These bundles receive signals from the nervous system that contract the fibers, which in turn generates force and motion.

这些聚合体收到了神经系统的指示之后就会告知纤维，由纤维完成力度和动作的控制。

You need a certain amount of calcium for your muscles to work, because it makes your muscle fibers contract.

您的肌肉需要一定量的钙才能发挥作用，因为它会使您的肌肉纤维收缩。

Mechanistically, this difference might be explained by caffeine's connection to improving motor unit recruitment, or the ability of your brain and muscles to communicate and contract your muscle fibers.

从机制上讲，这种差异可能是因为咖啡因与提高运动单位募集有关，或者是与大脑和肌肉交流和收缩肌肉纤维的能力有关。