Aging and the mechanisms underlying head and postural control during voluntary motion.

The quality of sensory information that is necessary for balance and postural stability will depend to a great extent on head stability as the body moves. How older persons coordinate head and body motion for balance during volitional activities is not known. The purposes of this article are to present a basis for understanding the influence of aging on head control during voluntary motion and to discuss some data that demonstrate how elderly people might control head movement to improve gaze and the quality of vestibular inputs. A "top-down" or "head-first" control scheme is proposed as the mechanism that elderly people without disabilities use to maintain head position during self-initiated motion. This type of control ensures that the angular position of the head in space remains relatively constant--through the use of a head-stabilization-in-space (HSS) strategy--regardless of the magnitude or direction of displacements in the body's center of force. The HSS strategy is thought to reduce potential ambiguities in the interpretation of sensory inputs for balance and is derived primarily from a geocentric (orientation to the vertical) frame of reference. Egocentric (orientation of the head with respect to the body) or exocentric (orientation to objects in the environment) frames of reference, however, refine the control of head stabilization. Preliminary research suggests that elderly people use the HSS strategy to control head pitch during difficult balance tasks. These findings, if supported by more definitive studies, may be useful in the treatment of patients with balance disorders. The treatment of patients with balance dysfunction is discussed within the conceptual framework of a "head-first" organization scheme.