Massarelli P S, Green H J, Hughson R L, Sharratt M T
Department of Kinesiology, University of Waterloo, Ontario, Canada.
J Appl Physiol (1985). 1989 Jul;67(1):210-20. doi: 10.1152/jappl.1989.67.1.210.
To investigate the hypothesis that the rate of fatigue development is not influenced by the absolute duration of contraction (train duration) and relaxation (off-phase of duty cycle) at constant duty cycle, strips of the diaphragm from 36 male adult rats (mean +/- SD wt 152 +/- 21 g) were stimulated directly for periods of 180, 250, and 320 ms at a constant duty cycle of 50%. The frequency of stimulation was adjusted to produce 40% of maximal tetanic tension at supramaximal voltages. After 30 min of stimulation, analysis of twitch characteristics between control and experimental groups indicated a prolongation of contraction time of 9% (P less than 0.05), an increase in relaxation time of 75% (P less than 0.05), and a decrease in twitch tension by 78% (P less than 0.05). Similarly, reductions (P less than 0.05) in isometric force output at high stimulation frequency (100 Hz) of 58% and at low frequency (20 Hz) of 67% were also noted. These changes were accompanied by an approximately 60% reduction in the maximal velocity of shortening. No difference was observed for any of the mechanical measures between experimental conditions. After 30-min stimulation, decreases of between 43 and 46% were noted for ATP (P less than 0.05) and increases of between three- and fourfold noted for IMP (P less than 0.05). No changes were found for either ADP or AMP. Total adenine nucleotide concentrations declined (P less than 0.05) an average of 24%. As with the mechanical data, no differences were found between the different stimulation conditions. It is concluded that for the conditions studied, fatigue mechanisms become manifest early in the stimulation period and are only minimally altered by the duration of specific contractions provided the relaxation period is of equal duration.
为了研究在恒定占空比下疲劳发展速率不受收缩（训练时长）和舒张（占空比的非工作阶段）的绝对时长影响这一假说，对36只成年雄性大鼠（平均体重±标准差为152±21克）的膈肌条带进行直接刺激，在50%的恒定占空比下分别刺激180、250和320毫秒。刺激频率调整为在超最大电压下产生最大强直张力的40%。刺激30分钟后，对照组和实验组之间的抽搐特征分析表明，收缩时间延长了9%（P<0.05），舒张时间增加了75%（P<0.05），抽搐张力降低了78%（P<0.05）。同样，在高刺激频率（100赫兹）下等长力输出降低了58%（P<0.05），在低刺激频率（20赫兹）下降低了67%（P<0.05）。这些变化伴随着最大缩短速度降低了约60%。在不同实验条件下，任何机械指标均未观察到差异。刺激30分钟后，ATP降低了43%至46%（P<0.05），IMP增加了三至四倍（P<0.05）。ADP和AMP均未发现变化。总腺嘌呤核苷酸浓度平均下降了24%（P<0.05）。与机械数据一样，不同刺激条件之间未发现差异。结论是，在所研究的条件下，疲劳机制在刺激期早期就表现出来，并且只要舒张期时长相等，特定收缩的时长对其影响极小。
