Displacement, types of Displacement andeg with human body
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displacement biomechanics definition types linear angular human movement
Displacement in the human body
Displacement is the change in position of a body part from its starting point to its final point, including the direction of that change.
It is a vector quantity, so it has:
- Magnitude: how far the body part changed position
- Direction: where it moved relative to the start
Formula:
[
\text{Displacement} = \text{final position} - \text{initial position}
]
Unlike distance, displacement does not describe the full path travelled.
Example: If the hand moves 50 cm forward and then 50 cm back to its starting point:
- Total distance travelled = 100 cm
- Displacement = 0 cm, because the final position is the same as the starting position.
Types of displacement in human movement
1. Linear displacement
Linear displacement occurs when a body part or the whole body moves from one location to another. It is measured in units of length, such as metres (m) or centimetres (cm).
It can be:
-
Rectilinear displacement: movement in a straight line
Example: During a sprint, the body's centre of mass moves forward in an approximately straight line. -
Curvilinear displacement: movement along a curved path
Example: The hand follows a curved path while throwing a ball, or the foot moves in a curved path during walking.
2. Angular displacement
Angular displacement is the change in angle of a body segment around an axis or joint. It is measured in degrees (°) or radians (rad).
[
\text{Angular displacement} = \theta_f - \theta_i
]
Examples:
- Flexing the elbow from 0° to 90° produces an angular displacement of 90°.
- Extending the knee from 90° flexion back to 0° produces an angular displacement of -90° if flexion was defined as positive.
- Rotating the head to look over the shoulder is angular displacement at the neck.
By convention, counterclockwise rotation is commonly recorded as positive and clockwise rotation as negative in biomechanics. Human biomechanics reference
3. General displacement
Most normal human movements combine linear and angular displacement. This is called general motion.
Examples:
- Walking: the whole body moves forward linearly, while the thigh, leg, and foot rotate angularly at the hip, knee, and ankle.
- Running: forward linear displacement of the body occurs along with repeated angular movement of the limbs.
- Throwing: the hand travels through space, while the shoulder, elbow, and wrist undergo angular displacement.
Quick comparison
| Feature | Linear displacement | Angular displacement |
|---|---|---|
| What changes? | Position in space | Angle of a segment/joint |
| Units | Metres, centimetres | Degrees, radians |
| Example | Body moves 2 m forward | Elbow flexes 90° |
| Human movement example | Walking forward | Knee extension |
In short: Linear displacement describes movement from one place to another, whereas angular displacement describes rotation of a body segment about a joint or axis. Most human movements involve both.
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