Huh, I find it interesting that your trainer has you do so many sit ups, or any at all. There has been a long growing trend that planks in all their many flavors promote core strength better. I know I swapped to planks well over a year ago and my core is far stronger, my balance better and my overworked hip flexors have stopped screaming so I can actually sleep on my back again.
http://www.health.harvard.edu/health...ip-the-sit-ups
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.