Mine are mostly as I relax into early sleep - I've had them all my life - and it always feels like I am falling off the bed, yet as I jerk back to awareness, I realize I am laying perfectly normally! I also sometimes have it happen in a chair, but not as often.
Huh, "nice" to know someone else is having the same symptoms as me. To this day I thought I was the only one with MC suffering from these.
I've been having different kind of jerks, ticks and cramps all over my body for 5 years now. They usually start as I relax and are rather painless. And I have also noticed that physical exercise provokes these symptoms. These jerks come and go, luckily now I only get them once or twice a week.