Combining feedforward control and series elasticity enables muscle-tendon units to rapidly and safely reject perturbations

Animals easily navigate complex terrain and can rapidly and safely reject unexpected perturbations. However, the underlying mechanism at the muscle-tendon level of this behaviour is unclear. To study this, we attach a real muscle-tendon unit to a novel bio-robotic interface that emulates interaction with a mass in gravity to generate stable hopping. We suddenly perturb the height of the ground. We find that the drop in the height of the ground causes an automatic shift in the phase of muscle activation with respect to time of ground contact. This causes the muscle to shorten, while the tendon stretches and rapidly absorbs the energy of the fall. The muscle subsequently slowly re-lengthens and safely dissipates the excess energy from the drop at shorter lengths and lower mechanical power. These results demonstrate in the presence of series elasticity, feedforward control strategies can impart robustness during dynamic locomotion tasks.