Regulation of feed-forward and feedback contributions to ankle stability during balance tasks

One of the fundamental problems of postural control is producing appropriate forces to counteract mechanical instabilities. The most common source of instability is the need to support the mass of our body segments against the force of gravity. This is most apparent during stance since a majority of our mass is concentrated at a considerable distance from our base of support, much like an inverted pendulum. In this simple description of stance, the problem of postural stability becomes equivalent to one of ankle joint stability. To maintain stability, the ankle must produce torque from the combined effects of passive muscle and joint properties, feedback mechanisms such as stretch reflexes, and feed-forward volitional activation. Though each of these mechanisms makes some contribution to joint stability, the relative roles of feed-forward and feedback mechanisms remain a topic of debate. We propose that it may be possible to better understand the relative contributions of these mechanisms by observing how the motor system responds to artificially imposed instabilities. This was accomplished by challenging ankle stability using haptic loads and quantifying the amplitude of antagonistic co-contraction and stretch reflexes as stability was compromised. We hypothesized that as stability was reduced, individuals would increase both co-contraction and stretch reflex amplitude to counteract the destabilizing forces generated by the haptic environment. Our results revealed that the amplitude of co-contraction did indeed scale with increasingly destabilizing loads, but this strategy was not entirely sufficient to counteract the imposed instabilities. Reflex amplitude, in comparison, was attenuated as stability was reduced. This is in contrast to similar studies which examined reflex contributions to joint and limb stability in the upper limb. We argue that in the lower limb, volitional contributions to joint stability may be preferred over involuntary mechanisms such as stretch reflexes despite the delays associated with volitional reactions.