The GRIND was designed with density points to help increase blood flow to the area you are rolling. The marks indicate there has been an increase in blood flow. But, if for any reason the marks last for longer than 1-2 days or if they become tender or bruised, reduce the frequency of use until the marks have diminished. If the marks continue, then reduce the amount of pressure on the GRIND while rolling. Additionally, be aware of your body’s hydration level – this can affect the marks left on your body after using our products.The GRIND is designed to elicit a variety of blood flow. While the foam density is consistent across the roller, the multi-surface points provide a variant of pressures as one rolls across. The blood brings fresh oxygen and nutrients to tissues helping speed up healing and recovery. To maximize myofascial release while rolling.