Recently, I studied a paper called "What Does Your Chair Know About Your Stress Level?
It can be download at the link below.
at page 209, Section IV-A mentioned that
In order to characterize the movement properties of the CoP, we have computed the corresponding spectra over the experiment stages 3–7. Fig. 5 shows the spectra of the norm of the CoP
I can understand that the meaning of Fig.4. However, I don't understand what does Fig.5 mean. How did they compute "the norm of the CoP" to "the spectra of the norm of the CoP"? And what does the graph's axis mean?