Did you know that when you take a step, a staggering 200 muscles work in perfect harmony to lift your foot, propel it forward, and set it down? This is just one of the countless tasks performed by the incredibly versatile muscular system. With over 650 muscles spanning our body, we can blink, smile, run, jump, and stand upright. It even powers the heart’s steady thump. So, what exactly is this marvel of nature, the muscular system?
It comprises three main muscle types: skeletal muscle, which attaches via tendons to our bones; cardiac muscle, which is only found in the heart; and smooth muscle, which lines the blood vessels and specific organs, like the intestine and uterus. All three types comprise muscle cells bundled tightly together, also known as fibres. These bundles receive signals from the nervous system that contract the fibres, generating force and motion. This produces almost all the movements we make.
Some of the only parts of the body whose motions aren’t governed by the muscular system are sperm cells, the hair-like cilia in our airways, and specific white blood cells.
Muscle contraction can be divided into three main types. The first two, shortening and lengthening muscle fibres, generate opposing forces. So, the biceps will shorten while the triceps will lengthen or relax, pulling up the arm and making it bend at the elbow. This allows us to, say, pick up a book or, if the muscle relationship is reversed, put it down. This complementary partnership exists throughout the muscular system.
The third type of contraction creates a stabilizing force. In these cases, the muscle fibres don’t change in length but instead keep the muscles rigid. This allows us to grip a mug of coffee or lean against a wall. It also maintains our posture by holding us upright. Skeletal muscles form the bulk of the muscular system, make up 30-40% of the body’s weight, and generate most of its motion.
Some muscles are familiar to us, like the pectorals and the biceps. The pectorals, for instance, are the muscles in your chest that allow you to push and pull things, while the biceps in your arms help you lift and carry. Others may be less known, like the buccinator, a muscle that attaches your cheek to your teeth and helps you smile or chew, or the body’s tiniest skeletal muscle, a one-millimetre-long tissue fragment called the stapedius deep inside the ear, which helps dampen loud noises.
Wherever they are found, skeletal muscles are under the control of the somatic nervous system, giving us an incredible level of control over their movements. This muscle group also contains two types of muscle fibres, slow-twitch and fast-twitch, to further refine our motions. Fast-twitch fibres react instantly when triggered but quickly use up their energy and tire out.
Slow-twitch fibres, on the other hand, are endurance cells. They react and use energy slowly to work for extended periods. A sprinter, through continuous practice, will accumulate more fast-twitch muscles in her legs, enabling her to pick up the pace quickly, if briefly. In contrast, back muscles contain more slow-twitch muscles to maintain your posture all day.
Autonomic control also contracts and relaxes smooth muscle in a rhythmic cycle. That pumps blood through the soft internal walls of blood vessels, enables the intestine to constrict and push food through the digestive system, and allows the uterus to contract when a person is giving birth. As muscles work, they also use energy and produce a vital byproduct, heat. Muscles provide about 85% of your warmth, and the heart and blood vessels spread evenly across the body via the blood. Without that, we couldn’t maintain the temperature necessary for our survival.
The muscular system may be largely invisible to us, but it leaves its mark on almost everything we do, whether it’s the blink of an eye or a race to the finish line.
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