Some study has went into comparing the physical strength of modern human beings versus the great apes and (to some extent) the great cats (the latter being some of the strongest animals, for weight, on the planet). Chimps of around seventy pounds are almost twice as strong as an average male, and great cats are about seven times as strong as humans for weight.
I didnt mean that the Cimmerians are embued with enchanted blood or anything......only that HYBORIA is a fantasy realm, and therefore has slightly different rules, allowing power and strength far beyond what we encounter in our modern world. That said, i go to a gym where dudes bench 500-600 pounds and can squat small automobiles! So I'm not confused by Conan's strength levels at all. Sorry if it came off that way. I have been defending ReH for years from naysayers so it just comes naturally!
There are many reasons for this difference. Leveraged limbs, for one, make a great cat incredibly strong. There are also differences in cellular design and muscular chemistry. You might ask 'why don't humans have cat strength?', the answer is that cat muscles are very expensive to maintain/repair from a chemical standpoint and humans and even apes do most of their hunting through group coordination rather than through explosive power (this is why wolves are not as big or strong as the great cats, for example). In addition, humans have a lot of uncontrolled and almost uncontrollable muscular breaking for fine motor behavior. When a chimp throws a rock he is doing it with the full force of his limb, when a human throws a rock he is performing a motion which may be more precise but is also slowed down by various stabilizing compensations and tensions in the body.
If Conan had the ability to somehow 'turn off' his muscular breaking he would probably be as strong as a 300lb chimp, which would make him Captain America-like. On the other hand this is hard to reconcile with his superb fine motor control. One might instead guess that the case of Cimmerians is more like that of great cats: their muscles have chemical and structural features that differ from those of modern humans, granting them a strength unparalleled.
As an athropological point, it is actually pretty unclear that ancient humans were that strong. Stronger for weight than modern humans, sure, but also rather smaller. An early man might have been more powerful and less inhibited in the use of strength than a modern human, but it's highly unlikely that he could match the strength feats of modern professional athletes such as power lifters.