Motor control pathways of the brainstem 1. Posture is a fundamental aspect of motor control. The maintenance of posture during movements of the body or parts of the body is as much part of motor control as the movements themselves. Postural mechanisms act to stabilise and maintain position of the body. It is a dynamic substrate of all movement and is continually acting to allow movements to occur without causing instability.
Postural mechanisms act by altering the efficacy of spinal reflexes.
Maintenance of posture is not, in general, a matter of simple reflex adjustments but typically involves plastic, adaptive modulation of reflex circuits to suit different requirements. Proprioceptive reflexes activated by the sway are rapidly and extensively adapted to provide optimal postural control. In the above experiment both manipulations of the platform have the result of changing the ankle angle, but the displacement of the centre of gravity is opposite in the two conditions. What is seen is that the gastrocnemis reflex is enhanced when it counteracts the displacement, but is silenced when it would be detrimental. Appropriate postural changes can not only compensate for disturbances in stability but can, via feed-forward control, pre empt changes.