A lifeless object is said to be in equilibrium, or in a state of balance, when all forces acting upon it cancel. The result is a state of rest. In an actively moving animal, internal as well as external forces have to be considered, and the maintenance of a balanced attitude in a body consisting of a number of parts that are loosely connected by movable joints is complex.
The maintenance of equilibrium is relatively easy in limbless animals. When the animal is turned on its back, the lack of contact pressure on the creeping surface and the tactile stimulation of the back initiate movements which return the animal to its normal position. This is known as the righting reflex. Free-swimming and flying animals are often in a precariously poised state of equilibrium, so that the normal attitude can be maintained only by the continuous operation of corrective equilibrating mechanisms. The same applies to a lesser degree to long-legged quadrupeds, such as many mammals, and to bipeds, such as birds and some primates (including humans).
For most species, there is a specific orientation of the whole body or of body segments (such as the head) with respect to gravity. This orientation is based on multisensory inputs and on a set of inborn reflexes acting on the musculature. These postural reflexes also stabilize the genetically defined body orientation against external disturbances. The sensory information relies on a number of sources: (1) static and dynamic information from the eyes; (2) static and dynamic mechanoreceptors incorporated in the various types of statocysts in invertebrate animals and in the vestibular organ or labyrinth of vertebrates; (3) proprioceptor organs such as muscle spindles, Golgi endings in tendons, Pacinian corpuscles and similar encapsulated endings associated with tendons and joints, and other pressure receptors in supporting surfaces (for example, the soles of feet); and (4) sensory endings in the viscera, capable of being differentially stimulated by changes in the direction of visceral pull on mesenteries, and on other structures. See Sensation, Sense organ
In higher terrestrial vertebrates, during stance the center of mass is usually situated high above the ground due to the support of the body by the limbs. A critical aspect of posture in quadrupedal and bipedal stance is equilibrium maintenance which is preserved only when under static conditions the projection of the center of mass remains inside the support base. This positioning of the center of mass is based on two main controls. A “bottom up” control is based on the afferent nerve impulses, cutaneous and proprioceptive, from the feet and the ankle joint muscles. These nerve impulse serve in building up posture from the feet to the head. A “top down” control starts from the head, and is predominant during dynamic activities such as locomotion. Due to labyrinthine afferent nerve impulses, the head axis orientation remains stable with respect to space. The movement-related visual afferents recorded by the retina monitor the head displacements with respect to space and adjust the body posture as a function of these inputs.
Two levels of control are involved in maintaining balance. A first level includes the spinal cord and the brainstem, where a set of inborn reflexes are organized for stance regulation and head orientation. Most postural reflexes rely on networks at that level. The cerebellum is involved in the adaptation of these reflexes to the external constraints. See Reflex
A second level of control includes cortical areas involved in multisensory integration and control as well as the basal ganglia. The postural body schema and the body orientation with respect to the external world are organized mainly at that level, with a predominant role in the right hemisphere. Coordination between balance control and locomotion or movements also depends on these higher levels.