What is the difference between a strain and a sprain?

Sprains affect the ligaments. These are fibrous bands that support the joints and prevent them over-extending in normal use. When you bend a joint unnaturally, such as twisting an ankle or a rugby player suffers a collision to their knee, the joint is forced way out of its normal range of motion. Pain, swelling and bruising may follow the pop or tear felt when it happened.

Strains affect the muscles and the tendons that connect the muscle to the end of the bone it pulls on. An impact injury to the muscle or overload might cause an acute (short term) strain, an example being lifting too heavy a weight, whereas repetitive movements can cause chronic (long term) strain, such as experienced by tennis players, golfers, hairdressers and people typing all day. Strangely enough, this is called Repetitive Strain Injury (RSI).