Muscle sprain and strain

A sprain is an injury to a ligament, caused by sudden overstretching. Treatment is by reducing any inflammation using an icepack, then resting the affected limb. A ligament is a tough band of connective tissue which connects bones together at a joint. The ligament is flexible, but is not designed to stretch in length because part of its purpose is to restrict movement in certain directions. This is not technically an injury of the muscle but it is of the connective tissue.
 
Strain
Muscle strain is excessive stretching or working of a muscle, resulting in pain and inflammation. Different tissues in the body have different tolerances to stretching. Ligaments, as above, are flexible but do not stretch well, leaving them prone to injury when overstretched. Muscles have a different response to stretching. The fibres within the muscles tear, which can cause muscle pain, but they regenerate, becoming larger and stronger. This happens to a greater or lesser extent depending on the activity - e.g. when bodybuilding the pain, recovery and muscle growth cycle is quicker and more noticeable, but this same process does go on with other repetitive activities we carry out in our jobs and daily lives. Hence the term 'keyboard athlete'. In RSI, repeated strain and the magnitude of the load exceed the muscle tissue's ability to repair. This may explain why workers often first report problems after a period of increased workload.