Anatomical Terms of Motion

Muscles in the body can only apply force by contracting. Most muscles in the body have muscle group pairs that allow muscle control in both direction, where one muscle is contracting and the other is relaxed.

Adduction vs. Abduction

There are many exercises that have the name “adduction” or “abduction” in them. Without understanding the terminology, it’s a lot of work to memorize the difference between exercises.

These terms are defined as limb motions relative to the midline of the body:

• abduction is “moving away” from midline
• adduction is “moving towards” the midline

An easy mnemonic way, albeit morbid, to remember the difference is that abduction (kidnappings) are when someone is “taken away”.

Examples of Adduction and Abduction

Shoulders

The shoulder abductors are later deltoids, they raise the arm away from the body. Example movement is the lat raise.

The shoulder adductors are pecs, lats, and triceps. Example movement is the decline cable fly.

Hands

Abduction of fingers is when the fist is opened up.

Adduction of the fingers is when closing or gripping.

Legs

Leg abductors are glutes. Example movement is pushing knees out when squatting.

Leg adductors are are group of muscles named adductors of the hip. This movement is not often trained directly.

Flexion vs. Extension

These terms describe motion that changes the angle between two body parts. Flexion is the movement that bends and decreases the angle. Extension is the movement that straightens and increases the angle.

Examples of Flexion and Extension

Elbow

Elbow flexors are the biceps. Example movement is a curl.

Elbow extensors are the triceps. Example movement is a tricep pushdown.

Shoulder

This one is a little weird, not sure what angle or body part this is supposed to be relative towards. It’s understood as “flexion moves the arm forward, extension moves it back”.

Shoulder flexors are the anterior deltoid. Example movement is a front raise.

Shoulder extensor are rear deltoids, lats, and back muscles. Example movement is pullover.

Trunk

Trunk flexor are the core. Example movement is a crunch.

Trunk extensor are the lower back. Example movement is back extension.

Knees

Knee flexor are the hamstrings. Example movement is hamstring curl.

Knee extensor are the quadriceps. Example movement is squat, near the bottom.

Hip

Same as shoulders, this should be understood as “flexion moves the leg forward, extension moves it back”.

Hip flexor is aptly named the hip flexor muscles. This movement is not often practiced directly.

Hip extension is glutes and hamstrings. Example movement is deadlift.

Elevation vs. Depression

This refers to movements above and below horizontal. For fitness, the predominant muscle movement is scapula.

Scapula elevator is the traps. Example movement is a shrug.

Scapula depressor are lats. Example movement is getting tight for deadlift. You don’t retract the scapula, you depress it.

Internal vs. External Rotation

These movements refer to rotation away or towards the center.

Examples

Shoulder Rotation

The muscles involved in both directions are hodgepodge of muscles delts, pecs, and back. Because the shoulder joint has so much mobility and freedom of movement, there’s no single path. This is why this movement is not very strong or stable.

In general, we prefer external rotation to internal rotation, to maintain stability of the join when performing other exercises. Think of bringing the elbow further in front than hands. Internal rotation is what we do most often in life because it’s less restrictive.

The exercises tend to be variants or related to shoulders movements but attempting to minimize the contribution from the larger delt muscles.

Hip Rotation

The muscles involved are glutes. Like the shoulder, we prefer external rotation for stability in movements.

The movement is twisting knees inwards (internal) or outwards (external). A common cue during squatting is to “screw your feet into the ground”, which is external hip rotation.