Stevens E D, Syme D A
Department of Zoology, University of Guelph, ON, Canada.
Can J Physiol Pharmacol. 1993 Dec;71(12):910-6. doi: 10.1139/y93-138.
Isolated rat diaphragm muscle was stimulated repetitively to induce fatigue, and the work done during each contraction was measured. Work per cycle was calculated by measuring force as the activated muscle was subjected to sinusoidal length changes (from 97 to 103% of L0, where L0 is rest length). Work was calculated from the loop formed when force was plotted against length. Work done was positive when the muscle was shortening and was negative when it was lengthening; net work was the difference. Work output was varied by changing the stimulus duty cycle (4, 8, or 16% of the total cycle duration) and cycle frequency (1, 2, or 4 Hz). The rate and extent of the decrease in power was influenced much more by changes in cycle frequency than by changes in duty cycle. Duty cycle and cycle frequency combinations that resulted in greater power in the prefatigue trials were associated with a more rapid rate of fatigue. However, net positive power at the end of the 15-min fatigue period was greater under these same conditions (i.e., high duty cycle and high cycle frequency). Fatigue in working diaphragm muscle depends more on cycle frequency than on duty cycle.
对分离的大鼠膈肌进行重复刺激以诱导疲劳，并测量每次收缩过程中所做的功。通过在激活的肌肉进行正弦长度变化（从L0的97%至103%，其中L0为静息长度）时测量力来计算每个周期的功。功是根据力与长度的关系图所形成的环线来计算的。当肌肉缩短时所做的功为正，肌肉拉长时为负；净功为两者之差。通过改变刺激占空比（占总周期时长的4%、8%或16%）和周期频率（1Hz、2Hz或4Hz）来改变功输出。功率下降的速率和程度受周期频率变化的影响远大于占空比变化的影响。在疲劳前试验中产生更大功率的占空比和周期频率组合与更快的疲劳速率相关。然而，在相同条件下（即高占空比和高周期频率），15分钟疲劳期结束时的净正功率更大。工作中的膈肌疲劳更多地取决于周期频率而非占空比。
