The hamstrings are made up of three muscles located on the forward thigh; from lateral to medial, the Biceps Femoris (both short and long head), the semitendinosus, and the semimembranosus. To be considered a hamstring, a muscle must originate from the ischial tuberosity, and insert over the knee into either the fibula or the tibula. Moreover, they must be innervated (controlled) by the Sciatic nerve’s tibial branch. The muscle must also cross the knee and hip joints.
Each of these muscles are tasked with knee flexion and hip extension. When the core is in a fixed position, the semimembranosus and Semitendinosus muscles extend the hip, flex the knee, and when the knee is bent, rotate it inward. In short, the hamstrings are critical for basic daily movements such and walking, running, and jumping, and to some degree, for managing slight core adjustments.