Your body is made up of 3 different types of muscles. Cardiac muscle is only found in your heart. It is an involuntary muscle that pumps in rhythm to transport blood and nutrients throughout your body. It is involuntary because no matter how hard your concentrate, your brain cannot directly control your heart, it just runs on autopilot.
Smooth muscle is found around your organs and is also involuntary. These muscles do such things as move your food throughout your digestive tract.
Thirdly, you have skeletal muscle. This is the (mostly) voluntary muscle that allows us to speak, move our hands, walk, etc.
You genetically have a set number of muscle fibers in your body that is set during puberty. You may gain more muscle fibers in childhood, but once you reach your teenage puberty years, the number of muscle fibers you have is set. Now that does not mean that your muscles will stay that size forever, as it is possible to grow the fibers you already have.
The number of muscle cells you are given mostly determines the way you look, but exercising your muscles and staying fit can lower body fat percentage, make your muscles more defined looking, and of course grow them a bit to look lean and toned, or even big and muscular if that is your thing!
Our bodies are constantly under construction. Work crews are transported around our circulatory system all day, every day, breaking down and disposing of broken or unneeded tissues. This includes muscles and ligaments. Unfortunately, our bodies take the "use it or lose it" approach, so as we age, if our muscles are not being used, we dispose of them over time. That is why it is of vital importance that everyone includes some sort of resistance training to their exercise programs a few times a week.
You see, when you cut your skin, your body creates a scab and then repairs the damage. It is the same for when you are lifting or pushing something heavy. Using your muscles causes tiny tears and rips in the muscle fibers. Your construction crews then swoop in and repair the damage, and leave your muscles stronger for the future.
Not only does resistance training help improve your tensile strength of your muscles and ligaments, but doing dynamic movements help coordinate your body better. Human beings are creatures of habit. We get good at the things we do often, so throwing a punch at a punching bag, or doing a jump squat, etc. is created by a wave of coordinated chemical releases from your brain to your muscles that allow your muscles to act in a synchronized manner. Athletes are perfect examples of this. If you shoot a basketball into a basket a few hundred thousand times, your brain is going to become really good at remembering how to do it and even becoming better at it. This is also known as "muscle memory".
This is important as we age, because like deteriorating muscles, our nervous system and balance also become less and less efficient as we age which can lead to injuries caused by muscles imbalances or uncoordinated movements. The more and more we train our muscles to be active, the more coordinated, balanced, and healthier we stay.
Having said all of that regarding "muscle memory", it is important to constantly be changing the way we exercise our muscles. If we were to bicep curls 3 days a week for the rest of our lives, only a specific portion of that muscle would adapt and stay "good" for that time, while the rest of the muscles in your arm and shoulder, etc. essentially melt away. Your muscles need dynamic, coordinated, multi-planar movements to stay fresh, youthful and healthy.
For the women out that that want to stay away from weights because they don't want to look like "she-hulk", remember that testosterone is what causes men to have bigger looking muscles. Men on average have about 20-30% more testosterone than women do, which allows them to be about 50% stronger and build bigger muscles. Having said that, muscles are muscles, so if you took a 130lb man and a 130lb woman with relatively similar muscle size, they should have the same strength. Pound for pound, women can be just as strong as men!
One final thing, just remember to push through the burn. We used to think that that burning sensation you get in your muscles during exercise (caused by lactic acid buildup) was your body telling you to stop exercising. But in fact, researchers have discovered that it is actually your body trying to eek out that last little bit of effort by supplying your muscles with a shot of (painful, burning) fuel. The thing is, the more you cause your muscles to burn like that, the more efficient they become at dealing with that pain. You may notice that the more you exercise, the less and less burn you get in your muscles, allowing you to push yourself further and further!