Keep challenging muscles by slowly increasing weight or resistance. The right weight for you differs depending on the exercise. Choose a weight that tires the targeted muscle or muscles by the last two repetitions while still allowing you to maintain good form. If you can't do the last two reps, choose a lighter weight. When it feels too easy to complete add weight (roughly 1 to 2 pounds for arms, 2 to 5 pounds for legs), or add another set of repetitions to your workout (up to three sets). If you add weight, remember that you should be able to do all the repetitions with good form and the targeted muscles should feel tired by the last two. [health.harvard.edu]
