Human balancing of an inverted pendulum: is sway size controlled by ankle impedance?

1. Using the ankle musculature, subjects balanced a large inverted pendulum . The equilibrium of the pendulum is unstable and quasi-regular sway was ob served like that in quiet standing. Two main questions were addressed. Can subjects systematically change sway size in response to instruction and ava ilability of visual feedback? If so, do subjects decrease sway size by incr easing ankle impedance or by some alternative mechanism? 2. The position of the pendulum, the torque generated at each ankle and the soleus and tibialis anterior EMG were recorded. 3. Results shouted that subjects could significantly reduce the mean sway s ize of the pendulum by giving full attention to that goal. With visual feed back sway size could be minimised significantly more than without visual fe edback. In changing sway size, the frequency of the sways was not changed. 4. Results also revealed that ankle impedance and muscle co-contraction wer e not significantly changed when the sway size was decreased. As the ankle impedance and sway frequency do not change when the sway size is decreased, this implies no change in ankle stiffness or viscosity. 5. Increasing ankle impedance, stiffness or viscosity are not the only meth ods by which sway size could be reduced. A reduction in torque noise or tor que inaccuracy via a predictive process which provides active damping could reduce sway size without changing ankle impedance and is plausible given t he data. Such a strategy involving motion recognition and generation of an accurate motor response may require higher levels of control than changing ankle impedance by altering reflex or feedforward gain.