Maintaining upright stance requires continuous integration of sensory information and motor output [1]. The size of the base of support influences postural stability, with a larger area (e.g. lying prone) easier to balance over than a small area (e.g. standing) [2]. Postural adjustments involve both reflexive and feedforward control mechanisms [3][4].

Sudden external perturbations elicit rapid, corrective motor reflexes that shift the center of mass back over the feet [4]. This involves sensory detection of sway via visual, vestibular, and proprioceptive systems, which send signals to motor nuclei in the brainstem and spinal cord to activate appropriate muscle contractions [5]. For example, lateral perturbations generate compensatory activity in the abductors on the opposite side to return to midline [4].

In contrast, feedforward control stems from the cerebellum, which makes anticipatory adjustments preceding voluntary movements [6]. Internal models predict the consequences of actions on balance and engage postural muscles prior to onset of disturbances. For example, when reaching to a shelf, early activation of back extensors counters the shift in weight even before the arm begins moving forward [6].

The vestibulospinal tract is also critical for posture, rapidly generating extensor muscle activity via lateral vestibular nucleus neurons to counter falling [4]. This sophisticated sensorimotor integration provides both reactive and predictive stability.