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Purpose The purpose of this study was to compare the development and etiologyof neuromuscular fatigue of the knee extensor muscles (KE) at differentmuscle-tendon unit (MTU) lengths during repeated maximal voluntary isometriccontractions (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 atshort (SHORT), optimal (OPT) and long (LONG) MTU lengths, consisting ofrepeating 5-s MVIC interspersed with 5-s passive recovery periods until torquereached 60% of the initial MVIC torque. The etiology of neuromuscular fatiguewas identified using non-invasive methods such as surface electromyography,near-infrared spectroscopy, magnetic nerve stimulation and twitch interpolationtechnique.Results The number of repetitions was significantly lower in men atOPT (14.8$\pm$3.2) and LONG (15.8$\pm$5.8) than boys (39.7$\pm$18.4 and 29.5$\pm$10.2, respectively; p<0.001), while no difference was found at SHORTbetween both age groups (boys: 33.7$\pm$15.4, men: 40.9$\pm$14.2). At OPT andLONG boys showed a lower reduction in the single potentiated twitch (Qtwpot)and a greater decrease in the voluntary activation level (VA) than men. AtSHORT, both populations displayed a moderate Qtwpot decrement and a significantVA reduction (p<0.001). The differences in maximal torque between boys and menwere 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 betweenchildren and adults are dependent on MTU length. Differences in maximal torquecould underpin differences in neuromuscular fatigue between children and adultsat OPT and SHORT. However, at LONG these differences do not seem to beexplained by differences in maximal torque. The origins of this specific effectof MTU length remain to be determined.

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