How do you strengthen stifles after an injury? Getting rid of scar tissue,  no known injury to the actual joint.

Most any exercise you google to strenghten hind end will work. Long trotting. Just make sure to start slow and build up. Cavalettis, or have them step over whatever you can find that is somewhat raised. Before I had cavalettis I just had them step over shavings bags or feed bags on the ground. Something not too high, like 6"-12". Backing, hills, etc