In order to build muscle during your horse's training, many small tears are made in the muscle fibers which are then fused back together to form new muscle protein strands -- this process happens after a exercise or training not during.
These repaired strands increase in thickness and number to create muscle hypertrophy (growth). In order for this growth to occur the muscle must undergo an activity that stimulates more than 75% of the muscle fibers to maximum tension.
For any movement to occur, muscles contract to generate the force to move the horse's body. Every muscle anchors to bone at its point of origin; then at the other end -- it's anchor point -- the muscle inserts on another bone through tendon attachments. As the muscle contracts, it folds into itself and shortens in order to pull the bone it attaches to back in the opposite direction.
Low-impact training that is performed at relatively slow speeds and intensities mainly use the slow-twitch muscle fibers. This type of exercise stimulates little tension within the muscle, and limits the growth of muscle fibers. Low-intensity workouts provide small increases in strength, but with higher increases muscle endurance. With higher impact or more intense exercises such as jumping, racing or speed events, most muscle fibers are stimulated and muscle tension is high. Because of this, muscle fibers grow and muscle strength is increased.
A horse's muscle structure makes up about 60% of their body mass. Sports massage therapy helps to break down the tissue build up, allowing muscles to be elastic, flexible and at their strongest. Well moving muscles without tension help keep joints aligned, puts less stress on tendons reducing wear and tear, and helps prevent injury.