Although I use youtube for YOGA I would still recommend this to users who want to stop, freeze and pay attention to each individual posture. Focus is key when getting stretching right and often counting till a pose is complete. You want to know what you're doing is correct to get the best out of it.
The step by step instruction is simple too. I would perhaps enlarge the fonts in this section so it's easier to read. As for other text the hand scribbles and the text around the back and forward arrows conflict. Not sure why this stands out but consistency here might be good. I get that the hand written typeface could relate to loosening up - stretching out. But one or the other just doesn't work in harmony well enough in appealing the eyes. Hope that makes sense.
Characters performing the stretches are great. High quality and diverse so everyone feels included. I would upgrade later to a gif of a ''before'' and ''after'' pose possibly. You did this well in upper body & core with two images, one above the other, showing a proper push up. This definitely helps. And, in time eliminate any duplicate images so you don't feel like you've seen this pose before but are starting fresh each time. Maybe familiarity is good here. Unsure.
Also, I know this is a work in progress but Progressive Muscle Relaxation's images do not progress. Same guy every 14 frames. Moreso, the character image sizes are not consistent. They are medium sized frequently then get a lot larger and take up the whole window from stretch to stretch. I would recommend pick a size and stick with it.
Colour wise looks lovely. Information below characters about the ''how'' to do each stretch is evident clearly ( Time, Sets, Reps )
I would look forward to customizing a routine in the future with a timer and avatar who gets more flexible as I use the product more. Fun stuff.
Awesome work overall. Can't wait to see more iterations come to life, Nick.