To investigate the involvement of supraspinal fatigue in the loss of maximal inspiratory pressure (PI_max) we fatigued the inspiratory muscles. Six participants performed 5 sustained maximal isometric inspiratory efforts (15-s contractions, duty cycle ~75%) which reduced PI_max, as measured from esophageal and mouth pressure, to around half of their initial maximums. Transcranial magnetic stimulation (TMS) delivered over the motor cortex near the beginning and end of each maximal effort evoked superimposed twitch-like increments in the ongoing PI_max, increasing from ~1.0% of PI_max in the unfatigued contractions to ≥40% of ongoing PI_max for esophageal and mouth pressures. The rate of increase in the superimposed twitch as PI_max decreased with fatigue was not significantly different between the esophageal and mouth pressure measures. The inverse relationship between superimposed twitch pressure and PI_max indicates a progressive decline in the ability of motor cortical output to drive the inspiratory muscles maximally, leading to the development of supraspinal fatigue. TMS also evoked silent periods in the electromyographic recordings of diaphragm, scalenes and parasternal intercostals. The duration of the silent period increased with fatigue in all three muscles, which suggests greater intracortical inhibition, with the largest change observed in diaphragm. The peak rate of relaxation in pressure during the silent period slowed as fatigue developed, indicating peripheral contractile changes in the active inspiratory muscles. These changes in the markers of fatigue show that both central and peripheral fatigue contribute to the loss in PI_max when inspiratory muscles are fatigued with repeated sustained maximal efforts.

中文翻译：

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反复持续最大努力下人吸气肌的上睑疲劳

为了研究脊髓上神经疲劳与最大吸气压力（PI _max）的降低有关，我们使吸气肌肉疲劳。六名参与者进行了5次持续的最大等距吸气努力（15 s收缩，占空比约为75％），这使食管和口腔压力下的PI _max降低至其初始最大值的一半左右。经颅磁刺激（TMS）在每个最大作用力的开始和结束附近在运动皮层上传递，在进行中的PI _max中叠加了类似抽搐的增量，从无疲劳性收缩中的PI _max的〜1.0％增加到进行中的≥40％_最大PI用于食道和口腔压力。在食管和口腔压力措施之间，随着PI _max的降低，随着疲劳强度的增加，叠加抽搐的增加率没有显着差异。叠加抽搐压力与PI _max的逆关系表明运动皮质输出最大程度地驱动吸气肌肉的能力逐渐下降，导致脊柱上疲劳的发展。TMS还引起diaphragm肌，斜角肌和胸骨旁肋间肌肌电记录的沉默期。沉默期的持续时间随着所有三块肌肉的疲劳而增加，这表明更大的皮层内抑制，在diaphragm肌中观察到最大的变化。随着疲劳的发展，沉默期间压力的峰值放松速率减慢，这表明活动的吸气肌肉周围出现收缩性变化。疲劳指标的这些变化表明，中央疲劳和外周疲劳均会导致PI _max降低 反复持续不断的最大努力使吸气肌肉疲劳时。