When recovering after exercise, the body does something called overcompensating. It doesn't just repair the damage done but actually strengthens the area. Muscles undergo hypertrophy and bone becomes more dense. So when you challenge the body physically a moderate amount, something called functional overreaching, your body responds by actually strengthening itself. Obviously inanimate objects don't have this system so they simply wear out over time

To go one "Why?" deeper.

The reason that our bodies do this, as opposed to making all muscles strong all the time, is to avoid wasting energy on muscles you aren't using. It is basically using what muscles were used recently as a heuristic for which muscles need to be strong to ensure survival.

It would be nice, though, if our minds could control that directly, rather than going through exercise. Perhaps evolution is not there yet.

1 - Evolution is only increased genetic contribution to the next generation. Random mutations that help with that will become more common. It's not a magical process that makes us better.

2 - It turns out that your mind does control it directly. The greatest predictor of improved muscular strength is how strongly you can intentionally contract that muscle. Unfortunately, you learn to do that through exercise :|

I happen to love the exercise so I’ll happily have it this way! Especially if (like me) you’ve crossed over the starting hurdle and you are building/maintaining real muscle - it’s awesome!

I would say since the primary goal of genetic life is to survive and pass on genetic code. Muscles which clearly ended up working so hard that they were damaged need to be strengthen so next time such and such situation occurs they do not need to be stressed to such an extent. They are prepare for the eventual re-occurring of that task. Eventually you will lose muscle mass if you do not use/need it.

With inanimate objects the same happens if you consider humans doing maintenance as part of the system: if some part gets damaged all the time because it's too weak we strengthen it or replace it with a stronger version.

There are a lot of low level mechanisms that improve health with exercise. The basic breakdown is some things like digestion where optimized by evolution around the fact our ancestors moved. For similar reasons UV light in childhood are a requirement for normal vision exercise was simply universal for a very long time.

A lower level example is feedback mechanisms. Mechanical stress from motion are used to maintain bone strength. The cells involved in bone breakdown and repair can detect damage and reinforce where needed, but they also remove bone where it’s not needed. Without that kind of feedback our bones would either be weak or heavy.

The same thing happens with your circulatory system, you grow new capillaries in response to demand. Or for a “negative” example fat cells reproduce when you can store more energy.

An extension to that question: Why do people say elevating your heart rate is good in relation to exercise, and bad in relation to cocaine? People frequently say things like "stimulants stress your circulatory system", but so does exercise.

Not a professional, but the way I reason it is that pushing your heart to the limit causes it to grow stronger, and that additional capacity means it doesn't have to work as hard during the rest of the day. You basically bring down your average heart rate by making your heart stronger.

That makes sense, but then why is it considered bad for drugs to raise your heart rate?

Just because the heart rate is elevated in both cases, that doesn't mean the context is the same. It's not the same cause, it's not the same result.

Exercise raising the heart rate is a consequence, that (the increased heart rate) is not the singular magic thing improving your health. With exercise there's a positive, body-comprehensive event going on.

Exercise strengthens the heart.

Cocaine (for example) damages the heart. It doesn't just elevate the heart rate, it does some vicious things to the heart, causing damage:

https://www.webmd.com/connect-to-care/addiction-treatment-re...

Caffeine raises your heart rate for a short(ish) time, but at least for moderate amounts of consumption that doesn't seem to have any negative effects. There are even some studies that suggest that one or two cups of coffee a day may have health benefits, including the cardiovascular system.

Cocaine on the other hand seems to raise your resting heart rate because it damages your cardiovascular system [1]. Your heart has to work more to achieve the same result, the opposite from what you want to achieve with exercise.

1: https://journals.plos.org/plosone/article?id=10.1371/journal...

Simple machines don't have internal processes that rebuild themselves to be stronger after wear and tear.

Bodies heal unlike machines. Maintaining capacity of tissues has energy costs. The body does not want to waste energy maintaining capacity that it does not need. Therefore if your day to day existence does not demand a high level of fitness to function, you will decondition . You must provide stressful stimulus to the body to let it know the level of condition it needs to maintain to be properly adapted.

Rubber band can't dynamically detect and repair weared off molecule clusters.

Mammals are highly adoptive by the design, a living organism consists of trillions programmed and partially controllable nanomachines.

Body has autonomous and centralised systems, which checks and sends hormonal signals and redistribute resources from other parts and could directly command specific cell groups to grow or change behavior in a chance to increase organism survivability.

The human body doesn’t shut down so much as idle. There are mechanical systems that don’t like to idle, like air cooled engines. Anything that maintains itself as part of vigorous operation (flushing out pipes, circulating lubricants or coolants) can get into trouble if not operated vigorously enough for too long. You need to “exercise” your car every few weeks or the battery dies.

Not exactly what you were asking for, but Nassim Taleb's "Antifragile" might be of interest to you. It's about how many human made things e.g. machines, the economy, etc. are fragile and get weaker/collapse under stress. In contrast many organic things (e.g. muscles) are antifragile, i.e., they gain from (small) stressors. Definitely an interesting read.

Your body has a hypertrophy response to exercise stress.

The low-level mechanism could be the endocrine system and its production of anabolic hormones, depending on how 'low-level' you are looking for.

Exercise is bad for your body. Recovery from exercise is very good.

If you have a simple machine that wears out then someone will be along to fix it, if it's of any use to do so and you have the budget. The body uses the same idea.

It is hard to give a short answer to your question but you will find the following books helpful (check reviews on Amazon and elsewhere on the Web);

1) Body by Science: A Research Based Program for Strength Training, Body building, and Complete Fitness in 12 Minutes a Week by John Little and Doug McGuff.

2) The Energy of Life: The Science of What Makes Our Minds and Bodies Work by Guy Brown.

Both human and simple machine wear out of exercise.

Human cells (controlled by DNA algorithm) repair the wear out parts via reinforcement learning.

My basic understanding is when we are strength training we are putting tiny tears in our muscle fibers. The body then uses protein to heal those tears and make the muscle fiber bigger as part of that process, allowing us to train even harder in the future.

Elastic bands are relatively just static objects.

In human body, new cells are created every day which is not case with elastic band. Best anology could be economy. If you keep creating wealth and not spending one day you reach your limitations. Continously churning and creating gives boost to economy.

Some tissues generate new cells every day, but tons of relevant things to exercise such as cardiac muscle cells and the fibrocytes making up things like ligaments and tendons really don't. This also goes for most of the other stuff on the inside of your body.

Why is a machine an appropriate analogy to a living body?

life eg. cellular growth