Muscle damage occurs when muscle fibers are injured during physical activity, especially during intense exercise or unfamiliar movements. This damage can lead to soreness, swelling, and a temporary decrease in strength. It is a normal part of the muscle-building process, as the body repairs and rebuilds these fibers to become stronger. The primary cause of muscle damage is eccentric contraction, where muscles lengthen while under tension, such as during weightlifting or running downhill. While some muscle damage is beneficial for growth, excessive damage can lead to conditions like rhabdomyolysis, which can be harmful and requires medical attention.