There's an easy peasy formula you can use. Measure your horse's weight, then measure your weight and your tack. The total shouldn't be more than 20% of your horse's mass.
Sounds Simple... But It's Not
The question we need to ask is, "Is my horse STRONG enough to carry me without causing damage? "
Whether your pushing the 20 percent rule or not, putting a rider on a horse's back transforms natural biomechanics related to posture, musculoskeletal function, and gait dynamics.
Find the Weak Spot
Your horse's back is an unsupported span hanging in space between two pillars, the hind end and forehand apparatuses- pelvis, shoulders, and limbs.
Horse's are designed to be strong and powerful, but only for short sprints when they are escaping a threat. They may run longer when they are having a playful romp.
They are NOT designed to carry weight and travel for extended periods of time at constant rates of speed or in a consistent frame.
What to Do
To help a horse to healthily manage a rider and the demands of riding, start with a balanced body - teeth and hooves are the cornerstones of unrestricted comfortable movement.
Next, condition the body to travel in skeletal alignment with all the body parts working in connection and supporting one another.
The back must be both stable and relaxed. It must be strong enough to carry the weight of the rider. Otherwise it will be strained and the body will transform healthy functioning tissue into "damage control" tissue that develops in an effort to protect itself.
Don't overload your horse. It will cause injury and pain, but think beyond the human:horse ratio. Any horse, no matter the size, must be physicallyprepared to carry a rider and handle the assoctated workload to avoid injury, pain, emotional distress, and maintain a great quality of life.