Ask your average musclehead, and he’ll tell you that the sheer number of plates you can lift on a bar is of critical importance. He’s not entirely wrong — how much weight and how many times you can lift it will have an effect on your physique. But equally — if not more — important is a variable called time under tension (aka TUT), or how long you force your muscle to hold a weight. In a recent study published in TheJournal of Physiology, subjects trained one leg with a very light weight (30 percent of their one-rep max) and executed the reps extremely slowly (six seconds up and six seconds down) until they reached failure. On the other leg, they trained with the same amount of weight and did the same number of reps but at a normal speed, meaning they didn’t reach failure. The “slow” leg experienced a greater increase in muscle protein synthesis, showing that even lighter weights can yield great results if you require your muscles to hold them for longer.