MUSCLE

How much a muscle is used is partially responsible for its size and lifting capability.  A muscle used frequently will maintain itself.  A muscle that is used frequently and also pushed to the limits of its capacity will grow and gain strength.   When you workout, remember to push yourself.  You can do this two ways.  First, by adding more weight.  The second way is to continue to use the same weight, but slow the exercise down to half speed.  This way you are less injury prone and will get the benefit of lifting a heavier weight.