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Anatomical planes and axes of movement

This is an excerpt from Kinetic Anatomy 5th Edition by Rachel M. Koldenhoven,Robert S. Behnke.

Human movement that takes place from the starting (anatomical) position is described as taking place in a plane (a flat surface) about an axis (a straight line around which an object rotates). Muscles create movements of body segments in one or more of the three planes that divide the body into different parts. These three planes are perpendicular (at right angles) to each other (figure 2.2). The sagittal plane (also known as the anteroposterior plane) passes from the front through the back of the body, creating a left side and a right side of the body. There could be any number of sagittal planes; however, there is only one cardinal sagittal plane. The term cardinal refers to the one plane that divides the body into equal segments, with exactly one half of the body on either side of the cardinal plane. Therefore, the cardinal sagittal plane divides the body into two equal halves on the left and right. The term cardinal plane appears in some texts as the principal plane. The terms are interchangeable.

Figure 2.2 The cardinal planes and axes: (a) sagittal, (b) horizontal, and (c) frontal planes.
Figure 2.2 The cardinal planes and axes: (a) sagittal, (b) horizontal, and (c) frontal planes.

The horizontal plane (also known as the transverse plane) passes through the body horizontally to create top and bottom segments of the body. There could be any number of horizontal planes, but there is only one cardinal horizontal plane, which divides the body into equal top and bottom halves.

The frontal plane (also known as the lateral plane) passes from one side of the body to the other, creating a front side and a back side of the body. Again, there could be any number of frontal planes, but there is only one cardinal frontal plane, which divides the body into equal front and back halves.

The point at the intersection of all three cardinal planes is the body’s center of gravity. When all segments of the body are combined and the body is considered one solid structure in the anatomical position, the center of gravity lies approximately in the low-back area of the spinal column. As body parts move from the anatomical position or as weight shifts through weight gain or loss or by carrying loads, the center of gravity also shifts. No matter what the body’s position or weight distribution, however, half of the weight of the body (and its load) will always be to the left and right, in the front and back, and above and below the center of gravity. The center of gravity of the body constantly changes with each movement, each change in weight distribution, or both.

Earlier, we defined an axis as a straight line around which an object rotates. In the human body, we picture joints as axes, and bones as the objects that rotate about them in a plane perpendicular to the axis. There are three main axes, and rotation is described as occurring in a plane about the axis that is perpendicular to the plane (figure 2.2). The sagittal plane rotates about a frontal horizontal axis (figure 2.2a).

Hands On

The knee joint is a frontal horizontal axis, and the lower leg is the object that moves in the sagittal plane when you bend your knee.

The horizontal plane rotates about a vertical (longitudinal) axis (figure 2.2b).

Hands On

As you turn your head to the left and right as if to silently say no, your head rotates in a horizontal plane about the vertical axis created by your spinal column.

The frontal plane rotates about the sagittal horizontal axis (figure 2.2c).

Hands On

When you raise your arm to the side, your shoulder joint is the sagittal horizontal axis, and your arm is the object moving in the frontal plane.

For a summary of the relationship between anatomical planes and associated axes, see table 2.1.

Table 2.1 Planes, Axes, and Fundamental Movements
Fundamental Movements

Again, remember that movement takes place in a plane about an axis. There are three planes and three axes, with two fundamental movements possible in each plane. In the sagittal plane, the fundamental movements are known as flexion and extension. Flexion is defined as a reduction of the angle formed by the bones of the joint (figure 2.3). In flexion of the elbow joint, the angle between the forearm and upper arm decreases. Extension is defined as an increase in the joint angle (figure 2.4). Returning a joint in flexion to the anatomical position is considered extension. Further extension beyond the anatomical position is referred to as hyperextension. At the ankle, we call these motions dorsiflexion and plantarflexion. Dorsiflexion is when the foot moves upward toward the anterior leg, and plantarflexion is when the foot moves downward. For example, when you press down on the gas pedal of a car, you plantarflex your ankle. Fundamental movements in the frontal plane are known as abduction and adduction. Abduction is defined as movement away from the midline of the body (figure 2.5). As you move your arm away from the side of your body in the frontal plane, you are abducting the shoulder joint. Movement toward the midline of the body, for example, returning your arm from an abducted shoulder position to the anatomical position, is defined as adduction (figure 2.6). At the ankle, we call these motions inversion and eversion. Medial (inward) movement of the ankle is known as inversion. Lateral (outward) movement of the ankle is known as eversion. The fundamental movement in the horizontal plane is simply defined as rotation (figure 2.7). The earlier example of shaking your head “no” is a rotation of the head. For describing movement in the upper (arm) and lower (leg) extremities, the terms external rotation and internal rotation are often used (figure 2.8). When the anterior (front) surface of the arm or leg rotates laterally (away from the midline of the body), the movement is defined as external rotation (or lateral rotation). When the anterior surface of the arm or leg rotates medially (toward the midline of the body), the movement is defined as internal rotation (or medial rotation). Two terms refer to actions of the forearm and ankle. The term pronation refers to the turning of the forearm toward the body, resulting in the volar, or palmar, surface of the hand facing the body, or, if the elbow is flexed, palm down. At the ankle, pronation involves combined motions of dorsiflexion, eversion, and external rotation. Supination, the reverse of pronation, refers to turning the forearm outward and palm upward from the pronated position. At the ankle, supination involves combined motions of plantarflexion, inversion, and internal rotation.

Figure 2.3 Flexion at the shoulder, hip, and knee.
Figure 2.3 Flexion at the shoulder, hip, and knee.
Figure 2.4 Extension at the shoulder, hip, and knee.
Figure 2.4 Extension at the shoulder, hip, and knee.
Figure 2.5 Abduction at the shoulder and hip.
Figure 2.5 Abduction at the shoulder and hip.
Figure 2.6 Adduction at the shoulder and hip as a return from the abducted position in figure 2.5.
Figure 2.6 Adduction at the shoulder and hip as a return from the abducted position in figure 2.5.
Figure 2.7 Rotation as a twisting movement of the spine.
Figure 2.7 Rotation as a twisting movement of the spine.
Figure 2.8 (a) Internal and (b) external rotation of the lower extremity.
Figure 2.8 (a) Internal and (b) external rotation of the lower extremity.

When a body segment (e.g., a forearm) moves through an arc of motion (e.g., at the elbow), the movement takes place in a sagittal plane about a horizontal (frontal) axis, and that arc of motion is referred to as that joint’s range of motion (ROM). Every diarthrodial joint in the human body that is capable of movement has a ROM. A joint’s ROM varies, depending on several factors, such as bone structure, ligamentous attachments, muscle health, trauma, and external restrictions (e.g., casts and bracing), but all unimpaired joints have a typical ROM.

Joints capable of creating movement in two (biaxial) or three (triaxial) planes are also capable of another movement, circumduction, which, because it combines two or more fundamental movements, is not considered a fundamental movement of any joint. When movement occurs in two or three planes in a sequential order, the joint is said to be circumducting. Moving your arm at the shoulder joint in a “windmill” motion is an example of circumduction (figure 2.9).

Figure 2.9 Circumduction at the shoulder.
Figure 2.9 Circumduction at the shoulder.

There are two other movements of the shoulder joint that some may consider as fundamental movements, but, by the definition of fundamental movements presented earlier in this chapter, they are not: horizontal flexion and extension, and horizontal abduction and adduction. The following “Hands On” example should demonstrate why.

Hands On

Stand in the fundamental position. Raise your arm in front of you to shoulder height (see figure 2.3). This movement is defined as shoulder flexion (movement in a sagittal plane about a frontal and horizontal axis).

From this position, move your arm to a position aligned with the side of your body (see figure 2.5). This movement places your arm in the position defined as shoulder abduction (movement in a horizontal plane about a vertical axis).

You just performed two fundamental movements of the shoulder joint (flexion and abduction), and, when you reversed the initial movements, you performed another two fundamental movements: shoulder joint adduction (movement in a horizontal plane about a vertical axis) and shoulder joint extension (movement in a sagittal plane about a frontal and horizontal axis).

By the definition presented earlier in this chapter, a fundamental movement takes place in a single plane about a single axis. The movements of horizontal flexion and extension, and horizontal abduction and adduction, do not qualify as fundamental movements of the shoulder joint.

FOCUS ON
The Subtalar Joint: A Triplanar Joint in Action

The subtalar joint is the articulation between the talus and the calcaneus. Due to the unique shapes of the articulating surfaces, the subtalar joint moves around an oblique axis that runs diagonally through the three planes. This makes the subtalar joint a triplanar joint despite it being a uniaxial joint. The oblique axis allows the motions of pronation and supination to occur. Pronation requires the combination of dorsiflexion, abduction, and eversion. Supination requires the combination of plantarflexion, adduction, and inversion.

More Excerpts From Kinetic Anatomy 5th Edition

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