Practice improves motor control in older adults by increasing the motor unit modulation from 13 - 30 Hz 4

26 Practice of a motor task decreases motor output variability in older adults and is associated with 27 adaptations of discharge activity of single motor units. In this paper we were interested in the practice28 induced modulation of multiple motor units within 13-30 Hz because theoretically it enhances the timing 29 of active motoneurons. Our purpose, therefore, was to determine the neural adaptation of multiple motor 30 units and related improvements in movement control following practice. Nine healthy older adults (6531 85 years) performed 40 practice trials of a sinusoidal task (0.12 Hz) with their index finger (10° range of 32 motion). Multiple motor unit activity was recorded intramuscularly from the first dorsal interosseus 33 muscle. The mean spike rate (MSR), spike rate variability (CVISI), and frequency modulation (5-60 Hz) 34 of the spike rate were calculated from the multi-motor unit activity, and were correlated with movement 35 accuracy and variability of index finger position. A decrease in movement trajectory variability was 36 associated with an increase in MSR (R=0.58), a decrease in CVISI (Christou et al. 2007b) (R=0.58), and an 37 increase in total power within 13-30 Hz band (R=0.48). The increase in total power within 13-30 Hz 38 band was associated significantly (P < 0.005) with an increase in MSR (R=0.75) and the decrease in 39 CVISI (R=0.70). We demonstrate that practice-induced improvements in movement control are 40 associated with changes in activity of multiple motor units. These findings suggest that practice-induced 41 improvements in movement steadiness of older adults are associated with changes in the modulation of 42 the motoneuron pool from 13-30 Hz. 43