The Hamstrings are a group of muscles in your posterior thigh that flex your knee and extend your hip. Hamstrings are actually three separate muscles: the semitendinosus, semimembranosus, and biceps femoris. Where are your hamstrings? The Hamstrings attach to your pelvis on one end and your tibia or fibula on the other. This stretches them across two joints, making them very long and powerful. Hamstrings play a strong role in squatting below parallel, jumping, and running. Lack of Hamstring flexibility and strength is the most common reason athletes cannot achieve a full squat with a neutral spine.