Quadriceps: Although the term "quadriceps" technically may refer to any four-headed muscle, it usually refers to and is synonymous with the quadriceps muscle of the thigh, the large muscle that comes down the?femur?(the bone of the upper leg), goes over the?patella?(the kneecap) and anchors into the top of the?tibia?(the big bone in the lower leg). The function of the quadriceps is to extend (to straighten out) the leg.