INTRODUCTION 
The locomotor tasks are particularly important for obese patients and yet are often profoundly affected by increased weight. This may lead to impairment in daily function but also limits lifestyle intervention strategies to address obesity. Walking in obese participants have been shown to differ from individuals within normal weight range in both linear and angular parameters. The purpose of this study was to evaluate the influence of obesity on the foot posture during walking by using a human obesity model.

METHODS 
Thirty healthy participants (19 males, 11 females; mean age of 32.9 ±4.6 years) were tested while walking with and without a weighted vest (obesity model and control). The obesity model represented 30% increase participant’s weight, with the weight distributed in “apple” shape. Whole body motion during gait was recorded using three-dimensional motion analysis system (Qualisys; 8 cameras, 2 force plates, 60 markers). Center of gravity (COG) and center of pressure (COP), step width during gait was analyzed together with ground reaction force. Change in longitudinal arch angles were also calculated using the positions of three markers on navicular bone, head of the first metatarsal bone and medial malleolus in order to investigate the effect of obesity on foot arch. Electromyography (EMG) of peroneus muscles and posterior tibial muscle was also recorded. The differences between obesity model and control condition were analyzed using Wilcoxon signed rank test. Correlation analyses were performed, and P values< 0.05 were considered to be statistically significant.

RESULTS
Longer distance from center of gravity (COG) to center of pressure (COP) was observed in obesity model than control (4.9±1.1 vs 4.1±0.9cm respectively, p<0.05, Figure 1). Shallower longitudinal arch angle of the foot (130.9±13.7 vs 136.0±10.4 degrees respectively, p<0.05, Figure 2) was observed also in obesity model. In addition, in obesity model, the distance from COG to COP was positively correlated with the step width normalized with body height (r=0.48, p<0.05, Figure 3), and with the ground reaction force towards internal direction (r=0.80, p<0.05, Figure 4). The EMG data did not differ significantly between obesity model and control, and was not correlated with any other data.

DISCUSSION 
The preferred strategy chosen by participants in this study to address the increase in weight was based on widening the step width. This strategy forced increase in COG to COP distance and ground reaction force towards internal direction, which resulted in decreased longitudinal arch height. These changes disturbed the integrity of truss mechanism. If truss mechanism is impaired, the load cannot be effectively distributed and shared. This in turn will affect walking efficiency and moreover may contribute to the development of flat foot. 
The other strategy to address the increase in weight would be to increase the muscles’ activation. The increased activation could allow for keeping the longitudinal arch angle higher, and COP closer to COG during walking. However, it was not able to be shown in this study, as no significant changes in muscle activities were observed. The main limitation of this study is using an obesity model instead of obese patients. In future studies, it is necessary to consider how weight affects the foot biomechanics during walking in patients with obesity.

CLINICAL RELEVANCE
From our obesity model, we have found that an increase in step width was a major strategy to adopt to weight gain. However, considering the risk for flat foot, and consequent planter pain, strengthening of trunk and lower limb muscles, need to be considered.

2000 - The Foot Posture Changes in Human Obesity Model