The biceps femoris is the big muscle at the back of the thigh.
Both heads of the biceps femoris are knee flexors, and the long head also helps to extend the hip.
Physical principles demonstrate that the biceps femoris muscle relative to the other hamstring muscles exerts the most force: implications for hamstring muscle strain injuries. Dolman B, Verrall G and Reid I. Muscles Ligaments Tendons J. 2014 Jul-Sep; 4(3): 371–377.