"The Domestic Horse: The Evolution, Development and Management of its Behaviour" by Daniel Mills and Sue McDonnell.
Damage attributed to bareback riding and pad saddles:
1) Spondylotic spurs of new bone on the vertebra.
2) Impinging dorsal spinous processes
3) Fissures through the epiphysis
4) Osteophytes on the vertebrae
I will note that horses found in archeological digs were not nearly as old as our horses get to be.
Research papers using modern testing techniques I'll just post the authors and the synopsis:
A comparison of forces acting on the horse's back and the stability of the rider's seat in different positions at the trot.
Peham C, Kotschwar AB, Borkenhagen B, Kuhnke S, Molsner J, Baltacis A.
Source
Movement Science Group Vienna, Clinical Department for Companion Animals and Horses, University of Veterinary Medicine, Vienna, Austria.
Abstract
The aim of the study was to compare the stability of the rider as well as the forces acting on a horse's back with different seating positions at the trot (sitting trot, rising trot and two-point seat). The same experienced rider was mounted on 10 sound horses trotting on a treadmill. The kinetic data were recorded with an electronic pressure mat, placed under a well-fitting dressage saddle with no saddle pad. The rider used three different seating positions, each for 20 s. Right forelimb motion was used to synchronise the pressure data with the stride cycles. To determine the rider's stability, the movement of the centre of pressure (COP) along the transverse (X) and longitudinal (Y) axes was calculated. The force was taken as the sum of all segments of the pressure pad multiplied by the area of the pressure pad. The maximum force and the X- and Y-deviations were evaluated using ANOVA for repeated measures with a Bonferroni Post hoc test. The stability of the rider in the Y-direction was significantly highest in the two-point seat, followed by the rising trot and the sitting trot, respectively. In the X-direction, there was no significant difference between the three positions. The significantly highest load on the horse's back was at the sitting trot (2112 N), followed by the rising trot (2056 N) and the two-point seat (1688 N). The rider was most stable in the two-point seat while transferring the lowest load on the horse's back. The rising trot was found to be more stable and less stressful for the horse's back compared to the sitting trot.
Copyright 2009 Elsevier Ltd. All rights reserved.
Forces and pressures on the horse's back during bareback riding.
Clayton HM, Belock B, Lavagnino M, Kaiser LJ.
Source
McPhail Equine Performance Center, Department of Large Animal Clinical Sciences, D202 Veterinary Medical Center, Michigan State University, East Lansing, MI 48824, USA.
claytonh@cvm.msu.edu
Abstract
The objectives of this study were to measure forces and pressure profiles when riding with a conventional saddle compared to bareback riding. An electronic pressure mat was used to compare contact area, mean total force and pressure variables for one rider riding seven horses at sitting trot with a conventional saddle or bareback. The use of a saddle was associated with a larger contact area and higher mean total force compared with the bareback condition. Mass normalized mean total force for bareback riding was lower than expected based on the rider's body mass, suggesting that shear forces exerted by the rider's thighs were not being registered by the pressure mat. In spite of the lower total force, the bareback condition was associated with higher average pressure, higher maximal pressure and larger area with mean pressure >11 kPa. Focal pressure concentrations were present beneath the rider's ischial tuberosities in the area of the horse's epaxial muscles when riding bareback but not when using a saddle. It was concluded that bareback riding was associated with focal pressure concentrations that may increase the risk of pressure-induced injury to the horse's epaxial musculature. The findings also emphasized that researchers should remain cognizant of shear forces, which may not be registered by the pressure mat, but may contribute to the effects of riding on the horse's back.
Copyright © 2012 Elsevier Ltd. All rights reserved.
Science for those who care. Anecdotes are nice but that's all they are.