Another factor is physical inactivity, as seen in astronauts in a zero-gravity environment where they just don't use their musculoskeletal system as hard as when they're on earth.

另一个因素是运动量小，见于宇航员在零重力环境下的状态，他们不会像和在地球上那样利用自身的骨骼肌系统。

Primary functions of the musculoskeletal system include support of the body, provision of motion, and protection of vital organs.

该系统的主要功能包括支撑身体，支持运动和保护重要器官。

Only skeletal and smooth muscles are part of the musculoskeletal system and only the skeletal muscles can move the body.

只有骨骼肌和平滑肌是肌肉骨骼系统的组成部分，且只有骨骼肌能够使身体运动。