Sensitivity of bulk leg tissue stiffness to muscle activation

Lower limb tissue stiffness is contingent on various factors, including body composition, loading rates, and the geometry of the indenting object. Being able to predict deformation of human soft tissue under different loading conditions could enable researchers and engineers to improve performance of biomedical devices through optimized human-device interfaces. In this study, we used a custom-built handheld indentation device to explore changes in leg (thigh and shank) tissue stiffness at rest and during isometric contractions. Force-displacement relationships were modelled using an exponential growth function. Averaged across 9 subjects and two indentation locations (thigh/shank), deformation forces during activation increased by a factor of ~1.7x over the same displacement as inactive data. Thus, bulk tissue stiffness varies dramatically with underlying muscle activation; this should be considered in developing novel orthoses.