Most lifters vary their workouts after a month because they know muscles adapt to stress. But two recent studies at Arizona State University show that when and how you vary your workouts is what determines whether you improve strength or endurance. In the first study, men who changed to fewer repetitions using more weight at every workout gained 50 percent more strength than those who made the same changes to their workout routine every 4 weeks. In the second study, men who increased repetitions while decreasing weight once a month, improved endurance. Improving endurance requires a gradual increase in volume, while strength responds to frequent changes.