The amount of work that muscles perform depends on volume (the number of times the weight is lifted) and time (the amount of time over which the exercise is performed). Intensity is a matter or increasing the rate that the exercise is performed, or increasing the weight and decreasing the repetitions. Increasing the rate is done by moving through the movements quicker (not recommended) or decreasing the amount of time resting between each set and each exercise (recommended). Why lift weights for two hours when the same amount of work can be accomplished in less than half the time by increasing the intensity of the workout? Adding more weight is also an option, but this is not recommended, as it is safer to use a controllable weight at which you can perform between six and 10 repetitions. A large amount of weight is difficult to control and may predispose the weight lifter to muscles strains or other injuries. More importantly, the weight lifter may not perform enough repetitions to adequately stress the muscle and cause it to adapt.