Anatomical terminology uses a standardized language of imaginary reference points, known as planes and axes, to describe and analyze human movement (kinematics). This fixed, universal framework allows scientists and professionals to communicate precisely about movement patterns. It identifies the specific two-dimensional slice through which a given motion, such as flexion or extension, predominantly occurs.
The Three Anatomical Planes
Human movement relies on three primary anatomical planes, which are imaginary flat surfaces that divide the body. The Sagittal plane divides the body vertically into left and right halves. It runs from front to back, creating a profile view of motion.
The Frontal plane (Coronal plane) is perpendicular to the Sagittal plane. It divides the body vertically into anterior (front) and posterior (back) sections. Movements in this plane are typically side-to-side motions, such as raising an arm out to the side.
The Transverse plane (Horizontal or Axial plane) is positioned horizontally, parallel to the ground. It separates the body into superior (upper) and inferior (lower) portions. Movements in this plane are generally rotational or twisting, like turning the head.
Defining Flexion and Extension
Flexion and extension describe changes in the angle formed by two body segments at a joint. These movements are defined relative to the anatomical position (standing upright with palms forward). Flexion causes the angle between two body parts to decrease, bringing the segments closer together.
For example, bending the elbow to bring the forearm toward the upper arm is elbow flexion. Other flexion movements include bending forward at the waist or bringing the heel toward the buttock by bending the knee.
Extension is the opposite of flexion, increasing the angle between the two body segments. This moves body parts further apart, often returning the limb to the straight anatomical position. Examples include straightening the elbow, standing up from a chair, or lifting a leg backward.
Mapping Movements to the Sagittal Plane
Flexion and extension occur exclusively within the Sagittal plane, which dictates their forward and backward nature. When performing an action like a bicep curl or a forward lunge, the movement tracks along this plane. This is because the movement does not cross into the side-to-side motions of the Frontal plane or the rotational motions of the Transverse plane.
Every movement in a specific plane rotates around an axis perpendicular to that plane. Since flexion and extension occur in the Sagittal plane, they rotate around the Mediolateral axis. This axis runs horizontally from side to side, piercing the body from left to right, and is sometimes called the Frontal or Transverse axis.
The joint acts as the pivot point for movement, maintaining the constant relationship between the plane and the axis. When a person walks, the hip, knee, and ankle joints are constantly flexing and extending, driving the body forward and backward in a Sagittal-plane-dominant pattern. A squat demonstrates this principle, involving simultaneous flexion and extension at the hips, knees, and ankles around the Mediolateral axis.
The consistency of observing flexion and extension within the Sagittal plane is why it is considered the plane of forward and backward motion. Whether performing a simple head nod or complex movements like running, the actions are largely confined to this single plane. Understanding this specific plane-movement relationship is fundamental to analyzing and training physical activities effectively.