1.Flexion RoM (active) 140º flexion if hip flexed, 120º if extended
Hamstrings are bi-articular and lose some efficiency in hip extension: Hamstrings are hip extensors and knee flexors so their action on knee depends on position of hip. In hip flexion, distance between hamstring attachments increases and hamstrings wrap around ischial tuberosity. The more the hip is flexed, more stretched the hamstrings become, so they become more efficient as knee flexors. However, short head biceps femoris and popliteus are one jointed and have same efficiency regardless of hip position
2.When hip flexion greater than 90º,it is difficult to keep the knee fully extended because of stretching of hamstrings.
3.Beyond 20º knee flexion, the knee becomes looser and ligaments between femur and tibia relax, allowing for a wider range of axial rotation (open kinetic chain)