Sorry for taking a while to respond Blaze. Here are more sources on the back injuries horses receive from carrying humans.

Results: At walk and trot, there was a significant influence on back kinematics in the 'saddle with weight' situation, but not in the other conditions. Overall extension of the back increased, but the range of movement remained the same. Limb kinematics changed in the sense that forelimb retraction increased. At canter, both the 'saddle with weight' and 'saddle only' conditions had a significant extending effect on the back, but there was no effect on limb kinematics. Conclusions and potential relevance: Weight and a saddle induce an overall extension of the back. This may contribute to soft tissue injuries and the KSS. The data from this study may help in understanding the reaction of the equine back to the challenges imposed by man when using the animal for riding.

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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.

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Muscle and Ligament Strain

Damage to the soft tissues is the most common cause of back soreness in the horse. This mostly involves the group of muscles along the back. Usually, all or parts of these muscles are strained while the horse is being ridden. The principal sites of damage are located just in front of and behind the saddle area. Signs include alteration of the horse’s performance and acute back pain. Most of these injuries respond to rest and physiotherapy, although several weeks may be needed for full recovery ( see also Muscle Disorders in Horses).

Another fairly common site of soft-tissue damage is the ligament that runs down the middle of the back. Signs of damage to this ligament typically persist longer, and the chances of complete recovery are not as good as for the uncomplicated muscle strains.

Kissing Spine Syndrome

The spinous processes are the bony structures that protrude upwards from the body of vertebrae. You can feel their upper points in the midline of the back between the large back muscles. They do not normally touch each other; however, with kissing spines syndrome, 2 or more do. This may be partially due to the effects of bearing a rider. When this occurs beneath the saddle area, some horses develop back pain. The condition may also cause a local bone reaction, small bone cysts, and false joint formation. Diagnosis can be aided by injection of local anesthetic into the affected spaces between the spinous processes. Many cases respond to rest and physiotherapy, but persistent cases may require surgically removing one or more of the tops of the spinous processes to relieve the crowding of the spines.

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