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 a shift in muscle function from more positive work during strides up from treadmill to obstacle to less positive work in strides down from obstacle to treadmill. Activation, strain trajectory, and activation*strain trajectory interaction had significant effects on all work loop variables, with the interaction having the largest effect on peak force and work per cycle. These results support the hypothesis that muscle is an active material whose viscoelastic properties are tuned by activation, and which produces forces in response to deformations of length associated with time-varying loads.