Please know that at no time should our boys do any sit ups or any pedaling as exercise. Usually PTs are not trained with knowledge of DMD boys. My son, Nicolas, sees Dr. Wong, and they suggested we stop going to PT altogether, and only go to have his range checked 3 to 4 times a year. I also know that are kids are kids, and will still play, but to have them get "tired" as he was doing while pedalling is detrimental to his muscles. Never should a PT tire Austin out.
In my opinion, and in my doctor's, the only thing a PT is good for is to check their range of motion and perform stretches.
Jenn... you can point your PT to this page where I organized a bunch of links on PT (PT LINKS)
If she reads through those links she'll find a lot of useful information to help her understand Austin's needs from PT.
In particular in one article from Laura Case on page 18 it states "Eccentric muscle and maximal resistive exercise are believed to be detrimental and should be avoided"
With regards to the bike - Laura also shows that bike riding is a possible exercise as long as it is "not against too much resistance"