Proper training and conditioning of the horse are essential for building topline in a dressage horse. This should include a balanced and varied workout routine that strengthens the muscles in the horse's back, neck, and hindquarters.
A good quality diet that provides the horse with the necessary vitamins, minerals, and nutrients for muscle development and maintenance can also help to improve topline. This may include feeding a balanced ration of hay, grain, and other supplements as needed.
Regular massage and other forms of bodywork, such as chiropractic and acupuncture, can help to increase blood flow and improve muscle tone in the horse's topline.
Proper conformation and confirmation can also play a role in the development of a dressage horse's topline. A horse with a naturally uphill build and a well-muscled back and neck may have an advantage in this regard.
Regular work on dressage movements and exercises that target the horse's topline, such as lateral work, can help to strengthen and develop the muscles in this area. This may include movements such as shoulder-in, haunches-in, and travers.