Vastus Intermedius

legsMusculus vastus intermedius

The vastus intermedius lies underneath the rectus femoris, directly against the thigh bone. It is the quadriceps muscle you cannot see and the only one with no second job.

Vastus intermedius arises from the front and side of the thigh bone itself and runs straight down beneath the rectus femoris to join the shared quadriceps tendon. It is sandwiched between the bone and the muscle above it, and lies between the lateralis and medialis on either side.

It is the simplest of the four. It crosses only the knee, has no rotational component, no second joint, and one action: extending the knee. It contributes to that at every angle and at every load.

It is completely invisible. It cannot be seen at any body composition and cannot be felt separately from the rectus femoris covering it. Its development shows up as thickness in the front of the thigh rather than as any distinct shape.

Its deepest fibres are sometimes described separately as articularis genus, a small muscle that pulls the knee joint capsule upward as the knee straightens so it does not get pinched. That is usually treated as a separate muscle rather than part of this one.

Training notes

Every knee extension trains it, and nothing trains it selectively. It has no distinguishing characteristic that any exercise could exploit.

Since it does not cross the hip, it is unaffected by whether the hip is flexed or extended — it contributes equally in a leg extension, a squat and a leg press.

Its contribution is to the thickness of the thigh rather than to any visible outline, which is why it is the quadriceps muscle nobody trains for.

Anatomy

Origin
  • Anterior and lateral surfaces of the upper two-thirds of the shaft of the femur
Insertion
  • Base of the patella, through the quadriceps tendon
Actions
  • Extends the knee
Innervation
Femoral nerve (L2–L4)

Exercises

Antagonists

Synergists