Adaptation and vision change the relationship between muscle activity of the lower limbs and body movement during human balance perturbations.

OBJECTIVE: Investigate the relationship between changes in lower limb EMG root mean square (RMS) activity and changes in body movement during perturbed standing. Specifically, linear movement variance, torque variance and body posture were correlated against tibialis anterior and gastrocnemius RMS EMG activity during perturbed standing by vibration of the calf muscles. METHODS: Eighteen healthy participants (mean age 29.1 years) stood quietly for 30s before vibration pulses were randomly applied to the calf muscles over a period of 200s with eyes open or closed. Movement variance, torque variance and RMS EMG activity were separated into five periods, thereby allowing us to explore any time-varying changes of the relationships. RESULTS: Changes of tibialis anterior muscles EMG activity were positively correlated with changes in linear movement variance and torque variance throughout most of the trials, and negatively correlated with some mean angular position changes during the last 2min of the trials. Moreover, the initial changes in Gastrocnemius EMG activity were associated with initial changes of mean angular position. Additionally, both tibialis anterior and gastrocnemius muscle activities were more involved in the initial control of stability with eyes closed than with eyes open. CONCLUSIONS: Visual information and adaptation change the association between muscle activity and movement when standing is perturbed by calf muscle vibration. SIGNIFICANCE: Access to visual information changes the standing strategy to calf muscle vibrations. Training evoking adaptation could benefit those susceptible to falls by optimising the association between muscle activities and stabilising body movement