肌肉是一种能收缩的动物组织，属于软组织，由胚胎的中胚层发育而来。肌肉细胞有收缩纤维，会在细胞间移动并改变细胞的大小。

肌肉分为骨骼肌、心肌和平滑肌三种，其功能皆为产生力并导致运动。心肌和平滑肌的收缩不由意识控制且为生存所必需，例如心脏的收缩或是肠胃道的蠕动等。骨胳肌的自主收缩用来移动身体且能够被精细地控制，例如眼睛的运动或大腿股四头肌的总体运动。自主肌肉纤维分成快慢两种，慢肌纤维可以持续较长的时间，但力量较小；快肌纤维收缩地较快，力量也较大，但也较快感到疲劳。

Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis.