In the beach body program it says to do 100 total reps of flexion/extension...is it okay to do all 100 reps in one set? I heard the neck benefits more from higher reps.
I'm not sure you understand what flexion/extension means. You can do one entire set of two different movements.
Sorry for the confusion, what I meant was if I decide to use the neck harness to train my neck extensors and go for 100 reps and I go for this by doing 1x100, would I still see size and strength gains in the neck or is 100 reps way too high?