"Although they offer softer surfaces, running on sand or grass may actually increase the risk of injuries in runners. Although softer than pavement, sports medicine experts report more sprains, strains and even tendonitis from running on uneven and inconsistent surfaces like the beach or grass. Because each step creates varying pressures and forces in the feet, ankles, knees and hips runners most constantly adjust to the surface. Not everyone has the skill or ability to do that easily.
These natural surfaces also tend to slope, and this also creates a dangerous off-center force on the ankles and feet. Over time, running on uneven surfaces can result in tendonitis, inflammation of joints and tendons and even fractures."