Without muscle, we would be pretty useless. The muscles of the body enable us to live life as we know it. They let us lift, carry, push, pull, walk, run, scratch, and hug. They can get stronger, weaker, bigger, smaller, tear, heal, tighten, and stretch. These movements and features are complex, and muscles are incredibly advanced to make it all happen.

Extra Science Alert: This one is a bit of a sciency post. I’ve done my best to make it as easy to read as I can. Even if you hate science, try to read through anyway. Understanding how the muscles work, just like learning how fat works, is super important. We have to know what something is in order to know how to use it to our advantage. Understand how to grow and maintain those beautiful muscles of yours is key to making your body work for you.

Different Types of Muscles

Not every muscle in the body is the same, and you have muscle in places you might not think of. There are 3 main types of muscles throughout the body: Smooth, Cardiac, and Skeletal. Skeletal muscle is what we usually think of first, the muscles that connect to your bones and drive your movement. These are primary muscles used during exercise, and are attached through connective tissue called tendons.

Smooth muscle is a little deeper, and it actually makes up the lining of many of the internal organs. These help our organs and internal systems function, and generally cannot be contracted voluntarily. Cardiac muscle is last. As it’s name implies, it’s what the heart is made of, and also cannot be directly controlled. Even though we can’t intentionally contract it, but like skeletal muscle, the muscles of the heart can be strengthened through exercise and training.

How Do Muscles Work?

Here’s the relatively simple version: Skeletal muscle is made up of chains of proteins called myofibrils. These overlap, and work to move the muscle. They pull together to contract and shorten the length of the chains, and they can relax to expand and lengthen. All of this needs energy to happen, and that energy comes from the breakdown of ATP molecules.

Those myofibril chains then come together to create full muscle fibers, or myocytes. These fibers also contain glycogen, a carbohydrate which helps provide more energy for exercise, and myoglobin which stores oxygen. Muscle fibers are surrounded and bundled together repeatedly by fascia, forming the complete muscle as we know it.

All of these muscle fibers work and move together across the entire muscle. Muscle action does not use every muscle fiber, since each fiber can be switched on or off independently. Depending on how much strength is needed in a particular situation, more or fewer groups of muscle fibers contract.

Muscles can contract in a few different ways. There’s the standard Concentric Contraction which leads to movement working against the direction of gravity. Next up is Eccentric Contraction which leads to movement working in the same direction of gravity as a sort of braking mechanism. Finally, there is Isometric Contraction, which engages the muscle but keeps it holding in one place without any movement.

Muscle Fiber Types

Even though they all do the same two movements, contract and relax, muscle fibers are not all equal. They come in two main forms, Type I and Type II, also known as slow twitch and fast twitch. Each muscle usually is made up of a combination of both fiber types. The proportions are different depending on the muscle group, and vary from person to person. The composition of Type I and Type II fibers can also change depending on your training and exercise.

Type I muscle fibers are called Slow Twitch because their myofibrils contract more slowly. They are aerobic, which means that they use oxygen for when they transfer energy. This helps them work at lower intensities and for a longer time. Because of this, they are great for bigger things like keeping up posture and stability.

Type II muscle fibers are known as Fast Twitch because of their short bursts of strength and speed. Because they can act quickly, they can’t use this power for long periods of time, and end up fatiguing sooner. They also access ATP faster, utilizing quick bursts of energy. Because of this, Type II fibers are not always activated, unless the muscle is putting out a large level of force. As a result, Type II fibers are responsible for a big chunk of the body’s movement, and play a big part in muscle size and definition.

How Do Muscles Change Size?

Because of all of this, muscles have the awesome ability to adapt and change in size over time. They can grow or shrink depending on the level of muscle activity. Exercise can spark more myofibril protein chains to be created, which increases the size of the muscle fibers. Because muscle fibers are mainly made of protein molecules, they need more protein to grow. If you don’t eat enough protein, your muscles won’t grow, and can even shrink through atrophy. While this can also happen as a result of injury or disease, the most common cause is inactivity. If muscles aren’t put through enough consistent activity and stress through exercise, they can waste away. This is generally not permanent, and rebuilding usually takes less time than it did to build up the first time.

Exercise is a complicated process using the entire body, with muscle as a key player. Knowing about the composition and function of the body’s muscles is key to understanding how to use them to your advantage. Now that all the science is out of the way, you can moving forward to start using those muscles to increase strength and overall fitness.