A group of three muscles situated from behind the knee to the pelvis, the hamstring muscles allow the knee to bend. Over-stretching can damage them as can vigorous sport or intense gym work. Not stretching prior or after exercise can cause them to become shortened.
How to tell if you have hamstring strain
A different walk
Stretching causes pain and the area feels tender
That area (where it hurts) feels weak and as if it’s about to collapse