Plantar pressure differences between obese and non-obese adults: a biomechanical analysis

Abstract

OBJECTIVE: To investigate plantar pressure differences between obese and non-obese adults during standing and walking protocols using a pressure distribution platform.

SUBJECTS: Thirty-five males (age 42.4±10.8 y; 67–179 kg) and 35 females (age 40.0±12.6 y; 46–150 kg) divided into obese (body mass index (BMI) 38.75±5.97 kg/m²) and non-obese (BMI 24.28±3.00 kg/m²) sub-groups, respectively.

MEASUREMENTS: Data collection was performed with a capacitive pressure distribution platform with a resolution of 2 sensors/cm² (Emed F01, Novel GmbH, München). The measurement protocol included half and full body weight standing on the left, right and both feet, respectively, and walking across the platform, striking with the right foot. Pressures were evaluated for eight anatomical sites under the feet.

RESULTS: For both men and women, the mean pressure values of the obese were higher under all anatomical landmarks during half body weight standing. Significant increases in pressure were found under the heel, mid-foot and metatarsal heads II and IV for men and III and IV for women. Foot width during standing was also significantly increased in obese subjects. For walking, significantly higher peak pressures were also found in both obese males and females.

CONCLUSION: Compared to a non-obese group, obese subjects showed increased forefoot width and higher plantar pressures during standing and walking. The greatest effect of body weight on higher peak pressures in the obese was found under the longitudinal arch of the foot and under the metatarsal heads. The higher pressures for obese women compared to obese men during static weight bearing (standing) may be the result of reduced strength of the ligaments of the foot.

Introduction

Subjective references are often made to the physical limitations, including movement difficulties, of the obese. Problems commonly cited include general discomfort in simple activities of daily living such as walking and stair-climbing, pain in the joints of the lower extremity, poor circulation including oedema, and soreness or numbness in the feet, particularly following periods of standing.

In a normal weight individual, the major joints of the lower extremity are exposed to reaction forces of approximately three to six times body weight during locomotion (single leg stance phase).^1,2 It may be reasonable to hypothesise that obese individuals experience greater absolute loads at these joints than individuals of normal weight.³

Persistent loading of the musculoskeletal system of the obese has been implicated in predisposition to pathological gait patterns, loss of mobility and subsequent progression of disability,³ to a range of orthopaedic conditions that include knee osteoarthritis^4,5,6,7,8 and diabetic foot pathology.⁹ Relatively few studies have considered issues related to biomechanics^3,10,11 and joints other than the knee.

A small number of studies have addressed the gait characteristics of the obese. These include studies of children during walking at different speeds using temporal, kinematic and electromyographic analyses.^12,13,14,15 Compared with normal weight children, obese subjects in these studies displayed asymmetry in temporal characteristics, particularly at the slow speed of walking, and a wider stance width. Flatfootedness and sub-talar pronation that contributed to a degree of out-toeing, particularly during the swing phase of gait, was also prevalent in the obese.

A gait analysis of obese middle-aged adults¹⁶ reported similar temporal and kinematic differences between obese and normal weight individuals to those found for children. The slower walking velocity and greater stride width are suggestive of a more tentative gait in the obese and adjustments in the base of the support due to both the size of the thighs and an attempt to maintain balance. The work of Messier et al¹⁷ also identified features that adversely affect gait in obese subjects. Independent of other co-morbid diseases, rear-foot movement variables and mean dynamic foot angles were significantly altered in a severely obese group (mean body mass index (BMI) of approximately 41.14 kg/m²).

In summary, relatively little research has addressed the movement capabilities and effects of loading on the musculoskeletal system during common weight-bearing tasks.^9,11,18 The present study is the first to present data on the peak pressures under the feet of obese adults who completed a protocol that utilised the basic locomotor tasks of standing and walking. The results provide an objective summary of the foot mechanics of this population compared with normal weight individuals. Potential physical consequences of increased or decreased pressures under the feet, including the prevention of pain and disability in the major structures of the lower extremity, are discussed.

Methods

Subjects

Thirty-five males and 35 females with a mean age of 41.2 y (s.d. 11.7) recruited from the greater Brisbane metropolitan area participated in this study. In both groups, non-obese and obese were included. For the males, BMI ranged from 20.2 to 55.8 and for the females from 17.1 to 48.4. A BMI of greater than or equal to 30 kg/m² was used to classify individuals as obese. Using this criterion, 17 men and 18 women were categorised as obese. All subjects were otherwise healthy with no locomotor limitation such as symptomatic osteoarthritis. The nature and purpose of the study were carefully explained to each prospective subject and a comprehensive subject information package provided at an initial familiarisation session prior to the signing of an informed consent. The Human Research Ethics Committee of the Queensland University of Technology approved the experimental protocol.

Procedures

Body weight, body height and foot dimensions were measured for each subject. Foot length was determined from the center of the heel to the anterior aspect of the second toe. Using a caliper, foot width was determined across the metatarsal break. Pressure data collection was performed using a capacitive pressure distribution platform with 1344 sensors, exhibiting a resolution of 2 sensors/cm² (Emed F01, Novel GmbH, München). Data were collected at a frame rate of 20 Hz.

All subjects were measured in two different experimental conditions. Data from the right foot were collected during barefoot standing (half body weight) as well as during walking across the platform and striking with the right foot. In the upright standing condition, subjects were asked to stand with their right foot (preferred foot in each case) on the pressure platform. The left foot was positioned on the surface of a wooden walkway, in which the pressure plate was mounted. The experimenter asked the subjects whether they felt stable and if the response was ‘yes’, data collection was triggered. This procedure also served to make the subjects aware of data collection and avoid large body sway amplitudes. The summed pressure values from standing represent the force on the pressure platform as calculated from the collected data. The force during standing was displayed on the screen of the Emed pressure plate system. Standing trials were only accepted when there was less than 5% deviation from half body weight.

Because variability in plantar pressures is higher during walking, five repetitive trials were performed in this experimental condition. All trials were undertaken barefoot. Subjects walked at a slow but controlled pace across the pressure platform, using two steps before striking the platform. A metronome was used to monitor subjects during an earlier familiarisation session (75 signals/min) and this was used as the baseline to achieve similar between-subject walking speeds. If an obvious deviation from the required walking speed was observed the trial was repeated. It has previously been shown that changes in walking speed have only a small effect on the peak plantar pressures during walking. Clarke¹⁹ found that, even with a significant increase in gait speed from 1.33 to 1.79 m/s, peak pressure values across all foot regions only increased by 7.2%. Systematic group changes in the gait pattern are unlikely because through visual inspection the experimenter instructed the subjects to adopt a similar speed. Consistency of speed was also determined by the time information from the pressure data. Ground contact times were measured and evaluated from the pressure data and no significant differences in ground contact times between the groups were found.

Vertical ground reaction forces in walking can be determined from the summed pressures, however limb dominance cannot be determined. Because the vertical ground reaction force is primarily determined by the subject's body weight, limb dominance is likely to play a minor role during steady walking. McCrory et al²⁰ found that, following hip arthroplasty, patients did not show any differences in vertical ground reaction force parameters between limbs, although limping was observed. In the current study pressures during standing and walking were evaluated for eight anatomical foot locations (heel, mid-foot, metatarsal heads I–V, hallux). Due to large differences between individual foot shapes an automatic identification of anatomical foot landmarks on the basis of a geometric algorithm was found to be unreliable. Therefore, a visual inspection of the foot-shaped peak pressure image output was chosen to identify the specific anatomical locations under the foot. From the identified sensors, pressure time data were evaluated.

Pressure distribution

The peak pressure analysis reveals information about the highest pressures under the foot as they occur at any point during foot contact. Not only the momentary loading of foot structures was analysed but also the pressure development over time was obtained. Regional impulses were calculated by determining the local force-time integral (force (F)=pressure×area) under a specific anatomical region. These impulse values were taken to further calculate a relative impulse distribution under the foot. The following equation was used to determine the relative load RL_i for foot region i.

This procedure allows a comparison of the load distribution pattern between individuals that is independent of the subjects' weight. Therefore, a better understanding of the load-bearing role of individual anatomical structures can be obtained by the relative load analysis.

Statistical analysis

Data were analysed using a two-way ANOVA and regression analyses. The repeated-measures factor used was the anatomical location under the foot and the independent factor was weight category. The post-hoc analysis for individual comparisons was performed using a Fisher's PLSD test. Differences were considered to be significant for P<0.05 and highly significant for P<0.01.

Results

Obese and non-obese groups demonstrated significantly different plantar pressures during both standing and walking protocols.

There were no significant differences between obese and non-obese groups of either gender for body height or foot length. However, foot width was significantly greater for groups of both male and female obese subjects (Table 1). The highly significant increased peak pressures for standing are displayed in Figure 1. Differences were pronounced under the heel and mid-foot plus various regions under the forefoot. For example, under the mid-foot, obese women displayed 7.7 times and obese men 3.1 times higher pressures than respective control groups.

Table 1 Characteristics of obese (O) and non-obese (N) subjects

Figure 1

Plantar pressures (kPa) during standing for obese (O) vs non-obese (N) men and women. *P<0.05; **P<0.01.

Figure 2 shows the peak pressure differences between the obese and non-obese male and female groups during walking. Significant increased peak pressures can be observed under most anatomical regions for the obese groups. Consistent with the standing posture, the largest pressure differences were found under the mid-foot. Further, a linear regression analysis for the 35 women revealed a determination coefficient of 0.66 between BMI and the peak pressures during walking (Figure 3).

Figure 2

Peak plantar pressures (kPa) during walking for obese (O) vs non-obese (N) men and women.

Figure 3

Relationship between mid-foot peak pressures and BMI for 35 obese women during walking.

Discussion

A small number of studies have considered aspects of gait in otherwise healthy obese children^12,13,14,15 and adults¹⁶ and obese older adults with osteoarthritis.³ In addition, healthy normal-weight toddlers,²¹ school children^11,22 and adults²³ have been assessed using various analyses of pressure distribution. Another plantar pressure study investigated healthy normal-weight adults carrying various additional weights.⁹ Earlier work of Hennig and Milani²³ and Clarke and Cavanagh²⁴ with normal-weight adults found low correlations between body weight and peak pressures under the feet. These studies did not consider peak pressures across a range of anatomical locations and attributed the lack of relationship between weight and peak pressures to either increased foot contact area during the stance phase of gait or the distribution of high loads to larger anatomical areas of the foot. Contrary to the findings in adults, body weight was a major factor in the magnitude of pressures under the feet of school children aged between 6 and 10 y.²²

The present investigation is the first to study the pressure distribution under the feet of obese adults whilst standing and walking. It is important to acknowledge that numerous factors influence the execution of these movement tasks of daily living and as a consequence, considerable individual variability exists. Despite this variability, otherwise healthy obese men and women in the present study displayed higher mean pressure values than the non-obese under all anatomical landmarks during half body weight standing. Significantly higher pressures were evident under the heel, mid-foot and metatarsal region in the obese and these differences were consistent with a greater foot width in the heavier subjects. Whilst not employing the technology used in the current study, the earlier work of Smahel,²⁵ using a footprint analysis technique, found increased contact areas in the mid-foot region for adults with higher body weight. In a more recent study⁹ of 19 adults whose weight was adjusted by adding known weights to a vest, similar results were found. In this study there was a significant increases in mean peak plantar foot pressures under the same anatomical landmarks for each incremental increase in weight (9.1 and 18.2 kg, respectively) beyond mean BMI values of 24.9 and 23.2 for males and females.

The results of the present study provide important objective information regarding functional limitations specific to foot mechanics in both static (standing) and dynamic (walking) situations. Given the marked differences in plantar pressures as a function of increased adiposity (BMI), it is interesting to speculate on the structural consequences of repetitive loading on the feet and other parts of the lower extremity.

The physiological manifestations of loading may be reflected in self-reported pain, soreness or discomfort in the lower extremity. For example, plantar heel pain is commonly characterised by pain and tenderness on the calcaneal tuberosity at the point of attachment of the plantar fascia.²⁶ Although the specific aetiology of plantar heel pain is unknown, a number of studies^26,27,28,29 have reported that increased body weight is an associated factor.

Messier et al³ have suggested that musculoskeletal pain in the lower extremity may cause people to alter their gait pattern in an attempt to avoid or minimise discomfort. Further, McGoey et al³⁰ reported that chronic musculoskeletal pain is common in morbidly obese individuals and that weight loss can lead to significant relief in self-reported pain. Other studies^9,31 have reported decreased sensitivity in the feet of diabetics, skin breakdown and a predisposition to foot ulceration associated with neuropathy in individuals with higher static pressures under the feet. Ease of walking, as determined by discomfort experienced in the feet, may be a major limiting factor in the predisposition of the obese (including diabetics who are obese) to participate in habitual physical activity such as walking.

Whilst not a component of the present study, pressure levels in this population may also be influenced by the type of footwear worn. The quality of the footwear used by an obese individual may be a further important factor in preparedness to be active, comfort during and following movement, plus predisposition to injury. An association between the nature of the shoe (for example, hardness of the sole) and pain, plus the ability to adjust running performance, has also been reported.³² These findings may also have implications for the obese.

Obesity has also been well established as a risk factor for knee osteoarthritis.^7,10,33 By comparison, little or no attention has been given to the influence of body weight on the foot and ankle. Hartz et al³⁴ suggested that mechanical stress resulting from obesity was the principal cause for the strong link between obesity and osteoarthritis. The mechanisms by which overweight may cause osteoarthritis might include local increased forces across the joint plus an as yet undetermined systemic factor.² The contrasting viewpoint, that obesity may be the result of pain and inactivity resulting from degenerative joint disease,³⁵ has been refuted. Research has addressed the relationship between level of overweight and osteoarthritis at different ages.⁴ Interestingly, Hochberg et al⁸ reported that percentage body fat and body fat distribution are not associated with knee osteoarthritis whereas body weight is. This finding suggests a stronger contribution of biomechanical factors to explain the relationship.

Given the significant differences between peak pressures under the feet of obese subjects in the present study, further analyses, with individuals of different body weight and level of adiposity, are required to assess whether increased adiposity or higher levels of body weight is the problematic factor. Similarly, an assessment of the changes in pressure distribution under the feet of obese individuals who are exposed to an intervention to substantially decrease adiposity is also warranted. Such work has important implications for co-morbidities of obesity including diabetes and osteoarthritis.

Conclusions

This study is the first to examine the plantar pressure differences between obese and non-obese adults. Obese subjects showed increased forefoot width and higher plantar pressures during standing and walking protocols. The highest-pressure increases in the obese were found under the longitudinal arch of the foot and the metatarsal heads. Compared to the non-obese groups, increases in pressure under the mid-foot and the middle of the forefoot were higher for the obese women as compared to the obese men during standing. This difference may be the result of reduced strength of the ligaments of the foot in obese women. These findings have implications for pain and discomfort in the lower extremity in the obese, the choice of footwear and predisposition to participation in activities of daily living such as walking. Further study in this area is warranted.