  Muscle fibres possess unique visco-elastic properties, capable of generating
a stabilising zero-delay response to unexpected perturbations. This
instantaneous response -- termed "preflex" -- is crucial in the presence of
neuro-transmission delays, which are particularly hazardous during fast
locomotion due to the short stance duration. While the elastic contribution to
preflexes has been studied extensively, research on the role of fibre viscosity
due to the force-velocity relation remains unexplored. Moreover, muscle models
predict conditions with saturated force-velocity relations resulting in reduced
viscous-like fibre engagement. The goal of our study is to isolate and quantify
the preflex force produced by the force-velocity relation. We use our approach
to analyse two perturbed vertical hopping conditions, differing in their
viscosity engagement at touch-down. Both cases showed stable hopping patterns
despite the reduced viscous-like response to ground perturbations in the
saturated case. Moreover, the force-velocity relation was not the predominant
factor driving energy adjustment to disturbance intensity. From a robotics
perspective, our results suggest that already a simple, constant damper mounted
in parallel to an actuator could provide stabilising preflexes at and shortly
after impact.
