Ultrasound imaging reveals differential triceps surae response to altered propulsive demand in walking

We investigated differences between medial gastrocnemius and soleus muscle contractile behaviour during walking under conditions strategically designed to augment the demand for forward propulsion. Ten young adults walked on a treadmill at 1.2 m/s both normally and with: (i) a 5% horizontal aiding force (decreases propulsive demand) and (ii) a 5% horizontal impeding force (increases propulsive demand). We reveal that medial gastrocnemius muscle shortening was significantly more sensitive than soleus muscle shortening to changes in propulsive demand. In addition, compared to normal walking, differences between peak gastrocnemius and peak soleus muscle shortening decreased during the aiding force condition and increased during the impeding force condition – findings consistent with purported differences in the biomechanical functions of these two muscles during walking.