Effect of Muscle–Tendon Unit Length on Child–Adult Difference in Neuromuscular Fatigue

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±3.2) and LONG (15.8±5.8) than boys (39.7±18.4 and 29.5 ±10.2, respectively; p<0.001), while no difference was found at SHORT between both age groups (boys: 33.7±15.4, men: 40.9±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.

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

“…We hypothesized that the similar fatigue between boys and men with PF muscles could be explained by reduced differences in the torque level of the PF, compared with the KE muscles. This proposition was also sustained by the fact that boys and men performed the same number of repetitions during an intermittent fatigue protocol with KE at short MTU length, where difference in torque level between boys and men is lower than at optimal MTU length (Piponnier et al 2019). However, in the present study, PF MTU length had no effect on differences in the development of the neuromuscular fatigue between boys and men, while it influenced differences in torque level between age groups.…”

“…This postulation is sustained by the statistical analyses conducted by Ratel and collaborators (2015), showing that the development and etiology of the neuromuscular fatigue were not different when the initial torque level was used as co-variate. In addition, our previous results showed that difference in torque levels between children and adults vary as a function of muscle groups (Piponnier et al 2018a) and knee extensors (KE) muscle-tendon unit (MTU) length (Piponnier et al 2019). In these studies, reduced torque level differences between children and adults (i.e.…”

Eccentric Resistance Training in Youth: Perspectives for Long-Term Athletic Development

“…The lesser fatigue in youths compared to adults following exercise has been shown within a range of exercise modalities with pre-pubertal boys displaying a greater maintenance of peak torque than adults following dynamic maximum isokinetic [ 67 , 73 , 74 , 75 , 76 , 77 ] and isometric [ 65 , 78 , 79 ] actions of the knee extensors and flexors, maximal isometric exercise of the elbow flexors [ 68 ], and isometric exercise of the plantar flexors [ 64 ]. The lower reductions in force-generating capacity following resistance exercise-induced fatigue in youth compared to adults have been shown to occur in conjunction with smaller alterations of neuromuscular properties including sarcolemma excitability (i.e., M-wave amplitude), excitation–contraction coupling (i.e., low-to-high frequency tetanic force ratio or low-frequency fatigue), muscle contractility (i.e., high-frequency torque) and muscle oxygenation [ 63 , 64 , 68 , 71 , 72 , 73 , 78 , 79 , 80 , 81 ]. Conversely, during submaximal isometric RT, differences in fatigue responses including torque and surface electromyography (EMG) have not been observed between children and men [ 82 , 83 ].…”

“…The variances in fatigue during exercise may be muscle-dependent as well as being influenced by the muscle-tendon unit (MTU) length. This has been observed during isometric fatiguing actions of the plantar flexor (PF) and knee extensor (KE) muscle groups in which boys fatigued similarly to men with the PF muscles but to a lower extent with the KE muscles than men [ 80 ]. With regards to the MTU length, neuromuscular fatigue at optimal and long MTU lengths are lower in children compared to adults, which are mainly accounted for by central fatigue (i.e., a reduction in drive to the muscle), rather than peripheral fatigue (i.e., failure in muscle contractility and excitation–contraction coupling (E–C).…”

“…With regards to the MTU length, neuromuscular fatigue at optimal and long MTU lengths are lower in children compared to adults, which are mainly accounted for by central fatigue (i.e., a reduction in drive to the muscle), rather than peripheral fatigue (i.e., failure in muscle contractility and excitation–contraction coupling (E–C). Conversely, at short MTU lengths, the differences in neuromuscular fatigue between children and adults are significantly reduced [ 80 ]. The lesser performance fatigability and peripheral fatigue at short MTU length could be partly explained by a lower torque level than at optimal length, where greater alterations in peripheral mechanisms (i.e., EC coupling) are observed.…”