Pump up the legs is the first thing you should do in the gym. If you are not too lazy and read an article on how to pump up muscles , then you should already know that the first thing to do is to pump large muscle groups. And the winner in this category is legs, yes, legs are the largest muscle group in the human body. Even not compare the back with the legs. Moreover, apart from the general muscle mass, the legs are larger, all the other muscle groups, affect the strength indicators.