From Laura Inverarity, D.O.,
Your Guide to Physical Therapy.
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About.com Health's Disease and Condition content is reviewed by Kate Grossman, MD

Strains are injuries that affect muscles or tendons, thick bands that attach muscles to bones. They occur in response to a quick tear, twist, or pull of the muscle. Strains are an acute type of injury that results from overstretching or over contraction. Pain, weakness, and muscle spasms are common symptoms experienced after a strain occurs.

Sprains are injuries that affect ligaments, thick bands of cartilage that attach bone to bone. They occur in response to a stretch or tear of a ligament. Sprains are an acute type of injury that results from trauma such as a fall or outside force that displaces the surrounding joint from its normal alignment. Sprains can range from a mild ligamentous stretch to a complete tear. Bruising, swelling, instability, and painful movement are common symptoms experienced after a sprain occurs.