肌肉 (jī ròu) literally means 'muscle tissue' and refers to the specialized soft tissue in animals—including humans—that contracts to produce movement or maintain posture. The character 肌 (jī) means 'muscle' or 'flesh' and often appears in medical or biological terms, while 肉 (ròu) means 'meat' or 'flesh'—here emphasizing the physical, organic substance of muscle. Together, they form a standard, unambiguous noun for skeletal, smooth, or cardiac muscle in anatomical, fitness, and health contexts.

This word is used in everyday speech when discussing exercise, injury, aging, or nutrition—for example, building muscle, muscle pain, or muscle weakness. It’s neutral in register, appropriate in both casual conversation and formal writing. Unlike English, Chinese rarely uses 肌肉 alone as a countable noun without measure words (e.g., 一块肌肉), but it commonly appears in compound terms like 肌肉力量 (muscle strength) or 肌肉酸痛 (muscle soreness).