By Elizabeth A. Murray, Mount St. Joseph University
There are mainly six main types of joints by structure, mostly categorized by the movement they allow. Our joints and their movements are examples of leverage systems. Let’s learn about the terminology for the forward-and-back and side-to-side movements that our bone joints allow.
Flexion and Extension
Let’s start with flexion and extension, which are easy to understand in limbs. Flexion is bending, such as at the elbow or knee, and extension returns us to anatomical position.
With regard to the joints within limbs, flexion always decreases the angle between the articulating bones while extension increases the angle between the articulating bones, bringing us back to anatomical position.
It must be noted that our knees and elbows are on backward, in that during flexion at the elbow, the direction of movement is anterior, while it’s posterior at the knee. This is due to the rotation that occurs in the lower limb during the development of vertebrates. However, this angular movement is similar at the ball-and-socket joints of the shoulder and hip, where forward is flexion in both.
Hence, forward equals flexion at ball-and-socket joints. And extension is a posterior movement, restoring us to standard anatomical position, regardless of upper or lower limb. But once you’re within the limb, flexion always decreases the angle of the two bones at the joint.
Not Standard
Flexion of the spine is a forward bend—whether only at the neck or when touching your toes. But we can also do a side bend or move the ear toward the shoulder, which is specifically called lateral flexion of the spine. Extension is returning to a straight spine, as in anatomical position.
There’s no absolute standard there; for instance, it’s pretty common to call looking at the ceiling or up at the stars hyperextension of the neck, while we use extension of the wrist for the complete range of posterior motion possible there, beyond anatomical position. Some sources even use the term extension for any movement in the opposite direction of flexion and reserve hyperextension for pathological states, following an injury.
Abduction and Adduction
Another angular movement pair are abduction and adduction—which are possible at ball-and-socket joints as well as at condyloid joints like the wrist and the knuckle joints between the fingers and palms.
Abduction takes the body part away from the midline, and adduction brings it back toward the midline of the body. Those words are helpful—since if someone is abducted, they are taken away—while adduction “adds” the body part back to the midline.
Some body parts can have their own midline for abduction and adduction; for example, with regard to the hand, the middle finger is the axis for abduction and adduction with reference to movements of the fingers.
And the special term horizontal abduction is used for taking an outstretched upper limb from pointing straight ahead, out to the side. Moreover, abduction and adduction are not possible at the hinge joints of the elbow or knee, since those are monoaxial joints and move only in an anterior and posterior direction, relative to anatomical position.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Rotational Movement
In the arc-like movement called circumduction, which only happens at ball-and-socket joints, we are really just combining a series of angular motions: we start that circuit with flexion, and then we gradually abduct away from the body, then we move into extension, until we finally adduct back toward the body, returning us to anatomical position.
That’s different from rotational movements, such as happen at pivot joints. So, let’s look at true rotational movements.
There is a pivot joint between our first and second vertebrae of the spine that looks and functions like a trailer hitch. The second cervical vertebra has a projection that sticks up, and the first cervical vertebra, with the skull on top of it, rotates around that projection, giving us a much greater range of side-to-side motion of the head than at other places in the spine.
Rotational movement also happens at the elbow, specifically at the part of the elbow joint between the radius and the humerus.
Other rotational movements happen at ball-and-socket joints. In the lower limbs, inward, internal, or medial rotation is performed when pointing the toes toward the midline, while lateral, outward, or external rotation is the act of pointing the toes out toward the sides.
The knee really can’t do that kind of rotation; that action is happening at the ball-and-socket joint of the hip. Same goes for the shoulder where moving the forearm and hand towards the body is inward or medial rotation, while swinging the forearm and hand out to the side is lateral or outward rotation.
Common Questions about the Terminology of Movement
Flexion is bending, such as at the elbow or knee, and extension returns us to anatomical position. With regard to the joints within limbs, flexion always decreases the angle between the articulating bones while extension increases the angle between the articulating bones, bringing us back to anatomical position.
Abduction and adduction are angular movement pairs which are possible at ball-and-socket joints as well as at condyloid joints like the wrist and the knuckle joints between the fingers and palms. Abduction takes the body part away from the midline, and adduction brings it back toward the midline of the body.
There is a pivot joint between our first and second vertebrae of the spine that looks and functions like a trailer hitch. The second cervical vertebra has a projection that sticks up, and the first cervical vertebra, with the skull on top of it, rotates around that projection, giving us a much greater range of side-to-side motion of the head than at other places in the spine.
