Strains and Sprains
Sprains generally happen when a twisting force is applied to a joint while it is bearing weight, which causes the ligament to stretch beyond its usual limit. Sprains tend to happen with sudden, unexpected movement.
Muscle strains happen when the weight on a muscle is greater than the weakest part of the muscle can bear. Strains tend to happen during activities that require your muscles to stretch and bear weight at the same time. Some evidence suggests that being injured before or having limited flexibility contributes to strains.