[Age-dependent changes in dynamic body balance as evaluated by the body tracking test (BTT)]

Stability of posture and gait is improved by growth and decays with aging. Maturational changes and deterioration of static posture with aging have been determined by using gravic body sway tests. In this study, we constructed a new system, the Body Tracking Test (BTT), to evaluate dynamic body balance function as opposed to static balance. Preliminary results of the study were previously reported elsewhere. Healthy volunteers of various ages (total, 516 persons) were subjects of the study. The principle of the BTT is for the subject to attempt to track an optical moving target displayed on the computer screen by shifting his or her body center of gravity that was also displayed on it. The target moved for a span of 15 cm horizontally on the 14-inch screen. The movement was lateral and antero-posterior (horizontal and vertical) in direction at a constant velocity of 0.125 Hz. The recording was for 60 seconds. In BTT the gain for target against tracking was fixed to 2.0 (target: tracking = 1: 2). The target was 100 cm anterior to the platform where the subject was to stand erect with the feet close together. The criteria for evaluation of the tracking function were determined by our preliminary study, the principle of which was to judge the rate of accordance of the tract of the moving target with that of body tracking. The score was divided into five ranks from A to E. We called it "The 5 grade evaluations of tracking ability". Age-dependent changes in the score were obtained and the statistical significance of the value was determined by chi 2 analysis. With regard to both the lateral and antero-posterior tracking abilities, there was an apparent improvement by maturation. The high school students reached the adult level in lateral tracking but not in antero-posterior tracking ability. Also, the tracking ability started to deteriorate after age 40. It is apparent from this study that the balancing or dynamic postural control differs between youth and the elderly, i.e.; lateral control seems to improve faster in youths and older persons tend to balance better in the antero-posterior directions. The BTT requires visual as well as proprioceptive information for its function. Moreover, dynamic maintenance and shifting of muscular tone and strength are prerequisite. In addition, for BTT, reflective postural control is strongly aided by training in terms of efficiency and skills that require higher cerebral functions. The BTT described here thus appears to represent a cumulative effect of the sensory, visual, motor and higher cerebral functions.