Why Muscle Strength Is the Strongest Predictor of Function in Advanced Knee Osteoarthritis
Introduction
Knee osteoarthritis (OA) remains one of the most common causes of pain and disability worldwide, affecting nearly 1 in 6 adults over age 45. As cartilage wears down and inflammation increases, patients experience pain, stiffness, and reduced ability to perform simple daily tasks — like standing up from a chair or climbing stairs.
While much of osteoarthritis treatment has focused on pain relief or joint preservation, a growing body of research highlights a different predictor of long-term function: muscle strength. A new 2025 study published in Musculoskeletal Science and Practice by Izadi et al. has found that lower-limb muscle strength — particularly in the quadriceps and leg press — is one of the strongest predictors of how well patients with advanced knee OA can move and function in daily life.
This study reinforces what we emphasize at Zero Point One Physical Therapy: strength is capacity, and capacity drives function. Whether you’re recovering from injury or living with chronic joint changes, stronger muscles mean a stronger life.
What the Study Found
The researchers recruited 50 patients (average age 65) with advanced knee osteoarthritis scheduled for total knee replacement. They measured several aspects of leg muscle performance:
Maximal strength using a leg press (1-rep max normalized to body weight)
Isometric knee extension strength (maximum voluntary contraction, MVC)
Rate of force development (RFD) – how quickly force could be generated
They compared these results to physical function tests:
30-second sit-to-stand (STS)
10-step stair climb (STC)
40-meter fast-paced walk (FPW)
Self-reported function using the KOOS-Physical Function Short Form (KOOS-PS).
Key results:
Leg press and leg extension strength predicted 29–38% of performance in sit-to-stand and stair climbing tasks.
Strength explained 7% of variance in self-reported physical function.
Rate of force development also predicted up to 22% of performance in functional tests.
Surprisingly, muscle strength did not predict fast-paced walking performance — suggesting that walking may rely more on joint comfort, balance, and endurance rather than maximal strength.
In other words, the stronger the leg muscles, the better patients performed real-life functional movements, regardless of their pain level or age.
Why Strength Matters in Osteoarthritis
Pain often leads people to move less — which in turn accelerates weakness, stiffness, and further pain. This “deconditioning cycle” is one of the most harmful aspects of osteoarthritis.
The study’s findings highlight the opposite effect: even small gains in muscle strength lead to clinically meaningful improvements in daily function. For example, improving leg press strength by 29 kg or knee extension force by 80 N could help patients perform two to three more sit-to-stand repetitions in 30 seconds — enough to move from moderate to high functional independence.
Strength acts as a protective buffer around the joint. It helps absorb impact, stabilize movement, and offload stress from cartilage and ligaments. Quadriceps and gluteal strength in particular are critical for climbing stairs, standing up, and maintaining balance — all movements strongly correlated with quality of life in older adults.
Bridging Research to Real Life: How Strength Training Helps
Progressive resistance training remains the gold standard for improving both muscle strength and physical function in OA. Randomized trials have shown that as little as 12–16 weeks of targeted strength training can significantly increase muscle capacity and reduce pain and stiffness (Baker et al., 2001; Ciolac & Greve, 2011).
At Zero Point One Physical Therapy, we see this daily. When patients with knee pain begin strength and conditioning programs that build progressive load tolerance — squats, split squats, step-ups, and controlled isometric work — they not only regain function but often delay or even avoid surgical interventions.
The takeaway from Izadi et al. (2025) is clear:
Muscle strength and rate of force development drive independence.
Functional tests like the sit-to-stand or stair climb are better indicators of improvement than pain alone.
Therapeutic programs must prioritize progressive loading — not just flexibility or rest — to rebuild functional capacity.
The Bigger Picture: Strength and Longevity
The implications of this study go beyond osteoarthritis. Muscle strength is increasingly recognized as a marker of biological age and longevity. Research shows that higher grip strength and leg power predict lower mortality, better metabolic health, and reduced fall risk (PMID: 38595265; PMID: 33237691).
In other words, strength isn’t just about muscles — it’s about maintaining your independence, vitality, and confidence to live life on your terms.
Clinical Applications
If you have knee osteoarthritis, here’s what the evidence suggests:
Assess muscle strength objectively. Tests like the sit-to-stand and stair climb are simple, reliable, and correlate strongly with overall function.
Train with purpose. Incorporate progressive strength training — starting with bodyweight and gradually increasing load.
Focus on both strength and speed. Building rate of force development helps with quick transitions like getting up or stabilizing during a stumble.
Track improvements beyond pain. Function — not just symptom relief — should be the primary goal.
Conclusion
The 2025 study by Izadi et al. reinforces what modern physical therapy and exercise science have long recognized: stronger muscles mean better movement, less pain, and greater independence.
For patients with advanced knee OA, improving lower-body strength — especially in the quadriceps and leg press — can predict better real-world function in stair climbing, sitting, and standing.
At Zero Point One Physical Therapy, our programs are built around this principle: Strength builds capacity — and capacity builds freedom.
References
Izadi, M. et al. (2025). Muscle strength is a key predictor of physical function in advanced knee osteoarthritis. Musculoskeletal Science and Practice, 78, 103339. [DOI: 10.1016/j.msksp.2025.103339]
Baker, K.R. et al. (2001). J Rheumatol, 28(8):1655–1665.
Ciolac, E.G., & Greve, J.M.D.A. (2011). Clinics, 66(12):2079–2084.
Holm, P.M. et al. (2022). J Strength Cond Res, 36(10):2635–2642.
de Zwart, A.H. et al. (2022). J Rehabil Med, 54: jrm00321.
FAQ: Muscle Strength and Knee Osteoarthritis
1. Why is muscle strength so important in knee osteoarthritis?
Muscle strength — especially in the quadriceps — is critical because it supports and stabilizes the knee joint. Strong muscles help absorb load, improve joint alignment, and reduce stress on cartilage. Research (Izadi et al., 2025, Musculoskeletal Science and Practice) shows that stronger legs predict better physical performance in activities like climbing stairs and standing up from a chair.
2. Can strength training reduce pain from knee osteoarthritis?
Yes. Numerous studies show that progressive strength training can reduce knee pain and stiffness while improving daily function. By building muscle capacity, strength training decreases the strain on painful joints and helps you move with more confidence and less discomfort.
3. What are the best exercises to strengthen knees with osteoarthritis?
Safe and effective exercises include:
Sit-to-stands from a chair (mimics daily function)
Wall sits or Spanish squats for quadriceps control
Step-ups to improve stair-climbing strength
Bridges and split squats to target glutes and hip stability
Always begin under the guidance of a physical therapist to ensure proper load progression and movement control.
4. Can I improve my knee function without surgery?
Yes — many people with even advanced osteoarthritis can significantly improve function through targeted physical therapy and progressive loading. The 2025 study by Izadi et al. found that small increases in muscle strength led to measurable, clinically meaningful improvements in daily function.
5. How long does it take to see improvement from strength training?
Most patients begin noticing changes in strength, balance, and confidence within 6–8 weeks of consistent, properly dosed exercise. Studies show that even modest gains in strength can translate to better mobility and independence in daily life.
6. What is the rate of force development (RFD), and why does it matter?
Rate of force development measures how quickly your muscles can generate force — critical for reacting to balance challenges or standing up quickly. The study found that faster force production predicted better stair-climbing and sit-to-stand performance, emphasizing the value of both strength and power training in knee OA rehabilitation.
7. How does Zero Point One Physical Therapy help people with knee osteoarthritis?
At Zero Point One Physical Therapy in NoMad, NYC, we take a progressive, fitness-forward approach that goes beyond symptom relief. Our 3-step process — Clarity & Relief → Function & Resilience → Performance & Longevity — helps rebuild muscle strength, confidence, and movement capacity through 1-on-1 physical therapy and individualized strength and conditioning.
8. When should I see a physical therapist for knee osteoarthritis?
You should seek physical therapy if you experience:
Persistent knee pain or stiffness
Difficulty standing, sitting, or climbing stairs
Weakness or instability during movement
Fear of movement or reduced activity due to pain
Early intervention helps slow disease progression, restore strength, and often prevents the need for surgery.
9. Can physical therapy delay or prevent knee replacement surgery?
In many cases, yes. Building strength, improving joint mechanics, and addressing movement patterns can significantly delay the need for surgical intervention — and in some cases, make surgery unnecessary. The earlier you begin strengthening under guidance, the more likely you are to preserve joint function.
10. How do I start physical therapy at Zero Point One?
If you’re ready to move with less pain and more confidence, you can start with a Free Phone Consultation to speak directly with our team.
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