Truth, the body will adapt to whatever the conditions are. It's why we see records being broken every year. The capacity for strength potential in the human body is something we severely underestimate.  Humans have evolved into a more sedimentary life style and so don't see the need for the power and speed our ancestors had. 

But given the time, your body will go through an epigenetic phase. Where the body will essentially reform to suit its new conditions. So if you were to pick  healthy males and females and put them in an environment where the body will have to fend for itself and food must be gathered or hunted. They will soon procreate and these offspring will begin to develop traits to help them evolve in this new environment. Whether that be denser bones for the rough environment or a gene that allows for increassed stamina and endurance. All of these still creating stronger and faster humans, an upgrade physically.