The hamstring muscle group consists of three muscles of the posterior region of the thigh:
the biceps femoris muscle (long head)
the semitendinosus muscle
the semimembranosus muscle.
They are mainly flexors and secondarily leg extensors of the thigh. When walking or running, they are also antagonists quadriceps, slowing the advance of the leg at the end of the step and preventing the brutal and full knee extension.