You may be surprised to learn that it is the resting time in between workouts that really matters the most when it comes to building muscle. And no, before you ask, this does not mean you should spend all your time resting. When you lift weights, or perform any resistance training you are actually causing almost imperceptible micro tears in your muscles. It is the new tissue lain in the rift between the muscle fibers that causes your muscles to get bigger and more defined. These micro tears can only occur when you challenge your current level of strength a little bit, by using your own body weight or the weight of a dumbbell or other piece of exercise equipment. And the healing can only occur when you give your muscles the chance to rest and recover. This is why it is recommended that you strength train two to three days a week, so that you can use the 48-72 hours between workouts to give your muscles the chance to recover and get stronger.
 
This rest and recovery time is not only for your muscles, you need to give your brain and central nervous system a little break. During a strength training workout your brain is continually sending signals to your muscles to control movement, balance and muscle contraction and needs time to recuperate before you begin again.