OBJECTIVES:To measure the magnitude and prevalence of motor overflow to the arm at rest during attempted unilateral arm movements. DESIGN:Cross-sectional assessment. SETTING:Motor physiology laboratory. PARTICIPANTS:Healthy young (n=20) and elderly (n=20) adult subjects. MEASUREMENTS:Surface electromyography (EMG) was obtained from bilateral forearm muscles during performance of 12 different unilateral finger-tapping tasks. RESULTS:For all subjects, faster movement rate (F=2.56-3.30, P<.05), cognitive distraction (F=4.09, P<.05), and fatigue (F=15.15, P<.001) were each associated with a significant increase in the magnitude of EMG in the arm intended to be at rest. In elderly subjects, tapping at maximum rate and fatigue were each associated with a further increase in motor overflow across the midline. In addition, better left hand dexterity correlated with greater motor overflow to the right hand during rapid left hand tapping (r=0.63, P<.005). Prevalence of motor overflow was also higher in older subjects for some tasks, for example during 1 Hz tapping by the right index finger (motor overflow present in 45%, vs 15% young subjects, P<.05). CONCLUSION:Several behavioral variables increase motor overflow across the midline in young and elderly adults. Motor overflow was even greater in elderly subjects with the most demanding tasks and was greater in those with better motor status, suggesting that this form of motor system change is a compensatory event of normal aging rather than age-related dysfunction. The results support the hypotheses that healthy aging is associated with an increase in the degree to which brain function is bilaterally organized
