History-dependent perturbation response in limb muscle

Muscle mediates movement but movement is typically unsteady and perturbed. Muscle is known to behave non-linearly and with history dependent properties during steady locomotion, but the importance of history dependence in mediating muscles function during perturbations remains less clear. To explore muscle's capacity to mitigate perturbations during locomotion, we constructed a series of perturbations that varied only in kinematic history, keeping instantaneous position, velocity and time from stimulation constant. We find that muscle's perturbation response is profoundly history dependent, varying by four fold as baseline frequency changes, and dissipating energy equivalent to  ~6 times the kinetic energy of all the limbs in 5 ms (nearly 2400 W Kg-1). Muscle's energy dissipation during a perturbation is predicted primarily by the force at the onset of the perturbation. This relationship holds across different frequencies and timings of stimulation. This history dependence behaves like a viscoelastic memory producing perturbation responses that vary with the frequency of the underlying movement. 

Methods
The necessary methods are included in the manuscript.