Normal aging affects unconstrained three-dimensional reaching against gravity with reduced vertical precision and increased co-contraction

Abstract

Reaching for an object in space forms the basis for many activities of daily living and is important in rehabilitation after stroke and in other neurological and orthopedic conditions. It has been the object of motor control and neuroscience research for over a century, but studies often constrain movement to eliminate the effect of gravity or reduce the degrees of freedom. In some studies, aging has been shown to reduce target accuracy, with a mechanism suggested to be impaired corrective movements. We sought first to explore the changes in control of shoulder and elbow joint movements that occur with aging during performance of reaching movements to different target heights with the normal effects of gravity, unconstrained hand movement, and stable target locations. Three-dimensional kinematic data and electromyography were collected in 14 young (25{+/-}6 years) and 10 older adults (68{+/-}3 years) during second-long reaches to three targets aligned vertically in front of the participants. Older adults took longer to initiate a movement than the young adults and were more variable and inaccurate in their initial and final movements. Target height had greater effect on trajectory curvature variability in older than young adults, with angle variability relative to target position being greater in older adults around the time of peak speed. There were significant age-related differences in use of the multiple degrees of freedom of the upper extremity, with less variability in shoulder abduction in the older group. Muscle activation patterns were similar, except for a higher biceps-triceps co-contraction and tonic levels of some proximal muscle activation. The path length of movements was not affected by age. These results show an age-related deficit in the motor planning and online correction of reaching movements against a predictable force (i.e., gravity). These results will facilitate interpretation of our forthcoming study of transcranial magnetic stimulation effects on the same task in these two populations, and is relevant to any study that seeks to measure the effect of pathological processes on upper extremity motor performance in the elderly.