Balance adjustments of humans perturbed while walking

Abstract

The earliest components of EMG [electromyogram] adjustments that followed the perturbation of walking human subjects were studied. Subjects were perturbed while walking at their own preferred rates of travel by the unexpected movements of a platform incorporated into the surface of a walkway. The forces and motions associated with perturbed steps were also analyzed to interpret the function of the observed EMG adjustments. Although the imposed perturbations were smaller in amplitude and rate than the normal walking movements, the resulting EMG adjustments were significantly larger than the background walking levels. The EMG adjustments were highly movement specific and closely resembled (in organization and latencies) those adjustments elicited by imposing the same platform movements on standing subjects. The principle effect of the EMG adjustments was to modify the parameters (rate, amplitude, phasing) of perturbed steps. Perturbing the rotational trajectory of the supporting ankle joint changed the torsional thrust exerted by the step and thereby helped to regulate the rate of forward progression of the body. Perturbing the vertical height of the support leg elicited an organizationally different adjustment; the amplitude of leg extension was changed to help regulate the height of the body above the rest of the walkway. A conceptual model combines the results with what is already known about the spinal generation of stepping movements. Several conclusions are supported: during locomotion, EMG adjustments are generated by discongruence between the anticipated (normal) and the actual stepping movements; during both stance and locomotion, postural adjustments are organized by generators that control the parameters of the normal stepping movements; and the continuity of perturbed locomotor movements is maintained by adjusting the parameters of the ongoing stepping movements rather than by introducing new unrelated patterns of movement.