The biceps femoris is one of the main muscles on the back of your thigh, and it’s part of a group of muscles called the hamstrings. You have a biceps femoris on each leg, running from the pelvis and the femur (thigh bone) down to the outside of the knee, where it attaches near the head of the fibula (a smaller bone of the lower leg). It has two parts, often called a long head and a short head. The long head crosses both the hip and the knee, while the short head only crosses the knee.
Its biggest jobs are bending your knee and helping extend your hip, which means it contributes when you pull your heel toward your glutes and when you drive your leg back behind you. It also helps rotate the lower leg slightly outward when your knee is bent, and it plays a key role in stabilizing the knee during running, jumping, cutting, and any fast changes of direction. Because it helps control how your knee moves, it’s important for protecting your joints and improving athletic performance.
For clients chasing visible abs, the biceps femoris matters more than you might think. Strong hamstrings help you train harder and safer in big calorie burning movements like deadlifts, hip hinges, sprinting, and loaded carries, all of which support fat loss and overall physique development. Also, if your hamstrings are weak or tight relative to your quads and glutes, it can tilt your pelvis and change how your core feels during ab work, sometimes making it harder to keep good form.
You’ll feel the biceps femoris working most in exercises like Romanian deadlifts, hip thrust variations, hamstring curls, glute ham raises, and sprint drills when done with solid technique and appropriate load.