Muscles are composed primarily of voluntary muscle fibers.
肌肉主要是由随意肌纤维所组成。
Scientist liken voluntary or directed attention to a mental muscle that can wear out over time.
科学家把有意识注意力必做一块脑部肌肉，随着时间的流逝，它会一点点失去力气。
Striated muscle, whose fibres appear striped under a microscope, is responsible for voluntary movement.
横纹肌的纤维在显微镜下成条纹状，负责随意运动。
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