Biceps femoris is two muscles

The biceps femoris hamstring muscle has two attachments: a long head and a short head. The long head attaches to the ischium (sitting bone), whilst the short head arises halfway down the femur (thigh bone) along the linea aspera.

They don’t even have the same nerve supply. This is telling. We know that that which wires together, fires together. Whilst the long head of the biceps femoris is innervated by the sciatic nerve with the other hamstring muscles, the short head of the biceps femoris is innervated by the common fibular nerve, which goes onto branch into the superficial and deep fibular nerves, which supply the anterolateral calf. In other words, think of the long head of the biceps femoris as the ‘true’ portion of the hamstring, and the short head of the biceps femoris as predominantly a knee flexor and external rotator.

The short head isn’t concerned with the hip at all. It doesn’t even share the same nerve supply as the other hamstring muscles.

It belongs to the lower limb group, which are predominantly focused with ankle movement (remember that as part of the ankle complex you also need to include the proximal tibiofibular joint, which the biceps femoris does cross as it inserts on the fibula).

I suspect that a lot of ‘ITB tension’ at the knee people complain about is actually a dysfunctional biceps femoris. People love to flex the knee to compensate for hip movement. On single leg stance, a lot of people have a hard time maintaining hip extension, instead flexing the knee for stability. This may be an overactive short head biceps femoris compensating for the true hamstrings group.

A large part of rehab is getting people to use their hips better. This includes learning to eccentrically strengthen the hamstrings at the hip joint on a stable knee joint. Hamstring curls will only get you so far. Learn to deadlift, lunge, squat and bear crawl instead and get your hamstrings working better as a functional unit. You might find that your hamstring tightness finally dissipates and your knee pain also goes away.