DIFFERENTIAL CONTROL OF LEG AND TRUNK MUSCLE-ACTIVITY BY VESTIBULOSPINAL AND PROPRIOCEPTIVE SIGNALS DURING HUMAN BALANCE CORRECTIONS

Knowledge about how proprioceptive signals trigger and modulate human balance corrections has important implications for the rehabilitation of postural and gait disorders, and increases our understanding of nor mal interactions between these sensory systems. We used combinations o f support-surface rotation and rearward translation to examine the tri ggering effects of ankle and knee movements on balance corrections. By comparing the responses in normal subjects to those in persons with a bilateral peripheral vestibular deficit, we determined the modulating influence of vestibular inputs on balance responses. Differences in n ormal and vestibular-loss responses under the different proprioceptive conditions revealed four general findings. First, ventral leg muscle responses are strongly modulated by vestibule-spinal inputs and by pro prioceptive inputs from the ankle and knee. Second, triceps surae musc le responses are initially dependent on ankle inputs, and after 100 ms are modulated by knee inputs; they are not altered by vestibular loss . Third, paraspinal responses in vestibular-loss subjects are enhanced because of unstable trunk sway induced by the lack of ventral leg-mus cle activity. Fourth, the earliest possible triggering signal for esta blishing the timing of interlink muscle activity appears to be knee fl exion and/or trunk rotation on the pelvis. These results indicate that a confluence of knee and trunk proprioceptive and vestibule-spinal in puts, rather than either input alone, is involved in establishing the muscle synergy underlying normal balance corrections.