At the bottom if the sea

As for calories- you will more or less burn roughly the same calories on strenuous activity as you always do at a given body composition. That is to say: untrained- doing a thing the first time- a 200lb person with 25% body fat will burn more calories doing 15 jumping jacks than a 110lb person who is 25% body fat. As you work out and lose or gain weight and fat the amount of calories you burn for any given activity change with your body composition: and as you repeatedly do the same activity over and over your body becomes more efficient at that activity. So in a magic example where you have a static body composition and consistent response- doing 1,000 push ups a day will make you really good at pushups.

The physics involved for the push up stay the same- but your body becomes better st doing push-ups and so you will see a slight decrease in the energy your body requires to do said push up. Meaning that after a while doing all those push ups daily, you won’t burn as much energy doing them as you did when you started by a somewhat marginal amount.

To increase the ability of the body to do a specific kind of work, and to mitigate this, all you have to do is increase the work you are asking your body to do. So the person doing 1,000 push ups won’t burn quite as many calories as they used to- but if they start doing say... 1,050 push ups and increase at set periods from there- they will not only counteract the efficiency they’ve formed at the task, but the additional pushups will cause changes in their body to accommodate the new work.

It is highly unlikely that they would see much if any visible changes from adding those 50 push ups to 1,000 however- because of the intensity stays the same you are more or less building endurance. There are cycles. As you gain muscle your body will have a higher basal metabolic rate. Just sitting doin nothing you will burn more calories than before. Likewise- your ability to store glycogen increases and regular activity of a certain sort will deplete your glycogen reserves. That means:

Glycogen is somewhat like your “reserve tank” of energy. Unlike fat which is also an “emergency reserve” but takes time to break down and requires caloric deficit- glycogen is easily used by the body and released when the muscles need energy and it isn’t being supplied. Some portion of the food you eat is broken down and converted to glycogen and stored for when you need that little extra push.

As you use more glycogen working out- your body adapts to store more. This more of the energy from your food goes to topping that up. You increase load, increase glycogen stores, run your glycogen lower- now you need a large amount to refill what you used plus that little extra bit you can store that you couldn’t before.

This is part of why high level pro athletes might eat 4-6 thousand calories a day! The body needs fuel to run their high performance physiques, to build new and repair muscle and other tissue, and to replenish their glycogen stores.

Once you reach a certain point- your body will no longer see any changes from doing the same workout the same way, same volume, same intensity, same workload. The body is trying to survive- so it tries to minimize use and need of energy. A huge body builder or svelt marathoner require more food just to stay alive. In nature- needing more food to live is a disadvantage. Keeping all that muscle and glycogen etc. hanging around takes lots of extra work and energy for the body. It wants to get rid of anything you don’t NEED to survive. But it also is designed to adapt to your environment.

So when you exercise your body thinks that you have some dire and regular need to run 20 miles or lift a bus- because why else would you do all that work and use all that energy if you didn’t need to? It responds to a pattern of this need by changing to make it easier to do those things.

It will do the bare minimum it has to and nothing more- so if you want more... you must force it to by asking more. If you don’t somehow increase the amount of WORK your body is doing but instead keep it the same- your body won’t change once it has optimized for that specific task.

So you can swim the same distance and see gains- but you have to increase the total work some other way than through distance. If you don’t- you will see a slight decrease in calories burned for the same work, but not drastic and to a limit. The body can only do so much, and it takes increasing effort to make smaller and smaller gains.

If you can increase from a baseline we will call “0 work units” and see a 30% increase in performance at what we will call “100 work units,” adding 100 more work units will not gain you another 30%, more likely 10% of base. If you train for years and are at 90% increase from base and 2500 work units, adding another 2500 work units over time might gain you .005% off base. So it will take you 3 years to make incredible change to bear super human, and then 3 more years to make what would be a semesters gains when you started.

Until eventually you reach your genetic limit. At that point, it you are either physically incapable of increasing the work units enough to make any measurable increases, or the amount of time such gains would takes is unmanageable. Ie: start working out a 20. By 30 be in pro shape. Say at 33 you hit your genetic limit. It would take you... 10 years to make a measurable increase using any natural training “tricks” available.

However- your physiological potential is decreasing. If your hormone levels etc. stayed the same you MIGHT be able to do it. But your hormones will fall off as you age reducing gains. Your bones and joints etc. will not be able to take the stress required. Hormonal and other changes will make it all but impossible to reach the maximum performance you could have at peak biological condition.

If you were training at maximum intensity to maintain your performance, you cannot train more than that. So as your body ages and naturally starts to lose muscle and store more fat, your same training will no longer maintain your shape. Some portion of your training will go towards offsetting the losses of age. The remaining portion for building and maintaining will be less- and thus your results will be less and you will see a decrease in results for the same intensity of training. Increasing intensity may not be an option for either health reasons or because your body is already at max capability.

Your body will not recover as quickly either- which means you can’t work out as hard and as often and get results. Working out damages the body. Rest and food- recovery, is where performance and muscle are made. If your work outs exceed your capacity to recover you do not build performance- you just damage your body and reduce your ability to perform until your body is healed.

So as you age and slower recovery sets in- your theoretical maximum productive workload decreases. This your theoretical ability to gain or maintain performance does as well.

This is why drugs like steroids are so widely used. In the aging- restoring hormone levels to those of a younger age can allow a workload higher than natural hormone levels would. There are other effects of hormones like protein synthesis, fat storage etc. in the young, higher than peak natural hormone levels allow more work than could be done naturally. Longer, more frequent work outs because of reduced recovery, increases results because of increased protein synthesis etc.

These substances have numerous complex health interactions of course- and part of the danger is that individuals can experience vastly different effects at the same dosages of the same substances- and many of those issues might not manifest for decades meaning that it’s common for people to believe these substances are “safe” because a guy they know has no problems or they are using and don’t see any negative effects.

Another danger of these substances is that connective tissues and bone take longer repair and strengthen than muscle. A person may be able to quickly go from new to exercise to lifting beyond their body weight 3x or more faster than a “natural” athlete- but their CNS, connective tissues etc. haven’t had time to adapt and optimize. They haven’t learned proper form either. At lower intensity the risk of injury from improper form is lesser, and increases with intensity. A small error in form that isn’t a real danger at 50lbs of Wright could cripple or kill a person lifting 500lbs of weight.

A person who trained 5 years to lift 150lb weights for an exercise has developed tendons that can take that abuse, bones denser to carry the weight. A person who gets there in a matter of months has not and the sheer amount of weight or stress from a level of volume or intensity could literally cause their body to fail under the load.

So I mention this academically and not as an endorsement of such drugs- and without condemnation either as there are those who use things like doctor administered hormone therapy for aging or other infirmity, and those younger athletes who have reached the limits the body can do naturally and wether justified or not see an imperative to go beyond to the maximum performance they are capable of without regards to natural limitation or risks.

The point there is just that the body has its natural limits and those limits are different for each of us and change with time. So you do see decreases in caloric usage from the same activity performed the same way- but not necessarily a meaningful amount. For most people- some minor and regular increase of overall work a concept of “progressive overload” is beneficial to training to see increasing results and long term maintenance of performance.