1 of 6

Structures of the Body

The Skeletal System

Makes up 15% of the body's weight. Ok, you don't need to know that for this course, but it's interesting.


You'll learn a lot about bones in this course because this is where muscles attach. Pound for pound, bone is strong as steel and 3x stronger than concrete.

Bony landmarks are the surfaces of bones. This includes bumps, grooves, hills, ridges, and other palpable (touchable) features of bone that will serve as helpful references when locating muscles.


A joint is the point of contact between bones, also known as an articulation. In other words, a joint is where structures join.

We group muscles together by the joint(s) they move. Some muscles will move more than one joint.

The Muscular System


This is what we're here for. This is the contractile tissue that moves the skeleton. There are over 600 muscles in the human body. Some numbers go as high as over 800. Only specialists will need to know that many. We cover 140, of which there are 2 of most. That's still a lot which is why we break them down into smaller categories.

Muscle contracts voluntarily for the most part. In other words, you make it contract. Keep in mind that muscle only shortens (contracts), it does not lengthen. Lengthening of a muscle is due to the shortening of another. Yes, it can be stretched, but this too is an entire course (we're working on it).

Muscle fibers are thinner than a human hair, but can support up to 1000x their own weight.

Muscles are named for their shape, location, fiber direction, action, or attachment sites. There's not a consistent pattern here, but the name gives you more clues to fill in the blanks.

Action: When muscles contract they create movement at a joint. This movement is called an action.

Origin/Insertion: Every muscle has an origin and insertion. This is where muscles attach. By knowing where a muscle attaches, you'll be better able to determine what muscle you've located, what joint it moves, and how it moves that joint (action).

Tip: The origin is typically the fixed point of a muscle, or the anchor, meaning there is little to no movement at the origin. The insertion is the part that moves the most. This can be easily confused so think that the origin is like a home you never move from or think that origin is the start.

Agonist: The muscle or group of muscles responsible for a joint movement (action) are called the agonist. Sometimes the agonist is referred to as the prime mover because it is the main muscle creating the action.

Synergist: Muscles that work to support the agonist, as in, they do the same action, are called synergists.

Antagonist: Muscles that do the opposite action are called antagonists.

Abbreviation for muscle: m.
Abbreviation for muscles: mm.

Note: You may encounter sources that list some information about a muscle slightly different. The human body is so complex that debate is ongoing and new data is constantly being presented. It's ok to have different sources because most information about a muscle is widely agreed upon. After this course you'll be able to understand and debate with the best of them. Our most trusted resources are from the people who spend their careers not only studying muscle, but interacting with it.


Tendons attach muscle to bone.

Abbreviation for tendon: t.
Abbreviation for tendons: tt.


Ligaments connect bone to bone.

Abbreviation for ligament: l.
Abbreviation for tendons: ll.

This course does not cover ligaments. Ligaments are an advanced subject and is recommended you learn after mastering muscles and bones.


Fascia is connective tissue below the skin and in between muscles and organs. It is a web or 3D matrix that is intertwined from head to toe. Think of it like the web located within an orange. We've only recently started to understand more about fascia. You don't need to understand fascia for this course, but if you are working with muscles, you will certainly learn about it. Very cool stuff.

Sprains vs. Strains

Two more terms that are easy to confuse. Strains occur with muscles and tendons. Sprains occur with ligaments. A memory device for this is strain = tendon or take a trip on the tendon train.

Scroll To Top