Motor unit firing rates in young and very old adult males during an isokinetic fatiguing task and short-term recovery in the anconeus muscle.

Abstract

Understanding motor unit (MU) properties with muscle fatigue in adult aging is limited to isometric tasks. The purpose was to investigate the effect of an isokinetic fatiguing task on MU firing rates between two adult age groups of males. Single MUs were recorded by intramuscular electrodes in the anconeus muscle of eight young (19-33 yr) and 11 very old adults (78-93 yr). Fatigue was induced by repeated isokinetic maximal voluntary contractions at 25% of maximum velocity (Vmax), until elbow extension power decreased by 35%. At baseline, the very old had lower maximal power (135 vs. 214 W, P = 0.002) and slower maximal velocity (177 vs. 196°/s, P < 0.001) compared with young. Despite a similar number of contractions (average 39 young and 44 very old) to task failure (P = 0.33), the older males were less fatigable as time under tension was ∼25% longer (P = 0.04). Maximum firing rates from ∼20 single MUs per age group were tracked continuously throughout the task and during 10 min of recovery. Similar rates were recorded (24.7 and 23.6 Hz, P = 0.18) at baseline 25% Vmax, and during the task (23.3 and 28.7% decrease, P = 0.21) between young and very old, respectively. Power and MU rates were recovered similarly by 2 min of rest in both groups (all P > 0.15). Despite differences in baseline capability, very old males in this relatively slow isokinetic task were more fatigue resistant, but the fatigue-related reduction and recovery in MU rates were similar between groups. Therefore, fatigue in this task between age groups is not differentially affected by alterations in firing rates.NEW & NOTEWORTHY Maximal motor unit firing rates were recorded during an isokinetic fatiguing protocol and short-term recovery in young and very old adult males. Prior studies were limited to isometric fatiguing tasks. Despite the old being ∼37% weaker and less fatigable, anconeus rates during elbow extension declined with fatigue and recovered similarly to young males. Therefore, it is unlikely that greater fatigue resistance of very old males during isokinetic contractions is related to differences in motor unit rates.