Hamstring

The hamstrings are made up of three muscles, semitendinosus, bicep - femoris and semimembranosus that run down the back of your upper thigh. Their main function is flexion of the leg at the knee joint - bringing your heel up towards your buttocks. 

Most sports people, especially sprinters and footballers will be prone to injury in these muscles, as they are often tight due to exercise demands. Likewise persons who spend a lot of time seated, will also find that these muscles become sore, due to long periods of inactivity.

Aim to spend time in your workouts to actually train these muscles, as they're often neglected, however always aim to spend time stretching the hamstrings to prevent injuries in this venerable muscle group.