Abstract
Knee movements play a critical role in most balance corrections. Loss of knee flexibility may
cause postural instability. Conversely, trained voluntary knee flexions executed during
balance corrections might help to overcome balance deficits. We examined whether bilateral
knee flexion could be added to automatic balance corrections generated by sudden balance
perturbations. We investigated how this could be achieved and whether it improved or
worsened balance control. Twenty-four healthy subjects participated in three different test
conditions, in which they had to flex their knees following an auditory cue (VOLUNTARY
condition), had to restore their balance in response to multidirectional rotations of a supportsurface (REACTIVE condition), or the combination of these two (COMBINED condition). A
new variable set (PREDICTED), calculated as the mathematical sum of VOLUNTARY and
REACTIVE, was compared to the COMBINED variable set. COMBINED responses
following forward rotations were close to PREDICTED, or greater, suggesting adequate
integration of knee flexion into the automatic balance reactions. For backward rotations, the
COMBINED condition resulted in several near-falls, and this was generally associated with
smaller knee flexion and smaller EMG responses. Subjects compensated by using greater
trunk flexion and arm movements. Activity in several muscles displayed earlier onsets for the
COMBINED condition following backward rotations. We conclude that healthy adults can
incorporate voluntary knee flexion into their automatic balance corrections, and that this
depends upon the direction of the postural perturbation. These findings highlight the
flexibility of the human balance repertoire, and underscore both the advantages and
limitations of using trained voluntary movements to aid balance corrections in man.