Join Date: Dec 2010
Location: southern Arizona
20% is based on having almost no effect at all on the horse. The one study I've seen used out of shape horses, and 20% was the guideline for no discernible effect on the out of shape horses (not ridden for 4 months, then tested with a 45 min ride with 2 weeks off between rides). The cavalry came up with that figure for riding daily, 8+ hours at a shot, with no effect after weeks of riding. Very few recreational riders use their horse like that!
I've read a number of studies, and I've never found anything suggesting a weight limit below 20%.
If 10% or 15% was needed, almost no men would be able to ride horses. That defies a few thousand years of history...
... Energy is an admirable thing, but the energy of stupidity seldom avails much..." - On Seats and Saddles (1868), Francis Dwyer, Major of Hussars (light cavalry)