The hocks are the joints most responsible for the forward motion of the horse. The hocks work in conjunction with the stifles. When the horse flexes the stifle, the hock automatically flexes. When the hock is straightened, the stifle automatically straightens.
There are two muscles responsible for these actions: the gastricnemius and the flexor metatarsi. The gastricnemius is a short muscle which works to straighten the hock. It is not always easy to massage, as it is tendonous and hard to reach , but I always include it in every massage session. The flexor metatarsi flexes the hock. This muscle is impossible to isolate in massage, but benefits when other leg muscles are worked, such as the quadriceps