How Your Muscles Work

Understand what our muscles are made up of and how we move.


By Maddy Bond


What did one traffic light say to the other?

Don’t look, I’m changing.

Cheesy joke, yes, but we bet it made you smile. Have you ever thought about what makes the sides of your mouth lift up like that? It takes a minimum of 10 muscles to form a smile, so imagine how many you use to go for a run or to just lift an apple off the table. Having a better understanding of how your muscles work will help you learn how to move them properly and safely, which in turn will improve your muscle development and training program.

Types of muscles

There are three primary types of muscles. They include cardiac muscle, smooth muscle, and skeletal muscle.

We’ll just focus on the latter today, as these are the ones you have control over; essentially the muscles that help you move. These are connected to your skeleton via tendons. They lengthen and shorten to help move your skeleton around. There are also three subtypes of skeletal muscle named Type I, Type IIa, and Type IIx, which are each tailored to a different form of exercise.

Type I muscles are more endurance focused. You can think of these as the marathon runners. Type IIx muscles are for power, so think more of the 100 meter sprinters. Thirdly, Type IIa muscles are a mix of the previous two; they can work for a while, but also have bursts of power behind them, sort of like a soccer player. Our bodies contain all three subtypes of skeletal muscles, and can never be completely one type. For example, your postural muscles, which hold up your torso while sitting or standing, are primarily Type I, while your legs may be more power trained with box jumps or your arms may be more endurance trained through momentum while running.

There are a couple of factors that play a part in determining how your muscles are made up and what sort of muscles you have. Of course, genetics is a large factor. While we are born with a somewhat predetermined setup for our muscle type, that doesn’t necessarily mean we’re only going to be good at one type of exercise — it just might be a little easier to train and see improvements in that area. Your workout and training schedule is the other factor that will decide your body’s muscle type. Training your body as a power athlete means that over time, your muscles will display more Type IIa or Type IIx traits. On the other hand, by focusing on endurance, your muscles are more likely to display Type I traits.

How muscles move your bones

The Sliding Filament Theory or the Cross-Bridge Cycle is the process of how muscles move on a microscopic scale. Your muscles are made up of microscopic filaments that detach from each other, shorten or lengthen, then reattach before repeating the process. Our muscles utilize energy, or ATP, to help the filaments release and change size.

There are four types of muscular tissue that work together to support movement. These are the agonists, the synergists, and the antagonists.

The agonistic muscles are the primary movers in a motion and do the most work, synergist muscles aid the agonists in either assisting the movement or stabilizing it, and antagonists perform the opposite movements of the agonist muscles. Let’s think about this in a movement you know well — the bicep curl.

The primary mover in a bicep curl is the biceps brachii, the outermost muscle on your upper arm. The synergist muscle during this movement is the brachialis muscle, found deep in your upper arm. The antagonist here is the tricep muscle group, found on the back of your arm. Each type of muscle is required to move your arm properly. The biceps brachii will shorten to bring the hand up towards the face. As it’s doing this, the brachialis muscle will shorten to assist the primary mover, while the tricep muscle is stimulated to relax and lengthen so you can move your arm further.

Think about the muscles you’re working during your next gym session. See if you can figure out which one is which when you’re lifting weights!

Maddy has worked in the health and fitness industry for 5 years. She has a bachelors in Exercise Science and has recently received her Masters in Exercise Physiology. She has worked with a wide demographic of clients as a Personal Trainer and loves helping people reach their goals and continue to grow.  She is an outdoor enthusiast and dedicates her workouts to rock climbing, hiking and whatever new experiences may come her way.

Main Photo Credit: Slava Zyrianov/; Second Photo Credit: PERO studio/; Third Photo Credit: cirkoglu/

Aug 17, 2021


May 10, 2024

Simple explanation. Now add mitochondria discussion