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Quadriceps Control of the Knee Joint

By Dr Kenneth Backhouse OBE

The quadriceps group of muscles form the major part of the bulk on the front of the thigh. Collectively these four muscles are the powerful, prime extensors of the knee joint but at the same time are vital for the stability and well being of the joint. Even if there is no wish to bounce around and so use the dynamic extensor role of these muscles, nevertheless it is important that they remain in good physical state or damage to the joint is likely to occur, whether at the time or in the long-run.

Anatomy of the Muscle Group

The name quadriceps means four heads but in this case, four individual muscles with a common purpose. Two of the four, rectus femoris and vastus intermedius, give the central power to extension of the knee, working through the patella as a lever, while vastus medialis and vastus lateralis assist in the extensor role but also give vital peripheral support to both movement and the joint itself.

Vastus intermedius is the largest of the four muscles. It is attached to and covers much of the front and sides of the femur. The central fibres run into the patella but on the two sides they convert into flattened tendon, or aponeurosis that run down on each side of the patella to the upper part of the tibia.

Rectus femoris runs down the thigh in front of the vastus intermedius and can easily be seen in a reasonably muscular person. It is the one part of the quadriceps that arises from the pelvis; immediately above the acetabulum of the hip joint. Because of this it is often said to be a flexor of the hip joint but in fact it has poor mechanical advantage over the joint, making it of minor importance in flexion under normal circumstances. Its prime role over the hip is in support of the joint, rather as a dynamic ligament. In practice the rectus muscle with the vastus intermedius, should be considered just as the power extensors of the knee. They both run onto the patella, from the lower end of which a thick, powerful tendon, the patellar tendon runs down into the tibial tubercle, the bony knob on the front of the upper part of the tibia.

The patella is usually called a sesamoid bone, i.e. a bone within a tendon where that runs over the major bone, not only reducing friction on the tendon but also lifting its line of pull, so improving the leverage and therefore efficiency. The patella is a very overgrown version, running over the lower end of the femur and so increasing the leverage of the quadriceps pull on the tibia. This is also improved by the tibial tubercle. The patella is always in relation with the lower part of the femur. With the knee straight it lies on the front, but as the knee bends it tracks round its lower end, its gently ridged under surface running in the groove between the two condyles of the femur. With the knee bent at 90 degrees the patella lies on the end of the femur and in full (grand) plié it is even further round. Because of the considerable movement of the patella on the femur, the patello-femoral joint has cartilage on both surfaces but the synovial membrane above also has to be quite extensive. This forms a pouch above the patella, under the vastus medialis and, with the knee straight, the supra-patellar bursa extends about three fingers breadths above the patella and controlled by a tiny slip of the intermedius muscle. The synovial membrane of the bursa unrolls as the knee is bent. It is this bursa that becomes filled with watery fluid, the so-called water on the kneee, in the synovitis of knee injury.

It is important to be aware that the femur and tibia are not in a straight line at the knee. Due to the upper end of the femur being set on the side of the pelvis the shaft runs centrally to the knee so that it meets the tibia at an angle; greater in a female due to the relatively wider pelvis. Thus, with the load of the body on the joint, the knee tends to be pushed medially towards a genu valgum (knock knee) unless effectively controlled, by pulling up of the muscles on the inside. As a result of the angle at the joint the line of pull of the muscles along the femur to the patella has to change to give a straight pull down to the tibia. If the patella does not have a compensating pull medially, it would move in line to the tibia: i.e. it would track or even dislocate laterally. This is in face a common problem; a spontaneous lateral dislocation of the patella is quite often seen in females, because of the greater angulation at the joint.

Vastus lateralis arises from the femur behind the vastus intermedius and runs down towards the outer side of the knee joint. It converts into an aponeurosis somewhat above the level of the patella where it joins that of intermedius over the outer side of the knee.

Vastus medialis runs down the inner side or the thigh, much as the lateral muscle. However it also has an oblique portion, arising lower down the thigh from the thick tendon of adductor magnus. This component forms the prominent bulge of muscle on the inner side of the patella, to which it is attached. In a way it can be considered almost as a separate muscle: it has its own branch of the femoral nerve that supplies the quadriceps. The oblique component is vital in ensuring that the patella tracks correctly over the femur and counteracts any tendency for a lateral shift. It acts in that important pull-up on the muscles on the inner side of the thigh that also control the tendency to knock-knee.

The lateral and medial muscles while, supporting the other two muscles in extension of the knee, also give valuable support to the inner and outer sides of the joint. In fact the quadriceps muscles together with the patella replace a joint capsule and ligaments over the whole anterior half of the knee joint . As such they are vital in producing stability in the joint. If they fail, as so often happens after injury or from disuse, synovitis (water on the knee) is most likely to occur. In the longer term degenerative and other problems of the knee can be put down to poor quadriceps control of the joint.

So important is it to ensure quadriceps control over the knee joint that anyone in hospital for more than the odd day is, where possible, given daily quadriceps exercises. Furthermore, even a minor problem around the knee joint automatically leads to central (i.e. the brain) inhibition of muscle control over the joint with rapid loss of muscle power and hence bulk. As an example a rugby-playing colleague of mine, some years ago, sustained a very minor knee injury and I suggested that he did not play the following week but keep exercising. At the end of a week he had lost an inch in circumference of the thigh, mainly due to loss of quadriceps muscle; this in spite of muscle activity.

Even after minor problems it is vital to try to ensure the quadriceps are kept as strong as possible. Firm regular pull-up of the quads, with the knee straight and held as hard as possible for a short time; repeated for about 5 minutes every hour should be the order of the day. This can be emphasised with the leg held out and a weight on the foot, to make the quads work even harder. Your whole future as a dancer (or even as a less physical person) depends on the control of the quadriceps over the knee; an otherwise unstable joint.