It’s only pain 
You’ll get used to it !
Which muscle category are you working on when going up to 50mA? I have different 'pb’s for different muscles. With glutes, I’ve gone up to 300, with hamstrings I’m usually in the 120mA range (sometimes higher, sometimes lower), and my ‘shameful’ category are my quads, which are in the 50-60mA zone, on a good day. I’m progressive with the cranking in the first 2 reps, but for example with hamstrings, I’ll start at 70-80.
Make sure you’re not lying flat on your stomach with the hamstrings, because you can cramp seriously… I’m on my feet, bent forward at 90 degrees when I do them, but the Compex booklet doesn’t describe such a position - I find it the most effective.
For explosive strength and strength, I believe that the rests are 40".
Please, post on youtube 