1. Recreational Runners Had Fewer Hip and Knee Joint Problems Than Non-Runners
Have you ever watched someone running in a park and wondered if their joints are safe? A study published in 2017 looked at the link between running and osteoarthritis. It combined data from 25 studies involving 125,810 people. When they calculated the results for 17 of these studies, the rate of osteoarthritis in the hips and knees combined was 3.5% for people who run as a hobby. For people who do not run, the rate was 10.2% [1][9]. If you divide 3.5 by 10.2, the result is about 0.34. This means the rate for hobby runners was about one-third of the rate for non-runners. This number goes against the common image that running wears down joints. However, we cannot decide that running protects joints based on this number alone. We need to look at another number from the same analysis in the next section.
2. Competitive Runners Had Higher Rates of Hip and Knee Osteoarthritis Than Non-Runners
The same analysis included another group: people who run competitively. For these runners, the rate of hip and knee osteoarthritis was 13.3%. This is higher than the 10.2% rate for non-runners [1][9]. If you divide 13.3 by 10.2, the result is about 1.3. Compared to hobby runners (3.5%), the rate for competitive runners is about 3.8 times higher. This shows that "runners" cannot be treated as one single group. The difference between enjoying running moderately and competing intensely is large. Another important point is that this analysis combined observational studies. These studies compare groups but do not prove cause and effect. The researchers noted that past injuries might affect the results [1][9]. For example, people who stopped running because of knee pain might be counted in the non-running group. Also, the numbers combine hip and knee problems, not just knees. So, we cannot say for sure that running protects joints or that it destroys them. What is happening inside the joints?
The authors state that a cause-and-effect relationship is not confirmed, and other factors like past injuries may influence the results [1][9].
3. How Joint Cartilage Gets Nutrition Without Blood Vessels
Adult joint cartilage does not have blood vessels. Yet, cartilage is living tissue that needs nutrition. It is explained that nutrition is exchanged mainly through the surface of the joint with joint fluid [2]. Joint fluid is a small amount of liquid made by cells in the synovial membrane. It delivers nutrition to the cartilage and also reduces friction to help the joint move [3]. Movement plays a role here. Experiments show that repeatedly putting weight on cartilage makes it easier for large molecules to enter the cartilage. However, this effect is small for small molecules [2]. This experiment used a model of cartilage. It does not show exactly how much nutrition increases when a person walks. We cannot say that exercise definitely increases nutrition. Instead, it is thought that movement and pressure are involved in the exchange of nutrition. What happens if the joint is not moved for a long time?
4. Cartilage Gets Thinner When Joints Are Not Used, But Exercise Does Not Make It Thicker
When a joint is fixed during fracture treatment or when the lower body is paralyzed, the load on the joint decreases. In these cases, it is thought that cartilage becomes thinner (atrophies) [4][11]. Not using the joint can affect its thickness. But does more load make cartilage thicker? The same review states that even for people with high loads, like elite athletes, the average thickness of the cartilage does not seem to increase [4][11]. Another report says that human cartilage hardly deforms during daily movements. Even if it dents under load, it returns to its original shape within 90 minutes [4][11]. This suggests that cartilage is not a tissue that "breaks easily" during normal movement. In short, cartilage is not like muscle, which can be trained to become thicker. It gets thinner if not used, but using it does not necessarily make it thicker. So, should people with painful knees avoid moving?
5. Why Exercise Is Recommended Even for People With Painful Knees
The Japanese Orthopaedic Association explains that in knee osteoarthritis, cartilage loses elasticity with age and wears down from overuse, causing joint deformity. Treatment includes exercise to strengthen thigh muscles and move the joint [5]. It seems strange that exercise is part of the treatment for a disease caused by "overuse." However, "overuse" and "lack of exercise" are different issues, and the association mentions both. The MSD Manual lists the role of exercise as keeping cartilage healthy, increasing the range of motion, and strengthening muscles around the joint to absorb load [6]. How big is the effect? A 2024 Cochrane Review combined 139 trials with 12,468 people who had knee osteoarthritis. It found that exercise probably improves pain, physical function, and quality of life in the short term, but the clinical importance is uncertain based on the criteria used [7][10]. We cannot say it "cures" or "definitely works." The effect is short-term, and the certainty is low to moderate.
6. How Many Steps Equal 60 Minutes of Walking? Calculating the Recommended Guideline
The Japanese Ministry of Health, Labour and Welfare recommends that adults engage in walking or physical activity of similar intensity for at least 60 minutes a day (about 8,000 steps or more). For older adults, it recommends at least 40 minutes a day (about 6,000 steps or more). It also recommends strength training 2 to 3 days a week [8]. Let us divide these numbers. 8,000 steps divided by 60 minutes is about 133 steps per minute. 6,000 steps divided by 40 minutes is 150 steps per minute. These numbers come from dividing the daily guideline by time. They do not set a walking speed. To see how many steps you take, try walking for 10 minutes while checking a watch and counting your steps, then compare this to the daily guideline. If you have knee pain, do not force yourself; consult a medical professional. You cannot directly train cartilage, but moving helps with nutrition exchange and strengthens surrounding muscles. Write down the numbers from the runner study: 3.5% (hobby), 10.2% (non-runner), and 13.3% (competitive) [1][9]. Compare them to see how the definition of "running" changes the result.