A multimedia project for the 2011 Student Bio Expo by Emma Sparrow. Equine Polysaccharide Storage Myopathy is genetic tendency that makes it hard or impossible for the animal to digest grains or sugar properly and leads to damage to the muscle tissue during physical exertion. EPSM is most commonly associated with draft horses while PPSM is commonly associated with Quarter horses or paints. In a normal horse, glycogen is the energy source used for muscle contraction. An EPSM horse cannot complete this process, resulting in excess stored glycogen which affects the horse physically.