How cartilage responds to load

Articular cartilage is avascular — no arteries deliver nutrients to it. It is fed by fluid exchange: cyclic compression squeezes fluid through the matrix, and re-expansion draws nutrients back in. Quantitative MRI in living humans shows cartilage deforming 3–6% during activity and recovering afterwards, with distinct behavior under static versus dynamic load. That pump is the reason this whole section exists: load is not the enemy of cartilage, it is the delivery system.

The pump, measured

In vivo imaging studies quantified the deformation directly: knee bends, running, and cycling compress cartilage by roughly 2.8–5.9%, and it re-expands during recovery. Notably, trained athletes deformed the same amount as untrained people — adult cartilage seems to adapt biochemically rather than by growing thicker. Chondrocytes transduce this cyclic load into matrix synthesis; without it, the tissue atrophies like unloaded muscle.

What happens when you load it

The flagship human trial is small but pointed: 30 adults at high OA risk after meniscectomy were randomized to supervised exercise three times weekly for four months or to control. Cartilage glycosaminoglycan content on dGEMRIC imaging improved in the exercisers and declined in controls (+15 vs −15 msec, p=0.036), and the GAG gain tracked how much each person's activity actually changed (r=0.70).

The safety side is broader. Across 9 RCTs (14 comparisons) in people at risk of or with knee OA, knee-loading exercise showed no evidence of cartilage harm on imaging. Across 12 RCTs measuring cartilage and inflammation biomarkers, exercise lowered them in 30% of comparisons, left them unchanged in 63%, and raised them in only 7%.

What happens when you don't

Disuse is not neutral. After spinal cord injury — complete unloading — knee cartilage thinned progressively, roughly 19–25% across compartments within 24 months. Even seven weeks of partial weight-bearing on crutches thinned healthy cartilage by 2.9–6.6% and shrank the quadriceps 11%. Total rest is an active harm state for this tissue, not a holding pattern.

The dose curve

A meta-analysis of 29 animal RCTs found an inverted-U dose-response: moderate daily exercise improved cartilage in 12 of 12 comparisons, while both low and high doses showed no benefit or worse outcomes in most. Animal doses don't translate directly into human prescriptions, but the shape of the curve — moderate, regular, progressive beats both rest and maximal loading — is the clearest dose-response data available, and it matches the human safety reviews.

The honest limits

The dGEMRIC trial is one small, single-center study with a surrogate imaging outcome, never replicated at scale. The exercise-cartilage systematic review found mostly null effects on cartilage morphology — "not harmful" is the defensible summary, not "regenerative." The disuse studies are observational and confounded (spinal cord injury involves more than unloading), and 7% of biomarker comparisons did move in an unfavorable direction. Open questions include whether the GAG gain is reproducible and durable, where the human "moderate dose" ceiling sits, and whether repaired cartilage after MACI or microfracture responds to load the same way.

The practical takeaway holds regardless: motion and load are inputs cartilage needs. The question is dose — covered in the load-management entry — not whether to load at all.