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Changes in the discharge characteristics of motor units as well as in the maximum force-producing capacity of the muscle are observed following training, aging, and fatiguability. The ability to measure the adaptations in the neuromuscular properties underlying these changes experimentally, however, is limited. In this study we used a computational model to systematically investigate the effects of various neural and muscular adaptations on motor unit recruitment thresholds, average motor unit discharge rates in submaximal contractions, and maximum force. The primary focus was to identify candidate adaptations that can explain experimentally observed changes in motor unit discharge characteristics after 4 wk of strength training (Del Vecchio A, Casolo A, Negro F, Scorcelletti M, Bazzucchi I, Enoka R, Felici F, Farina D. J Physiol 597: 1873-1887, 2019). The simulation results indicated that multiple combinations of adaptations, likely involving an increase in maximum discharge rate across motor units, may occur after such training. On a more general level, we found that the magnitude of the adaptations scales linearly with the change in recruitment thresholds, discharge rates, and maximum force. In addition, the combination of multiple adaptations can be predicted as the linear sum of their individual effects. Together, this implies that the outcomes of the simulations can be generalized to predict the effect of any combination of neural and muscular adaptations. In this way, the study provides a tool for estimating potential underlying adaptations in neural and muscular properties to explain any change in commonly used measures of rate coding, recruitment, and maximum force.

Purpose: This study aimed to compare the effects of four resistance training (RT) programs with different velocity loss (VL) thresholds: 0% (VL0), 10% (VL10), 20% (VL20) and 40% (VL40) on sprint and jump performance, muscle strength, neuromuscular, muscle hypertrophy and architectural adaptations. Methods: Sixty-four young resistance-trained men were randomly assigned into four groups (VL0, VL10, VL20, and VL40) that differed in the VL allowed in each set. Subjects followed an RT program for 8 weeks (2 sessions per week) using the full-squat (SQ) exercise, with similar relative intensity (70-85% 1RM), number of sets (3) and inter-set recovery period (4-min). Before and after the RT program the following tests were performed: 1) muscle hypertrophy and architecture of the vastus lateralis (VLA); 2) tensiomyography; 3) 20-m running sprint; 4) vertical jump; 5) maximal voluntary isometric contraction in SQ; 6) progressive loading test in SQ; and 7) fatigue test. Results: No between-group differences existed for RT-induced gains in sprint, jump and strength performance despite the differences in the total volume performed by each group. VL20 and VL40 showed significant increases (P < 0.001) in muscle hypertrophy (group × time interaction, P = 0.06). However, only VL40 exhibited a significant slowing (P < 0.001) of the delay time in the VLA muscle (group × time interaction: P = 0.05). Moreover, VL40 showed a significant decrease in the early rate of force development (P = 0.04). Conclusions: Higher VL thresholds (i.e. VL20 and VL40) maximized hypertrophic adaptations, although an excessive VL during the set (i.e. VL40) may also induce negative neuromuscular adaptations. Therefore, moderate VL thresholds should be chosen to maximize strength adaptations and to prevent negative neuromuscular adaptations.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) The aim of the study was to compare the effect of resistance training (RT) performed to muscle failure (RT-F) or non-failure muscular (RT-NF) on muscle mass, muscle architecture and activation in trained young men. Thigh lean mass (TLM), vastus lateralis muscle cross-sectional area (CSA), architecture and 1-RM test were assessed at baseline (pre) and after ten (post) RT weeks. EMG amplitude assessment was performed after 10W RT. 14 young trained men participated in the present study were (23 ± 2.3 years, 171.7 ± 5.0 cm, 73.9 ± 5.8 kg). All protocols were similarly effective in increasing TLM (RT-F: 6371.7 ± 629.7 g to 6867.7 ± 657.8 g, RT-NF: 6401.1 ± 587.8 g to 6798.4 ± 610.0 g; P < 0.0001) and CSA (RT-F: 32.9 ± 5.3 cm2 to 37.7 ± 5.6 cm2, RT-NF: 33.1 ± 5.8 cm2 to 37.5 ± 6.6 cm2; P < 0.0001) from pre- to post-training. Both protocols showed significant increases in 1-RM values from pre- to post-training for 45° leg press (RT-F: 237.5 ± 21.6 kg to 290.0 ± 40.2 kg, RT-NF: 237.5 ± 32.9 kg to 299.9 ± 41.4 kg; P < 0.0001), leg extension (RT-F: 55.6 ± 8.6 kg to 73.2 ± 9.8 kg, RT-NF: 56.3 ± 9.5 kg to 73.8 ± 8.4 kg; P < 0.0001), and leg flexion (RT-F: 53.5 ± 8.0 kg to 64.3 ± 6.4 kg, RT-NF: 51.57 ± 7.9 kg to 63.2 ± 6.8 kg; P < 0.0001). EMG amplitude showed no significant differences between protocols (P > 0.05). In relation to the muscular architecture, both the pennation angle (PA) (TF-F: 22.5 ± 3.8º for 25.5 ± 3.8º, TF-NF: 23.1 ± 2.5º for 26.0 ± (P 0,05) entre os protocolos (TF-F: 92,2 ± 24,9% vs. TF-NF: 100,3 ± 25,6%; P > 0,05). Em relação à arquitetura muscular, tanto o ângulo de penação (AP) (TF-F: 22,5 ± 3,8º para 25,5 ± 3,8º; TF-NF: 23,1 ± 2,5º para 26,0 ± 2,9º; P < 0,0001) quanto o comprimento do fascículo (CF) (TF-F: 5,5 ± 0,5 cm para 6,0 ± 0,5 cm; TF-NF: 5,5 ± 0,5 cm para 6,0 ± 0,6 cm; P < 0,0001) apresentaram aumentos significativos do pré para pós-treinamento. Em conclusão, TF-F ou TF-NF são igualmente eficientes em promover aumentos na hipertrofia muscular, força muscular, amplitude de EMG e arquitetura muscular.