Muscle strength and osteoarthritis of the knee: a systematic review and meta-analysis

Key points

1. 01Knee extensor strength is quadriceps power to straighten the knee, and knee flexor strength is hamstring power to bend it; both are modifiable with training.
2. 02Low knee extensor strength raised the risk of worsening in the main weight-bearing (tibiofemoral) compartment by about 18%.
3. 03Low knee extensor strength raised the risk of patellofemoral (kneecap) worsening by about 62%, a larger but less precise estimate.
4. 04This is the first review to test knee flexor strength, which was also linked to tibiofemoral worsening.
5. 05The extensor-strength link was significant in women but not men, and stronger in people at risk of osteoarthritis than in those already diagnosed.

How it was conducted

Design

Systematic review with random-effects meta-analysis of longitudinal studies, GRADE certainty, prospectively registered (OSF)

Search

MEDLINE, EMBASE, CINAHL, SPORTDiscus, Scopus to April 2022; 14 studies included (mean age 27 to 72 years)

Participants

Adults with osteoarthritis (8 studies), at risk of osteoarthritis (3 studies), or a combination (3 studies), followed 1 to 7 years

Intervention

Exposure was low knee extensor or flexor strength (lowest vs highest tertile, or two lowest vs two highest quartiles)

Outcomes

Structural worsening on X-ray (Kellgren-Lawrence grade, joint space narrowing) or MRI (cartilage lesion or thinning)

Analysis

Risk ratios pooled with REML random-effects models, stratified by sex/gender, compartment, and population where possible

What they found

• Low knee extensor strength and worsening tibiofemoral OA: RR 1.18 (95% CI 1.04 to 1.35; 12 studies).
• Low knee extensor strength and worsening patellofemoral OA: RR 1.62 (95% CI 1.01 to 2.61; 4 studies).
• Low knee flexor strength and worsening tibiofemoral OA: RR 1.16 (95% CI 1.07 to 1.26; 5 studies).
• By sex: significant for women (RR 1.25, 95% CI 1.04 to 1.51; 4 studies) but not men (RR 1.10, 95% CI 0.87 to 1.39; 4 studies).
• By population: higher risk in those at risk of OA (RR 1.49, 95% CI 1.09 to 2.05) than those with OA (RR 1.04, 95% CI 0.91 to 1.18).
• All pooled estimates graded very low certainty of evidence under GRADE.

Limitations

• Ten of 14 studies (71%) were at high risk of bias, and only one was low risk.
• All estimates were very low certainty due to high risk of bias, heterogeneity, and imprecision, so future studies may change them.
• Only one study followed people after ACL injury, limiting conclusions in younger post-traumatic populations.
• Several effect measures were converted from odds ratios or group summaries to risk ratios, and excluding middle strength tertiles may have inflated effect sizes.

Why it matters

For patients

Keeping the muscles around your knee strong may help protect the joint from getting structurally worse over time.

For clinicians

Address low knee extensor and flexor strength in people with or at risk of knee osteoarthritis, recognizing the evidence is associative and low certainty.

For readers

Weaker knee muscles precede joint deterioration on imaging, but the evidence is too uncertain to prove cause.