Involuntaries– which muscles?

I’ve been enjoying my aneros collection for about 6 months and each session has become more and more enjoyable.

I do experience involuntary muscle contractions, but only in my pc muscle. When I do, it feels like my dick is being pumped so hard, that it tests the expandability of my shaft skin. It feels SO good and carries over into my every day life. While sitting at my desk at work, I’ll experience a muscle jerk in my left leg. This means that my pc muscle is about to go into steady, repetitive contractions for the next half hour or so, which even outside of sessions feels very nice, pounding at my prostate while seated.

For people who experience the illusive involuntaries: which muscles are the ones that contract for you?