It's an accepted fact that resistance training builds muscle. It doesn't happen while you're working out, although you'll be able to see what appears to be an increase in size during a workout. That is due to the "pump" you get from the increased blood flow to your muscles while you're exercising. The actual growth a muscle experiences as a result of resistance training happens in the hours and days after you've worked out.
Hypertrophy is what anyone with the goal of increased muscle size is aiming for, whether they're familiar with the term or not. It is when the overall muscle size increases because of an increase in the muscle fiber size. Hypertrophy occurs after a strenuous workout, when your muscle fibers have been broken down from the exercise. During an aggressive workout, your muscles actually sustain tiny tears. Your body naturally responds to this damage with an over-reaction that repairs the tears and rebuilds the fibers larger and thicker, kind of like when scar tissue results from a cut.
Your body's response to outside stimuli, positive or negative, is to release hormones. The hormones your body releases in response to the stress of a challenging workout is to release, among other substances, insulin-like growth factor, testosterone and growth hormone. The testosterone acts as an indicator that tells your muscles to rebuild. The growth factor and growth hormones increase the flood of nutrients to your muscles so that they can do so.
Importance of Protein
Anyone giving advice on muscle building always recommends assuring adequate protein intake. That is because protein is vital for cell repair and growth as well as hormone production. What is rarely expounded on is the maximum your body will be able to use efficiently at any one time. Depending on how active you are, only 20 to 30 grams are useful, leaving any excess protein in your system to be stowed away in the form of fat. Because of this, you should spread the protein you eat or drink throughout your day. Make sure to get a couple of servings in before your workout, so that it is available for energy and to assist in the muscle growth after exercising.
Timing is Everything
Your body will naturally rebuild and repair itself at an estimated rate of every two weeks to a month. When you work out, you accelerate that natural progress causing your body to (re)build muscle in as little as 24 hours, although it can take up to 72 hours for a muscle to fully recover depending on the level of stress and break-down you've subjected it to. It's these known factors that lead most personal trainers to recommend a 3 day split workout that allows 3 to 4 days before you work a specific muscle group again. This gives your muscles a chance to recover and make the most of your body's natural rebuilding process.