Effect of body mass index on heel pad mechanics: loading and unloading dynamics

Plantar heel pain (PHP) is the most predominant foot pathological condition persisting among people with obesity. Conventional techniques measure the heel pad's (HPs) thickness, stiffness, and elasticity by subjecting a known force under static conditions. However, it falls short of assessing the HP under natural physiological loading conditions. Therefore, the present study investigates the biomechanical characteristics of the HP during dynamic plantarflexion movement (body weight unloading and loading) using a 3D motion capture system and a multiaxial force plate. Participants were selected and divided into three categories based on body mass index (BMI): healthy weight (HW), overweight (OW), and obese (OB). Each category consisted of thirty male and thirty female participants. The retroreflective markers were placed on the HP, and participants were instructed to perform the plantarflexion movement on the force plate. The biomechanical output parameters, such as HP deformation and compliance, were calculated for different phases of plantarflexion movement along the vertical and horizontal directions. Statistical analysis was performed at a 95% confidence level. The study revealed that male and female participants in the OB category exhibited less HP deformation than those in the HW and OW categories. Male and female participants in the HW category showed higher HP compliance than those in the OB category. The magnitude of HP compliance was higher in the vertical direction than in the horizontal direction. HP stiffness increases with higher BMI (OB > OW > HW) and leads to PHP. The study revealed that males have lower HP compliance and deformation than females. The findings provide a better understanding of HP characteristics across different weight groups, which could be used to develop customized soles that absorb shock during dynamic activity and reduce heel pain among the population with obesity.