You might want to try swapping out the planks for l-sits. I personally find that they are more effective, but you can decide for yourself which works best for you.
btw, forgot to ask. i assume i-sits are isometric holds halfway up a situp?[/quote]
I tend to do ‘conventional’ l-sits on the floor. Although, as skw said, they can also be done on dipping bars (or push up handles).
I find that this takes the stress off the upper body : doing them on the dipping bars would be a better choice if your upper body is the limiting factor in performing the hold.
Please bear in mind that I’m not suggesting that you perform l-sits with a dip added into the movement, at least not in this case : if you are going to do Coach Thibaudeau’s program, just stick with the l- sits holds, as they are quite similar to planks.
Changing out the movements too much is going to reduce the effectiveness of the program.