This is something that you hear a lot. Low reps with heavy weights to gain muscle mass, and low reps with light weights to get definition. Yes, low reps with heavy weights is the way to go for building pure muscle mass. Many reps with light weights to get definition however is something to think about. There are two pros when doing low reps with heavy weights.
Most muscles are made of fast contracting fibers. And by doing low reps with heavy weights you will target those fibers. Also, by doing a low amount of reps you will not build up as much lactic acid in your muscles as if you were doing a high amount of reps. Even if getting your body used to a higher amount of lactic acid can be a good thing for endurance workouts, this is counter productive for gaining muscle. If you want to gain lean body mass, this is not something you want to do.
Exceptions?
So, are there any exception?
There are a few muscles in your body that are made up of a bit more long slow muscle fibers. This includes abs, forearms, calves, and upper traps. If any of your exercises should be higher reps, this is the muscles you want to target. Other than that, try not to build up to much lactic acid, and do not hunt too much for the “pump” that the acid gives you.
Alternate?
This is also something that is included in many workout routines. After a couple of weeks they change to higher amount of reps, or completely change to different exercises. While this can be good for someone that has been working out frequently for quite some time I strongly recommend getting a correct lean body workout routine and using that one until you actually hit a plateau. If the routine works, stick to it. If you want to put on muscle mass don’t change to higher amount of reps. Keep doing heavy weights and low amount of reps to make sure your not building up lactic acid.
Alternating exercises can be useful to avoid adaptation and target a slightly different part of the muscle. But again, if the exercise works and you are getting results then stick to that exercise. Adaptation depends a bit on your age and genes, but if you haven’t been working out for many years chances are that it will take up to a year of frequent workout before you don’t get the results you could have because of adaptation.
In short, The grass is Not always greener on the other side of the fence. If it works, why alternate?