Henu is correct here. Mars is useful specifically for the athleticism and competitive drive here, which applies to all sports, yet some sports call for other energies to be used besides just Mars. We can see how horse jockeys are not necessarily ripped, but have a small and nimble frame.
The Mars Square would improve your sense of confidence against other riders, it can give a drive and energy to train harder, and it can also improve your strength in regards to handling the horse.