IFT are medium frequency currents, from 2000-10000 Hz. The base means the carry frequency. And spectrum is used to make the difference between the two carry medium frequencies currents. Eg: you have a carry frequency of 4000 Hz and spectrum 0-10 Hz. It means you have one medium frequency current of 4000 Hz and another medium frequency current of 4010 Hz,the difference between the two carrying medium frequency currents= from 0 to 10 Hz (the spectrum). So when you need to set up the parameters, you need to set up the carry frequency and the spectrum. As low as the carry frequency and the spectrum are,the effect concern stimulation and muscle contraction (normal muscle, only atrophy). As higher the carry frequency and spectrum (eg: carry fq: 9000 Hz,Spectrum: 90-100 Hz), the effect is concerning analgesia.
Correct me if I am wrong, please!