I know a few people that use or have used treadmills in conditioning their horses.It is beneficial when the horse is not of riding age & /or are limited to areas to exercise due to weather etc.It is less stress on joints than longing
I have never used one but I know someone had posted pics of her horses trying out her new treadmill,she was working them 20min on it Anyway she got some comments that that was too long!! 10min was more appropriate. Also when starting them on it you work up the time.
For those that don't show halter that are questioning the OP,why this, why that :proper conditioning is from both inside & out. Diet for good weight gain & hair coat, regular grooming & regular excercise to tone up muscles/fitness. This proper conditioning is what some may term as Bulking them up,but more appropriately it is actually result of having them fit/conditioned.