I'm trying to find a reasonable medical explanation for a normal-looking human having around 10x the strength of the general population, defined for the sake of easily-measurable goals as the amount of weight one could move at a gym. For example, the average untrained man can lift a barbell from waist-level to chin ("upright row") carrying about 50 pounds. I'd like my character to be able to do the same with about 500 pounds, the equivalent of being able to lift one or two adults on each arm.
I think 10x is reasonably conservative (not aiming for Superman or anything like that), as strength training can increase weight lifted by around 3x on most exercises, and by more in many exercises. Basically, the goal is someone who is strong enough for most reasonable purposes, where there aren't many practically useful feats of strength that are beyond someone living a fairly average life that aren't achievable by my character.
I have a very strong background in science and medicine, and I'm looking for fairly technical, here are some of my thoughts so far:
- Anything beyond around 10x and the physical robustness of the human body becomes a problem. I wonder if there are simple changes that can be made to improve the ability of the spine to carry weight. Perhaps swapping out the hydroxyapetite used in bone with a mineral with higher yield strength, though I'm not sure if bones fail before joints
- Bigger muscles isn't a solution, because I'm not looking for someone who looks like a body builder. Should look fit like a male model, but not bulky. Otherwise something like mutations in the MSTN gene for less effective myostatin would be promising
- Is there an animal with muscle that's more efficient than ours that we could copy?
- Heat management is an issue, as is delivery of oxygen and glucose. Powerful muscles produce a lot of metabolic heat, so there may need to be more vascularization close to the surface of the skin to more efficiently reject heat. Oxygen means potentially more surface area in the lungs, which would also help with eliminating CO2. Glucose means eating more, and potentially a speed up of glycogenolysis to liberate glucose more quickly
- This is starting to seem like a few big changes might be necessary to accommodate anything beyond short, occasional bursts of strength. Perhaps I should focus this question on short, occasional bursts, lest we end up having to re-engineer humans entirely
EDIT: Added some more thoughts to the section above