The frequency depends on the goal. The pulsewidth for glutes and hamstrings needs to be around 400 to have a strong contraction. Feet have smaller muscles and would require less, around 200-250 microseconds. I suggest you take a look at the meaning and values of these parameters in an EMS Digest I have on-line. After you read the section on chronaxie, which determines pulsewidth, go on to the next section for the role of frequency in EMS.