Steroids, blood doping, and probably other unnatural means like hormone treatments, prosthetics (a.k.a. bionics, a.k.a. cyborgs), mechanical enhancements (e.g. exoskeltons), gene therapy, etc. can enhance athletic performance. There is an active effort today to keep these unnatural enhancements out of sport as they are viewed as unfair. But, is it only a matter of time before a no holds barred sports league that permits these enhancements and has athletes proudly proclaiming how they have modified themselves for their sports arises?
Initially, it might be vaguely disreputable, like professional wrestling or mixed martial arts competitions. But, could it come to have mainstream appeal?
What about the military? It is cliche in science fiction for the military to want to develop corps of soldiers who are enhanced. But, aside from glorified "no doze," this doesn't appear to have happened so far. Will it? Is not, why not?
From a military perspective, the question, put more broadly, is "does braun really matters in modern warfare?" For the most part, one doesn't have to be super strong to fly an airplane, operate a tank, direct an artillery battery, crew a submarine or destroyer, or what have you. Even for infantry, the main physical requirement is the need to carry around gear and to be quick in evading enemy fire, not the need to be particularly physically strong while operating a rifle. The few areas where physical ability is important are increasingly being automated. For example, high G aircraft manuveurs are more likely to come from drone aircraft than enhanced pilot phsyiology. Modern materials science is also making the gear load of soldiers lighter.
Perhaps there are still jobs in the military where braun really does matter. But, the portion of the total force in that kind of job is much smaller than you'd expect, despite a long tradition of physical fitness in armed services personnel that is at least partially a matter of esprit de corps. The low percentage of jobs that are closed to women in modern military forces is a gauge of that trend. And, if the military had a need or desire to, it could probably design some jobs that require braun as they are designed now to be less physically demanding without seriously compromising military effectiveness.
Still, for those jobs were braun does provide an edge, special forces perhaps, is it only a matter of time before their is an "enhanced brigade" of people who a modified chemically and physically to make them more able than a normal human being, even an extraordinary one? I find it hard to believe that anyone would find this to be contrary to the laws of war, and I am sure that there is no shortage of gung ho young people who would volunteer for this kind of service, even knowing that there were serious drawbacks associated with that choice. If people will dope up to ride bicycles or play football, surely there are others who will do it to serve their country in the military.
Also, of course, the enhancements needn't be limited to "braun". What do ordinary soldiers lack or have only in small numbers, that would give them an edge militarily? Whatever that is, perhaps it could be enhanced artificially by actually changing a person. For example, perhaps someone could be wired to have faster reaction times, to have a better memory, to be immune to PTSD, or to heal faster.