http://www.equinews.com/article/race...rences-studied
When horses negotiate a turn, they usually choose to gallop with the leading leg corresponding to the direction of the turn; hence, a racehorse turning to the left is most balanced and comfortable on the left lead.
On a straight course, either lead can be used, and horses tend to change leads several times during a race. The author suggests that the choice of lead may be linked to biomechanical factors affecting ease of breathing (each complete gallop stride is accompanied by one inhalation and exhalation) or “handedness” (individual preference for one lead over the other).
