Postural mechanisms

Postural mechanisms avert the body from being destabilized by forces (involving gravity and those generated by limb movements) acting to shift the center of mass. Muscles either assist or oppose gravity whenever contracting; those that oppose are explained as antigravity muscles. Most antigravity muscles, like the leg extensors and the short deep extensor muscles of the back are included in keeping posture. In humans, the flexor muscles of the arms are also antigravity muscles. As antigravity muscles are usually more powerful than muscles assist by gravity, in human limbs the strongest muscles are the leg extensors and arm flexors.

Motor commands from the forebrain and cerebellum involve those for the postural adjustments essential during the execution of a movement. For illustration, when a standing dancer abducts her leg she should shift her center of mass above the other leg to avoid falling over. This requires feed forward postural adjustments to trunk and arms that anticipate the unbalancing forces which act during the movement. The adjustments depend accurately on the initial position and nature of the intended movement and they should be learnt.

Tough, postural adjustments frequently have to be made to compensate for unexpected disturbances in body position and movement. This cannot be completed by feed forward as, by definition, the disturbance is not known ahead of time. Therefore, these adjustments are made by postural reflexes, organized by the brainstem, that are negative feedback methods as shown in figure below. The sensory input to the reflex circuitry is from three sources:

- Vestibular, from the otolith organs

- Proprioceptor, from muscle spindles, Golgi tendon organs, and joint receptors

- Visual, from the superior colliculus

Such inputs are highly integrated to recruit the needed series of corrective muscle activity.

Figure: Postural reflex negative feedback. The circuitry generates a motor output that decreases the mismatch among the desired position of the body and its real position. The error is detected by the sensory systems that feed information into the reflex circuitry.

The precise nature of postural reflex adjustments made by humans based on context, that is, the primary position of the body and the size and direction of the destabilizing force. Swaying, generated by sudden displacement of the surface on which a person is standing, will activate quite distinct sets of muscles depending on the direction of sway, but in common distal muscles are excited before proximal ones, with most movement taking place at the ankle joint. Rotation or tilt of the surface, though, results in bending at the hips. In extremis postural reflexes try to sustain the center of gravity to prevent falling, or to put the limbs in a place to brace against falling.

Numerous distinct postural reflexes can be seen in animals by surgically transecting the brainstem (i.e., decerebration) and in humans who have suffered severe brain damage. These reflexes cannot easily be extracted in isolation in healthy behaving humans as motor functions are usually so highly integrated. They might be seen in newborn infants, in whom the motor systems are immature.