Control of roll and pitch motion during multi-directional balance perturbations.

Does the central nervous system (CNS) independently control roll and pitch movements of the human body during balance corrections? To help provide an answer to this question, we perturbed the balance of 16 young healthy subjects using multi-directional rotations of the support surface. All rotations had pitch and roll components, for which either the roll (DR) or the pitch (DP) component were delayed by 150 ms or not at all (ND). The outcome measures were the biomechanical responses of the body and surface EMG activity of several muscles. Across all perturbation directions, DR caused equally delayed shifts (150 ms) in peak lateral centre of mass (COM) velocity. Across directions, DP did not cause equally delayed shifts in anterior-posterior COM velocity. After 300 ms however, the vector direction of COM velocity was similar to the ND directions. Trunk, arm and knee joint rotations followed this roll compared to pitch pattern, but were different from ND rotation synergies after 300 ms, suggesting an intersegmental compensation for the delay effects. Balance correcting responses of muscles demonstrated both roll and pitch directed components regardless of axial alignment. We categorised muscles into three groups: pitch oriented, roll oriented and mixed based on their responses to DR and DP. Lower leg muscles were pitch oriented, trunk muscles were roll oriented, and knee and arm muscles were mixed. The results of this study suggest that roll, but not pitch components, of balance correcting movement strategies and muscle synergies are separately programmed by the CNS. Reliance on differentially activated arm and knee muscles to correct roll perturbations reveals a dependence of the pitch response on that of roll, possibly due to biomechanical constraints, and accounts for the failure of DP to be transmitted equally in time across all limbs segments. Thus it appears the CNS preferentially programs the roll response of the body and then adjusts the pitch response accordingly.

The role of vision in maintaining heading direction: effects of changing gaze and optic flow on human gait.

How is heading direction maintained in human gait? This question was investigated with respect to the role of optic flow and in the context of different movement strategies. While walking on a treadmill the deviation from the ideal straight path was measured in terms of lateral sway induced by a lateral gaze shift (by looking at a moving visual target). The role of the focus of expansion (FOE) within a radially expanding optic flow pattern was investigated by varying its relative velocity of expansion from 0- to 4-fold (the equivalent of walking speed), thus increasing the perceptibility of FOE. If FOE was a relevant cue for maintaining heading direction, a reduction of lateral sway amplitude was expected with increasing flow velocity. The presence of a radially expanding flow pattern did not reduce lateral sway. Lateral sway was least when the visual background remained stable without any flow pattern. Increasing the velocity of the flow pattern resulted in an increase in lateral sway. If the relative velocity of the flow pattern was raised beyond that corresponding to walking speed, lateral sway amplitude approached the maximal values observed in the dark. In all experiments, sway amplitude increased linearly with the increasing excursion of the visual target. Different strategies to perform the gaze shift (eye or head turns) only resulted in minor differences in lateral sway amplitude. The results show that gaze shifts during locomotion induce lateral sway, which depends upon the presence, and characteristics, of background optic flow. Under the present conditions, the FOE within the flow field seems not to be a dominant cue to control heading. However, the systematic increase in lateral sway induced by high flow velocities indicates that motion parallax has an effect on heading during locomotion.

Dynamic posturography using a new movable multidirectional platform driven by gravity.

Human upright balance control can be quantified using movable platforms driven by servo-controlled torque motors (dynamic posturography). We introduce a new movable platform driven by the force of gravity acting upon the platform and the subject standing on it. The platform consists of a 1 m2 metal plate, supported at each of its four corners by a cable and two magnets. Sudden release of the magnets on three sides of the platform (leaving one side attached) induces rotational perturbations in either the pitch or roll plane. Release of all magnets causes a purely vertical displacement. By varying the slack in the supporting cables, the platform can generate small (0.5 degrees ) to very destabilising (19 degrees ) rotations. Experiments in healthy subjects showed that the platform generated standardised and reproducible perturbations. The peak rotation velocity well exceeded the threshold required to elicit postural responses in the leg muscles. Onset latencies were comparable to those evoked by torque motor-driven platforms. Randomly mixed multidirectional perturbations of large amplitude forced the subject to use compensatory steps (easily possible on the large support surface), with little confounding influence of habituation. We conclude that this gravity-driven multidirectional platform provides a useful and versatile tool for dynamic posturography.