What determines how much muscle will you gain / maintain?

 A lot of factors come into play.

1)The natural level of testosterone a person has.

Testosterone is without question strongly related to muscular size. One of the more relevant studies is one published in the New England Journal of Medicine 1996. The study showed that men who injected 600 milligrams a week of testosterone gained significant amounts of muscle, compared to ones who didn’t. We can jump to the conclusion to say it was due to training effects but in fact training just added to the effect, but even if the men didn’t train, they still gained more muscle mass compared to the group that trained but didn’t inject testosterone !!!

Study: http://www.nejm.org/doi/full/10.1056/NEJM199607043350101#t=article. 

2) Another factor that determines response to resistance training exercise is the genetically determined ratios of your muscle fibers. 

If you are born with more muscle fibers in any particular muscle, that muscle will be more prone to gaining muscle mass and strength. Although the proportion of muscle fibers would need to be determined by muscle biopsies, , you can get a rough idea by looking at the length of your muscle bellies. The longer the muscle between its points of origin and insertion, the greater the number of muscle fibers. Take your bicep for instance, the longer it runs along the humerus, the more muscle you will potentially be able to build. 

3) Performance – doing complete repetitions;

that is moving a weight through a full range of motion. Studies show that complete repetitions are the best way to ensure gains in muscular size and strength. This pretty much means executing the exercise with proper technique. 

4) Performance – tension.

Time under tension is arguably the second most important factor when it comes to performance. Sets, reps and everything else are just a proxy for how much tension you are generating on a specific muscle. So, if there is one thing you could do to improve your hypertrophy training productivity – it’s doing slower repetitions. 

5) Performance – damage

When you lower the weight, the muscle lengthens, and more muscle fibers come into play. In addition, because of this greater involvement of muscle fibers, a greater degree of damage is imposed on the trained muscle fibers. This damage incurred by eccentric muscle contractions is repaired following exercise. The idea is that the body not only repairs the damage but compensates for future muscle stress by adding increased protein to the fibers, causing them to thicken – grow. This mechanism is being questioned today by new scientific research, that is largely showing that damage doesn’t apriori lead to hypertrophy, but I’ll still leave it here because we can’t be certain yet. v

6) Metabolic stress 

Metabolic stress refers to an accumulation of metabolic muscle waste products that act as intramuscular signalling factors, which in turn promote muscle protein synthesis reactions that lead to added muscular growth. This is the reason why increased muscle hypertrophy can be seen, even when lighter weights are used. Again, this is caused by an interference with blood circulation to the trained muscle, causing an increase in the metabolic muscle waste products that equals metabolic stress. 

Stay strong!