Noooooo. Walking correctly and briskly is the best way to build muscle. Add hills and you have a great exercise.
Walk walk and more walk is the best thing for your horse!! Make sure he is on turnout in a field which encourages him to walk (ie food and water on opposite ends). And when you do ride him, walk walk walk! And walk don't doddle. You need a clear four beat, energetic walk what is in a frame. Walking outside on natural slopes is absolutely great for them, body and mind.
I would have the vet out to see why he wont trot or canter undersaddle.
Absolutely! Jim Wofford wrote a great article about muscling up an Eventer *my Coach showed it to me recently because I've been wanting to get Nelson muscled up* and he states this exact theory.
Walk, walk, walk and walk some more. Then, when you are done walking, add some more walk.
It isn't just a poddly walk though *poddly...is that a word?* just as Anabel said, you need an energetic, engaged walk. The back legs need to be reaching the front legs, or surpassing them.
Then, get your horse to work on long and low, that really helps alot, especially for the topline, neck and rump. You need to sit on all 3 points of your seat, tall upper body, chest open, looking up, legs lifting the ribs up into your seat and that 4 beat energetic walk will really start taking effect.
Just as already stated, hills are amazing - being an Eventer, I ride outdoors as much as possible when the weather permits. Hack, hack, hack, hack and then do more hacking when I finish hacking.
My daily riding routine, consists of majority of walk - long and low, energetic, engaged walking. I do so much flippin' long and low, that's all Nelson wants to do now...lol. Takes alot of work for me to keep him on his hind end when we do though, especially at the trot and canter - but it is working!!
I was going to add all this fandangled muscle building suppliments to my guys daily feed regime, but after talking with Anabel and my Coach - I decided to just stick to traditional way of conditioning and save my pennies on more important things.