Muscle cramping is usually caused by dehydration and an imbalance of electrolytes causing spasm and stiffness. That is why it is important during long, intense workouts to replace lost electrolytes and stretch. If you find yourself cramping stop immediately and stretch the muscle.
At the Winchester Institute, Dr. Haggerty stresses the importance of proper stretching and hydration. He provides many of his patients with sport specific stretching routines.