WHY WE SHOULD TALK ABOUT RIDER FORCES AND NOT RIDER “WEIGHT”!
Rider weight is a topic that has been debated extensively in recent years. So let’s first assume that we have sound horses and riders with good core stability riding in appropriately fitted saddles. Does it make a difference if the rider is 50kg or 100kg? Notice I have avoided using the term weight.
Weight is actually mass (kg) by acceleration which is equal to a force. So how much of us there is should be expressed as our mass (kg) and weight should in scientific terms be expressed as a FORCE (mass x acceleration). When we weigh ourselves on scales we are actually measuring how much FORCE our body MASS is exerting on the scales. Under gravity if we “weigh” 70kg on the scales then we are actually exerting a FORCE of 70kg x 9.81 m/s/s (1g) which would be equivalent to 687 N (Newtons). This become important as we move up and down because although our mass stays the same, the amount of FORCE we put on the saddle and therefore the horse’s back increases. In effect we “weigh” more when moving up and down and the faster we move the more we “weigh”.
TENDON AND JOINT LOADING
A 500kg horse carrying a 50kg rider would be carrying 10% of its own bodyweight. At 100kg that’s 20% of its own bodyweight. Horses evolved over hundreds of thousands of years to carry … 0% of their bodyweight. We could argue that in the past few thousand years, we have selectively bred them to carry weight, although perhaps not that successfully. Any weight added on the horse loads the back muscles, which were not designed to carry weight and loads the horse’s shock absorbing system in the limbs – the joints and tendons.
There is a big difference between sitting on a saddle stationary and moving up and down in the saddle. This is because when in motion the forces on the back are increased due to acceleration. This is why we should talk of FORCE and not WEIGHT.
WEIGHT = mass (kg) x gravity (g). Mass is how much of us there is and weight is what we measure due to our mass being pulled down to the earth by gravity. On the surface of the earth we take gravity to be 1g or 9.8 metres/second/second. CLARIFICATION – Weight is a confusing term because in everyday use we measure it in kg but in scientific terms we refer to it in Newtons (Force). In the context of when there is a static mass on the Earth’s surface, weight and mass are actually the same. They only become different once the mass starts to move.
FORCE = mass (kg) x acceleration (g). So we also measure acceleration in g, the same as gravity. So a rider sitting still on a non-moving horse applies a Force equal to their mass (equivalent to weight x 1g) but as a rider moves up and down on a horse then the Force on the horse’s back increases. So if the rider is moving up and down at 2g then the Force on the horse is doubled. In this case we are looking at the total Force on the horse’s back. We typically measure Force in Newtons. If we want to know what that is per unit of area, then we have to look at Pressure (which we save for another post).
So why are WEIGHT, ACCELERATION AND FORCE important?
50kg RIDER at CANTER
Downwards maximum acceleration of 3g
PEAK FORCE on the horse’s back = 3 x 9.8 x 50
= 1470 Newtons
100kg RIDER at TROT
Downwards maximum acceleration of 1.5g
PEAK FORCE on the horse’s back = 1.5 x 9.8 x 100
= 1470 Newtons
So the impact on the horse’s back and limbs is a combination of many factors including…
• MASS (Weight) of Rider
• VERTICAL MOTION OF Rider
WEIGHT (as in common use to mean kg of rider) is therefore only one factor to consider! We should really be discussing FORCE! Not to mention Saddle Contact Area and Pressure which we will save for another day.