Effect of MTU length on child-adult difference in neuromuscular fatigue

TL;DR

This study investigates how muscle-tendon unit length influences neuromuscular fatigue differences between children and adults during repeated maximal contractions, revealing that fatigue patterns vary with MTU length and are linked to maximal torque differences.

Contribution

It demonstrates that neuromuscular fatigue differences between children and adults depend on MTU length, highlighting the role of maximal torque in fatigue development at specific lengths.

Findings

Children show greater fatigue at optimal and long MTU lengths.

Differences in maximal torque are more pronounced at optimal length.

Fatigue mechanisms vary with MTU length and age group.

Abstract

Purpose The purpose of this study was to compare the development and etiology of neuromuscular fatigue of the knee extensor muscles (KE) at different muscle-tendon unit (MTU) lengths during repeated maximal voluntary isometric contractions (MVIC) between boys and men.Methods Twenty-two pre-pubertal boys (9-11 years) and 22 men (18-30 years) performed three KE fatigue protocols at short (SHORT), optimal (OPT) and long (LONG) MTU lengths, consisting of repeating 5-s MVIC interspersed with 5-s passive recovery periods until torque reached 60% of the initial MVIC torque. The etiology of neuromuscular fatigue was identified using non-invasive methods such as surface electromyography, near-infrared spectroscopy, magnetic nerve stimulation and twitch interpolation technique.Results The number of repetitions was significantly lower in men at OPT (14.8$\pm$3.2) and LONG (15.8$\pm$5.8) than boys (39.7$\pm$18.4 and 29.5 $\pm$10.2, respectively; p<0.001), while no difference was found at SHORT between both age groups (boys: 33.7$\pm$15.4, men: 40.9$\pm$14.2). At OPT and LONG boys showed a lower reduction in the single potentiated twitch (Qtwpot) and a greater decrease in the voluntary activation level (VA) than men. At SHORT, both populations displayed a moderate Qtwpot decrement and a significant VA reduction (p<0.001). The differences in maximal torque between boys and men were almost twice greater at OPT (223.9 N.m) than at SHORT (123.3 N.m) and LONG (136.5 N.m).Conclusion The differences in neuromuscular fatigue between children and adults are dependent on MTU length. Differences in maximal torque could underpin differences in neuromuscular fatigue between children and adults at OPT and SHORT. However, at LONG these differences do not seem to be explained by differences in maximal torque. The origins of this specific effect of MTU length remain to be determined.