Thanks for the ideas with the bridge exercise, I appreciate the pointers. So far I haven't really been doing a systematic sets/reps kind of approach, though I might start that now. I have managed to increase the number of times I can do it though, like at first I could only manage it about three or four times properly before I started trembling, and now I can manage eight times with no problem even on the swiss ball, though that often is still hard on my upper legs when I add the difficulty of pulling the ball towards me.

So your back is arching and your head and chest are lfting up off the floor a bit, right?

Er, no. I the one I was shown the head remains low. Not really resting, it's another one of these tense things, but the nose remains towards the mat so the back isn't arched, and you have to lift your arms up from the mat and move those. Like in the class we did different things, forward/backward like I mentioned, also a swimming kind of motion and chopping motion, but I think the main difficulty comes from not letting your arms down while you do all this stuff. At first it doesn't seem so hard to just lift your arms to not touch the floor just this little bit, but for me keeping them up gets harder quickly and becomes quite difficult, and then I just can't hold them up anymore and collapse like a wet noodle. Probably to measure progress with that one you'd have to time how long you can keep up this kind of arm position.