$(a)$ If the cerebellum is not functioning properly,what are the activities of our body affected?
$(b)$ How do muscle cells move?

(N/A) If the cerebellum is not functioning properly,the voluntary actions of our body will be affected,specifically those related to maintaining the posture and balance of the body.
$(b)$ Muscle cells move by changing their shape through the contraction of specialized proteins,which causes the cells to shorten.