Why in the name of God would a boxer want body mass? that is the real reason fighters should not lift weight. If you are 5'8" and weigh, when in top shape (fighting trim?), 145 you are a welter weight. Isometrics, pushups, the good old heavy bag are for strength; you lift to get stronger and not only to you compromise your flexibility but you add 12 pounds of muscle mass and suddenly you are a middleweight. a short middleweight. Which of course means you need more strength and next you know you are a light heavy.