伸直 is a verb meaning 'to straighten (a limb or the body)' — for example, extending an arm, leg, or spine fully so it forms a straight line. The first character 伸 means 'to stretch' or 'to extend', emphasizing active movement outward or lengthening; the second character 直 means 'straight' or 'direct', specifying the resulting posture or alignment. Together, they form a compound verb that describes the physical action of removing bends or curves from a part of the body.

This word is commonly used in health, fitness, and medical contexts — such as physical therapy instructions, yoga cues, or doctor’s advice after injury. It often appears with body parts like 手臂 (arm), 腿 (leg), 背 (back), or 腰 (lower back). While it can occasionally describe non-human subjects (e.g., a bent wire being straightened), its primary usage relates to human posture and movement control.