Axis Reference Systems
 Overview

A coordinate system, commonly used for two-dimensional applications, can be defined by dividing a plane into four quadrants with two perpendicular lines that intersect at a point O, called the origin. Click here to see an example of a two-dimensional coordinate axis system. While a two-dimensional axis system is adequate to define motion occurring on a flat surface, it is not adequate to describe the position and motion of an anatomical structure. To be able to describe relationships between anatomical structures we need to use a three-dimensional coordinate axis system.

You are probably already familiar with the terms sagittal (or median), coronal, and transverse planes. They refer to imaginary reference planes that have been defined to pass through the body in such a way as to be perpendicular to one another. A coordinate axis system functions in much the same way, but allows us to more specifically describe position and motion than is possible with simple reference planes. The following three-dimensional coordinate systems are the most commonly used in medical and scientific applications:

We will be using the rectangular (or right handed) orthogonal coordinate system, defined by the intersection of three mutually perpendicular straight lines that intersect at a point called the origin, to describe position and motion of components of the musculoskeletal system. The three perpendicular lines are called axes, and are conventionally named X, Y, and Z. The right-handed coordinate system relates to commonly used anatomical terminology in the following ways: The Z axis defines anterior (+) and posterior (-) directions; The Y axis defines superior (+) and inferior (-) directions; The X axis defines right (-) and left (+) lateral directions.

We will be using vectors to describe force, velocity, and acceleration. The characteristics of a fixed force vector include:

  1. Its magnitude (a signed quantity).
  2. Its direction (defined by its slope and sense).
  3. Its point of application (where a force contacts an object).
Any two forces having a common point of application can be replaced by a single force called the resultant. The resultant force will have the same effect at the point of application as do the original two forces. Conversely, a resultant force can be represented by two (or more) forces. The magnitude of the 2 component vectors that combine to form the resultant is equal to Fx = F cos(θ); Fy = F sin(θ). In order for an object to be in a state of equilibrium, the sum of forces acting upon it needs to be equal to zero.

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