Reference Frames
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A reference frame is a particular perspective employed by the analyst to describe and/or observe a motion effectively. There are different types of reference frames. A fixed frame is a reference frame that is fixed. A moving frame is a reference frame that moves with the body. A moving reference frame can translate and/or rotate. When a reference frame is either fixed or moving with a constant velocity, it is an inertial frame. An accelerating reference frame is a non-inertial frame. A fixed reference frame fixed to the environment, not to the moving subject, and used commonly in describing the motions of different body parts is the global frame while all the reference frames fixed to the moving body parts are called the local frames.

Among the perspectives presented in Axis Transformation as example, the spectators' perspective is an inertial, fixed, and global frame. The TV watcher's perspective is a moving (translating), non-inertial, and local reference frame. The gymnast's perspective is a moving (rotating), non-inertial, and local reference frame.

The term "coordinate system" is slightly different from "reference frame". The coordinate system determines the way one describes/observes the motion in each reference frame. Two types of coordinate systems are commonly used in biomechanics: the Cartesian system and the polar system. See Coordinate Systems for details of these coordinate systems. One can describe a motion differently in the same perspective depending on the coordinate system employed. Figure 1 shows examples of different reference frames used to describe the human body motion. One can easily define a local reference frame for each body segment.

f09.gif (3915 bytes)     Figure 1


Young-Hoo Kwon, 1998-