Knee Muscles

The knee joint muscles are those which cause the knee to either bend or straighten. They include the hamstrings and gastrocnemius at the back, which bend (flex) then knee. The the quadicep muscles at the front straighten (extend) the knee.

Muscles at the back of the knee (posterior)

These muscle are located at the back of the knee and primarly work to flex (bend) the knee when they contract. The hamstring muscles at the back of the thigh consist of the biceps femoris, semitendinosus, and semimembranosus. Other muscles are the sartorius, gracillis, popliteus and gastrocnemius.

Semitendinosus muscle

Semitendinosus muscle

When running the hamstrings act eccentrically to slow down the knee extension motion. Hamstring strains are common in individuals with chronically tight hamstrings or who do not warm-up thoroughly.

  • Origin: Ischial tuberosity.
  • Insertion: Upper medial surface of the tibia.
  • Actions: Hip extension, Knee flexion, Internal rotation of the hip when the knee is flexed.
  • Innervation: Tibial part of the sciatic nerve.
  • Daily uses: Bending the knee to step over something.
  • Example strengthening exercises: Knee curl with resistance band, Nordic curl.
  • Example stretches: Sitting hamstring stretch.
    Standing hamstring stretch.

Go to: Hamstring exercises.


Semimembranosus muscle

Semimembranosus muscle

Semimembranosus is the most medial of the three hamstring muscles. Chronically tight hamstrings are often a contributory factor to lower back pain and knee pain.

  • Origin: Ischial tuberosity.
  • Insertion: Posterior part of the medial condyle of the tibia.
  • Actions: Hip extension.
    Knee flexion.
    Internal rotation of the hip when the knee is flexed.
  • Innervation: Tibial part of the sciatic nerve.
  • Daily uses: Bending the knee to step over something.
  • Example strengthening exercises: Knee curl with resistance band.
    Nordic curl eccentric exercise
  • Example stretches: Sitting hamstring stretch.
    Standing hamstring stretch.

Biceps Femoris muscle

Biceps Femoris muscle

Biceps Femoris is one of the three muscles which form the hamstring group forming the back of the thigh. The muscle is often described as having a long head (the attachment from the ischium) and a short head (attached to the femur).


Sartorius

Sartorius Muscle

The sartorius originates on the outside of the hip and inserts on the inside of the knee. When it contracts it helps to flex the knee and rotate the hip outwards.


Gracilis

Gracilis Muscle

The gracilis is one of the long adductor muscles.


Popliteus

The popliteus is a small muscle at the back of the knee.

Popliteus Injury

Gatrocnemius

Gastrocnemius Muscle

The gastrocnemius muscle is the large calf muscle at the back of the lower leg. It originates above the knee. As a result, it does flex the knee but only very weakly when compared to the hamstring muscles.


Knee muscles at the front – Quadriceps

These muscles make up the quadriceps muscle group. Their main function is to extend the knee (straighten it).

Vastus Medialis

Vastus Medialis knee joint Muscle

Vastus Medialis is the most medially (inner) located of the quadricep muscles. The portion of the muscle just above the knee is known as VMO (vastus medialis oblique). This is important in stabilising the knee joint and often becomes inhibited following injury.

  • Origin: Intertrochanteric line (between the greater and lesser trochanters of the femur).
    Medial lip of the linea aspera of the femur.
  • Insertion: Patella via the quadriceps tendon and then the tibial tuberosity via the patella tendon.
  • Actions: Knee extension.
  • Innervation: Femoral nerve.
  • Daily uses: Cycling.
    Walking upstairs.
  • Example strengthening exercises: Knee extension with a rehab band.
    Squats.
  • Example stretches: Standing quad stretch.
    Laying quad stretch.

Vastus Lateralis

Vastus Lateralis Muscle

Vastus Lateralis is the most lateral (outer) of the four quadriceps knee muscles and is felt on the outside top of the thigh.

  • Origin: Outer surface of the greater trochanter of the femur.
    Upper half of the linea aspera.
  • Insertion: Patella via the quadriceps tendon and then the tibial tuberosity via the patella tendon.
  • Actions: Knee extension.
  • Innervation: Femoral nerve.
  • Daily uses : Cycling.
    Walking upstairs.
  • Example strengthening exercises: Knee extension.
    Squats.
  • Example stretches: Standing quad stretch.
    Laying quad stretch.

Vastus Intermedius

Vastus Intermedius is one of four quadricep muscles, located deep in the thigh underneath the Rectus Femoris muscle.

Vastus Intermedius
  • Origin: Anterior and lateral surfaces of the shaft of the femur.
  • Insertion: Patella via the quadriceps tendon and then the tibial tuberosity via the patella tendon.
  • Actions: Knee extension.
  • Innervation: Femoral nerve.
  • Daily uses: Cycling.
    Walking upstairs.
  • Example exercises: Knee extension with a band.
    Squats.
  • Example stretches: Standing quadriceps stretch.
    Laying quadricep stretch.

Rectus femoris (hip & knee muscle)

Rectus Femoris

The Rectus Femoris muscle is part of the Quadriceps muscle group. It is the only of the quadriceps group knee muscles which also crosses the hip joint. It is a powerful knee extensor when the hip is extended (back), but is weak when the hip is flexed (forwards).

  • Origin : Anterior Inferior Iliac Spine (AIIS).
  • Insertion: Top of the patella and the patella tendon to the tibial tuberosity.
  • Actions: Flexion of the hip.
    Extension of the knee.
  • Innervation: Femoral nerve.
  • Daily uses: Kicking a football.
  • Example strengthening exercises: Standing hip flexion using resistance band.
    Sitting hip flexion – isometric.
  • Example stretches: Hip flexor stretch.
    Laying quadricep stretch.
    Standing quadricep stretch.

References & further reading

This article has been written with reference to the bibliography.