How do i build their muscle up?

i have a colt that is 8 nearly 9 months old. He lacks muscle in his hind and for quarters. he is an Arappaloosa. how can i build up his muscle? i have one hill that is fairly steep, i can't do trotting poles and am not aloud to leave the property. i don't have a round yard and i don't have an arena. (i don't ride him because he's too young.) Do you have any suggestions on building up his muscles?

also i have an appaloosa mare she's 5yrs old. she lacks muscle in her neck, back, hind and fore quarters (basically all over).
again i have the same resources as i do the young colt. (i can only jump her over logs). in the saddle she only knows basic commands (go,stop, turn) No half halts, half pass etc...
do you have any suggestions?
please help

Weighted padding, straps etc like you would put a weighted collar on a dog to build his neck and back muscles

lots of trotting. this will make them gain muscle and become more balance. trotting up and down hills is also a great way to build muscle.

horse owner.

Don't worry about it now. He still has lots of growing to do. Don't do any training until he's at least 2 and this is unmounted work. A horse shouldn't go undersaddle until they are AT LEAST 4.

supplements, lunge for 30 min every day, increase the feed, for the mare, teach her how to collect & she will gain back/neck/hind muscle. just lunge them on a line in the field, its a good skill for every horse to have.

EDIT- show horses are broken to saddle & rider at 2 years old, and are show ready for the flat at 3 years old. jumping & other extreme training shouldn't start until 5 or 6 years because bones setting in the legs.