Dressage requires the horse to be supple through the thoracolumbosacral region, and movement should allow for efficient locomotion and expression, but excessive thoracolumbosacral movement is likely to be detrimental to the soft tissues of the vertebral column. It is not known how development of the musculature relates to thoracolumbosacral movement in the ridden dressage horse. The aim of the study was to investigate the relationship between grading of muscle development and back kinematics. Thirty-five horses (Novice to Grand Prix level) in active dressage training were ridden in sitting trot in a straight line by their normal rider on an artificial surface. Thoracolumbosacral angles were derived from high-speed motion capture. Muscle scores were assigned based on visual assessment and manual palpation of the left and right sides of the neck, abdomen, thoracic and lumbosacral (LS) regions, pelvis, and hindlimbs. Our findings suggest that there is a relationship between muscle scores and kinematics of the back in ridden dressage horses. There was an association between neck trunk, thoracolumbar, LS angles, and dorsoventral difference between withers and tuber sacrale markers and muscle scores. Muscle scores assigned during clinical examination were related to the back kinematics of dressage horses ridden at a collected trot. Observations from this study suggest that thoracic, abdominal, and LS muscle development is important for achieving gait patterns which are desirable, according to equitation texts, at the collected trot.