This is an excerpt from Massage for Sport Performance by Michael McGillicuddy.
A muscle spasm is the failure of a muscle fiber to return to its normal resting length. Muscle spasms can occur in any muscle of the body at any time. In fact, most people are walking, sitting, and standing every day with many of their muscles in spasm. When enough of the fibers of a muscle go into spasm, the muscle shortens and fails to function. This condition is called a cramp. A cramp is an involuntary, spasmodic, painful contraction of skeletal muscle. Muscle spasm and cramps occur for many reasons, such as stress, overuse, dehydration, electrolyte imbalance, low mineral levels, and injuries such as herniated disks and disease.
While swimming, running, or bicycling, athletes often experience cramps, especially to the lower extremities. Cramps that occur in the legs are often referred to as charley horses. Most of these types of cramps will ease with a decrease in activity level, stretching, and massage. Another common site for muscle spasms and cramping in athletes is the erector spinae muscles that run up and down the back. The primary job of the erector spinae muscles is to hold the body in an upright position when sitting or standing. These muscles seldom have an opportunity to rest. If they worked perfectly when a person lies down, the back muscles would relax and soften. But when you touch the back muscles of the average person who is lying down, the muscles feel tight and ropy. Sometimes they are so tight they feel like steel cables.
After intense workouts, the length and width of muscles should be reestablished by massage and stretching. Prolonged periods of heavy exercise without massage and stretching lead to less efficient muscle contraction, decreased range of motion, less power, pain, and greater likelihood of injury. Trainers should always be aware of the possibility that an athlete will cramp while lying on a massage table. Most cramps can be relieved with the application of therapeutic stretching, but when an athlete experiences cramping in more than one muscle group, medical attention may be required. Cramping in more than one muscle group can be the sign of severe dehydration and should be addressed by the proper medical personnel.