How often have you heard someone say that their knees are worn out from years of exercise? The idea that the knees can just wear out from use seems quite intuitive. After all, this is what happens to the myriad of machines we use throughout our lives. Bicycles, cars and blenders all wear out with use.
But when you consider that the tissues of the human body are constantly being replaced, the idea that they wear out becomes something of a puzzle. Muscle is completely replaced in five months, bone in ten years and cartilage in one to three years depending on the joint. This means that every ten years you essentially have a new leg. The muscle tissue that powered you up a mountain in your thirties has been completely replaced by the time you are biking across California in your forties.
Osteoarthritis is a degenerative condition characterized by a progressive loss of joint cartilage. This loss of cartilage is sometimes painful. The knee joint is a very common location for osteoarthritis and can cause significant disability. Does exercise wear out the cartilage in your knees?
To answer this question I found a systematic review titled “What is the Effect of Physical Activity on the Knee Joint” published in 2010 in the Journal of Medicine & Science in Sports & Exercise.1 The Australia based research team led by Donna Urquhart found 28 relevant articles, 16 of which had a prospective design.1 The studies were conducted all around the world with the US and Australia comprising the majority.1The severity of knee osteoarthritis was determined by joint space narrowing and bone spur presence on x-ray and by cartilage thickness and cartilage defects on MRI.1
The authors found strong evidence that more physical activity is associated with more bone spur formation and fewer cartilage defects. They found no association between physical activity and joint space narrowing. There is also moderate evidence that more physical activity is associated with thicker cartilage.
My conclusion from this meta-analysis is that regular physical activity promotes knee health. The idea that knee arthritis is the result of simple wear and tear is not supported by the current evidence.
Bone spurs have historically been included in the definition of osteoarthritis but their clinical significance is being questioned as they do not correlate with any symptoms. Bone spurs are an adaption to increased force and probably don’t deserve to be as scary as they sound.
But if regular exercise is not wearing out knees, why are so many people getting knee osteoarthritis? Other factors are likely at play here such as inactivity, genetics and injury history.
The finding of more cartilage defects in individuals with a sedentary lifestyle fits well with the bench research on cartilage growth. Cartilage is a tissue that grows in response to mechanical stimulation. Growth is limited when there is a lack of stimulation, such as in bedrest, spaceflight or a sedentary lifestyle. A lack of exercise results in weakening of cartilage, making it more prone to injury.
Twin studies comparing monozygotes to dizygotes have demonstrated that one half to two-thirds of the incidence of osteoarthritis can be attributed to genetics. While many studies have been published linking a variety of genes to osteoarthritis, the limited replication of these findings makes it too early to come to any specific conclusions.2
People who have a traumatic knee injury such as an ACL tear, patellar dislocation or meniscus tear are much more likely to get knee osteoarthritis.3Thismay be due to the result of abnormal joint mechanics secondary to the loss of the stabilizing force of the torn ligament.
A final and important consideration when interpreting the results of this review is the fact that radiographic osteoarthritis was investigated and not symptoms such as pain, stiffness or disability. The link between these symptoms and degeneration on x-ray and MRI is tenuous at best. The relationship between exercise and osteoarthirtis on imaging is only one piece of the puzzle.
So keep up with your regular physical activity. Your joints need it.
- Urquhart DM, Tobing JFL, Hanna FS, et al. What is the effect of physical activity on the knee joint? A systematic review. Medicine & Science in Sports & Exercise. 2011;43(3):432.
- Ryder J, Garrison K, Song F, et al. Genetic associations in peripheral joint osteoarthritis and spinal degenerative disease: A systematic review. Ann Rheum Dis. 2008;67(5):584-591.
- Magnussen RA, Mansour AA, Carey JL, Spindler KP. Meniscus status at anterior cruciate ligament reconstruction associated with radiographic signs of osteoarthritis at 5-to 10-year follow-up: A systematic review. J Knee Surg. 2009;22(4):347-357