Motor Unit Activity at Recruitment When Young and Old Adults Support Compliant Loads

Abstract

Two variables known to influence the integration of synaptic inputs and thereby modulate the discharge characteristics of motor units are the age of the individual and the compliance of the load supported by a limb. PURPOSE: To compare the influence of load compliance on the discharge characteristics of biceps brachii motor units at recruitment in young (Y) and old (O) humans. METHODS: The discharge times of single motor units were recorded from the biceps brachii in 16 Y adults (28 ± 4 yr) and 14 O adults (75 ± 4 yr). Two sustained contractions were performed with the elbow flexors using loads set at either small (11.7 ± 4.4) or large (17.8 ± 6.5% maximum) differences below the motor unit recruitment threshold. The task required subjects to maintain elbow angle at 1.57 rad. Motor unit activity was compared with data from Y and O adults who performed a similar protocol with the wrist fixed in a rigid restraint. RESULTS: Twenty motor units were recorded for each group of subjects. The average recruitment threshold forces did not differ between groups (Y: 25.9 ± 12.6%; O: 30.0 ± 10.9% maximum, p = 0.07). Y adults had a greater number of motor units that discharged action potentials intermittently at recruitment (small: 60%, large: 75% of tasks) than O adults (small: 20%, large: 20% of tasks). The time to recruitment for both Y and O was longer for the large difference (187 ± 227 s vs. 23 ± 46 s, p < 0.001). Discharge rate was similar for the two target-force differences in O (9.13 ± 2.9 vs. 9.29 ± 4.8 pps), but was less in Y for the large difference (8.57 ± 2.5 vs. 10.9 ± 3.3 pps, p = 0.04). The coefficient of variation for interspike interval was similar between target-force differences for Y and O (p = 0.43), but was less in O (22.4 ± 9.4% vs. 27.6 ± 12.3%, p = 0.07). Discharge rate did not differ between small and large differences in either Y or O for either load condition (p = 0.06). However, discharge variability was greater for the large target-force difference in Y when performing the rigid-restraint tasks (p = 0.002) and did not differ in O. CONCLUSION: Discharge rate and discharge variability at recruitment did not differ for either load for the old adults, suggesting that the integration of synaptic input differs between young and old adults. Supported by AG-09000 to RME.