Quadriceps femoris

What it is

Four muscles sharing one insertion, treated as a group because they finish together. Rectus femoris runs down the middle from the pelvis; vastus lateralis, vastus medialis and vastus intermedius arise from the shaft of the femur itself and surround it on the outside, inside and front.

All four converge into the quadriceps tendon, which envelops the patella. The patella is a sesamoid bone — a bone formed inside a tendon — and it is not a passive lump. It holds the tendon away from the joint, increasing the leverage the muscle has on the knee. Below it, the patellar ligament continues to the tibial tuberosity, the bump you can feel below your kneecap.

As a group it is the largest muscle mass in the body, though gluteus maximus is the largest single muscle.

What it does

Extends the knee — straightens the leg. All four heads do this and it is the only action three of them have.

Rectus femoris is the exception, and it is the one worth understanding. It originates on the pelvis rather than the femur, so it crosses the hip as well as the knee, and it flexes the hip in addition to extending the knee. That makes its length depend on the position of two joints at once, which is why the same muscle can feel entirely different in a lunge and in a seated leg extension.

What loads it

Any movement where the knee straightens against resistance, and any movement where it bends under load and has to control the descent — which is the same thing running backwards.

The goblet squat and box squat both load it substantially. In the box squat, lowering under control is the quadriceps working while lengthening, which is a demand people frequently mistake for "the easy part".

What is commonly got wrong about it

"The teardrop muscle" and selective inner-quad training. Vastus medialis has a distinct lower portion, often labelled VMO, and a long tradition holds that it can be trained preferentially to improve kneecap tracking. Whether it can be selectively activated at all is disputed, and the wider claim built on it is on shaky ground. See the vastus medialis record.

That the knee extends only when you are extending your knee. Most quadriceps work in real life is control of bending — going down stairs, lowering into a chair, absorbing a landing.

That deep knee bends are inherently damaging. This was received wisdom for decades on the basis of very thin evidence, and it has not held up well. What is true is much duller: loads and ranges need to be built up rather than jumped into.