AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
Reaching movements have been widely studied to investigate movement patterns and neuromotor control mechanisms for the past two decades (Georgopoulos, 1995). Many of those studies have focused on reaching movements while seated that require less postural control than during standing (Moore et al, 1992). Other studies have investigated reaching towards a target, but not returning from it (Duren et al, 1990). Few studies have investigated the more natural movement of quickly reaching towards and returning from a target while standing. These reach-reversal movements may be particularly challenging for the elderly since they have less dynamic balance control.
The purpose of this study was to investigate the ability of young and elderly subjects to perform fast reach-reversal movements to targets in different directions. Reach-reversal movements are defined as the motion towards a target and return to the start position. Results indicated that both age and target direction affected the reach biomechanics.
Decreased postural stability is associated with an increased risk for falling in the elderly. The maximum excursion of the center of pressure (MCOPE) decreases with age (Duren et al, 1990). Recently Blaszczyk et al. (1994) reported that the decrease in MCOPE with age depends on movement direction. In that study, the elderly had significantly smaller MCOPE than young subjects while leaning backwards or to the left side but not while leaning to the front or to the right side. However, they examined static leaning movements only, however, rather than the disturbance of balance associated with functional tasks such as reaching movements. In this study, we examined both age and target direction effects in order to have a more clear understanding of the factors affecting control of reaching movements in the elderly.
Eight healthy, right-handed elderly subjects (74.1(5.4 yrs; 4 male and 4 female;) and eight young subjects (23.2(4.1 yrs; 4 males and 4 females) volunteered to participate in the study. They all reported no history of neurologic or muscular abnomalities.
Each subject performed reach-reversal movements in 4 directions (0, 45, 90, and 120 deg from body midline) and at 2 distances (arm-length and maximum reach) while standing on the force platform. Markers were placed on the hand, wrist, elbow, shoulder, hip, knee, ankle and metatarsophalangeal joints, and kinematic data were recorded with a 3D video-based motion analysis system. An accelerometer was placed on the wrist to determine movement onset. Subjects reached to the target as fast as possible and then brought their arm back to the starting position immediately. The order of reach direction was randomized. Each dependent variable was analyzed separately in a 2 ( 2 ( 4 ANOVA with repeated measures for age, gender,and target direction. The significance level was set at ( = .05.
Subjects were similar in body weight and body height. The base of support (BOS) was significantly greater in the elderly than in the young subjects (p<.01).
Reach distance (difference between maximum reach and arm-length reach distance) decreased with increase in target direction for both young and elderly subjects. Although young subjects reached farther than the elderly subjects in all directions, the difference was significantly only for the 0 deg direction (p<.001).
Fig. 1: Reach distance in different target directions.
Movement time depended on age and reach distance. Movement time was similar for both age groups and all directions in the arm-length reaches. For the maximum distance reaches, however, movement time was significantly longer for the elderly than for the young subjects (p<.01). Thus, the elderly took longer to move the same distance as the young. In contrast, the movement reversal time showed age and gender effects during arm-length reaches but no differences occurred during maximum reaches. Reversal time increased with age (p<.01) and was greater for female than for male subjects (p<.05). No target direction effect occurred.
Shoulder joint angular velocity depended on age and target direction. Both age and target direction affected the peak shoulder flexion angular velocity (SFAV) (p<.05 and p<.01), but not the peak shoulder extension angular velocity. SFAV decreased with age (p<.05) and target direction (p<.01).
Maximum excursion of the center of pressure (MCOPE) showed no gender or age effects during arm-length reaches. In maximum reaches, however, MCOPE was significantly greater in the young subjects (0 and 45 deg targets) (p<.05) (Fig. 2). MCOPE depended on target direction in both young and elderly subjects (p<.001). To determine relative stability between age groups, MCOPE was normalized by both BOS and reach distance (RD). MCOPE/BOS was greatest for the 45 deg target for both elderly (2.06%) and young subjects (3.46%). MCOPE/BOS was significantly greater in the 45 deg target direction than for the other directions (p<.0005). When normalized by reach distance, however, MCOPE/RD increased with direction (p<.005).
Fig. 2: Maximum center of pressure.
The results of this study supported the hypothesis that elderly subjects generated reach-reversal movements differently than young subjects, especially in the maximum reaches. Elderly subjects moved relatively more slowly, and spent more time near the target during the reversal portion of the movement. Shoulder flexion velocity was also slower in the elderly subjects.
There was no difference, however, in how elderly and young subjects reached to the different target directions. When normalized by reach distance, MCOPE/RD changed in the same way with target direction for both age groups, suggesting that subjects used the same strategy to modify their MCOPE as a function of reach distance. When normalized by base of support area, MCOPE/BOS was greatest at the 45 deg target direction, indicating that all subjects were statically most stable when reaching to the 45 deg target, regardless of age.
Blaszczyk, J.W., Lowe, D.L. Hansen P.D. Gait Posture 2, 11-17, 1994.
Duran, P.W. et al. J Gerontol. Med. Sci. 45, M192-197, 1990.
Georgopoulos, A.P. Trend Neurosci, 18, 506-510, 1995
Moore, S., et al. Phys. Ther. 72, 335-343, 1992
