Balance process during repeated surface perturbation: Adaptation response of joint stiffness and muscle activation

It is believed that humans are keen to learn and initiate more efficient and less energy consumption strategies, especially when they desire repetitive work or motion. However, in human's balancing process, the ability to adapt a repeated surface movement and its response towards imbalance, due to less sensory input, is still unclear. In this study, adaptation behaviours of joint stiffness pattern and muscle activation were observed during limited sensory inputs. Seven young subjects participated in this study. Two different surface perturbations (tilt up-tilt (TT) down and translation (T)) at four different sensory manipulation conditions (include vision and vestibular system) were introduced to the subject. Then, they were asked to maintain their position as long as possible. The results have shown that amplitude of joint stiffness decreased by almost 1.2 percent at the ankle over 10 cycles. However, there is almost no adaptation at the hip. Even though average the adaptation percentage increased as sensory inputs became better (r²>0.3), no significant difference between sensory conditions was recorded (p>0.05). Meanwhile, different adaptation patterns were observed among five different muscles at both types of perturbation, with adaptation at almost 1 percent on average. The findings have shown that adaptation behaviour is able to describe motor learning functions of the balancing process in humans. It helps to enhance human posture control model and muscle dynamic model especially related to continuous repeated motion or force applied to the system.