Investigating the effects of activation state and location on lower limb tissue stiffness

Lower limb tissue stiffness is contingent on various factors, including location, tissue composition, loading rates, and the geometry of the indenting object. Previous studies demonstrated that tissue stiffness varies greatly between individuals and between locations on an individual. Additionally, some studies have shown that activation of underlying muscle tissue increases bulk soft tissue stiffness. Yet, few studies have simultaneously considered both location and activation; this could be particularly important for measuring and predicting the function of devices such as prostheses and exoskeletons that interact with limbs at various locations during dynamic movement. In the present study, a custom handheld indentation device was used to explore changes in bulk leg tissue stiffness at rest and during isometric contractions. The indentation force-displacement curves were modelled using a Hertz model. At each level of activation (active/inactive), the shank had dramatically (∼150%) greater tissue stiffness than the thigh (p < 0.001). However, results suggested location independence for stiffness ratio (active/inactive, p = 0.42); for either location, stiffness was approximately 2x greater for active vs inactive muscle. These results should be considered during the development of biomechanical models to simulate human tissue indentation stiffness across a range of activation states and locations.