While most healthy horses can easily carry a rider and saddle, they do have their limits. Now researchers have identified a threshold for when a rider is too heavy for a horse to comfortably carry.

The scientists base their findings on detailed measurements taken of eight horses that were ridden while packing anywhere from 15 to 30 percent of their body weight. The horses ranged in size from 400 to 625 kilograms (885 to 1375 pounds).

When carrying 15 and 20% of their body weight, the horses showed relatively little indication of stress. It's when they were packing weights of 25% that physical signs changed markedly, and these became accentuated under 30% loads.

The horses had noticeably faster breathing and higher heart rates when carrying tack and rider amounting to 25% or more of their body weight. A day after trotting and cantering with the heftier weights, the horses' muscles showed substantially greater soreness and tightness. Those horses that were least sore from the exercise had wider loins, the part of a horse's back located between their last rib and croup.

Based on these results, the study's authors recommend that horses not be loaded with greater than 20% of their body weight. A 545-kilogram (1200 pound) horse, then would be best off carrying no more than 109 kg (240 lbs) of tack and rider.

Interestingly, this research from the Ohio State University Agricultural Technical Institute has concluded with the same weight guideline as the US Calvary Manuals of Horse Management published in 1920.