Right a TENS unit is just a low powered Estim device that typically won't deliver enough current to recruit motor units, but depending on the power of the unit and the location of the nerves relative to the electrodes sometimes you can recruit muscles with one.

I'm interested in how you can select which muscle fibers to recruit with the Compex?

I agree you're almost certainly going to be recruiting more Type IIs than Type I (and 2a's than 2b's) at any given force level given the nature of estim recruitment versus your nervous system's recruitment of muscle and if your frequency is adequate you'll be getting 100% of whatever force generating ability the recruited muscles fibers have, but I don't understand how you can selectively recruit different fiber types?

Thanks Thisisit, I love chatting about this stuff. You are correct SOME TENS devices offer high HRZ which can excite the ST fibers depending on their HRZ and intensity levels (remember TENS is providing thousands of short bursts so the muscle is truly not be contracted rather than excited at a rapid rate). Keep in mind that none of the brands use a biphasic current like Compex. Their current is uncontrolled and does not go below parallel (what goes in must come out). Here is a quick break down of why and how we can select the fibers per program keep in mind Compex goes from 0-999 on intensity, 1-125hrz for the NMES programs and 150hrz on TENS (again TENS provides short bursts so you are not achieving performance gains).

Break down-
1-9 hrz - is pure blood flow programs found in Massage (1hrz), Pre-warmup (8hrz), Training Recovery (8hrz - 1hrz), Competition Recovery 1hrz - 5hrz)
10 - 18 hrz - at 10hrz we create a tetanic contraction working the ST muscle fibers found in Endurance program (10 hrz - 18hrz). The hrz levels increase as you select level 1-5 on wired or 1-3 on wireless
50- 70 hrz - at 65hrz we excite and fire the FT type 2a fibers found in the Resistance program (50hrz - 70hrz). Again the hrz levels increase as you select level 1-5 on wired or 1-3 on wireless
75-100 hrz - at 90 hrz we excite and fire the FT type 2b fibers found in the Strength program (75hrz - 100hrz). Again the hrz levels increase as you select level 1-5 on wired or 1-3 on wireless
104-125hrz - at 104 hrz we excite and fire the deep FT type 2b fiber found in the Explosive Strength Program. Most humans do not have the ability to fire this group or benefit from this program, however we work with world class athletes who can and have the program available.

To sum this up 10-18hrz is great for people just starting to train most active athletes have a large bank of ST fibers stored the program contracts the muscle for 8 seconds and rests for 2 seconds at a 40 min work time, 50-70hrz is entering the FT muscle fibers or similar to a HIT workout with a 8 second (contraction) work time and 4 seconds of rest for 13 minutes, 75-100hrz is similar to heavy 3x3 lift (squat, bench, dead lift, etc), most people are not doing this often, using a 4 second work time and 19 second rest for 22 minutes, 104-125hrz is similar to a 1 rep max only used for certain training situations using a 3 second work time and 34 second rest with a 18 minute work time.

Hopefully this explains everything further, again ping me with any questions.

CompexEdu - PM sent