How does the amount of weight I lift affect my muscles?

In general, lifting a heavier weight for 2-8 repetitions per set primarily exercises your type II muscle fibers, and will produce large muscles capable of lifting heavier loads for short periods of time. If you lift a lighter weight for 8-15 repetitions, you are exercising primarily your type I muscle fibers. This type of exercise will result in smaller, leaner muscles that are capable of lifting light loads for longer periods of time. If you lift a very light weight for more than 20 repetitions, you are really doing aerobic exercise rather than strength training. If you are trying to build strength, the weight needs to be heavier than you can comfortably use for 20 repetitions.

The answer is quite simple. The heavier the weight and the more your body will have to work to move that weight. However, be sure when lifting weights to lift the appropriate amount of weight to avoid injury. Also, be sure if lifting heavier than normal to have a spotter.

When you lift heavier, your type II muscle fibers kick into gear. These fibers are fast-twitch and will help produce a more explosive motion. When you lift heavy, you will not be able to produce as many repetitions as if you were to lift lighter. When you lift light your type I muscle fibers kick into gear. These fibers help with building endurance in the muscle.