General Features of Vibration-Induced Effects on Balance

Tests on healthy adults and patients showed that vibration of certain leg and trunk muscles caused involuntary inclination movements. These vibration-induced falling reactions (VIFs) were recorded by letting the subjects stand on a force platform signalling changes in the body inclination. The equilibration contractions in the leg muscles during voluntary counteraction of such falling were shown by the irregular oscillations also included in the signal, which could be separated from the slower sway signals. The amount of these oscillations in the supportive forces served to indicate the unsteadiness produced by vibration. Analysis of how various factors influenced the strength of the postural reactions, VIF-latency measurements and studies of VIF-responses in certain patients lend support to the following conclusions. 1) Certain leg and trunk muscles are particularly important as afferent sources for equilibrium during standing. 2) Vibration-induced afferent inflow from these muscles tends to bring the body out of balance, not because of local changes of tension in the muscles stimulated (segmental reflex effects) but because of widespread postural adjustments from supraspinal structures. 3) The muscle end organs mainly responsible for the reactions described are probably the secondary spindle endings.