Lets start with the simplest case: a single joint moving periodically (think about your hip joint while walking in uniform speed):

This is the time series of the angle. The color represents the phase of movement:

Plotting the movement in the phase plan (angle vs. angular velocity) shows why phase is properly coded by color – color is cyclic like phase:

We’ll measure phase by parts of a cycle, from 0 (code red: angle is 0 and angular velocity is at peak moving left) going clockwise on the phase circle to 0.25 (cod purple: angle is at peak right and angular velocity is 0) to 0.5 (code blue: angle is 0 again but angular velocity is at peak moving right) to 0.75 (code green: angle is at peak left and angular velocity is 0) to 1 cycle which is 0 again. Here is the periodically-moving joint again, color coded by its phase:

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