Syme Douglas A, Tonks Dillon M
Dept. of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada.
Am J Physiol Regul Integr Comp Physiol. 2004 May;286(5):R916-26. doi: 10.1152/ajpregu.00347.2003. Epub 2004 Jan 15.
Muscle fatigue reflects alterations of both activation and cross-bridge function, which will have markedly different affects on steady-state vs. dynamic performance. Such differences offer insight into the specific origins of fatigue, its mechanical manifestation, and its consequences for animal movement. These were inferred using dynamic contractions (twitches and cyclic work as might occur during locomotion) and steady-state performance with maximal, sustained activation (tetani, stiffness, and isokinetic force) during fatigue and then recovery of frog (Rana pipiens) anterior tibialis muscle. Stiffness remained unaltered during early fatigue of force and then declined only 25% as force dropped 50%, suggesting a decline with fatigue in first the force-generating ability and then the number of cross bridges. The relationship between stiffness and force was different during fatigue and recovery; thus the number of cross bridges and force per cross bridge are not intimately linked. Twitch duration increased with fatigue and then recovered, with trajectories that were remarkably similar to and linear with changes in tetanic force, perhaps belying a common mechanism. Twitch force increased and then returned to resting levels during fatigue, reflecting a slowing of activation kinetics and a decline in cross-bridge number and force. Net cyclic work fatigued to the degree of becoming negative when tetanic force had declined only 15%. Steady-state isokinetic force (i.e., shortening work) declined by 75%, while cyclic shortening work declined only 30%. Slowed activation kinetics were again responsible, augmenting cyclic shortening work but greatly augmenting lengthening work (reducing net work). Steady-state measures can thus seriously mislead regarding muscle performance in an animal during fatigue.
肌肉疲劳反映了激活和横桥功能的改变，这对稳态与动态性能会产生明显不同的影响。这些差异有助于深入了解疲劳的具体起源、其力学表现及其对动物运动的影响。通过对青蛙（豹蛙）胫前肌在疲劳及随后恢复过程中进行动态收缩（如运动时可能出现的单收缩和周期性工作）以及最大持续激活下的稳态性能（强直收缩、刚度和等动力）来推断这些差异。在力量早期疲劳阶段，刚度保持不变，随后随着力量下降50%，刚度仅下降25%，这表明随着疲劳，首先是力量产生能力下降，然后是横桥数量减少。疲劳和恢复过程中刚度与力量的关系不同；因此，横桥数量与每个横桥的力量并非紧密相连。单收缩持续时间随着疲劳增加，然后恢复，其变化轨迹与强直力量的变化非常相似且呈线性关系，这可能揭示了一种共同机制。单收缩力量在疲劳时先增加然后回到静息水平，这反映了激活动力学的减慢以及横桥数量和力量的下降。当强直力量仅下降15%时，净周期性工作疲劳到变为负值的程度。稳态等动力力量（即缩短工作）下降了75%，而周期性缩短工作仅下降了30%。激活动力学减慢再次起作用，增加了周期性缩短工作，但大大增加了延长工作（减少了净工作）。因此，稳态测量可能会严重误导对动物在疲劳期间肌肉性能的判断。
