Originally Posted by Sweeney Road
I've been lunging my underweight rescue mare at a walk. She's at least 200 hundred pounds underweight, but she's lost a lot of muscle tone. Should I be exercising her until she sweats?
Absolutely not - Keep steadily increasing her food so that she is able to put on weight. Once she is at a reasonable weight then you can start to increase her work.
A horse sweats when the work load reaches a point where muscles get hot and the body reacts by sweating to cool it down. You don't need to sweat the horse up to get it fit - in fact getting them fit slowly is far better than sweating them up.
An overweight horse does tend to start sweating faster than a trimmer horse. Once the horse starts to sweat the muscle fitness breaks down, the reason that you would give an eventer a day or two off after a 3 day and then bring back up again to the required level of fitness.