During a workout your muscles undergo minor tearing as we are purposely overloading them to create a change. Once you finish your training session, while your body is resting, the muscle then starts to rebuild and repair itself, growing bigger and stronger. During this rebuilding process the muscle utilizes your fat stores as an energy source. As well as fat the muscles also require other nutrients such as protein and certain vitamins and minerals that you must try and receive through a well balanced diet. Which is all part of the lasting benefits of including resisitance training in your exercise regime. Lastly it is vital to give your muscles time they need to grow and develop, therefore recovery is a key issue that you must always address when planning your training schedule.