I have a rescue that is very undermuscled along his back/topline. He was very underweight when I got him and now he is starting to put on weight and fill out, but he is very undermuscled along his back/topline. We haven't started riding him yet because of the lack of weight so I am looking for groundwork exercises that could help. Thank you in advance :)

Babes I got you I was in the same boat and my boy is still lacking a bit, but hes so close to having a great topline

Depending on how bad the neglect/abuse was, it might take awhile for their body to recover. It took Maple (my boy) almost a year to just get back to being okay, and hes finally shining and doing awesome (had him for 2 years now)

Nutrition is a big part of it, Ace Hi Complete + Golden Ground flaxseed it takes about 60 days for you to really see progress with Ace Hi, but by day 30 you'll be seeing progress already :) (im on day 35) Make sure to ease onto it

Alfalfa and bermuda and grazing is what my boy gets for forage

Plus mineral block and loose salts! Ive also heard excellent things for topline progress using Purina Super Sport, but have not used it myself because I cant seem to find it anywhere near me lol

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For exercises, neck stretches with cookies or carrots are great!

Teaching them to lunge with their head low and lifting their core and working hind quarters is super important! I taught mine to do that on the lunge line, and now he does it free lunging too

Using target training can be very helpful, but I taught it by gently pressuring behind him with the lunge whip, and gently tugging on the lunge line and then releasing and lots of praise when he did it right

Lunging over ground poles and cavalettis is awesome

If you dont have the money for a fancy set up, PVC pipes are awesome for poles (and light, so easy to move), and you can use cardboard boxes to lift the poles up

Small jumps are also good :)
Backing up with their neck tucked and hill work (up, down, back, side stepping) is also really good

If you have a pond near you, that is an awesome work out for them, and also really fun

In hand trail walking is fun because it works their muscles, desensitizes them, and is fun to mix things up :) also helps them be more surefooted

Also a book my vet highly recommends and I will be purchasing soon is "Activate Your Horses Core"

Good luck and have patience, it can be a long road, but super rewarding when you see results :D

Surcingle work is great for building top lines! Making sure your horse really engages their hind end to get the best results. This also really helps for teach collection and balance. I like to add a few poles to my lunging also!

Lots and lots of lateral work. Sidepassing, leg yields, forehand and haunch turns, etc etc help develop those muscles on their back end. Zippy's currently super fit but is missing a little muscling on his hindquarters and we do lots of lateral work