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I posted Why is exercise good for you? But I wanted to dig a little deeper into one specific point. I can grasp the concept that the body responds to the microdamage of exercise by building up the muscles/tendons to be stronger and better able to withstand future exercise. So it makes sense that a suitable level of exercise done in a safe manner would be good for your body's overall health.

I can accept that this continues to hold true when sustained over many years, even into old age. But I've also heard that there are some parts of the body which don't respond to exercise in the same way that muscles do. For example, I heard that cartilage in the knees will slowly erode over decades of life and will wear out faster with heavy use. I have long legs and an older tall guy once told me to take care of my knees. He said he used to play a lot of basketball but had to stop because it ruined his knees.

Another example would be back injuries like a herniated disc caused by improper heavy lifting. You can read about how Christian Bale suffered a herniated disc just from gaining 43 pounds (19.5 kilograms) and adopting bad posture for his character role in the movie American Hustle.

So it seems like there are certain parts of the body that respond well to exercise, and other parts of the body that we should try not to strain too much. I am wondering about the difference between parts of the body that repair their own damage and other parts which just wear down and never repair. If the goal is to gain the health benefits of exercise, how do we do that without also getting the harmful effects? Like doing permanent damage to your body that will not heal, or slowly wearing out important areas like joints over the decades and leaving you debilitated in your old age?

One approach I tried is to avoid "high-impact" exercise like jogging and instead I got a rowing machine which I understand is easier on the knees and ankles. I wonder if there is someone who knows more about parts of the body which do and do not regenerate well, and how to exercise in a way that provides the benefits of a workout without doing lasting damage to their body. Maybe there are some users here who are in their 60s or older and have found good, sustainable workout practices?


Note
I am aware of this question: Do joints wear out from exercise? But I would like ask in a more general sense about how to develop a healthy workout routine that can be sustained for many decades while avoiding not only joint injury, but also back injury and indeed any workout-related injuries or ill physical effects in general that can develop over the years.

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    I’d like to note that examples like Christian Bale’s are anecdotal. Most herniated discs come from rotating the spine under heavy load but are often blamed on “lifting heavy 20 years ago.” Doctors, Physios and chiropractors are amazingly terrible at informing people of such things. – Frank Sep 27 '20 at 9:57
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Consider a knee:

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The meniscus absorbs much of the shock of jumps and landings. Tears in the meniscus is a common cause of surgery. Sports with high risk of meniscus tear include american football, soccer, basketball and wrestling. Altough running is high impact this injury do not seem to be common here. Rather it seems that rotation of the knee and unexpected movement due to an opponent is the cause of the injury. Therefore strengthening the muscles around the knee may help prevent this injury.

In the middle aged population degenerative tearing seem to be more common. Osteoarthritis is the gradual thinning of cartilage such as articular cartilage and meniscus in the figure and may predispone one for meniscus tears.

Traditionally this has been considered as "wear and tear". Loosing weight has been showed to be effective for overweight people. This can be viewed as reducing the "wear and tear". This seem to be the mental model you have adopted. It is probably partially wrong and simplistic.

Frequent moderate physical activity now seems to be the advice for patients suffering from osteoarthritis. Pumping synovial fluvid that help repair the cartilage (albeit at a slow rate) seems to be the mechanism here. From the book Human Anatomy referenced in the related question you mentioned: "Lack of exercise causes the articular cartilage to deteriorate more rapidly from lack of nutrition, oxygenation, and waste removal." Interestingly cartilage does seem to have some limited ability to regenerate itself.

In fact osteoarthritis may be an inflammatory disease. The mechanism by which loosing weight helps against osteoarthritis may not be a mechanical one but rather a hormonal one. It could be that reducing body fat fights the metabolic syndrome that cause low grade inflammation. Overeating and being fat seems harmful in itself. Experiments on mice also seem to indicate that circulating fat acid composition is important (with omega 3 being good and omega 6 bad): "This study provides further evidence that circulating FA composition and systemic metabolic inflammation, rather than body weight, may be the major risk factor for obesity-associated OA.".

The last point together with the knowledge that saturated fat and simple carbohydrates (eg. sugar) may also cause low grade inflammation, whereas nonsaturated fat (olive oil, fish etc) and complex carbohydrates (whole grains) reduce inflammation. So what you eat may to some extent affect your joint health.

Osteoarthritis do not seem to be more common among runners than non runners. This also go against the "wear and tear" model.

The fact that arthritis affects women more so than men, particularly after menopause seems to indicate that hormone levels is a factor.

My conclusions based on all this:

  • Try to avoid sudden unexpected (rotational) forces to the knee.
  • Strengthen the muscles around the knees to protect against the above point.
  • Exercise frequently. Short walks daily is probably great. This will nutrify and oxygenate the cartilage.
  • Warm up properly. "When synovial fluid is warmed by exercise, it becomes thinner and more easily absorbed by the articular cartilage. The cartilage then swells and provide a more effective cushion against compression." ("Human Anatomy")
  • Use correct technique when lifting weights.
  • When lifting weights stay within your limits. Do "not" go for 1RM PR lifts.
  • Have a fairly low bodyfat percentage and a moderate amount of muscle mass. Remember that muscles are an endocrine organ. So is fat. Muscles send good hormonal signals. Fat send bad signals (that causes inflammation).
  • Avoid extreme endurance activities such as marathon running etc and excessive amounts of training. Partially because of "wear and tear" but also because it is bad for the hormonal profile and may cause low grade inflammation (think aging) that in turn harms the cartilage. Eccentric muscle contractions seem to be particularly bad wrt to inflammation. The key point here seems to not train more than you can recover from, and with age the ability to recover diminshes.
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    Thank you for all this insightful info. I think there is a sort of "traditional" or "uneducated" view of the human body as something like a piece of wood, where things like knee cartilage wear down the more you use them. But the articles you linked depict a more complex picture of the body than I was aware of, particularly about joints like the knee. I'm getting the picture that the key thing is to stick to an appropriate amount of exercise for your body. And combined with adequate rest and exercise, that is better overall for your long-term health than inactivity. Your details really helped! – peacetype Sep 24 '20 at 1:51
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    I do not understand much of the body but I understand that it is incredible complex. Common sense and moderation, like you have shown in your comment, is probably the best approach. I do not think you should shy away from running unless you are overweight. Personally I am a bit big (100 kg). I run slowly downhill to reduce the impact on the knees. I also try to run regularly but not very often or long. Say 5 km 1-2 x a week. I also train with weights. I think variety is important. Some people fall into the trap of only (over)doing 1 activity. I think of health as a chain (the weakest link) – Andy Sep 24 '20 at 6:54
  • That sounds like good sense, and the links you provided support that variety is important, and also consistency. In the past I’ve had a bad habit of sporadic exercise: long periods of nothing, then a sudden & intense effort to start but overdoing it. But the research you linked suggests that sort of thing is almost as bad as doing nothing at all. Consistency and moderation as you say sounds like the way to go! – peacetype Sep 24 '20 at 9:18

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