How can I build up my horse's top line as well as build up some more muscle in myself?

lunging, grids. no stirrup work and doing squats while holding stuff.
For yourself - excercises on a gymnastic ball are nice ☺️
For the horse you can try lunging or riding with side reigns. I wouldn't recommend always doing this but one or two times a week will help build the top line. And for you just basic stretches or a basic work or routine. Crunches, push-ups, planks, holding your legs off the ground with your back and but flat to the ground, ect.
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