Motor Unit Firing Rates Of Maximal Lengthening And Shortening Elbow Extensions Across 3 Angular Velocities

PURPOSE: Motor unit firing rates (MUFRs) have been reported to be lower during lengthening compared to static or shortening contractions. However, comparisons of MUFRs between shortening and lengthening contractions have mostly been limited to sub-maximal efforts. We compared MUFRs of the triceps brachii during maximal lengthening and shortening elbow extensions across 3 angular isokinetic velocities. METHODS: Nine young adults (25.0 ± 4.5 years, n = 2 females) attended 1 to 3 data collection sessions, seated on an isokinetic dynamometer with their non-dominant arm firmly secured to an elbow extension attachment. Elbow joint range of motion was set to 60° (from 60° to 120° of elbow flexion). Three pairs of intramuscular fine-wire electrodes (50 μm, adhered together) were inserted into the lateral head of the triceps brachii. A single lengthening and shortening contraction was performed in series (~3 seconds between, randomized order) at isokinetic velocities of 30, 60, and 90 °/s (6-9 sets/session). Three minutes of rest was provided between sets. Motor unit trains (minimum of 4 inter-spike intervals per train) were analyzed and compared across contraction mode (lengthening vs. shortening) and isokinetic velocities. RESULTS: Across all velocities, peak lengthening and shortening torque was ~114% and ~ 84% of isometric torque, respectively. From the 411 motor unit trains analyzed, there was an average inter-spike interval count per train of 10.7 ± 7.6. The mean MUFR for shortening contractions at 30, 60, and 90 °/s was 32.4 ± 10.7, 37.0 ± 12.6, and 40.2 ± 12.0 Hz, respectively; whereas for lengthening contractions at 30, 60, and 90 °/s, they were 31.3 ± 11.5, 35.5 ± 12.4, and 35.5 ± 10.1 Hz, respectively. There were no differences in MUFRs between shortening and lengthening contractions (p = 0.760), but rates were lower (p = 0.002) at 30 °/s than those during 60 and 90 °/s. CONCLUSIONS: These preliminary findings indicate mean MUFRs are similar during maximal lengthening and shortening isokinetic contractions, which contrasts with previous reports during sub-maximal efforts. However, higher angular velocities were associated with higher MUFRs, reinforcing the role of angular velocity on motor unit control mechanisms. Supported by NSERC. NSERC