Muscles extending the knee joint

Rectus femoris	Quadriceps femoris
Vastus lateralis
Vastus medialis
Vastus intermedius

Tensor fasciae latae

Quadriceps femoris is the large muscle bulk on the anterior surface of the thigh. As its name implies, it is composed of four main parts. One part, rectus femoris, has its origin above the hip joint, while the other three parts take origin from the shaft of the femur. All four join together around the patella to form a thick strong tendon called the ligamentum patellae, which inserts into the tibial tuberosity.

Vastus lateralis

Vastus lateralis is situated on the anterolateral aspect of the thigh, lateral to rectus femoris. It has an extensive linear attachment from the upper lateral part of the intertrochanteric line, the lower border of the greater trochanter, the lateral side of the gluteal tuberosity and the upper half of the lateral lip of the linea aspera. It also attaches to the fascia lata and lateral intermuscular septum. From this origin the muscle fibres run downwards and forwards with those at the top passing almost vertically downwards.

The muscle bulk is mainly situated in the upper half of the lateral side of the thigh, from which a broad tendon arises which narrows down as it approaches the lateral side of the patella. This tendon inserts into the tendon of rectus femoris and the base and lateral border of the patella. Some fibres pass to the front of the lateral condyle of the tibia blending with the iliotibial tract helping to form the expansion which finally attaches to the line running towards the tibial tuberosity. To a large extent this part of its attachment replaces the knee joint capsule in this region.

Vastus medialis

Vastus medialis is situated on the anteromedial aspect of the thigh, medial to rectus femoris, with most of its bulk showing at the lower third just above the patella. It has an extensive linear origin from a line beginning at the lower medial end of the intertrochanteric line, running downwards around the medial aspect of the upper end of the shaft on the spiral line, the medial lip of the linea aspera, continuing on to the upper two-thirds of the medial supracondylar line, the medial intermuscular septum and the tendon of adductor magnus.

Its upper fibres pass mainly downwards, whilst its lower fibres tend to pass almost horizontally forwards. These two sets of fibres which make up vastus medialis are considered by some to be anatomically and functionally distinct, with the oblique fibres being called vastus medialis obliquus. The muscle attaches to the tendon of rectus femoris, the medial border of the patella, and the front of the medial condyle of the tibia. The expansions which pass across the knee joint to attach to the tibia replace the joint capsule in this region and become fused with the deep fascia. This attachment also runs to the tibial tuberosity.

Vastus intermedius

Vastus intermedius is the deepest part of quadriceps femoris lying between vastus lateralis and medialis, and deep to rectus femoris.

It arises by fleshy fibres from the upper two-thirds of the anterior and lateral surfaces of the femur, its fibres pass downwards to form a broad tendon on its more superficial aspect. This attaches to the deep surface of the tendons of rectus femoris and the other vastus muscles, and to the base of the patella. In the middle of the thigh, vastus intermedius is difficult to separate from vastus lateralis, while lower down it is impossible to separate from vastus medialis. 

Articularis genus. Some of the deep fibres of vastus intermedius, arising from a small area on the inferior third of the anterior surface of the femur, pass downwards to attach to the upper part of the suprapatellar bursa of the knee joint which lies deep to vastus intermedius. These fibres are the articularis genus. Its main function is to prevent the synovial membrane becoming trapped and interfering with the normal movements of the knee joint.

Ligamentum patellae

All four of quadriceps tendons contribute to the formation of the ligamentum patellae. It runs from the apex of the patella to the upper part of the tibial tuberosity acting as the tendon of insertion of the quadriceps femoris muscle. The patella is really a sesamoid bone in the tendon of rectus femoris and vastus intermedius helping to relay the pull of the quadriceps over the front of the femur.

Nerve supply

Quadriceps femoris, including articularis genus, is supplied by the femoral nerve, root value L2, 3, 4. The skin covering the quadriceps is supplied by L2, 3.

Quadriceps femoris

Action

Although rectus femoris is part of the quadriceps femoris group, by crossing anterior to the hip joint it also flexes the thigh.

Quadriceps femoris is the main extensor of the knee joint. Rectus femoris crosses in front of the hip joint and therefore is also a flexor of that joint. Each muscle appears to have its particular role in extension of the knee and often comes in at different ranges of the movement. For example, vastus medialis is more obviously active in the final stage of extension and is believed to resist the tendency of the patella to move laterally caused by the angulation of the femur. Specific exercises designed to strengthen the oblique fibres of vastus medialis are advocated by some practitioners in order to affect tracking of the patella, resisting dislocation and possibly helping reduce anterior knee pain in certain circumstances.

Rectus femoris works particularly strongly in straight leg rising or in the combined movement of flexion of the hip and extension of the knee.

Functional activity

The quadriceps are used strongly in stepping activities, for example stair climbing and squats. Rectus femoris will perform its function particularly in the swing phase of walking when the lower limb is being carried forward and the knee is being extended. Vastus medialis, in the final stages of extension of the knee, will help in the locking mechanism of the joint when the femur is allowed to rotate medially.

Surprisingly, in the standing position very little or no action is recorded in the quadriceps as in this position the knees are in the closepacked position. It is at these times that if the knees are knocked from behind, forward collapse will almost certainly occur. However, when standing on a moving vehicle the quadriceps will be active. When standing on one leg, all the muscles around the knee will work statically to provide stability at the joint.

The quadriceps is a powerful and an important muscle. It must work strongly throughout its full range. It will lose strength and bulk rapidly if there is any injury to it or to the knee joint. It may take months to regain power, but only days to lose it.

Palpation

When sitting on a chair and straightening the knee joint, particularly against resistance, the separate parts of quadriceps femoris, except vastus intermedius, can easily be palpated: medialis on the lower medial aspect, lateralis in the upper half of the lateral side and rectus femoris running down the centre. Stand with the knees semiflexed, place your hands on the front of each thigh; the three parts of the muscle (as above) can be readily palpated.

In straight leg raising, the muscle should be able to extend the knee into a few degrees of hyperextension, this being the extra range required for the knee to be able to lock. Patients not being able to do this will often complain of their knee giving way during walking.