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I'm willing to accept the common wisdom that exercise is good for you. But at the same time, I wonder how this can be true. Man-made objects tend to wear out faster with more use. For example, if you jog one hour every day your shoes will gradually wear down and develop holes. But if you were to only jog one hour a month, those shoes would last much longer before you need to buy new ones. So why don't we believe that we should "conserve" our bodies by limiting heavy usage and preserve the body's condition to keep it in better shape into old age?

Obviously, living organisms are not the same as inanimate objects. But why is exercise supposed to be better for your long-term health instead of wearing out your body faster? When I read advice about exercise, I see a lot of benefits mentioned but I also see a lot of potential problems to be avoided. You hear about people developing an injury because of their workout. And I've heard about exercise releasing more "free radicals" into your body which are supposedly bad for you. As well as "increasing inflammation" and other bad-sounding things.

I don't know much about biology or medical science. But if exercise causes these problems for people, why do we believe that it is somehow healthier for you? Is it because studies indicate that exercise helps avoid things like heart disease and cancer? Do those benefits come at the lesser cost of those downsides to exercise? So we might say, "Years of jogging gave me a bad knee but it helped me avoid heart disease, so it's a net win."

It just strikes me as odd logic that heavier usage on your body somehow makes it last longer than a sedentary lifestyle. If you use your legs more they last longer than if you conserve them? That just seems strange to me. Don't get me wrong... I'm not challenging the claim that exercise is good for you. I hear all the time that exercise is good and inactivity is bad, so I figure that it must be true. But I'm wondering if somebody can point to why it's true. Rather than just taking it in good faith.

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    IMO you only need to try it to understand why. Your body immediately feels better. Everything is easier when you regularly exercise. It's self-explanatory, really. I find that most people who complain about exercising don't actually exercise much, if at all. – user91988 Sep 11 at 15:14
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    Well asked and clear. I would add just one thought: you should also consider atrophy. As being sedentary doesn't save anything, instead your body decides "hey, don't need that anymore" eg less thick and strong bones, aerobic capacity, etc. Like the old phrase "use it or lose it". – Jester Sep 11 at 18:35
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    Try not driving your car often and see how fast it wears out from the few infrequent drives you do take. Your body is worse in that it actively removes resources from unused areas to put towards areas that are being used so it actively degenerates "non-vital" parts that are unused. – DKNguyen Sep 13 at 0:54
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    @user91988 That does not give you any information as to why it works, it just gives you evidence that it does work, which OP already stipulated to in the first sentence – duckmayr Sep 13 at 2:05
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    Don't get me wrong. Asking the question "why" is a sign of intellectual curiosity which is a good thing. And plenty of common knowledge that is wrong. But there are a lot of things about the body we don't have a good explanation for even though the effects are real. A psychiatrist can treat depression with SSRIs without knowing exactly what mechanisms in the body cause depression, which is good because nobody knows what causes depression. We just know that whatever it is SSRIs modulate it. Don't set the bar too high. Anecdotal evidence is worth a lot, especially if it's your own body. – Jared Smith Sep 13 at 10:46
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You hit on some common misconceptions, but you also hit on some truths. For instance, you compare the human body to inanimate objects with respect to damage, but you also accurately point out that

Obviously, living organisms are not the same as inanimate objects

So, what's the difference?

The repair process

Man-made objects tend to wear out faster with more use.

Imagine you have an axe. The axe undergoes wear and tear and becomes dull and damaged. Dullness can be worked around by grinding the blade even more, but eventually, the blade will get cracked or even break. And that's not something that can be trivially repaired.

But imagine if the blade got a crack, and all you had to do was throw some iron ore on it, and let it rest for a couple days, and come back to a blade that was even stronger than before. The crack is not only repaired, but that's now the strongest part of the blade. It was rebuilt more dense than before.

That sounds farfetched for an axe blade, but that's how our body copes with exercise. Exercise is the act of damaging your tissue - be it muscle, tendons, or what have you - then giving your body time to eat and rest while it repairs itself under the assumption that "we'll be doing this exercise more, so let's prepare for that". And that's where we get our longevity too. With wear and tear, your muscles get stronger, your tendons get more durable, your lung capacity increases, and more.

More acute damage

You hear about people developing an injury because of their workout

So when you go to the gym and lift some weights, you damage the muscle tissue, which is then repaired stronger. What about if you actually injure yourself during training?

Anyone who works out for any non-trivial period of time will need to spend some time learning how to do each exercise in a way that minimizes or outright prevents unexpected injuries like falling, breaking bones, dislocating a joint etc. But at the end of the day, it only takes a momentary lapse in judgement to sustain an injury like that. So every individual has to make that decision for themselves. But it's not like you go to the gym and immediately start doing dangerous things. One thing we preach harder than anything else is "start light, and use proper form". Proper form is what prevents injuries, and if you're unable to do an exercise with proper form, then you're using weights that are too heavy. Exercising with proper form is and feels safe. At any and all points, you should feel like you're controlling the weight, and at no point should you feel like you're being yanked around by the weight, or having to catch it falling from a height. That would make you more injury-prone.

Other concerns

And I've heard about exercise releasing more "free radicals" into your body which are supposedly bad for you.

Pretty bad. It causes cancer! But then again, we subject ourselves to so many things that cause cancer. So why is it that so few of us actually get cancer? Because our body has ways of dealing with things that might develop in that direction.

I use the term "cause" pretty loosely here in an attempt to mimic alarmists. Like a lot of things that "cause" cancer, free radicals are an important part of our bodily functions. I'm no expert on free radicals, so I'm not going to spend too much time on it, but I found this Q&A by Dr. Harvey B. Simon of Harvard, who is probably more suited to explain it.

As well as "increasing inflammation" and other bad-sounding things.

I'm not sure this is a concern, per se. An inflammation is your body's response to something. It's how it deals with certain types of injuries or infections. It would be like being worried about how blood is leaking from your skin, rather than being worried about the knife stuck in your arm.

There are diseases that cause inflammations where it's not needed, but if you have problems with inflammations after exercising, it's likely you've been injured in some minor way, and we're back to preaching proper form.

But if exercise causes these problems for people, why do we believe that it is somehow healthier for you? Is it because studies indicate that exercise helps avoid things like heart disease and cancer? Do those benefits come at the lesser cost of those downsides to exercise? So we might say, "Years of jogging gave me a bad knee but it helped me avoid heart disease, so it's a net win."

Yeah, I think you hit the nail on the head on this one. I would argue that most runners do NOT suffer from bad knees though, so the vast majority simply enjoy the improved cardiac functions. Most of us do NOT drop weights on our heads, so we simply enjoy having stronger muscles compared to before we started training.

There are pains we go into the gym expecting, though. Like delayed-onset muscle soreness and "the burn". But these are the harmless, temporary pains that are your muscles' way of saying "phew, that was exhausting, I need a break". And then that, combined with a proper diet and plenty of rest, is what gives us that repair that separates us from inanimate objects.

But I'm wondering if somebody can point to why it's true. Rather than just taking it in good faith.

I just had to quote this one too. In the fitness sphere, it's very easy to find some guru on youtube who says "do this exercise, and don't do that one", and millions will just take it for granted that this must be great advice, because this guru has six-pack abs. Meanwhile, if you make sure to learn WHY an exercise is good or bad, not only are you equipping yourself to work out smarter, you're also learning anatomy.

Working out is an endless learning process about how the body works. Hell, I'm not an expert by any means, and probably never will be.


EDIT: @Jester, in a comment on the question, points out a truly good point that I overlooked. The body's "use it or lose it" response to inactivity. You're not using your muscles much? Then your body won't spend energy preserving them either, and they atrophy.

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    Thanks for the thoughtful reply. Is there stuff that doesn't regenerate, like cartilage in the knee? An older tall guy once told me he used to play a lot of basketball but had to stop because it ruined his knees. So I have tried to avoid high-impact exercise like jogging and got a rowing machine which I understand is easier on the knees. Should we conclude that there are parts of our body that respond well to exercise, and other parts that we should try not to strain? Now I am wondering about the difference between parts of the body that repair their own damage and parts which just wear down. – peacetype Sep 11 at 20:59
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    @peacetype - I'm afraid I can't go into detail on what does and doesn't regenerate well. I don't have that expertise. But I would say that could be its own, valid question on this site, if you angle it as an "injury-prevention" type question. In fact, I too would love to read answers from other users who do know about that stuff. – Alec Sep 11 at 21:09
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    @user3153372: Muscles need resources even when you don’t use them. It’s just a beautiful mechanism to get muscles just as big and strong as you need them. – Michael Sep 12 at 8:16
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    @user3153372 Good question. Michael's response makes sense to me (i.e. muscles consume the body's resources so if you don't need them then the body will reduce the muscles and use the resources for something else). But you hit on a line of questioning which I wonder about sometimes, but it is a total rabbit-hole so best not to get into it here. But when I think about it, why can't your body just be perfectly strong always? Why do organisms need to age and die? Why can't cells keep repairing themselves forever? This gets into the realm of existential questioning, so I think it's off-topic here. – peacetype Sep 13 at 4:28
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    There's one important point to why we can't be perfectly strong: because then everyone would need to eat like a bodybuilder, and people who need specialized strength would loose out. A swimmer exercises different muscles, and can get stronger muscles for swimming partly because they aren't also simultaneously training to be a boxer. If the body automatically kept every muscle strong, it'd be the same as trying to train for every sport and every activity at the same time. There are animals whose bodies do this, but humans are built to be more specialized. – daboross Sep 13 at 8:35
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This is an interesting question that had me thinking for a while. It's difficult to answer "why" exactly, other than saying "it is that way", so I'll try to describe the need for exercise and a few benefits it gives. @Alec's answer has neatly addressed your questions about wear and tear so I won't address those.

One of the things humans have evolved to do well is output a lot of work over a long time, most likely for persistence hunting. The metabolic systems that allow us to do that require at least some work (exercise) to function properly. They make up the core part of our physiology - exchange of energy with the environment - so they can't be neglected. It seems that for biological systems, the price of being able to do something is that you must do it, and none of us can now opt out of this trade.

You can think of metabolism as having two sides - the Exercise side and the Feeding side. On the Exercise side, the body uses stored energy and incurs damage to its tissues. This is balanced by the Feeding side, where consumed food is used to replenish the stores and lay down new tissue. Both are obviously happening all the time, but the balance swings back and forth as you exercise, eat, sleep etc.

The point is that the chemistry of each side doesn't work particularly well when the other doesn't happen enough. They require each other: to oscillate between the two sides is better than to stay in the middle. If you burn 300 extra calories in a day and eat an additional 300, there will be no effect on your weight, but you will be healthier than if you did neither.

Type-2 diabetes happens when the body becomes insensitive to insulin, i.e. becomes unable to reduce its blood sugar level. This has a number of causes, but the most important one is persistently high blood sugar (over-Feeding). The most effective way to increase insulin sensitivity is regular strenuous exercise.

Also, atrophy (use it or lose it) is absolutely a concern. The atrophy associated with inactivity extends beyond losing muscle size, to decreased bone mass (osteoporosis) and risk of Alzheimer's.

They say that you don't stop moving when you get old; you get old when you stop moving:

Atrophy vs training

These are thigh cross-sections. Body composition like the middle images - with low muscle mass and lots of fat tissue - is highly correlated with all sorts of diseases, especially cardiovascular disease. Conversely, muscle strength and mass in old age is a good predictor of remaining healthy lifespan.

Is it because studies indicate that exercise helps avoid things like heart disease and cancer? Do those benefits come at the lesser cost of those downsides to exercise?

This is a much easier question to answer - overwhelmingly yes, and there are many more benefits than that.

If you want a far better explanation than mine I'd recommend reading Survival of the Fittest by Mike Stroud.

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  • "The price of being able to do something is that you must do it." "You don't stop moving when you get old; you get old when you stop moving." Terrific quotes! I like your philosophy. Those 3 images are a powerful visual. Are those leg or arm cross-sections? You make a strong case for why the body needs to be exercised. Avoiding atrophy and the associated risks are a big incentive. The question that remains for me now is how to exercise without doing any harm to your body? At @Alec's suggestion I tried to expand on this in a separate question. – peacetype Sep 11 at 23:33
  • +1 for the pictures; it's quite eye opening! – Thomas Sep 12 at 17:15
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    @peacetype good point, forgot to say - they're thigh cross-sections. – Eddie Summers Sep 13 at 10:05
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It could be partly related to resource conservation. For example, if your body waits for activity in a particular muscle group to cue the growth of those muscles, it doesn't waste calories building muscle mass in excess of the strength that you actually need, which would be particularly important in times of scarcity.

Another hypothesis, perhaps better illustrated by mental exercise such as playing video games (which has been demonstrated to have mental health benefits in controlled studies such as this one), is that it gives you immediate feedback on problem-solving faculties, which then helps shape their development over time. While a business venture may take years to come to fruition, a game may be won or lost in the span of an hour.

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  • Thanks. I realize this question doesn't have a single, obvious answer. So I think it's worth hearing a variety of perspectives on it. Your response jives with something that @Eddie Summers said, about how if you neglect your body's systems then they won't function well. It's like your body is tuned to respond to exercise by making itself better at being exercised. So if you don't use it, the systems that perform physical activity will become weak. In your comparison, is exercise more like the short-term game feedback, or the long-term business venture? – peacetype Sep 12 at 0:28
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    Oh, I also wanted to say how @Eddie Summers' post had some great quotes: "The price of being able to do something is that you must do it," and "You don't stop moving when you get old; you get old when you stop moving." I think you can say the same thing about mental activity and life in general. If you learn a new skill and then don't apply it, you forget much of what you learned. And when people get old if they "retire" to the effect of no longer pursuing goals/ambitions or other meaningful pursuits, it seems they are more likely to die earlier. I think it pays to remain active in all ways. – peacetype Sep 12 at 0:34
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    I like this answer more than the others, as it gets to the "why is this seemingly insane property of animals the case". It might be improved though by noting that while in the environment we evolved for "optimising for short sprints" and "optimising for long runs" etc all were common (and so work quite well) "optimise for sitting still doing almost nothing" was very rare, so that "optimisation" doesn't really work leading to all the problems associated with a sedatory lifestyle – Richard Tingle Sep 12 at 10:19
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I think you're barking up the right tree: The body is made of many different chemicals, so logically some parts will wear faster than others. And the body can try to repair some parts, but not others (no new eyeballs, for example).

Following, certain exercises might be good for certain body parts while be arguably bad for others. And the degree of exercise also matters in efficacy and damage. For example, some heavy weight-lifters actually wear wide straps to keep their abdomens from herniating outward, which would require surgery to correct!

As to the specific benefits to specific areas of the body, one could spend a long time studying the theories and clinical studies. Here are a few ideas that I know about...

The lymphatic system proximal to the intestines has been proposed to benefit from any muscular contractions there, helping to propel the lymph along.

Altering blood pressure is believed to help keep the vascular walls remain pliable, making them less prone to damage and blockage.

Stronger heart muscle can rise to the occasion in times of emergency instead of failing or becoming prematurely exhausted. Other benefits include better blood pressure.

There is a growing school of thought that the fascia (sheets) that surrounds many organs can restrict those organs if it becomes stiff/fibrotic, actually causing the organs to under/malfunction (directly or indirectly) and/or causing referred nerve pain.

If you begin looking up medical definitions and following link after link, you could spend years studying it; I know because I have.

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    Even eyeballs repair themselves to a very significant degree. – jpa Sep 13 at 15:52
  • It's relative, of course. Macular degeneration is a big problem. And spontaneous retinal detachments often require medical intervention. – kackle123 dances with Monica Sep 13 at 16:22
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Exercising is good for you for a myriad of reasons that act through an impressive numbers of processes.

Understand that mental and physical health are closely related and intertwined. You cannot separate one from the other. They affect each other in a way that I think we severely underestimate nowadays. However, for the sake of a clearer explanation, I will split the two aspects.

Also, you must understand that living things are not objects. We don't really have a certain amount of material that we can spend and then it's over. Like a car that has a finite amount of fuel and then stops when empty. Humans don't have like e.g. a "spine currency" that if you use it too much then it's worn out and you cannot use it ever again. Rather, we have processes that when you age become slower, less efficient, ... And that is what causes "issues" when you become older (you repair slower, you lose muscle density, ...). However, exercising will rather help those processes remain efficient as long as possible because you stimulate them. Wearing due to exercising happens because people do things in an incorrect way (too much, too intense, improper technique, using bones instead of muscles, ...)

Now, for the explanation to your question. For the physical aspects.

First, exercise is good for you physical health because it makes your body more resilient to external aggression. By exercising you become more resilient to shocks, falls and everyday fatigue (think walking, carrying your groceries, carrying your kids, ...). This happens through thickening of the muscles, bones, connective tissues but also transformation of those same tissues e.g. endurance training creates capillaries through your muscles to better transport oxygen to them and better remove waste products from them.

Second, connecting to my previous point, exercise is good for your physical health because it helps your body move and remove waste. Indeed when you exercise you circulate blood flow & oxygen faster and in a greater amount. This acts like a cleansing operation allowing your body to more easily remove accumulated wastes products (due to exercise but also daily-life stress).

Thirdly, it's good for your physical health because you avoid turning your body into a non-optimal structure by accumulating fats which causes a lot of health problems : cardiovascular issues (think clogging, harder for your hearth to push blood through clogged vessels, ...), pulmonary issues, mechanical constraints on your structures (bones, cartilage) because you don't develop the muscles to support you and instead rely on your structure to carry you (which makes them wear out faster).

Next, there are the mental health aspects. It has been proven and is now coming more and more in the studies that inactivity causes mental health issues (anxiety, depression). I am not knowledgeable enough to provide a clear and detailed explanation but this is something I truly believe will come in force in the coming years. Also, it recently (some years) became apparent that some chemicals are really important for cognitive functions e.g. it seems that lactate (which was before thought to be a bad thing) is one of the primary fuel for your brain. And when you don't move/exercise... You diminish your capacity to produce and use lactate as a fuel. So there are studies that analyze the impact of lactate production on your cognitive functions.

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  • Thanks, this really helps me get to the why about the pros and cons about exercise or the lack thereof. One particular point of concern is what I've heard about free radicals. About how exercise creates more of these in your body, and that they are bad for you, but can be counteracted by eating foods with antioxidants. I think I may have read/heard that they play a role in premature aging? Or contribute to disease risk? Honestly I'm not sure, but wondering if you have any remarks about that. Do you think it's something to be concerned about in relation to planning your workout routine? – peacetype Sep 15 at 0:00
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    @peacetype: As I understand there is a balance in the body between free radicals and antioxidants. Hard and long endurance training in particular can upset this balance: file:///C:/Users/andre/Downloads/antioxidants-07-00119.pdf. I would think not training for much more than 1 hour and allowing sufficient time for recovery should alleviate this problem, but I also see recommendations regarding consuming antioxidants for endurance athletes. I think the problem is that many endurance athletes go overboard and start running say > 100 km a week, which I do not think is healthy. – Andy 6 hours ago
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In addition to what other answers already say, I think there's a point to be made: Our bodies DO wear out with heavy use. If you push your body to its limits every day, it does tend to wear out. If you go to gym every day without a proper training regimen designed to alleviate these kinds of issues, you start to accumulate muscle damage, joint damage and so on. And not all of that will be healed, so you can end up causing significant permanent damage to yourself.

However, our bodies are the results of a billion years of evolving in hostile conditions. Light to moderate damage is not only expected, but that expectation is built into the design.

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The body is mainly a biochemical "machine", that can at least partially regenerate itself, which have evolved trough 100 000s of years. However our socitety has the last 100s years progressed much faster than evolution can keep up with. To the point where one hardly have to use ones body at all. However if you do not exercise you run the "machine" outside the area of operation it was "built for". It seems that since this "machine" has evolved under the condition of constant use certain design "decisions" ("shortcuts") has been made that requires use. An example of this is cartilage that do not have blood supply but rather is dependent on movement to pump nutrients into the joint. Another example may be blood supply to the brain. A low heart rate causes less blood and therefore less energy and repair materials to the brain. Increasing the heart rate, eg. by going for a short walk in the middle of the workday, is good for the brain. Strength is part muscle mass but also part the ability inside the brain to generate a strong high frequent firing pulse. Therefore strength training improves cognitive function.

It seems highly likely that prehistoric man lived with very uncertain food supply. Maybe he could kill an animal consume it and gain a lot of calories, but then have to go for say a week without consuming many calories.

White fat stores calories. When feasting on the animal his body would use some of the calories to rebuild muscle mass and store the rest as white fat. He would then burn this fat during the coming week. (Re)building muscle and tendons is very energy costly. The body therefore only (re)builds the muscle that is needed. The body determines this based on what have been used: "use it or loose it".

In our modern society acquiring calories is a simple and certain process. The bodys predisposition to store energy as fat instead of (re)building muscles and tendons have therefore become a problem.

This is a biomechanical problem: if you do not train strength you gradually get weak, which is not a problem until you suddenly need strength and then risk getting injured. Your muscles and tendons stabilizes and protect your bones and cartilage. Since cartilage almost do not regenerate this is very important. Likewise having a strong heart protect you from cardiac damage if you suddenly should need to eg. sprint.

It is also biochemical problem. The body is a chemical "machine". It signalizes using different hormones. Both muscle mass and fat send hormonal signals. If one have a lot of fat and little muscle mass the hormone balance becomes wrong.

Physical activity in itself also releases hormones. Among these are neurotrophins proteins that promote growth and repair of brain cells. Physical activity also releases serotonin hormone which makes you sleep better, which in turn is important for the brain. Along with serotonin physical activity also stimulates the release of dopamine and norepinephrine. These brain chemicals regulates the mood and prevent depression. The increased blood flow to the brain caused by elevated hearth rate is also important for energizing and repairing the brain. Physical activity also increases insulin sensitivity which is important in order to avoid type 2 diabetes.

The result of a sedentary lifestyle may be wrong amount of signal hormons. Which in turn may cause many lifestyle diseases.

When one exercise the body responds to the stimulus by adopting. Eg. the muscles become stronger. That is unless you exceed the bodys ability to adopt, in which case you risk injury.

A simplified conclusion from this may be: Use it or gradually loose it, but do not overuse it or you may suddenly loose it.

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