MECHANISMS CONTROLLING HEAD STABILIZATION IN THE ELDERLY DURING RANDOM ROTATIONS IN THE VERTICAL PLANE

Frequency-related response characteristics of the mechanisms controlli ng stabilization of the head in 10 elderly subjects were compared with response characteristics in 8 young adults. Angular velocity of the h ead with respect to the trunk and EMG responses of 2 neck muscles were recorded in 10 seated subjects during pseudorandom rotations of the t runk in the sagittal plane at frequencies of 0.35 to 3.05 Hz. Subjects were required to actively stabilize their heads with (VS) and without (NV) visual feedback so that voluntary mechanisms and the influence o f vision could be tested. Reflex mechanisms were examined when subject s were distracted by a mental calculation task during rotations in the dark (MA). Age emerged as an influential factor in the performance of head stabilization mechanisms, and decrements in performance were eve n more pronounced in the older as compared with younger elderly subjec ts. Age effects could be seen in the (a) diminished ability to volunta rily stabilize the head, particularly with the absence of vision, (b) impaired ability to stabilize the head when cognitively distracted, an d (c) appearance of a resonant response of the head. Control of head s tabilization shifted from reflex mechanisms to system mechanics, proba bly as a result of age-related changes in the integrity of the sensory systems. The elderly's system mechanics could not effectively compens ate for the disturbances, however, and instability was the result.