imo its the lats, lower back and legs working together, when sidehopping you pull the handlebar so you need strong back. Ive improved my legs alot, i can jump 20cm more on feet than i could do 3 months ago and ive gained 6kg of weight with that time. but not much improvement on the bike as my hands and back arent really strong so im focusing more on them now but i will still train my legs stronger i set some goals for myself for spring 2x bodyweight squat for 1 rep 2.2x BW deadlift for 1 rep and +30 or more kilos for barbell rows and bench press good luck with your training and let me know about your programme when its ready