Quadriceps
The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large fleshy mass which covers the front and sides of the femur. The name derives . Structure. Parts. The quadriceps femoris muscle is subdivided into four separate muscles (the 'heads'), with the first superficial to the other three over the femur (from the trochanters to the condyles): Attachments. The rectus femoris arises from the anterior inferior iliac spine and from  the superior edge of the acetabulum. It is thus a biarticular muscle. The other parts of the quadriceps arise from the surface of the femur. All four parts of the quadriceps muscle ultimately insert into the tuberosity of the tibia via the patella, where the quadriceps tendon becomes the patellar ligament. Other muscles. There is a small fifth muscle of the quadriceps complex — the articularis genus muscle — that is not often included. In addition, cadaver studies have confirmed the presence of a sixth muscle, the tensor vastus intermedius. While this muscle has a variable presentation, it consistently originates at the  proximal femur, runs between the vastus lateralis and vastus intermedius muscles, and inserts distally at the medial aspect of the patellar base. Historically considered a part of the vastus lateralis muscle, the tensor vastus intermedius muscle is innervated by an independent branch of the femoral nerve and its tendinous belly can be separated from the vasti lateralis and intermedius muscles in most cases. Innervation. The quadriceps femoris is innervated by the femoral nerve, which originates from L2, L3, L4. Function. All four quadriceps are powerful extensors of the knee joint. They are crucial in walking, running, jumping and squatting. Because  the rectus femoris attaches to the ilium, it is also a flexor of the hip. This action is also crucial to walking or running, as it swings the leg forward into the ensuing step. The quadriceps, specifically the vastus medialis, play the important role of stabilizing the patella and the knee joint during gait. Clinical significance. The quadriceps femoris muscle is a target for manual therapy and physical therapy from repetitive strain injuries, such as from skiing. The quadriceps femoris muscle is the most common site of myositis ossificans. Society and culture. Training. In strength training, the quadriceps are trained  by several leg exercises including the squat, leg press, and leg extension. Etymology. The proper Latin plural form of the adjective "quadri" would be "quadricipites". In modern English usage, "quadriceps" is used in both the singular and plural form. The singular form "quadricep", produced by back-formation, is frequently used.