Compensation of translational and rotational perturbations in human posture: stabilization of the centre of gravity.

EMG responses in the leg muscles, head acceleration and joint movements induced by dorsiflexing rotation of the ankle have been analysed. Perturbations were induced while subjects were standing on a platform with the ankle joints co-linear with the axis of rotation or while standing above the axis (between 5 and 25 cm), which introduced an additional translational component to the displacement. The former condition was followed by a monosynaptic gastrocnemius reflex potential and a late tibialis anterior activation (latency about 100 ms); in the latter condition the monosynaptic reflex response became smaller and a longer latency gastrocnemius response (latency about 70 ms) appeared, the strength and duration of which increased in parallel with the translational component, while the amplitude of the tibialis anterior activity decreased. Neither vestibulospinal nor muscle proprioceptive mechanisms can solely account for this effect as the biomechanical parameters were little changed in the different conditions. It is suggested that the controlled variable in the investigated task is to hold the bodies' centre of gravity over the feet which makes the change in the pattern meaningful: pure dorsiflexion of the feet is followed by a backwards sway of the body, thus a tibialis anterior activation is functionally essential to hold the centre of gravity over the feet. A backwards translation of the feet withdraws the supporting surface, thus a gastrocnemius activation is needed to restore the centre of gravity over the feet. The inhibition of the monosynaptic reflex with increasing translation is suggested to arise from a reciprocal modulation of mono- and polysynaptic gastrocnemius reflex responses.