Sweat patch on hindleg??

[FowlerstoneArizona]

Hi all, my 17yo exracer has this random sweat patch on his hindquarters. He's kind of always had it (noticed it once or twice in the 7 years I've owned him) however it seems to be getting bigger and more angry within the last year
He isn't 'lame' but I'm sure he is weak on it as struggles to get a right canter lead and often disunites (unless I work really hard!) the muscle that the sweat patch covers seems to be much firmer than on the opposite side, he's had chiro look who was stumped and vets didn't seem to bothered (didn't really get much of an answer out of them!).
Has anyone else experienced this? I worry in case it could be an early sign of stringhalt or similar

Thanks in advance

X

 

[Nocturnal]

I may be mistaken, but I thought these indicated muscle damage.

 

[FowlerstoneArizona]

Ooh yea forgot to say have been told they're a sign of muscle damage but have never known them to get bigger or to sweat more