There’s a reason why you’re spending countless hours in the gym but still looking scrawny and not brawny: muscle mass isn’t built solely in the weight room. In fact, what you’re doing at the kitchen table–and even in the bedroom—may be more important for gaining muscle mass than what you’re lifting. (Of course what you’re lifting, and how often you’re lifting it, matters a lot too.) For any athlete looking to add size fast, the following four factors are critical.