Even quiet stance appears to involve knee and hip movements (Hsu et al. 2007). The main
action of knee movements in response to sudden support surface perturbations could be to
absorb its impact, thereby reducing tilt of the trunk. For example, sudden lateral tilts of a
support-surface are largely absorbed by flexion of the uphill knee and extension of the
downhill knee (Allum et al. 2003; Carpenter et al. 1999). The importance of this “absorption
function” is underscored by the marked instability that ensues when normal knee flexion
movements are pathologically reduced, as occurs in patients with cerebellar ataxia,
Parkinson’s disease or peripheral vestibular loss (Allum et al.2003; Bakker et al. 2006; Horak
et al. 2005). Thus, it appears that active voluntary knee flexion could contribute to enhanced
stability following support surface perturbations.