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PURPOSE The purpose of this study was to compare the development and etiology of neuromuscular fatigue of the knee extensor muscles at different muscle-tendon unit (MTU) lengths during repeated maximal voluntary isometric contractions (MVIC) between boys and men. METHODS Twenty-two prepubertal boys (9-11 yr) and 22 men (18-30 yr) performed three knee extensor 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 noninvasive 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), whereas 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 than men. At SHORT, both populations displayed a moderate Qtwpot decrement and a significant voluntary activation 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). CONCLUSIONS 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.

Epub ahead of print; PURPOSE: To compare the effects of 8-week eccentric (ECC) versus concentric (CON) training using downhill and uphill running in rats on whole body composition, bone mineral density (BMD), and energy expenditure (EE). METHODS: Animals were randomly assigned to one of the following groups: 1) control (CTRL), 2) +15% uphill-running slope (CON), 3) -15% downhill-running slope (ECC15) and 4) -30% downhill-running slope (ECC30). Those programs enabled to achieve conditions of iso-power output for CON and ECC15 and of iso-oxygen uptake (VO2) for CON and ECC30. Trained rats ran 45min at 15 mmin five times per week. Total (TBM), fat (FBM) and lean (LBM) body mass measured through EchoMRI, and 24-hour EE including basal metabolic rate (BMR) assessed using PhenoMaster/LabMaster cage system were obtained before and after training. At sacrifice, the right femur was collected for bone parameters analysis. RESULTS: While TBM increased in all groups over the 8-week period, almost no change occurred for FBM in exercised groups (CON: -4.8+/-6.18 g; ECC15: 0.6+/-3.32 g; ECC30: 2.6+/-6.01 g). The gain in LBM was mainly seen for ECC15 (88.9+/-6.85 g) and ECC30 (101.6+/-11.07 g). ECC was also seen to positively affect BMD. An increase in BMR from baseline was seen in exercise groups (CON: 13.9+/-4.13 kJ.day; ECC15: 11.6+/-5.10 kJ.day; ECC30: 18.3+/-4.33 kJ.day) but not in CTRL one. This difference disappeared when BMR was normalized for LBM. CONCLUSIONS: Results indicate that for iso-VO2 training, the impact on LBM and BMD is enhanced with ECC as compared to CON, and that for iso-power but lower VO2 ECC, an important stimulus for adaptation is still observed. This provides further insights for the use of ECC in populations with cardiorespiratory exercise limitations.