...Good running posture during a 30 minute jog is going to do more for your stability than 100 sit ups, IMO.
Love jogging...and would as soon be emasculated as do endless sit-ups. I have no idea if you are right, but I sure want
to think you are!
On a serious note: I've got an exercise physiology text somewhere that says muscles train for about 30 deg of motion. Think of a bicep curl. According to the text, the muscle fibers used during the first 30 deg of motion are different than the muscle fibers used during the last 30 deg of motion. I suspect this is true based on my experience back in the days where I went to a gym and lifted weights.
If true, then for riding purposes I suspect the value of a crunch would be the first 20-30 deg of motion. I also suspect a lot of what we need in riding is lateral balance and stability, and those come from a side-to-side motion. Looking at the exercises, I'd guess
the Floor Bridge and Russian Twists might be the most helpful, but they wouldn't require extremes of motion.
Me? I've been working with my exercise partners Mr Pick and Mr Shovel to break up rocky soil and place railroad ties in it to slow erosion.
Is it helping?
Well, I'm reminded of when I was in college. My roommate said he was told once that snorting a pinch of cayenne pepper would stop sneezing. "Did it work?", I asked. "Well", he said, "I was crying so hard from the pain that I forgot about sneezing, so I guess it did..."