Motor Unit Discharge Rates during Maximal Shortening and Lengthening Contractions at Three Angular Velocities in Human Elbow Extensors.

Abstract

Motor unit discharge rates (MUDR) have been reported to be lower during lengthening compared with shortening contractions, but these comparisons have been limited to submaximal, relatively slow angular velocity contractions. The purpose of this study was to evaluate MUDRs from the triceps brachii during maximal-effort shortening and lengthening contractions across multiple angular velocities. Young males ( n = 10) and females ( n = 5) completed maximal isokinetic shortening and lengthening elbow extensions at 30, 60, and 90 deg·s -1 through a 60° joint range of motion. Fine-wire electrodes were inserted into the triceps brachii for recording MUDR. Surface electromyography was recorded in a monopolar setup from the triceps brachii. In total, 302 and 335 motor unit trains were analyzed during the shortening and lengthening phases, respectively. No differences ( P = 0.686) in mean MUDRs were observed for the shortening and lengthening phases at 30 deg·s -1 (36.4 ± 11.7 vs 34.4 ± 14.2 Hz), 60 deg·s -1 (38.9 ± 14.2 vs 39.5 ± 13.3 Hz), and 90 deg·s -1 (41.3 ± 12.5 vs 41.6 ± 11.7 Hz). In agreement, surface electromyography amplitude showed no difference between shortening and lengthening contractions across all angular velocities ( P = 0.173). MUDRs were lower at 30 deg·s -1 compared with 60 and 90 deg·s -1 ( P < 0.001), but no differences were present between 60 and 90 deg·s -1 ( P = 0.539). Contrary to prior comparisons at submaximal levels, we report no differences in MUDRs during maximal shortening and lengthening contractions, indicating no deficit in motor unit activation between dynamic contraction phases.