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Most role playing games have an upper limit to how strong a character can become usually by means of a level limit or item power limit, players simply reach a point where no matter what they do they simply can't get stronger.

The same seems to be for most humans, as the majority tend to reach either a mental or physiological limit of how strong they can become.

Some athletes have such a powerful mental strength that allows them to re-write the limits of what is considered possible for humans by simply training more, and better.

For example, Iron crosses were considered the coolest and hardest thing a human could ever do, until upside down iron crosses were invented, which for years were considered impossible, until weighted upside down iron crosses became a thing. But eventually this will reach a limit.

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My question is, what needs to change in the biology of humans in order to remove strength limits? to make it so that the more you train the stronger you get.

Obviously said humans are already immortal, but what other thing needs to change in order to delete the limit?

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    $\begingroup$ Repeal of the second law of thermodynamics. Otherwise, there will always be a limit $\endgroup$ – nzaman Nov 4 '18 at 19:41
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Oh, wow. This is fortunate. I've actually been studying this for use in a roleplaying game.

Peak human strength is based on the strength of three components : the bone, the sinew, and the muscle fiber itself (which is very rarely the limiting factor).

Sinew Limit

The circumference $ ~ 2 \times pi \times radius $ of the smallest load-bearing bone, multiplied by the length of the bone connecting to a muscle by means of sinew gives you the highest force concentration on the person's frame. You can estimate sinew bone length a faction of the person's height (generally 1/4), and the it seems like 5% to 10% of that bone length is about right (but haven't checked it against an anatomy text). Sinew has a maximum tensile strength of 150 MPa.

$ MassLimit_{sinew} = TensileStrength_{sinew} \times (2 \times \pi \times Radius_{smallest load bearing bone} \times (Height \div 4 \times 0.1) \div Acceleration_{gravity}$

So, there are places you can tweak: the bones can be thicker (this gives the most performance increase), but not any thicker than 100% of the whole body thickness. More of the bone can be given over to sinew (but there is a cost in loss of range of motion), or you could invent some sort of better sinew. You can increase height, width, or depth (which increases limb length), but this is limited by the bone limit, below.

Bone Limit

The whole structure, plus any weight that is being carried, is ultimately held up by the bones. Bone's compressive strength (170 MPa) is ultimately born by the smallest load-bearing cross section in the body (bone area ($~ pi \times radius \times radius$), multiplied by number of legs). This area holds the entire weight of whatever is being carried, plus the body's own weight. It establishes a maximum limit to things such as increasing height and bone thickness.

The closer to the limit you try to load the bone, the more fragile the whole structure will be to shocks like falling and jumping. You could increase the strength of the bone to increase the bone limit.

Muscle Limit

Exercise raises the amount and strength of muscle to be applied. From what I have read, multiple-repetitions exercises to exhaustion increase the amount of muscle, and high-intensity / few-repetition exercises to failure increase the density of the muscle fibers in a unit of muscle. It seems that both are recommended in cycles of growing muscle mass, then building muscle density for peak strength. This will continue until you hit one of the sinew or bone limits.

If you do the math, some of the world record lifters are at these limits, so human strength is already close to the best it can be without some sort of structural change. You see examples (Michael Phelps comes to mind) where either genetics or training has changed the shape of the frame to allow more muscle and sinew to connect to it.

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humans already can get stronger, but diminishing returns means you have spend more and more time to get stronger. Our muscles get bigger and bigger, but this starts to impact flexibility, many of the largest body builders can't reach their own feet. The limit on our strength is not due to muscle, we don't reach the limit of muscle, we reach the limit of what our bones, cartilage, and tendons can support. There are just limits based on the shape of our skeleton. As long as human have a finite form we will have limits. The most common injuries for weight lifters are detached muscles and joint herniation, (tendon failure).

I would say you could let those things get stronger too, tendons and cartilage can't change as fast as muscle. But even if they could you quickly run into the issue of where to put it all. there is only so big the attachment for a tendon or joint can be and still function.

So either you need a way to handwave the strength of material so attachments and such can get stronger without changing, or you humans have to get bigger in extra ways as they get stronger, that is working out also makes you taller so you skeleton can get bigger. Or course this has its own problems, you can only scale up an animal so far before you have to start changing its proportions and organs, so even with this there is a limit.

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    $\begingroup$ This is the simple reality: all materials have their limits. Muscles, ligaments, and tendons tear. Bones break. One limit we haven't discovered yet is that cardiovascular can only deliver so much oxygen so quickly no matter how well you train. There are always limits: and from the perspective of a good writer, those limits are important. They make characters relatable... understandable.... Because gods (from a literary standpoint) are basically boring. We like reading about the old Greek/Roman gods because they were fallible. The moment you remove the limits, your characters are boring. $\endgroup$ – JBH Nov 4 '18 at 14:02
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    $\begingroup$ Well Popeye the Sailor was literally a god strength wise but the cartoon was fun, Same for One Punch Man... $\endgroup$ – user56803 Nov 4 '18 at 14:07
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    $\begingroup$ Superhuman characters can be just as relatable as long as you give them limits on other ways. The classic heroic character is often superhuman in some aspects and completely normal in others. $\endgroup$ – John Nov 4 '18 at 14:11
  • $\begingroup$ Re "many of the largest body builders can't reach their own feet", this is true of many other people as well. It's not (at least in my experience) due to muscle bulk, but lack of flexibliity training. $\endgroup$ – jamesqf Nov 4 '18 at 17:49
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    $\begingroup$ Yes the back may be a bad example but your talking about moderate body building which can actually increase flexibility if done correctly. Extreme bodybuilding results in some strange things. lower flexibility in the shoulder is the most extreme of the changes, many of the muscles interfere with the range of movement for the arm. to the point the most bulky bodybuilders have a hard time wiping themselves. $\endgroup$ – John Nov 4 '18 at 20:16