Foot arch height affects the effect of BMI on gait and lower-limb joint kinematics

Introduction

Obesity is a major public health concern worldwide and a complex disease involving excessive amounts of body fat. Obesity is a medical problem that increases the risk of other health problems such as heart disease, stroke, type-2 diabetes, osteoarthritis, and certain cancers that may cause premature death (Jensen et al., 2014). The prevalence of obesity in the United States is 42.4% in 2017-2018 among adults aged over 20 years old and has increased 12% over the past 20 years (Hales et al., 2020). To combat obesity, increasing energy expenditure via increasing physical activity level has been strongly recommended; physical activity promotes weight loss, prevents weight gain and regain, and can help maintain cardiovascular and metabolic health (Jensen et al., 2014). Walking is a common and cost-effective intervention used to increase overall physical activity and to meet the recommended 150 minutes of weekly moderate-to-vigorous physical activity (WHO, 2010). However, most obese adults fall short of these recommendations (Blanchard et al., 2005).

One major contributor to decreased physical activity in individuals with obesity includes impaired gait stability due to the excessive body weight with the greater girth of the thigh and trunk (Alonso et al., 2012). Walking requires coordinating motor actions specific to constraints such as body weight (Hung et al., 2013). Compared to adults with normal weight, adults who are classified as obese show differences in spatiotemporal gait parameters. They take shorter but wider steps by decreasing step length and increasing step width, walk more slowly by reducing velocity, and spend more time with their feet on the ground by reducing swing time as well as increasing double-limb support time (Forhan & Gill, 2013; Lai et al., 2008). Moreover, obesity affects gait kinematics and kinetics in adults. Obese adults, who have slower gait velocity than normal-weight adults, have greater knee abduction (McMillan et al., 2010; Peyrot, 2012) and greater absolute ground reaction forces (Browning & Kram, 2007) compared to normal-weight adults. These differences in walking, especially slower gait velocity, are attributed to obese adults’ attempt to increase stability because of impaired balance (Singh et al., 2009), to minimize mechanical external work (Malatesta et al., 2009), to decrease load at the knee (Browning & Kram, 2007), and to curb energy cost and relative effort (Peyrot et al., 2010). However, the differences in walking are actually associated with increased safety risks such as tripping (Gill & Narain, 2012).

Obese adults also tend to have lower arches or “flat feet” based on footprint and plantar pressure measures (Azarfam et al., 2014; Gravante et al., 2003; Wearing et al., 2006). Their feet tend to be less flexible during the propulsive phase of walking due to fallen arches (DeSilva & Gill, 2013; Walters & Mendicino, 2014). Individuals with lower arches based on weight-bearing static measurements are sometimes characterized as exhibiting a phenomenon known as excessive foot pronation during standing (Figure 1). Data on static weight-bearing foot position in obese adults suggests that those with overpronated feet in standing are more likely to develop foot pain (Butterworth et al., 2012) such as chronic plantar heel pain (Dicharry et al., 2009). The combination of differences in walking and arch height in obese adults is thought to contribute to musculoskeletal injuries due to soft tissue damage (Churchill & Sferra, 1998) such as posterior tibial tendon dysfunction (Timm et al., 2005), ankle sprains (Lai et al., 2008), and plantar fasciitis (Frey & Zamora, 2007). Obese adults also showed greater toe-out angle during walking than normal-weight adults (Lai et al., 2008; Messier et al., 1994).

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Figure 1.

Representative excessive foot pronation during standing.

Although spatiotemporal gait parameters, lower-limb joint kinematics, and arch height have been shown to differ in obese adults and to contribute to their increased injury risks, to our knowledge, no studies have examined the relationship between walking and arch height in these populations. That is, we have no knowledge to confirm whether both excessive weight and low arches together result in altered gait compared to the effects of either excessive weight or low arch height alone. Anthropometrics contribute to obesity-related gait characteristics (Lai et al., 2008), but only the effects of some weight-related anthropometrics have been examined. The purpose of the current study is to determine whether arch height affects obesity-related changes in gait using standard three-dimensional gait analysis. We hypothesized that foot arch height would affect gait characteristics in obese adults as measured by spatiotemporal gait parameters and lower-limb joint kinematics.

Methods

Arch Height Measures

We estimated participant’s arch height using two methods: digital footprints with a plantar pressure mat and navicular height measures. First, foot pressure was measured with a digital pressure mat (Tekscan Inc., South Boston, MA). The mat (488 mm × 447 mm) collected data vis 8,448 sensing elements at 185Hz. When participants stand barefoot on the right leg on the digital foot pressure mat, Tekscan software locates peak pressure distributions from each sensor to create a digital footprint (Figure 2). These digital footprints estimated arch height using the Chippaux-Smirak Index (CSI) (Fascione et al., 2012; Mathieson et al., 2004). The CSI is the ratio between the midfoot's smallest width and the largest width of the metatarsal head area, CSI = B/A 100 (Figure 2b). The CSI has commonly been used to measure the arch height (Gill et al., 2014; Queen et al., 2007), which is correlated with skeletal measures of arch height, such as the navicular height (Forriol & Pascual, 1990; Onodera et al., 2008).

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Figure 2.

Example of digital foot pressure data.

Results

BMI Classification, Arch Height, and Gait Kinematics

Results from the ANCOVA showed that the covariate, CSI, was significantly related to the participant’s step width (F(1,43)=20.15, p<0.01, ηp²=0.33) and double-limb support time (F(1,43)=8.75, p<0.01, ηp²=0.18; Table 2). CSI was also significantly associated with knee extension angle (F(1,43)=4.79, p=0.03, ηp²=0.12) and abduction angles (F(1,43)=5.89, p=0.01, ηp²=0.21; Table 3). Therefore, step width, double-limb support time, knee extension, and knee abduction angles were influenced by CSI. However, with the effect of CSI removed, BMI classification alone could not predict step width, double-limb support time, knee extension angle, and abduction angle (ps>0.05; Table 2 & 3). Although CSI did not predict, BMI classification predicted knee adduction and internal rotation, and hip internal- and external-rotation angles in the absence of CSI (ps<0.01; Table 3).

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Table 2.

F-values (significance) for main effects of BMI on spatiotemporal gait parameters, with and without controlling for arch height

			with Controlling for CSI
Gait Parameters	BMI	CSI	BMI
Gait Velocity (m/s)	6.26 (<0.01)	3.21 (0.08)	0.41 (0.67)
Stride Length (m)	1.61 (0.21)	1.70 (0.20)	0.10 (0.91)
Step Length (m)	0.19 (0.83)	1.96 (0.17)	0.48 (0.62)
Step Width (m)	4.29 (0.02)	20.15 (<0.01)**	4.69 (0.01)
Single-limb Support Time (s)	2.91 (0.07)	0.02 (0.89)	0.89 (0.42)
Double-limb Support Time (s)	17.33 (<0.01)**	8.75 (<0.01)**	0.29 (0.75)

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Table 3.

F-values (significance) for main effects of BMI on foot progression angle and lower-limb joint angles, with and without controlling for arch height

				with Controlling for CSI
Joint	Angle (°)	BMI	CSI	BMI
Ankle	Toe-Out	9.54 (<0.01)**	3.96 (0.06)	2.74 (0.08)
	Plantar-Flexion	1.26 (0.29)	1.98 (0.17)	0.74 (0.49)
	Dorsi-Flexion	1.52 (0.23)	0.53 (0.47)	0.40 (0.68)
	Inversion	1.98 (0.15)	3.23 (0.08)	0.92 (0.41)
	Eversion	1.57 (0.22)	0.89 (0.35)	1.61 (0.21)
	Adduction	0.12 (0.89)	0.28 (0.60)	0.26 (0.78)
	Abduction	0.13 (0.88)	0.16 (0.69)	0.01 (0.99)
Knee	Flexion	0.58 (0.57)	0.04 (0.83)	2.85 (0.07)
	Extension	14.92 (<0.01)**	4.79 (0.03)*	3.31 (0.08)
	Adduction	20.13 (<0.01)**	0.22 (0.64)	8.98 (<0.01)**
	Abduction	31.21 (<0.01)**	5.89 (0.01)*	3.45 (0.09)
	Internal-Rotation	7.91 (<0.01)**	2.35 (0.13)	6.84(<0.01)**
	External-Rotation	1.98 (0.15)	1.77 (0.13)	1.75 (0.19)
Hip	Flexion	2.18 (0.13)	1.69 (0.20)	3.04 (0.06)
	Extension	1.96 (0.15)	1.73 (0.18)	0.49 (0.49)
	Adduction	4.08 (0.02)*	0.04 (0.85)	1.24 (0.30)
	Abduction	1.66 (0.20)	0.30 (0.59)	0.95 (0.40)
	Internal-Rotation	8.05 (<0.01)**	1.71 (0.20)	7.84 (<0.01)**
	External-Rotation	12.60 (<0.01)**	0.07 (0.80)	5.58 (<0.01)**

Participants with higher BMI scores demonstrated a relationship between arch height and gait parameters and lower-limb joint angles. As illustrated in Figure 3, Pearson’s correlations indicate that participants with higher BMI scores who had lower arches had a higher toe-out angle, higher gait velocity, wider step width, longer double-limb support time, higher knee extension angle, and higher knee abduction angle than participants with lower BMI scores (ps<0.01).

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Figure 3.

Relationships between lower-limb joint kinematic measurements and BMI.

Discussion

The purpose of this study was to determine whether arch height would be a mediator of obesity-related characteristics in walking among adults with different BMI classifications. Our findings revealed that obese adults had lower arches than normal weight and moderately obese adults. Arch height predicted step width, double-limb support time, and knee joint extension and abduction angles. Participants with severe obesity who have pronated feet exhibit a more cautious gait by walking more slowly with greater toe-out angles, wider steps, longer double support periods, and greater knee joint extension and abduction angles.

Our findings that arch height affects knee joint kinematics in the obese population suggest that future investigations on the direct relationship between BMI classification and musculoskeletal injury should include examining the arch height and musculoskeletal injury risk or osteoarthritis during walking in the obese population. These results are essential because no previous studies have focused on the effect of arch height on lower-limb joint kinematics during walking in the obese population. However, conditions that coexist with obesity and flat feet can cause a misalignment in the lower body, subsequent damage to the joints over time, and a devastating impact on postural stability. It is unclear whether lower arch height observed in adults with obesity represents musculoskeletal problems. However, given our findings, the effect of arch height on changes in knee joint range of motion was evident in individuals with obesity, suggesting that relationships between arch height and knee mechanics may increase the risk of musculoskeletal injuries such as knee osteoarthritis and subsequent instability during walking, which may cause fall risks.

This study is unique in examining the relationship between arch height and gait kinematics in adults with varied BMI classifications. In particular, our findings highlighted relationships between arch height and lower-limb joint kinematics in obese adults. Our results suggest that body mass and gait alterations that increase biomechanical stability during walking (e.g., increasing step width and double-limb support time) are associated with lower arches for obese adults. Lower arches lead to excessive knee extension and abduction during the heel-strike phase, which provides an insufficient propulsive force that enables faster steps with limited foot contact on the ground (Yan et al., 2013). Although a lower arch via a wider footprint may increase balance, a lower arch may be less able to provide a forceful push off during walking.

Our results have practical implications for aiding increased physical activity for obese adults and for gaining a better understanding of how arch height may affect gait kinematics based on BMI classification. In particular, these findings suggest that intervening in obese adults’ arch height may be necessary for supporting increased walking, a common and cost-effective method for increasing physical activity. Adults who are obese and who have lower arches may be more likely to respond to activity modification if provided with increased arch support. For example, orthotics designed to induce higher arches may facilitate physical activity for overweight or moderately obese individuals and prevent them from transitioning to becoming severely obese. The current findings are congruent with research suggesting that it is critical to address factors that contribute to walking instability in the obese population to increase participation in physical activity (Hulens et al., 2003). These findings, most importantly, provide insight into how increased physical activity through walking can be supported in adults with obese BMI scores.

One limitation of the current study includes using BMI to determine body composition and subsequent classification. We chose BMI to categorize our groups because it has been used for the same purposes in previous studies. Other methods for determining body composition and classification, such as body fat percentage, can yield more precise information about individuals’ weight status. However, to ensure proper reliability, sophisticated equipment is usually required, whereas BMI only requires height and weight measurements. Second, we recruited less participants with class 3 obesity than participants with normal-weight and class 1 & 2 obesity due to the lower participation rate. We acknowledge that the presence of unequal variance between groups might have resulted in an incomplete comparison between groups. A third limitation includes that we did not capture non-weight bearing arch height in participants. This did not allow us to capture a measure of foot flexibility. However, the focus of the current study was on foot structure during weight-bearing activity. A fourth limitation involves testing direct links between spatiotemporal gait measures, lower-limb joint kinematics, and arch height. With this being one of only a few studies to test relationships between spatiotemporal gait, lower-limb joint motion, and arch height in obese adults, we would expect questions to arise from the findings, which would lead to future studies. Future studies that use different methodologies (e.g., recruiting subjects who have similar BMI scores but different arch height) are needed to investigate direct links between changes in joint kinematics and kinetics during walking mediated by arch height in obese individuals.

Conclusion

In conclusion, the findings suggest that arch height affects obesity-related changes in spatiotemporal gait parameters and knee joint kinematics. The data reported here are the first to find that kinematic changes in lower-limb joint associated with excessive weight may be contingent on foot anatomy and, more specifically, the height of the foot arch.