Casolo Andrea, Del Vecchio Alessandro, Balshaw Thomas G, Maeo Sumiaki, Lanza Marcel Bahia, Felici Francesco, Folland Jonathan P, Farina Dario
Department of Bioengineering, Imperial College London, London, United Kingdom.
Department of Biomedical Sciences, University of Padua, Padua, Italy.
J Appl Physiol (1985). 2021 Nov 1;131(5):1584-1598. doi: 10.1152/japplphysiol.00192.2021. Epub 2021 Oct 7.
Neural and morphological adaptations combine to underpin the enhanced muscle strength following prolonged exposure to strength training, although their relative importance remains unclear. We investigated the contribution of motor unit (MU) behavior and muscle size to submaximal force production in chronically strength-trained athletes (ST) versus untrained controls (UT). Sixteen ST (age: 22.9 ± 3.5 yr; training experience: 5.9 ± 3.5 yr) and 14 UT (age: 20.4 ± 2.3 yr) performed maximal voluntary isometric force (MViF) and ramp contractions (at 15%, 35%, 50%, and 70% MViF) with elbow flexors, whilst high-density surface electromyography (HDsEMG) was recorded from the biceps brachii (BB). Recruitment thresholds (RTs) and discharge rates (DRs) of MUs identified from the submaximal contractions were assessed. The neural drive-to-muscle gain was estimated from the relation between changes in force (ΔFORCE, i.e. muscle output) relative to changes in MU DR (ΔDR, i.e. neural input). BB maximum anatomical cross-sectional area (ACSA) was also assessed by MRI. MViF (+64.8% vs. UT, < 0.001) and BB ACSA (+71.9%, < 0.001) were higher in ST. Absolute MU RT was higher in ST (+62.6%, < 0.001), but occurred at similar normalized forces. MU DR did not differ between groups at the same normalized forces. The absolute slope of the ΔFORCE - ΔDR relationship was higher in ST (+66.9%, = 0.002), whereas it did not differ for normalized values. We observed similar MU behavior between ST athletes and UT controls. The greater absolute force-generating capacity of ST for the same neural input demonstrates that morphological, rather than neural, factors are the predominant mechanism for their enhanced force generation during submaximal efforts. In this study, we observed that recruitment strategies and discharge characteristics of large populations of motor units identified from biceps brachii of strength-trained athletes were similar to those observed in untrained individuals during submaximal force tasks. We also found that for the same neural input, strength-trained athletes are able to produce greater absolute muscle forces (i.e., neural drive-to-muscle gain). This demonstrates that morphological factors are the predominant mechanism for the enhanced force generation during submaximal efforts.
神经和形态学适应共同作用，为长期进行力量训练后肌肉力量的增强提供支持，尽管它们的相对重要性仍不明确。我们研究了运动单位（MU）行为和肌肉大小对长期进行力量训练的运动员（ST）与未训练对照组（UT）次最大力量产生的贡献。16名ST（年龄：22.9±3.5岁；训练经验：5.9±3.5年）和14名UT（年龄：20.4±2.3岁）用肘屈肌进行最大自主等长力量（MViF）和斜坡收缩（分别为MViF的15%、35%、50%和70%），同时从肱二头肌（BB）记录高密度表面肌电图（HDsEMG）。评估从次最大收缩中识别出的运动单位的募集阈值（RTs）和放电率（DRs）。根据力量变化（ΔFORCE，即肌肉输出）相对于运动单位放电率变化（ΔDR，即神经输入）的关系估计神经驱动-肌肉增益。还通过MRI评估了BB的最大解剖横截面积（ACSA）。ST的MViF（比UT高64.8%，P<0.001）和BB的ACSA（高71.9%，P<0.001）更高。ST的绝对运动单位RT更高（高62.6%，P<0.001），但在相似的标准化力量下出现。在相同的标准化力量下，两组之间的运动单位放电率没有差异。ST中ΔFORCE-ΔDR关系的绝对斜率更高（高66.9%，P=0.002），而标准化值没有差异。我们在ST运动员和UT对照组之间观察到相似的运动单位行为。对于相同的神经输入，ST更大的绝对力量产生能力表明，形态学因素而非神经因素是他们在次最大努力中增强力量产生的主要机制。在本研究中，我们观察到从力量训练运动员的肱二头肌中识别出的大量运动单位的募集策略和放电特征与未训练个体在次最大力量任务中观察到的相似。我们还发现，对于相同的神经输入，力量训练运动员能够产生更大的绝对肌肉力量（即神经驱动-肌肉增益）。这表明形态学因素是次最大努力中增强力量产生的主要机制。
