Rate coding is compressed but variability is unaltered for motor units in a hand muscle of old adults

The discharge of single motor units (n = 34) in the first dorsal interosseus muscle and the fluctuations in force during steady contractions were measured across a range of index finger abduction forces in old adults (77.1 +/- 6.9 yr, n = 20). These results were compared with previously reported data on 38 motor units from young adults (25.7 +/- 5.7 yr). Both minimal and peak discharge rates increased with recruitment threshold, but the strength of these relations was notably weaker for the old adults. Minimal discharge rates were similar for young and old adults (P = 0.77), whereas peak discharge rates were lower for old adults (P < 0.01). Consequently, the range of rate coding for each motor unit was substantially less for the old adults (7.1 pps) compared with the young adults (12.1 pps, P < 0.01). However, the variability in motor-unit discharge was similar for young and old adults; the coefficient of variation of the interspike intervals was similar at recruitment (old: 25.4%, young: 27.1%, P = 0.39) and declined with an increase in discharge rate (old: 13.2%, young: 14.2%, P = 0.21). Furthermore, the fluctuations in force during steady isometric contractions (2-95% of maximal force) were similar for young and old adults, except that the relative variability at the lowest force was greater for the old adults. A computational model of motor-unit recruitment and rate coding incorporated the experimental observations and was able to match the measured and simulated values for force steadiness across the operating range of the muscle.