^^ You may be thinking about this article:
This study was designed to test the weight-carrying ability of light-breed horses performing 45 minutes of light to moderate exercise, simulating a riding lesson for a student at an intermediate skill level. In this study, weight loads of 25 and 30% of the horse's body weight were shown to influence work rate, heart rate, and lactate concentrations. No differences were found in heart rate, plasma lactate concentration, respiration rate, rectal temperature, and work rate for horses carrying weights of 15 and 20% of body weight. Serum creatine kinase activity, commonly used as a measure of muscle damage in exercising horses, was not changed when the horses carried 15 and 20% of their body weight. Post-exercise creatine kinase activity was greater in horses carrying 30% of body weight, and levels remained elevated 24 and 48 hours after exercise.Horses
However, what is not discussed is how much various levels rose or the impact. For example, I spent a couple of hours yesterday breaking up caliche with a pick and setting some 50 lb blocks and a railroad tie into the ground. I give you my word: my lactic acid levels rose significantly, mostly in my shoulders and lower back. Tomorrow, tho, I'll be at it again.
If 20% means there was no buildup of by products, then that is the level at which a horse isn't doing any significant work. With saddle, I'm at 205-210 lbs, and my 900 lb Arabian mare has no trouble carrying me for a 3 hour trail ride. Might she be a little sore that evening? Maybe - but we don't do 3 hour trail rides daily. And if we did, I'd bet she'd get in better condition - just as my jogging (after a 3 year lay-off) is slowly getting ME in better condition.
BTW - my 800 lb Appy used to carry a 200 lb rider plus a 40 lb saddle for 10 hour days in the mountains of Utah. He's not nearly in that shape right now, but he also didn't show any signs of soreness back when he WAS a ranch horse. At least, not until he was loaned to another ranch and spurred bloody...