Understanding muscle function during perturbed in vivo locomotion using a muscle avatar approach

To investigate in vivo mechanics of the guinea fowl lateral gastrocnemius (LG) muscle during obstacle negotiation while running on a treadmill, we used mouse extensor digitorum longus (EDL) muscles in ex vivo experiments with in vivo strain inputs from perturbed and steady strides obtained in a previous study. In vivo strain trajectories from a stride down from obstacle to treadmill, two strides up from treadmill to obstacle, and a level stride with no obstacle, as well as a sinusoidal strain trajectory at the same amplitude and frequency, were used as inputs in work loop experiments. With five strain trajectories and three activation patterns, each muscle was used in a total of 15 work loop experiments. EDL forces produced using in vivo strain trajectories were more similar to in vivo LG forces (R2 = 0.58 – 0.94) than to forces produced using the sinusoidal trajectory (average R2 = 0.045). Given the same activation, in vivo strain trajectories produced consistent work loops that showed...