Muscles Around the Knee

For Young Learners

Structure and Location

Four groups of muscles move and protect the knee.

At the front, the quadriceps. Four muscles that join into one tendon and straighten the knee. This is the group that goes soft and thin surprisingly fast if you stop using your leg.

At the back, the hamstrings. Three muscles that bend the knee and also help control the shin.

Below, the calf. The big calf muscle actually starts above the knee, so it has a small part to play there too.

Above, the hip muscles. They do not cross the knee at all, and they still matter, because they control where your thigh points when you land and turn.

Function

Muscles do two things here. They move the joint, and they protect it.

Protection is the part people forget. A strong muscle contracting absorbs force that would otherwise land on the joint surface. Muscles also act faster than you can think, steadying the knee when you step on something uneven.

That is why almost every knee rehabilitation programme is mostly strength work, whatever the original problem was.

For Patients

Structure and Location

The quadriceps group has four parts: rectus femoris, which crosses the hip as well as the knee, and the three vasti, which arise from the femur itself. They converge into the quadriceps tendon.

The hamstrings are biceps femoris on the outer side, and semitendinosus and semimembranosus on the inner side. They cross both the hip and the knee.

Gastrocnemius, the bulky calf muscle, arises from the back of the femur above the joint. Popliteus is a small muscle behind the knee that helps unlock it at the start of bending.

The hip abductors and the gluteal muscles do not cross the knee but strongly influence it.

Function

The quadriceps straighten the knee and control it during bending, acting as a brake as you sit down or walk downhill. The hamstrings bend the knee and resist the shin sliding forwards, sharing work with the ACL.

Muscles also act as shock absorbers. A contracting muscle takes load that would otherwise pass through the joint surface, and it responds faster than conscious reaction, stabilising the knee on uneven ground.

Clinical Relevance

Quadriceps weakness develops remarkably quickly after any knee problem, sometimes within days, because the joint reflexively inhibits the muscle when it is swollen or painful. That weakness then reduces protection of the joint, which is why it is the first target of nearly every rehabilitation programme.

Hip strength matters more than most people expect. Weak hip muscles let the thigh roll inwards when you land, which loads the kneecap and the inner side of the joint, so hip work is standard in knee rehabilitation.

Specific muscle and tendon injuries are covered in the conditions section.

For Medical Students

Structure and Location

Four groups act on the knee. Quadriceps femoris lies anteriorly and has four heads, of which only rectus femoris also crosses the hip. The hamstrings lie posteriorly, cross both hip and knee, and divide into a lateral and a medial group. Gastrocnemius arises above the joint and contributes to it. Popliteus is a small muscle behind the joint.

The hip abductors and external rotators do not cross the knee and still influence it.

Origins, insertions and the detailed innervation of each muscle belong to the Anatomy Academy.

Function

The quadriceps extend the knee and act eccentrically during flexion under load, controlling descent in sitting and in downhill walking. The hamstrings flex the knee and extend the hip, and act as agonists to the anterior cruciate ligament by resisting anterior tibial translation.

Popliteus unlocks the extended knee by internally rotating the tibia on the femur at the initiation of flexion.

Muscle also attenuates load reaching the articular surface and provides reflex stabilisation at latencies shorter than voluntary reaction.

Clinical Relevance

Arthrogenic muscle inhibition causes rapid quadriceps weakness in the presence of effusion or pain. It is reversible but requires deliberate retraining, and it is a principal target of rehabilitation across the whole range of knee pathology.

The extensors and flexors have different nerve supplies, which is clinically useful: isolated quadriceps weakness with preserved hamstring power points towards the femoral nerve or the upper lumbar roots rather than towards the knee itself.

Proximal control matters. Hip abductor and external rotator weakness permits femoral adduction and internal rotation in stance, increasing dynamic valgus and patellofemoral load.

Muscle and tendon injuries themselves are covered in the conditions section.