Leg strengthening

The quadriceps are the knee's shock absorbers. Weakness increases the rate of loading on cartilage, and in pooled prospective cohorts totalling 46,819 people, knee-extensor weakness raised the odds of developing knee OA (pooled OR roughly 1.4–1.9). Strengthening improves load distribution and neuromuscular control — and does so without any imaging or biomarker signal of cartilage harm.

The headline evidence

The Cochrane review of 54 RCTs found land-based exercise reduces knee OA pain by a standardized mean difference of about 0.49 — roughly 12 points on a 0–100 scale, comparable to NSAIDs — with moderate-quality evidence for improved function and benefits persisting 2–6 months after programs end.

Dose analysis across 48 RCTs sharpens the prescription: single-focus programs (quadriceps-specific among the most effective), supervised, at three or more sessions per week outperform mixed low-dose programs. Aerobic and resistance training deliver similar pain relief, so strengthening is one strong route, not the only one.

How hard, and how much

A meta-regression of 45 trials (n=4,699) found that programs following ACSM progressive resistance-training principles produced larger strength gains (SMD 0.448), and its modeling suggests a 30–40% strength increase may be needed for clinically meaningful symptom change. That is a real training target, not token quad sets.

A wording note that matters: the trials behind this tier used progressive resistance per ACSM principles, without tempo prescriptions, and pain-monitoring evidence permits acceptable pain — up to about 5/10, provided it settles by the next morning — rather than requiring a strictly pain-free range. "Controlled tempo, tolerable range" is what the evidence supports; "slow tempo, pain-free only" is not what was tested. Isometrics can serve as the entry ramp when joint irritability limits range.

What strength does not do

Honesty requires the null results. In the MOST cohort, baseline knee-extensor strength did not predict incident knee symptoms at 15–30 months across 2,275 knees. In a companion MOST analysis, higher quadriceps strength protected against symptomatic OA over 30 months but not against radiographic joint-space narrowing. No trial shows strengthening changes cartilage structure. Strength is protective for how the knee feels and functions — it is not a proven shield for the cartilage itself, and it is not a complete shield for symptoms either.

Two further caveats: participants can never be blinded in exercise trials, so effect sizes include expectation effects; and the ACSM meta-regression itself found strengthening per se was not required for symptom benefit — other exercise also works.

Safety

Exercise therapy shows no cartilage harm on imaging across RCTs, and biomarkers of cartilage breakdown do not rise with structured programs. The one signal worth respecting: in an RCT using heavy 80% 1RM training in knee OA, 4 of 16 participants dropped out with exercise-induced knee pain, versus none in the low-load blood-flow-restriction arm. Heavy loading works, but it needs a graded introduction in painful knees — start lower, progress by the load-management rules, and use isometrics or BFR as on-ramps when direct heavy loading flares symptoms.

Open questions

Whether actually achieving the modeled 30–40% strength gain causally delivers the predicted symptom benefit, and whether strength gains slow structural progression over horizons longer than five years — the trials to answer both have not been run.