from previous study [21]. Planus foot type showed significantly lowered standing AHI and greater MVI (pronatory foot posture), greater AHF, and smaller CPEI than rectus and cavus foot types. Significant difference was observed between rectus and cavus foot types, all except foot elongation and CPEI. Given significantly greater arch drop, commensurate foot elongation was expected in planus foot type. Surprisingly, planus feet demonstrated a significantly reduced foot elongation than rectus and cavus foot types.

Fig. 1.

Standing arch height index (AHI), arch height flexibility (AHF,mm/kn), and center of pressure excursion index (CPEI, %) are compared across (a) foot types, (b) AHF categories, (c) race, and (d) gender.

Groups with the same letter denote a significant difference. Comparison across foot type and arch flexibility categories showed that planus feet exhibited flexible arch and over-pronation in gait. Comparison across race and gender, however, demonstrated that over-pronation doesn’t always occur in feet with low arch.

Fig. 2.

Prevalence of Foot Types.

Prevalence of foot types is shown across (a) race and (b) gender. Black participants demonstrate a significantly greater prevalence of pes planus (92.0% versus 73.4% overall). No significant difference in prevalence of foot types is observed between genders.

Pearson correlation analysis showed a significant (p < 0.01) linear correlation between standing AHI and sitting AHI (r = 0.924); standing AHI and MVI (r = −0.196); standing AHI and AHF (r = −0.174); standing AHI and CPEI (r = 0.130); standing AHI and peak pressure (r = −0.013); and standing AHI and BMI (r = 0.085). No significant correlation was noted between standing AHI and age (p = 0.312) and between standing AHI and body weight (p = 0.539). A significant but weak correlation was noted between AHF and body weight (r = −0.373, p < 0.01); and AHF and BMI (r = −0.164; P < 0.01); AHF and foot elongation (r = 0.223, p < 0.01); AHF and CPEI (r = −0.106, p < 0.01); AHF and peak pressure (r = −0.090, p < 0.01), but not between AHF and age (p = 0.262).

5. Discussion

This cross-sectional study describes biomechanical foot characteristic of healthy at-risk cohort of USMA cadet at the beginning of their training. Comprehensive objective measures of foot, including AHF, distinguish this study from previous investigations. No statistically significant difference in static foot posture (standing AHI and MVI) was observed between genders, consistent with the findings of Levy et al. [11]. Yet, female participants demonstrated a significantly greater AHF and over-pronation. It is well-established that female military personnel are at higher risk for lower extremity injuries than males [10,11]. Additional studies are needed to explore the significance of AHF and pronatory foot function as relating to higher incidence of lower extremity injuries in female.

Nearly three-quarters of study volunteers demonstrated a planus foot type in this study, which is similar to a 74% prevalence in the Framingham Foot Study of older subjects [26]. Although different criteria are used to classify the foot types, both studies showed that planus foot type was associated with pronatory foot function (lower CPEI). This study, however, demonstrated that the pronatory foot posture does not always correlate with pronatory foot function. Specifically, over-pronation (reduced CPEI) was expected in Black participants, who displayed the lowest AHI and largest MVI. However, it was the Asian participants who displayed over-pronation in gait. Similarly, female participants showed over-pronation despite not having lowered AHI. This is contrary to the conventional belief that pronatory foot function would be associated with flatfoot.

Greater foot elongation is expected in those feet with flexible arch. Such association was observed in Asian participants and those with flexible arch category. Female participants and those with planus foot type failed to show such association. In fact, individuals with planus foot type showed a significantly reduced foot elongation when compared with rectus and cavus foot types even though they had the greatest AHF and arch drop. Reasons for this discrepancy are not clear. It is possible that these flexible planus feet may have compensated with subluxation of joints. Alternatively, these flexible feet may have abducted in the transverse plane instead of foot elongation in the sagittal plane. Unfortunately, this study quantified neither change in forefoot width nor joint subluxation from sitting to standing.

Bivariate linear correlation analysis showed a significant but weak correlation between static foot postural measures (AHI and MVI) as well as between static foot postural measures and AHF. Also, both AHI and MVI showed a significant but weak correlation with dynamic foot function (CPEI). This is consistent with findings of Teyhen et al., which showed a modest relationship between static and dynamic arch index [6]. While over-pronation was noted in female and Asian participants with greater AHF, there was only a weak inverse correlation between arch flexibility and dynamic foot function (CPEI). Multiple factors are likely associated with dynamic foot function [27]. Analysis of the same group of cohort by Zifchock et al. noted that planus foot types are more likely to exhibit flexible arch than cavus foot type while cavus foot type showing stiff AHF [28]. Findings of this study illustrate the complexity of foot biomechanics and further support for a need of comprehensive approach.

Due to time constraints, the standing AHI threshold (i.e., bisection of mean values for each foot type) [21] was used to categorize foot types, instead of the typical clinical goniometric measures of forefoot to rearfoot relationship, subtalar joint alignment, hindfoot alignment as well as the degrees of compensation. This study was also limited to a group of selected active healthy young individuals who completed a set of rigorous physical tests as a part of their entrance requirements. Consequently, the findings may be only generalizable to the physically active college-age participants.

This study provided a comprehensive characterization of at-risk healthy cohort at the start of rigorous military training. Based upon measures of foot structure, function, and arch flexibility in a large cohort of active healthy young individuals, important differences were noted for dependent variables including race, gender, arch flexibility and foot types. A majority (73.7%) of active healthy young physically fit participants demonstrated a pronatory foot structure and function. Long-term effects of these observed differences in the presence of lower extremity loading due to repetitive prolonged weight-bearing physical activity, typical to military training, remain unknown. Additional studies are needed to determine if specific injuries or foot disorders are associated with these dimensions of foot structure, function, and flexibility.