This is a very succinct and informative video from TED-ed on what makes our muscles grow. Here are a few of the key aspects discussed that we can elaborate on.
0:40 - Here the video touches briefly on what is known as neural adaptations. One often-overlooked improvement from exercise is not only the increase in size of our muscles, but he improved neural patterns from our brains "learning" how to trigger muscular contractions better. Or, to get super technical, our ability to recruit what is known as high-threshold motor units. Strength training with heavy weights (the heavier the better) improves our neural patterns allowing us to produce more force with our current muscle fibers. In other words, you can get stronger without necessarily increasing the size of the muscle itself.
1:00 - Here the video explains the difference between isolation exercises and compound functional movements. When the door is light, the character in the video can pull it open with just the muscles in his arm, this is isolation. However, once the character tries to open the heavier iron door, he must "recruit" more muscles, joints, and body parts to generate more force. This is a compound, functional movement. This is what makes our CrossFit Combine classes so effective, because we train individuals to move better through heavy compound functional movements. Think of the difference between a biceps curl (isolation) versus a deadlift (compound, functional).
1:30 - Here the video discusses the microscopic mechanism that is causing change within your body. Remember that exercise and resistance training is a stimulus to promote growth in your muscles. The video goes on to explain at how overloading the muscle constantly is what causes muscular growth (hypertrophy). In other words, you must have enough intensity in your training to cause a sufficient hormonal response, i.e. create enough damage. Not only that, but as you get stronger, you must continue to challenge yourself more and more - a training stimulus known as progressive overload. Just being "active" is not enough to promote continual growth and improvement. Intense resistance training, consistently, is what is required to improve and age gracefully.
3:02 - Protein, IGF (insulin-like growth factor), and testosterone are all needed for muscular growth. Active individuals should aim for at least 1g/protein per pound of body weight to ensure proper recovery. In other words, a 160lb individual should aim for 160g of protein every day. Also note that testosterone plays a big part in muscular growth, so the truth is, a lot of men and essentially all women don't have the required levels of testosterone for measurable amounts of muscular growth. This doesn't mean you can't get strong though! (refer to #1 on this list)
3:18 - Recovery is when your muscles re-build. Too much volume (repetitions, sets) or intensity (weight), especially too soon can cause your body to fail to properly recover. With exercise, the "this is good, so more must be better" principle does not always apply.
3:30 - Genetics plays a big part in how you look. Just as your eyes, size of your nose, how long your arms are, etc. is unique to you, so is the amount of muscle you can build, and where. Don't discount the fact that the amount of muscle you can build is individual to you. Don't compare yourself to others as everyone has strengths and weaknesses. Be the best you can be (even as cliched as that sounds).