A muscle strain, also known as a pulled muscle, occurs when a muscle or tendon is overstretched or torn. The fibrous tissue that connects muscles to bones, called tendons, can also be injured in a strain. Strains can happen in any muscle, but they are most common in the lower back and in the muscles at the back of the thigh (hamstrings). The difference between a strain and a sprain is that a strain involves an injury to a muscle or to the band of tissue that attaches a muscle to a bone, while a sprain injures the bands of tissue that connect two bones together.

Muscles are made of thousands of small fibers woven together, and when these fibers are stretched beyond their limit, they tear apart, resulting in a muscle strain. Strains can be caused by one event, such as using poor body mechanics to lift something heavy, or by repetitive injuries when you stress a muscle by doing the same motion over and over. Acute strains can also occur suddenly and unexpectedly due to injuries or trauma.

The key symptoms of a muscle strain include sudden pain that worsens while contracting the muscle, swelling and bruising, loss of strength and range of motion, soreness, limited range of movement, bruising or discoloration, swelling, a “knotted-up” feeling, muscle spasms, stiffness, and weakness. Initial treatment includes rest, ice, compression, and elevation. In mild cases, only a few muscle fibers are stretched or torn, and the muscle remains intact and strong. In severe cases, however, the strained muscle may be torn and unable to function properly.

If your symptoms worsen despite treatment, especially if your pain becomes intolerable, or you experience numbness or tingling, see a doctor. Regular stretching and strengthening exercises for your sport, fitness, or work activity, as part of an overall physical conditioning program, can help minimize your risk of muscle strains.