Steven Wilding and Dr. Kenneth Knight, Professor of Exercise Sciences
Muscle cramps are the most common heat-related illness experienced by athletes (Cooper et al, Journal of Athletic Training, 2006, 41, 332-336). Muscle cramps also occur frequently at night (“night cramps”) and in the elderly. Because the cause of muscle cramps is unknown, our treatments for them are often times unsuccessful. In fact, less than 50% of athletic trainers felt they were very successful in treating muscle cramps (Stone et al, J Sport Rehab, 2003, 333-342).
One technique to induce muscle cramps is to stimulate motor nerves with a mild electrical current. The minimum frequency of electrical stimulation at which a person develops a cramp, termed “threshold frequency”, is an indicator of a person’s risk of developing muscle cramps. Lower threshold frequencies indicate an increased predisposition to cramp, while higher threshold frequencies indicate a decreased predisposition to cramp. Recent studies of muscle cramps have been testing subject’s minimum threshold frequency (i.e. the lowest frequency of electrical stimulation a person needs to receive for a cramp to occur). In our previous work we were able to demonstrate that the intensity of these electrically induced muscle cramps are mild and usually last no longer than 15 seconds.
In order to sufficiently study muscle cramp treatments and prevention strategies, we first needed to induce a cramp which lasts for a sufficient period of time (60-90 seconds). Because cramps induced at a person’s threshold frequency do not last very long, it is difficult for researchers to determine if the induced cramp stopped because of the treatment or rather stopped spontaneously. Therefore, the purpose of this study was to: 1) determine the intensity of an electrically induced muscle cramp at the minimum threshold frequency and at 5, 10, 15, and 20 Hz (stimulations per second) above threshold frequency and 2) determine the duration of cramps induced at various frequencies above an individuals’ threshold frequency.
My main task in this study was to perform the data collection by testing the subjects on 5 consecutive days, and then analyze the findings to determine what the most effective method is in inducing long-lasting, intense cramps. Although we would not be discovering how to most effectively treat cramps, we would be paving the way for further research to find treatments.
Twenty healthy, cramp-prone subjects came in on 5 consecutive days for testing. Using the technique mentioned above, we electrically stimulated the subjects’ tibial nerve to induce a cramp in the subject’s big toe on their dominant leg. On the first day, the stimulation began at a frequency of 4 Hz, and we raised it in 2 Hz increments until a cramp was induced (it was typically about 14-16 Hz when a cramp occurred on the first day). When a cramp occurred, we told the subject not to fight the cramp, but to let it last until it alleviated by itself. We measured the intensity of the cramp by EMG analysis, which measured muscle activity, and we also measured how long the cramp lasted in seconds.
Each subsequent day, the subject received electrical stimulation at a frequency of 5, 10, 15 or 20 Hz above their threshold frequency. We again measured the intensity of the cramp by EMG analysis and measured the duration of the cramp in seconds.
We found that increasing electrical stimulation frequency resulted in more intense and longer lasting cramps. Cramps induced at 20 Hz above threshold frequency were 68% more intense than cramps at threshold frequency. Cramp duration was 91% longer when stimulated at 15 Hz above threshold frequency. Cramp duration was 70% longer at 20 Hz above threshold frequency. It was an interesting finding that stimulation 20 Hz above threshold frequency resulted in a more intense, but shorter duration cramp than when stimulated at 15 Hz above threshold frequency. (See figure 1) This may have been because the subjects found the stimulation more uncomfortable at the higher stimulations and therefore were not able to relax as much.
We concluded that 15 Hz above threshold frequency was the ideal stimulation frequency for scientists studying muscle cramp treatments to use in the future. It created an intense cramp that lasted for an average of 78 seconds. It was also comfortable enough for the subject to handle.
This was a significant find in the world of science. Now further research can be conducted on finding treatments and prevention strategies for muscle cramps.
I was able to present these findings in poster format at the National Athletic Trainer’s Association Symposium 2009 in San Antonio Texas. There is also a journal article currently under the peer review process in the Journal of Sport Sciences.