Positive force feedback in bouncing gaits?

Abstract

During bouncing gaits (running, hopping, trotting), passive compliant structures (e.g. tendons, ligaments)
store and release part of the stride energy. Here, active muscles must provide the required force to withstand the developing tendon strain and to compensate for the inevitable energy losses. This requires an
appropriate control of muscle activation. In this study, for hopping, the potential involvement of afferent
information from muscle receptors (muscle spindles, Golgi tendon organs) is investigated using a two-segment leg model with one extensor muscle. It is found that: (i) positive feedbacks of muscle-fibre length
and muscle force can result in periodic bouncing; (ii) positive force feedback (F⫹) stabilizes bouncing
patterns within a large range of stride energies (maximum hopping height of 16.3 cm, almost twofold
higher than the length feedback); and (iii) when employing this reflex scheme, for moderate hopping
heights (up to 8.8 cm), an overall elastic leg behaviour is predicted (hopping frequency of 1.4–3 Hz, leg
stiffness of 9–27 kN m⫺1). Furthermore, F⫹ could stabilize running. It is suggested that, during the stance
phase of bouncing tasks, the reflex-generated motor control based on feedbacks might be an efficient and
reliable alternative to central motor commands.