IF is a particularly weak current to use for mm contraction, although it is more comfortable. You would find the lower beat frequencies (say 25Hz, motor range 10-50 Hz I think?) more effective in eliciting a contraction at lower current/voltage than higher settings. The lower the beat the more twitch contractions, the higher the more tetanic contraction.

At a treatment frequency (not beat frequency) of 3200Hz the skin impedance is 50 Ohms, whilst at 4000Hz it is reduced to 40 Ohms, thus letting more energy pass through the tissue to deeper muscles. This in turn means that less intensity is required to get the same effect. If you have an option for a higher frequency then perhaps take that option as well.

It might be better to use Russian Stim or other EMS currents. Working out the ramp time and on/off time to account for fatigue and PT effort should also be explored. The machines are getting pretty good now. I saw one only this week that has combined EMG/mm stim that you can use to make the patient work to a certain level of EMG before the stim. kicks in and helps them on there way. Pretty nifty