Overstretching produces microscopic tears in the tissues that can lead to scarring and the eventual loss of elasticity, the opposite of your goal. A certain amount of sensitivity is necessary to carry out a successful stretching program. The stretch should be done slowly until a slight amount of tightness is felt. Human athletes work up to holding the stretch for 30 seconds. No research has been done to indicate how long a stretch should be held with a horse, so it is up to you to be aware of what he is feeling.
Human athletes repeat each stretching maneuver several times for the best results.
Be aware of your horse's comfort level before repeating the exercise. Ignorance and lack of attention to task will be the downfall of an effective stretching program. Never use a bouncing or jerky stretch. The muscle spindle is activated when the muscle is overstretched, producing a protective tightening. You will be working against the muscle with a bouncing stretch, increasing the likelihood of microtrauma.