Knee muscles

The quadriceps - the muscles that straighten the knee

The main muscle that straightens the knee is the quadriceps. The 'quadriceps' or 'lap' muscle is so named because it has four 'heads', that is four separate muscles that join together at the bottom into a common tendon.

The four muscles of the quadriceps include -

rectus femoris
vastus lateralis
vastus medialis
vastus intermedius (underneath the rectus femoris, so you don't see it here)

The vastus intermedius is not shown here as it lies underneath the rectus femoris.

Illustration of the front of the thigh to show the rectus femoris muscle, the vastus medialis and the vastus lateralis, and how they have a common tendon at the lower end.

The rectus femoris part of the quadriceps

This illustration shows how the rectus femoris originates on the pelvis and reaches all the way to the tibia.

The other heads of the quadriceps originate on the femur.

Physiotherapists tend to focus a lot of attention on the vastus medialis, especially at the lower end where the muscle fibres are more oblique. Here the muscle is referred to as the vastus medialis obliquus or VMO. Weakness in the VMO is considered to be a factor in failure to regain a good extension, although there is quite a lot of controversy about this.

Illustration of the rectus femoris component of the quadriceps muscle group

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The 'extensor mechanism'

The lower end of the quadriceps at the patella and patellar tendon splays out into a fibrous soft tissue network called the retinaculum (or fascia), and the whole functional unit is referred to as 'the extensor mechanism'.

This allows maximum force to be applied when the knee is straightened.

Illustration of the extensor mechanism of the knee, showing how the lower end of the quadriceps groups spays out into a retinaculum that supports the patella.

The muscles that bend the knee

The main muscles that bend the knee are the hamstrings muscles at the back of the thigh and the gastrocnemius muscles at the back of the calf.

The hamstrings group consists of three individual muscles, and are so-called because of the long tendons at the lower end, which a butcher uses to string up a ham for smoking...

Illustration of the muscles at the back of the knee. — Back of knee

The three hamstring muscles are -

In the calf you can see the bulky gastrocnemius muscle. Under it lies the soleus muscle (not visible on image). The tendons of the two combine to form the strong Achilles tendon at the back of the ankle.

Note that the long thin tendon of the semitendinosus muscle is frequently 'harvested' by the surgeon for use in cruciate ligament reconstruction. It is commonly referred to as one of -the 'pes' or 'pes anserinus' tendons (semitendinosus, gracilis, sartorius) which insert on the medial (inner) aspect of the tibia in a compound tendon which looks a bit like a goose's foot (hence the latin name 'pes anserinus').

The 'gracilis' muscle is another whose tendon is often used, together with that of semitendinosus, to make a replacement cruciate ligament.The gracilis muscle is, however from a different group of muscles known as the 'adductors' and is not technically a hamstrings muscle.