I was the same way. So I worked on my planks, added weight..and progressed that a way. I also changed using the bosu and stability ball..worked on holding the plank longer..so I feel that I made progress without doing the prone jack knife ..
Well the book states that doing modifications means that if the program isn't working you won't know if it's due to the program or the modifications.
There are 2 schools of thought on the prone jackknife. A. Just struggle through as best as possible..even if it means just getting your feet up on the ball and hanging out there and then slowly gain balance to start moving the ball. B. Try a modification.
Personally I think balance in all things is key. I'd say keep trying it. Not being able to do the prone jackknife usually means you need to gain more balance and more core strength. But sticking with it WILL increase both of those.
So do this.. if you are doing plank..do it on the ball and then see if that increases your balance to then try moving your feet in.
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I don't know the answer but I sure hope so as I have the very same issue and question. I have to imagine that doing something that targets the same muscle group is better than leaving it out altogether. At least that's what I'm hoping.
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