Knee

What it is

The largest and one of the most complex joints in the body, and really three joints in one capsule: between the femur and tibia on the inner side, the same on the outer side, and between the patella and the femur at the front.

Like the shoulder, it is a joint with poor bony fit. The rounded condyles of the femur sit on the relatively flat top of the tibia. Stability comes from soft tissue:

• Cruciate ligaments — anterior and posterior, crossing inside the joint, controlling front-to-back movement and rotation
• Collateral ligaments — medial and lateral, resisting sideways forces
• Menisci — two crescents of fibrocartilage that deepen the contact area, spread load and absorb shock

The patella sits in the quadriceps tendon and increases the muscle's leverage.

What it does

Flexes and extends. It also rotates, but only when flexed — with the knee straight the joint locks and rotation is not available.

Straightening fully involves the screw-home mechanism, a small rotation that brings the joint into a close-packed, locked position where you can stand with little muscular effort. Popliteus unlocks it before flexion can begin.

What loads it

Everything on your feet. Squatting, walking, stairs, running, and landing. The goblet squat and box squat load it through substantial range.

What is commonly got wrong about it

"Knees should not travel past the toes." One of the most repeated cues in fitness, and it does not hold up. Restricting forward knee travel reduces load at the knee and increases it at the hip and lower back — it moves the load rather than removing it. Plenty of ordinary movements, stairs among them, take the knee past the toes.

That squatting deep wears out the knee. Long-held received wisdom on thin evidence. There is no good support for the idea that deep squatting damages healthy knees, and the loads involved in daily life are substantial anyway.

That meniscus tears need surgery. The FIDELITY trial randomised 146 people aged 35 to 65 with a degenerative medial meniscus tear and no osteoarthritis to arthroscopic partial meniscectomy or to sham surgery, and found no significant difference in any primary outcome at twelve months.¹ Traumatic tears in younger people are a different situation, and were excluded from that trial.

That knee pain means damage. Cartilage and meniscal changes are common on imaging in people with no pain. Knee pain is a clinical matter and specifically outside this site's scope.

Sources

1. Sihvonen R, Paavola M, Malmivaara A, et al. Arthroscopic partial meniscectomy versus sham surgery for a degenerative meniscal tear. New England Journal of Medicine 2013; 369(26):2515–2524. doi:10.1056/NEJMoa1305189 PMID 24369076 146 patients aged 35–65 with a degenerative medial meniscus tear and no osteoarthritis, randomised to partial meniscectomy or sham surgery. No significant between-group difference in any primary outcome at 12 months.

Every source here was checked against PubMed rather than recalled. Where this site says something is disputed or unsupported and no source appears, that is deliberate: it means the claim is being conceded rather than cited.