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Introduction to the muscular system


The human body consists of hundreds of muscles, which come in all different shapes and sizes. Each muscle’s particular structure allows it to perform a specific function.

The muscles are attached to bones or other tissues, to help us maintain position, perform movements and even protect some organs.

Ok, now muscle tissue is made up of contractile cells, often called muscle fibers. Muscle tissue can be grouped into 3 types; skeletal, cardiac and smooth muscle.

Skeletal muscles connect to the skeleton and other structures like the eyes to help with movement and stability of the body.

These muscles are voluntary, meaning that we have active control of them to perform movements, like flexing your elbow.

Cardiac muscle is the muscle tissue that makes up the walls of the heart. These muscles contract in a rhythmic way to pump blood to the whole body and they are involuntary meaning that we can’t consciously control this type of muscle.

Lastly, is smooth muscle, which mainly lies in the walls of blood vessels and hollow organs. In blood vessels, smooth muscle helps contract the vessel walls to alter their diameter, which helps control blood flow.

In hollow organs, smooth muscles perform rhythmic contractions called peristaltic contractions, which moves the contents of these organs in one direction, like food in the stomach or small intestine.

Smooth muscle is also under involuntary control. Alright, now muscles come in a variety of shapes that help serve their specific functions.

For example, a flat muscle has parallel fibers, and often has a flat sheet-like tendon called an aponeurosis - as is the case for the external oblique muscle covering the abdomen.

Next is a quadrate muscle, which describes a square muscle with four equal sides. An example of a quadrate muscle is the famous six pack, anatomically called the rectus abdominis, which is a long paired muscle that is divided into square-like portions by bands of connective tissue.

Pennate muscles, on the other hand, have their fibers attaching obliquely to a tendon. These muscles can be grouped into unipennate, bipennate or multipennate muscles depending on the relationship between the muscle fascicles and the tendon.

Unipennate muscle fibers go in one direction, and merge on one side of its tendon, like the extensor digitorum longus muscle in the leg.

Bipennate muscles look more like a feather, having oblique fibers on both sides of the tendon, like the rectus femoris of the anterior thigh.

And multipennate muscles have fascicles in different directions, attaching to a branched central tendon, like the deltoid muscle, covering the shoulder.

Next are fusiform muscles, which have a thick muscle belly that becomes tapered at both ends. An example of a fusiform muscle would be the biceps brachii.

Speaking of bi-ceps brachii, multiheaded or multibellied muscles have more than one head of attachment or more than one contractile belly.

Both the biceps brachii and triceps brachii muscles, have two and three fusiform heads, respectively, and thus could also be referred to as multiheaded.

Examples of multibellied muscles include the gastrocnemius muscle in the leg, or the digastric muscle under the jaw which both have two bellies.

Next up are the convergent muscles, which are large muscles that arise from multiple points, but their fibers converge to insert into a single point.

A good example is the pectoralis major muscle of the anterior chest wall. This muscle arises from the sternum, ribs, and clavicle, but inserts into a single spot on the humerus.

Lastly, are circular or sphincteral muscles, which are indeed shaped like a circle. Typically, these muscles surround a body opening, and their circular shape causes constriction of the opening during contraction.

For example, the orbicularis oris surrounds the mouth and when contracted, it helps constrict the oral opening, seen when puckering your lips when whistling.

Okay, now let’s take a deep breath and have a quick quiz! Can you identify the shapes of these muscles? Alright, now muscles attach to different body parts, including bones, cartilage, skin or even other muscles.

For example, many facial muscles attach to the skin of the face, which allows facial muscles to move the skin of the face to produce facial expressions like smiling.

Now, every muscle arises from a point, called the origin, and inserts into a point, called the insertion. Typically, the origin is proximal meaning that it is closer to the trunk of the body.


Humans' muscular system consists of hundreds of muscles that carry out many different functions. It is made up of skeletal muscles, which are voluntary muscles that we can control, and smooth muscles, which are involuntary muscles that we cannot control. Skeletal muscles are attached to bones by tendons, and when they contract, they pull on the bones and move the body. Smooth muscles line the walls of blood vessels and organs such as the stomach and intestines, and they contract to move substances through these vessels or organs.

  1. "Clinically Oriented Anatomy" Lippincott Williams & Wilkins (2013)
  2. "Atlas of Human Anatomy" Saunders/Elsevier (2014)
  3. "Anatomy, Bone Markings"  (2020 Jan)