Motor-unit synchronization is not responsible for larger motor-unit forces in old adults

Motor-unit synchronization, which is a measure of the near simultaneous discharge of action potentials by motor units, has the potential to influence spike-triggered average force and the steadiness of a low-force isometric contraction. The purpose of the study was to estimate the contribution of motor-unit synchronization to the larger spike-triggered average forces and the decreased steadiness exhibited by old adults. Eleven young (age 19-30 yr) and 14 old (age 63-81 yr) adults participated in the study. Motor-unit activity was recorded with two fine-wire intramuscular electrodes in the first dorsal interosseus muscle during isometric contractions that caused the index finger to exert an abduction force. In a separate session, steadiness measurements were obtained during constant-force isometric contractions at target forces of 2.5, 5, 7.5, and 10% of the maximum voluntary contraction (MVC) force. Mean (+/-SD) motor-unit forces measured by spike-triggered averaging were larger in old (15.5 +/- 12.1 mN) compared with young (7.3 +/- 5.7 mN) adults, and the differences were more pronounced between young (8.7 +/- 6.4 mN) and old (19.9 +/- 12.2 mN) men. Furthermore, the old adults had a reduced ability to maintain a steady force during an isometric contraction, particularly at low target forces (2.5 and 5% MVC). Mean (+/-SD) motor-unit synchronization, expressed as the frequency of extra synchronous discharges above chance in the cross-correlogram, was similar in young [0.66 +/- 0.4 impulses/s (imp/s); range, 0.35-1.51 imp/s; 53 pairs) and old adults (0.72 +/- 0.5 imp/s; range, 0.27-1.38 imp/s; 56 pairs). The duration of synchronous peaks in the cross-correlogram was similar for each group (similar to 16 ms). These data suggest that motor-unit synchronization is not responsible for larger spike-triggered average forces in old adults and that motor-unit synchronization does not contribute to the decreased steadiness of low-force isometric contractions observed in old adults.