Muscles contract and relax as we move. However, sometimes a muscle (or a few fibers in a muscle) will involuntary contract. When the contraction is sustained and does not relax, it becomes a cramp.
Watch the video below for simple tips to help avoid muscle cramps!
MedicineNet.com. (2012). Muscle Cramps. http://www.medicinenet.com /muscle_cramps/article.htm
McArdle, W. D., Katch, F. I., & Katch, V. L. (2006). Essentials of exercise physiology. Baltimore, MD: Lippincott Williams & Wilkins.