三个地方不明白？

The diaphragm, like other striated muscles, is subject to fatigue, which is defined as an inability to sustain tension with repeated activity. For nonrespiratory skeletal muscle, fatigue may be ‘central’—that is, the subject is not trying hard enough (either consciously or subconsciously)—but this is unlikely to be significant in respiration because subjects with respiratory failure usually have a high central respiratory drive. Peripheral fatigue occurs when the frequency of motor nerve action potentials becomes chronically increased in an attempt to increase muscle tension.Eventually, when working against an unsustainable load, relaxation of the muscle fibre, the energy-requiring part of contraction, becomes excessively prolonged, and the muscle is unable to respond to the next action potential to generate the required tension. In the diaphragm, resistive loads less than 40% of maximum may be sustained indefinitely, but loads greater than 40% of maximum can only be sustained for a short time.

膈肌与其他横纹肌一样，也会疲劳，表现为重复活动时无法维持张力。对于非呼吸性骨骼肌而言，疲劳可能是“中枢性”的——即有意无意的“未竭尽全力”——但呼吸肌的“中枢性”疲劳可能不明显，因为呼吸衰竭患者通常有很高的中枢呼吸驱动力。当试图增加肌张力，而运动神经动作电位的频率持续提高时，就会发生外周性呼吸肌疲劳。最终，非持续性负荷时，呼吸肌纤维耗能的松弛期会过度延长，直至无法对下一动作电位做出反应，也就无法产生所需张力。对膈肌而言，当阻力负荷小于最大负荷的40%时可以持续维持肌张力，但大于最大负荷的40%时只能维持很短时间的肌张力。

大家看看理解的对吗？我有点不明白，

1.“负荷不可持续时，呼吸肌纤维耗能的松弛期会过度延长”，呼吸肌松弛反而是耗能的？

2.还有“松弛期为什么会过度延长”？

3.这个维持，应该指“维持肌张力”吧？对膈肌而言，当阻力负荷小于最大负荷的40%时可以不间断维持肌张力，但大于最大负荷的40%时只能维持很短时间的肌张力。