Department of Physiological Sciences, Universit`a degli Studi di Firenze, Viale G.B. Morgagni 63, 50134 Florence, Italy.
J Physiol. 2011 Jul 1;589(Pt 13):3371-81. doi: 10.1113/jphysiol.2011.209874. Epub 2011 May 3.
Fatigue occurring during exercise can be defined as the inability to maintain the initial force or power output. As fatigue becomes pronounced, force and maximum velocity of shortening are greatly reduced and force relaxation is prolonged. In principle, force loss during fatigue can result from a decrease in the number of cross-bridges generating force or a decrease of the individual cross-bridge force or to both mechanisms. The present experiments were made to investigate this point in single fibres or small fibre bundles isolated from flexor digitorum brevis (FDB) of C57BL/6 mice at 22-24◦C. During a series of 105 tetanic contractions, we measured force and fibre stiffness by applying small sinusoidal length oscillations at 2.5 or 4 kHz frequency to the activated preparation and measuring the resulting force changes. Stiffness data were corrected for the influence of compliance in series with the cross-bridge ensemble. The results show that the force decline during the first 20 tetani is due to the reduction of force developed by the individual cross-bridges and thereafter as fatigue becomes more severe, the number of cross-bridges decreases. In spite of the force reduction in the early phase of fatigue, there was an increased rate of tetanic force development and relaxation. In the latter stages of fatigue, the rate of force development and relaxation became slower. Thus, the start of fatigue is characterised by decreased cross-bridge force development and as fatigue becomes more marked, the number of cross-bridges decreases. These findings are discussed in the context of the current hypotheses about fatigue mechanisms.
运动过程中的疲劳可定义为无法维持初始力量或功率输出。随着疲劳的加剧，力量和最大缩短速度会大大降低，而力量松弛时间会延长。原则上，疲劳过程中的力量损失可能是由于产生力量的横桥数量减少，或者单个横桥的力量减少，或者这两种机制同时发生。本实验旨在研究 22-24°C 下从小鼠屈趾短肌（FDB）分离的单个纤维或小纤维束中出现这种情况的原因。在一系列 105 次强直收缩中，我们通过在激活的制备物上施加 2.5 或 4 kHz 频率的小正弦长度振荡来测量力量和纤维硬度，并测量产生的力量变化。硬度数据经过与横桥集合串联的顺应性的影响进行了修正。结果表明，在前 20 次强直收缩期间力量下降是由于单个横桥产生的力量减少，此后随着疲劳加剧，横桥数量减少。尽管在疲劳的早期阶段力量下降，但强直收缩的发展和松弛速度加快。在疲劳的后期阶段，力量的发展和松弛速度变得更慢。因此，疲劳的开始阶段的特征是横桥力量的发展减少，随着疲劳的加剧，横桥的数量减少。这些发现与关于疲劳机制的当前假设进行了讨论。
