Previous research has not examined the effect of voluntary knee movements on balance
correcting strategies. Specifically, we were interested to see if healthy subjects alter or even
delay pre-programmed balance correcting strategies, when implementing a voluntary
movement that may or may not fit their automatic balance correcting strategy. We posed two
main questions. First, can standing subjects implement voluntary knee flexion movements
when their balance is simultaneously perturbed by a moving support surface? Second, would
incorporating such voluntary knee flexion into the postural strategy alter the intersegmental
shaping of automatic balance corrections? One possible outcome is that voluntary knee
flexion would be purely added to the directional sensitivity of automatic balance corrections.
Alternatively, voluntary knee movements might suppress the initial automatic responses and
thereby also alter the pre-programmed balance correcting synergy in latency or directional
sensitivity. This may cause a disruption of the kinematic strategy leading to postural
instability (Alexandrov et al. 1998b; Hughey and Fung 2005).