DESCRIPTION: (Investigator's Abstract) The objective of this proposal is to determine what strategies aged adults use to control motorneuron firing. Preliminary work by the Principal Investigator suggests that the elderly may use alternative schemes for controlling motorneurons. For example, unusual derecruitment behavior of motor units, increased synchronization in the firing of motor units and unusual occurrence of doublet firing have been observed. In addition, work from other laboratories has shown loss of motor units with age despite the need to maintain force and precision in daily movements. Experiments are proposed to address the following key questions: 1) What strategies do older adults use to activate motor units during voluntary muscle contraction? 2) Do synergistic and antagonistic muscles adopt alternative strategies for motor action in older individuals? 3) What motor unit control strategies do older adults use to respond to sudden distrubances in balance? The experiments described in this proposal will be conducted in young and old adults using the first dorsal interosseous and anterior tibialis muscle groups during voluntary effort ranging up to 100 percent of maximal voluntary strength. Discharges of single motor units will be recognized using a specialized quadrifilar needle electrode and a computer aided technique to decompose the composite signal into individual motor unit firing trains. The long term goal is to determine whether an alternative motor control strategy is used to maintain optimal motor activity in the elderly.