  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.
