Foot Type Biomechanics Part 1: Structure and Function of the Asymptomatic Foot (2024)

Background

Differences in foot structure are thought to be associated with differences in foot function during movement. Many foot pathologies are of a biomechanical nature and often associated with foot type. Fundamental to the understanding of foot pathomechanics is the question: do different foot types have distinctly different structure and function?

Aim

To determine if objective measures of foot structure and function differ between planus, rectus and cavus foot types in asymptomatic individuals.

Methods

Sixty-one asymptomatic healthy adults between 18 and 77 years old, that had the same foot type bilaterally (44 planus feet, 54 rectus feet, and 24 cavus feet), were recruited. Structural and functional measurements were taken using custom equipment, an emed-x plantar pressure measuring device, a GaitMatII gait pattern measurement system, and a goniometer. Generalized Estimation Equation modeling was employed to determine if each dependent variable of foot structure and function was significantly different across foot type while accounting for potential dependencies between sides. Post hoc testing was performed to assess pairwise comparisons.

Results

Several measures of foot structure (malleolar valgus index and arch height index) were significantly different between foot types. Gait pattern parameters were invariant across foot types. Peak pressure, maximum force, pressure-time-integral, force-time-integral and contact area were significantly different in several medial forefoot and arch locations between foot types. Planus feet exhibited significantly different center of pressure excursion indices compared to rectus and cavus feet.

Conclusions

Planus, rectus and cavus feet exhibited significantly different measures of foot structure and function.

2.1 Subject Recruitment

Sixty-one asymptomatic healthy adults, that had the same foot type bilaterally, were recruited for enrollment into this investigation. Subjects were between 18 and 77 years old, had no current symptoms of pain, had no foot or ankle pathology, and were able to ambulate independently. Individuals with neuromusculoskeletal disease, uncontrolled cardiovascular disease, or lower extremity surgery within the past year were excluded. Each foot of each participant was categorized by clinical exam into a foot type group: planus, rectus or cavus. Resting calcaneal stance position (RCSP) and forefoot to rearfoot relationship (FF-RF) based criteria were used to classify foot types [5]:

• planus: RCSP ≥ 4° valgus OR FF-RF ≥ 4° varus

• rectus: 0° ≤ RCSP ≤ 2° valgus AND 0° ≤ FF-RF ≤ 4° varus

• cavus: RCSP ≥ 0° varus AND FF-RF ≥ 1°valgus

RCSP and FF-RF were measured with a 1° resolution goniometer.

2.2 Measures of Foot Structure

Each measurement was taken once per foot. To avoid the issue of inter-rater reliability one rater (HJH) measured all foot structure parameters.

Malleolar valgus index, MVI (%), is a measure of standing hind foot alignment (Fig.1a) ^[8]. MVI is the deviation of the transmalleolar midpoint relative to the longitudinal foot bisection, normalized to ankle width.

Arch height index, AHI (%), is a measure of the dorsal arch height normalized to foot length (Fig 1b). Maximum foot length, truncated foot length (heel to 1^st metatarsophalangeal joint), and arch height at one half of foot length were measured on each foot while standing. AHI was calculated as the ratio of arch height to truncated foot length expressed as a percent in both sitting and standing postures [9].

Arch height flexibility, AHF (mm/kN), is a measure of the change in arch height between sitting and standing conditions, normalized to change in load, estimated to be 40% of body weight.^[9] AHF was calculated as follows:

2.3 Measures of Foot Function

Foot function incorporated plantar loading and gait pattern parameters. Five acceptable trials per foot were collected for each measure while subjects walked at their self-selected speed. The means of the five trials were used for statistical analysis.

2.4. Statistical Analysis

The hypotheses were formulated to assess if foot structure (MVI, AHI_sitting, AHI_standing, AHF) and foot function (plantar loading and gait pattern parameters) were significantly different across foot type (planus, rectus, and cavus). All descriptive and comparative statistical analysis was performed using SPSS software (IBM, Chicago, IL, USA). Generalized Estimation Equation (GEE) modeling was employed with a linear identity link function to determine if each dependent variable of foot structure and function was significantly different across foot type. The GEE model permitted the inclusion of bilateral data (122 feet) by accounting for the covariance between potentially dependent measures (right and left feet). The general Chi-square (χ²) was calculated for each dependent variable with significance set at p<0.05. If significant, a standard least significant difference (LSD) post hoc test was performed with Bonferroni correction (p<0.0167) to account for multiple comparisons between the three foot types.

3.1 Test subject demographics

Sixty-one test subjects were enrolled in this study with the following characteristics:

• planus - 22 subjects, 44 feet, 12 females, height =1.7±0.1 m, weight=675.4±146.9 N, BMI=23.3±4.3 kg/m², age=35.6±11.0 years

• rectus - 27 subjects, 54 feet, 19 females, height =1.7±0.1 m, weight=662.6±138.7 N, BMI=24.4±4.1 kg/m², age=33.1±9.8 years

• cavus - 12 subjects, 24 feet, 6 female, height =1.7±0.1 m, weight=720.8±155.0 N, BMI=24.0±3.5 kg/m², age=42.8±16.2 years

The subjects in each group had similar characteristics with the exception that individuals with cavus feet were 7-9 years older than those with rectus and planus feet (p=0.004).

3.2 Measures of Foot Structure

MVI, AHI_sitting, and AHI_standing were significantly different across foot type (Table Ia). Post hoc analysis revealed that AHI_sitting and AHI_standing parameters were significantly different between planus, rectus, and cavus feet. Also, MVI distinguished planus from rectus and cavus foot types. There were no significant differences in AHF across foot types.

3.3 Measures of Foot Function

Table Ib shows the absolute gait pattern parameter values were invariant across foot types. When normalized by gait cycle time, double support and stance times were significantly different across foot type. Post hoc analysis revealed a statistically significant difference between planus and cavus feet for both parameters. The minimum clinically important difference (MCID) has been set at 15% by the investigators and none of the normalized gait pattern parameters exceed this threshold.

Peak pressure distribution across foot types is shown in Figure 2. CPEI, PP, and MF parameters are summarized in Tables IIa and IIb. The following parameters were found to be significantly different across foot types: CPEI, hallucial peak pressure (PP-hallux), normalized PP-hallux, PP-toe2, PP-metatarsal head (MH) 2, normalized PPMH2, MF-hallux, normalized MF-hallux, MF-toe2, normalized MF-toe2, normalized MF-MH1, normalized MF-MH5, MF-medial arch, and normalized MF-medial arch. The post hoc analyses for each significant parameter are shown in Tables IIa-b.

PTI, FTI and area parameters are summarized for each masked region in Tables IIIa-c. The following parameters were significantly different across foot types: PTI-hallux, normalized PTI-MH5, normalized PTI-medial arch, FTI-hallux, normalized FTI-hallux, FTI-MH4, normalized FTI-MH5, FTI-medial arch, area-toe2, normalized area-MH4, area-MH5, normalized area-MH5, area-medial arch, and normalized area-medial arch. The post hoc analyses for each significant parameter are shown in Tables IIIa-c.

4.1 Why study foot types?

Given that individuals with excessive pronation (planus) or supination (cavus) tend to be more vulnerable to certain pathologies, an understanding of the biomechanics associated with different foot types is warranted [13]. In military and athletic populations, cavus and planus foot types are associated with increased risk of tibial stress injuries as compared to rectus foot type [6, 14]. Individuals with diabetes and peripheral polyneuropathy are at risk for developing neuropathic foot ulcers. Diabetic patients with planus feet tend to ulcerate in the medial forefoot, while those with cavus feet ulcerate in the lateral forefoot [15]. This study was conducted 1) to reveal the structural and functional differences in asymptomatic individuals with planus, rectus, and cavus foot types and 2) to serve as a basis for future pathomechanical investigations.

4.2 Foot type recruitment

Based on our cohort, the prevalence of each foot type may not be equal among the general population. The greater percentage of planus (36%) and rectus feet (44%) in this study may reflect the higher prevalence of these foot types in the asymptomatic population; cavus feet (20%) were relatively rare and therefore most difficult to recruit. In a study of 398 individuals with diabetes, 51.5% were rectus, 29% were planus and 19.5% were cavus.[13] While our findings were similar, it is not clear whether foot type prevalence associated with other pathologies may be different than that of asymptomatic healthy individuals.

4.3 Foot Structure

Several structural parameters (MVI, AHI_sitting, and AHI_standing) were sensitive to foot type. Both AHI_sitting and AHI_standing parameters distinguished planus, rectus and cavus feet. These parameters were obtained using a simple and reliable mechanical device [16]. Although there were no differences across foot types, AHF was clinically interesting. Flexible pathologic feet may be treated with foot orthoses, but rigid feet may require surgical reconstruction. From this data, AHF was not a function of foot type and there was an equal likelihood of having a flexible or stiff arch for planus, rectus, or cavus feet.

MVI could distinguish planus from rectus and cavus feet. This result corroborates the findings of Song et al. where asymptomatic planus and rectus feet exhibited a significant difference in hindfoot alignment [8]; however, MVI could not discriminate between cavus and rectus feet. This was expected because MVI is conceptually like RCSP, which was not different between asymptomatic cavus and rectus feet. In severely inverted pathologic cavus feet, RCSP and MVI may be different than in healthy rectus and cavus feet. Although MVI measurements require specialized equipment and software they have demonstrated high reliability in a previous investigation [8].

In light of this information, the first hypothesis, measures of foot structure (MVI, AHI_sitting, AHI_standing, and AHF) will be different across foot types, was accepted.

4.4 Foot function

None of the temporal-spatial parameters during gait were significantly different across foot type. Normalized stance and double support times were statistically, but not clinically, significantly different. Hence, gait pattern was invariant to foot type.

The following functional parameters were sensitive to foot type: CPEI, PP, MF, PTI, FTI, and area. In other studies, functional measures (CPEI, PP, MF) have demonstrated excellent test-retest and between-day reliability [8, 17]. Our data show some foot function parameters with substantial variability (Tables III and IV). This may be due to the inherent anatomical complexity of the foot and the lack of precise control humans have over its biomechanical behavior. This was accounted for by averaging each functional variable over 5 trials.

Most differences in foot function were found between planus and cavus feet. One exception was CPEI, where planus feet were described by lower CPEI values (indicating a less concave COP trajectory and medially directed GRF) compared to both rectus and cavus feet. Rectus and cavus feet could not be separated using CPEI. This may have resulted from excluding pathologic cavus feet which display a more severe concave and lateral COP trajectory than rectus and asymptomatic cavus feet.

For plantar loading parameters, planus feet had higher medial pressures and forces than cavus feet. This was expected because of the difference in COP trajectory between these two foot types.

Many individuals with planus feet can have excessive dorsal translation of the first ray (i.e. medial cuneiform, 1^st metatarsal, and hallux) [18]. This phenomenon, referred to as a ‘hypermobile first ray’, is supported by the findings that planus feet had lower loads in MH1 than MH2. As an individual with planus feet and a hypermobile 1^st ray walks, the 1^st metatarsal elevates during stance, leaving MH2 to bear a larger load. These data suggest that 1^st ray hypermobility may be present in individuals with planus feet, even if they are asymptomatic. Although 1^st ray mobility was not directly measured in this study, 1^st ray hypermobility in planus feet was expected, given the degree of forefoot varus alignment.

In this study of asymptomatic individuals, peak load and load-time parameters were higher in planus and cavus feet compared with rectus feet. Thus, foot type should be considered as a covariate in studies of pathologic feet. For example, in a diabetic population, the combination of foot type and loading parameters may be important factors in the development of plantar ulceration.

Based on this information, the second hypothesis, measures of foot function will be different across foot types, was accepted for plantar loading but not for gait pattern parameters.

4.5 Future work

Future studies are needed to evaluate the effect of foot type, structure, and function on foot pathologies, such as osteoarthritis and diabetes, in comparison to this normative data set to better understand their origin and progression. These tools may also be used to evaluate and develop conservative and surgical treatments to alleviate symptoms and improve foot function through basic modifications of foot structure.

4.6 Limitations

The unequal number of subjects in subgroups (primarily a result of fewer cavus feet) is a potential limitation as it may lead to unequal group variances. However, the variances in this study were not different for the vast majority of parameters.

4.7 Significance

This is the first study to identify structural and functional parameters that are able to distinguish planus, rectus, and cavus foot types in asymptomatic individuals spanning a wide age range. Furthermore, these measures are noninvasive and do not require ionizing radiation to distinguish planus, rectus, and cavus feet. Foot structure parameters can be cost-effectively measured within a clinician's office with basic instrumentation. Plantar loading parameters require a plantar pressure measuring device although there are several commercially-available systems which could be used in a clinician's office.

Highlights I.

• We investigated the structure and function of different foot types.

• 61 healthy individuals with 44 planus, 54 rectus, and 24 cavus feet were recruited.

• Measures of foot structure and function were different across foot types.

• Gait pattern parameters were invariant across foot types.

• Foot type should be used as a covariate in studies of lower extremity function.

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