In the previous section, we defined distance (length of the path travelled) and displacement (change in position – shortest distance from pf to pi). Displacement is the variable most often used in biomechanics. The reason is that the tools used to quantify kinematic variables typically take frames of information. Think of a camera used to assess a squat technique. A researcher would place a camera to film the squat in the sagittal view. Recording the movement with a camera produces a series of still pictures. If the camera records at a frequency rate of 30 Hz, it produces 30 pictures per second. Let’s say we are interested in the movement of the barbell. We could place a marker on the end of the bar and record its position relative to a coordinate system in each frame of data (each still picture). To quantify change in position, you can report on the movement between frame 1 and frame 2 but you cannot report what happened between the frames because technically, you don’t know what happened between them. You cannot calculate the length of the path travelled (distance) because of this unknown but you can calculate the displacement (p2-p1) since you know the position in frame 2 and 1.
Since we don’t know what happened between the frames of data, we can only represent the change between the two frames to represent the change in position (displacement), velocity and acceleration. These are the variables most often used in biomechanics.