Hoffman Ben W, Cresswell Andrew G, Carroll Timothy J, Lichtwark Glen A
Centre for Sensorimotor Performance, School of Human Movement and Nutrition Sciences, The University of Queensland, Brisbane, Queensland, AUSTRALIA.
Med Sci Sports Exerc. 2016 Aug;48(8):1495-505. doi: 10.1249/MSS.0000000000000920.
The repeated bout effect characterizes the protective adaptation after a single bout of unaccustomed eccentric exercise that induces muscle damage. Sarcomerogenesis and increased tendon compliance have been suggested as potential mechanisms for the repeated bout effect by preventing muscle fascicles from being stretched onto the descending limb of the length-tension curve (the region where sarcomere damage is thought to occur). In this study, evidence was sought for three possible mechanical changes that would support either the sarcomerogenesis or the increased tendon compliance hypotheses: a sustained rightward shift in the fascicle length-tension relationship, reduced fascicle strain amplitude, and reduced starting fascicle length.
Subjects (n = 10) walked backward downhill (5 km·h, 20% incline) on a treadmill for 30 min on two occasions separated by 7 d. Kinematic data and medial gastrocnemius fascicle lengths (ultrasonography) were recorded at 10-min intervals to compare fascicle strains between bouts. Fascicle length-torque curves from supramaximal tibial nerve stimulation were constructed before, 2 h after, and 2 d after each exercise bout.
Maximum torque decrement and elevated muscle soreness were present after the first, but not the second, backward downhill walking bout signifying a protective repeated bout effect. There was no sustained rightward shift in the length-torque relationship between exercise bouts, nor decreases in fascicle strain amplitude or shortening of the starting fascicle length.
Protection from a repeated bout of eccentric exercise was conferred without changes in muscle fascicle strain behavior, indicating that sarcomerogenesis and increased tendon compliance were unlikely to be responsible. As fascicle strains are relatively small in humans, we suggest that changes to connective tissue structures, such as extracellular matrix remodeling, are better able to explain the repeated bout effect observed here.
重复运动效应是指单次不习惯的离心运动诱发肌肉损伤后出现的保护性适应。肌节生成和肌腱顺应性增加被认为是重复运动效应的潜在机制，其通过防止肌束在长度-张力曲线的下降支（被认为是肌节损伤发生的区域）上被拉伸来实现。在本研究中，我们寻找支持肌节生成或肌腱顺应性增加假说的三种可能的力学变化的证据：肌束长度-张力关系持续右移、肌束应变幅度降低以及起始肌束长度缩短。
10名受试者在跑步机上以5 km·h的速度、20%的坡度向后下坡行走30分钟，共进行两次，两次之间间隔7天。每隔10分钟记录运动学数据和腓肠肌内侧肌束长度（超声检查），以比较两次运动之间的肌束应变。在每次运动前、运动后2小时和运动后2天构建超最大胫神经刺激下的肌束长度-扭矩曲线。
第一次向后下坡行走运动后出现最大扭矩下降和肌肉酸痛加剧，但第二次运动后未出现，这表明存在保护性的重复运动效应。两次运动之间的长度-扭矩关系没有持续右移，肌束应变幅度也没有降低，起始肌束长度也没有缩短。
重复的离心运动产生了保护作用，但肌肉肌束应变行为没有变化，这表明肌节生成和肌腱顺应性增加不太可能是其原因。由于人类的肌束应变相对较小，我们认为结缔组织结构的变化，如细胞外基质重塑，更能解释此处观察到的重复运动效应。
