Plantar flexor muscle-tendon unit length and stiffness do not influence neuromuscular fatigue in boys and men

Purpose The twofold purpose of this study was (1) to compare differences in development and etiology of neuromuscular fatigue at different plantar flexor (PF) muscle-tendon unit (MTU) lengths between boys and men, and (2) to examine the relationship between musculotendinous stiffness and peripheral fatigue. Methods Nineteen pre-pubertal boys and 23 men performed three intermittent fatigue protocols at different PF MTU lengths (short: S, neutral: N and long: L), consisting of repeating maximal voluntary isometric contractions (MVIC) until the torque reached 60% of the initial value of MVIC. The etiology of the neuromuscular fatigue and the gastrocnemius medialis aponeuroses and tendon stiffness (K-GM) were investigated using non-invasive methods. Result The number of repetitions did not differ between men and boys, regardless of the PF MTU length (S: 16.5 +/- 5.4 and 17.6 +/- 5.8; N: 15.8 +/- 4.5 and 13.3 +/- 3.6; L: 13.6 +/- 4.8 and 12.6 +/- 4.6, respectively). Boys displayed a lower decrement of potentiated twitch torque (Qtw(pot); p < 0.001) and greater decrease of voluntary activation level than men (p < 0.001). Although boys showed lower K-GM values than men at S, no significant correlation was found between K-GM and Qtw(pot). Conclusion PF MTU length had no effect on differences in the development and etiology of neuromuscular fatigue between boys and men. Although both groups displayed similar development of fatigue, central mechanisms mainly accounted for fatigue in boys and peripheral mechanisms were mainly involved in men. Additionally, musculotendinous stiffness did not account for difference in peripheral fatigue between children and adults.