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HOTSHOT Science: What Causes a Muscle Cramp?

By: Dr. Bob Murray, Managing Principal at Sports Science Insights and advisor to HOTSHOT

Sheila had been training hard for the past six months in preparation for an Ironman qualifying event. Despite a bout of Achilles tendonitis, a twisted ankle, a week-long case of the flu, and disruptions in her training schedule due to unexpected work travel, Sheila was in great shape and ready to go. On race day, she quickly settled into her swim and was pleased with her time. The bike leg had some challenging hills and she could feel her quads starting to fatigue during the final 30 miles. Sheila started slowly on the run, taking care not to push too soon. By mile 5, she had eased into a pace she hoped to hold for the remainder of the marathon. When she reached the bottom of the first hill at mile 7, she stumbled a bit, but quickly caught her balance. Three strides later, her left hamstring muscles seized into a painful cramp, dropping Sheila to her knees. After a few minutes of stretching, the cramp dissipated and she gingerly walked, then jogged before attempting to regain her run pace. Only a few minutes later, the cramps struck again. After suffering through three more episodes of cramp-stretch-walk-jog-run-cramp, Sheila gave up in great frustration. All that hard training and preparation down the drain because of muscle cramps!

Athletes in all sports can relate to Sheila’s story—and that’s particularly true for athletes who have a history of muscle cramps. Even though Sheila was well hydrated and had consumed plenty of carbohydrate during the race, cramps that she had never experienced during training literally brought her to her knees. Why did she cramp? Why just her left hamstrings? What could she have done to prevent it?

Although it’s likely that muscle cramps have been around since the dawn of man, only recently are scientists beginning to understand what causes a muscle to cramp and how that cause has opened the door for effective interventions to prevent and treat cramps. The cramps that afflict athletes are called “exercise-associated muscle cramps” or EAMCs for short.

What causes a muscle to cramp in the first place? In healthy athletes, muscles don’t cramp on their own. Constant input from the nerve that controls individual muscle fibers (muscle cells) is required to spark and sustain the constant contraction of thousands of muscle fibers. So what causes those nerves to become hyperactive? To answer that question requires a little background in the structure and function of motor nerves—also called alpha motor neurons, the nerves that innervate skeletal muscles.

Motor nerves extend from the spine all the way to the muscle fibers they innervate. In some cases, the motor nerve might be three-feet long. Take a look at the figure and you’ll see that motor nerves have four main parts: the dendritic branches, the cell body, the axon, and the terminal branches. In leg muscles, one motor nerve might attach to and control over 1,000 individual muscle fibers. For example, the gastrocnemius muscle in the calf has 580 motor nerves and each nerve connects to roughly 1,700 muscle fibers. If one motor nerve fires, all 1,700 muscle fibers contract in unison; the more motor nerves that are activated, the stronger the contraction.

Motor nerves are controlled in part by nerves that extend from the brain to the spinal cord so that whenever we voluntarily decide to move, those nerves signal the motor nerves to cause the appropriate muscle contractions. Pretty straightforward. But motor nerves also receive input from thousands of other nerves located in the spinal cord, along with nerves that deliver information from muscles to the spine. In fact, two-thirds of the nerves associated with muscles provide this type of sensory feedback to the spine. Long story short, each motor nerve receives input from tens of thousands of other nerves via its dendritic branches. All that incoming information is sorted by the cell body; and, if the conditions are right, impulses are sent down the axon to the terminal bulbs that connect to individual muscle fibers.

If one or more motor nerves are constantly stimulated, all the connected muscle fibers contract, causing fasciculations (twitches) when just a few motor nerves become hyper-excited or a full-blown cramp when many motor nerves are activated. Motor nerves can become hyper-excited if they are overwhelmed by incoming information from other nerves around the body. In Sheila’s case, the fatigue of the race, the gradually increasing muscle damage, her elevated body temperature, and perhaps even the brief stumble she experienced, all conspired to make many motor nerves serving her left hamstring muscles hyper-excited. Stretching the muscle relieves the cramp by increasing the amount of inhibitory input sent from the cramped muscle to the spine. Sheila used stretching to temporarily stop her cramping, only to have the cramps reoccur whenever she began exercising again.

In summary, exercise-associated muscle cramps are caused by hyperactive motor nerves and are prevented or treated by restoring the nerve’s normal function. For those reasons, athletes should follow sound recommendations for training, pacing, nutrition, and hydration to prevent motor nerves from becoming hyperactive. If cramping still occurs, stretching the affected muscles will stop the cramp, at least temporarily.

Of course, for Sheila and other athletes, preventing cramps is much better than having to treat muscle cramps during training and competition. HOTSHOT® is formulated to keep motor nerves from becoming hyperactive by stimulating sensory nerves in the mouth which send signals to the brain and spinal cord to help maintain normal nerve and muscle activity. This neural connection is now helping athletes avoid the pain and frustration of muscle cramps. It’s also possible that the science of TRP channels might unlock other benefits to Neuro Muscular Performance that could aid training, racing, and recovery, possibilities now under laboratory study.

Motor nerves extend from the spine all the way to the muscle fibers they innervate (see Figure A).

alphamotorneurons-2


 

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HOTSHOT Science: What causes a muscle cramp? Read here

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