Society For Neuroscience Abstract | 2008

MOTOR UNIT RECRUITMENT IN THE BICEPS BRACHII OF OLDER ADULTS DURING A FATIGUING CONTRACTION. Pascoe MA, Holmes MR, Gaw ME, and Enoka RM The purpose of the study was to examine the influence of age on the discharge characteristics of motor units in the biceps brachii that were recruited during a sustained submaximal contraction. The discharge times of ten single motor units were recorded from the biceps brachii in five old adults (75 ± 4.7 yr). The task was to sustain an isometric contraction at a target force below the recruitment threshold (RT) of an isolated motor unit until it was recruited and discharged action potentials for ~ 60 s. The average RT for these motor units during ramp contractions was 18.1 ± 5.2% MVC force (range; 9.9 – 24%), and the discharge rate at recruitment averaged 9.45 ± 2.4 pps. Subjects performed a sustained contraction with the elbow flexors at a target force that was set at either a relatively small (4.8 ± 2.3% MVC) or large (10.2 ± 2.7% MVC) difference below RT force; these are subsequently referred to as the 5% and 10% groups, respectively. The target force was significantly less (P = 0.004) for the 10% group (7.3 ± 1.7% MVC) compared with the 5% group (12.9 ± 3.5% MVC). The average RT for these motor units was similar for the 5% group (17.8 ± 3.8%) and 10% group (17.5 ± 3.8%). The discharge rate at recruitment during the ramp contractions was also similar for the 5% group (9.2 ± 1.9 pps) and 10% group (9.3 ± 2.4 pps). The time to recruitment during the sustained contraction did not differ (P = 0.201) for the two groups of units: 5% group = 190 ± 190 s, 10% group = 505 ± 758 s. All trials were ended after 81.3 ± 26.2 s of continuous discharge, and were not accompanied by at change in RT after the task (before 18.1% MVC, after 18.3% MVC; P = 0.823). Trials were divided into five equallength epochs, and the discharge characteristics were compared with repeated-measures ANOVA. The mean discharge rate did not change (5% group: 10.8 ± 2.3 pps, 10% group: 10.0 ± 3.6 pps), but the coefficient of variation (CV) for interspike intervals decreased similarly for both groups (5% group: 16.9 to 11.6%, 10% group: 16.7 to 13.5%). There were no differences between groups in discharge rate during the ramp (9.15 and 9.34 pps for the 5% and 10% groups, respectively) and first five interspike intervals of the sustained (10.2 and 10.2 pps) contractions. Similarly, there were no group differences in CV for interspike interval for the ramp (20.3 and 20.0%) and first five interspike intervals of the sustained (20.6 and 16.9%) contractions. These results contrast with findings for young adults, who demonstrated reduced discharge rates and elevated CV for interspike interval values for 10% units during sustained contractions. These data suggest that the integration of synaptic input during sustained contractions differs for young and old adults. Supported by NIA AG09000 to RME. Key Words: motor unit, aging, discharge variability, recruitment threshold