Muscle Cramps During Endurance Activities
Many endurance athletes such as runners, triathletes and cyclists will experience muscle cramps at some point during their training or racing. Using a definition from Dr. Martin Schwellnus a cramp is "a painful, spasmodic, involuntary contraction of skeletal muscle that occurs during or immediately after exercise"(1). While most athletes understand what a cramp feels like there is much confusion as to what causes cramps and how they can be prevented.
Dating back to the 1930s a theory was put forth that dehydration and electrolyte depletion were the primary causes of exertional cramps. This is still a popular theory that has come under fire recently. In 1996 Manjra, Schwellnus and Noakes did a study of over 1300 marathon runners and found the following criteria to be primary risk factors associated with muscle cramps during exercise:
* Older age
* Longer history of running
* Higher body mass index
* Shorter daily stretching time
* Irregular stretching habits
* Family history of cramping
They found no evidence of a large electrolyte imbalance in runners with cramps. Nor was dehydration deemed to be a causative factor. Other studies done throughout the 1980s and 1990s came to similar conclusions.
New Theory
Schwellnus and Noakes put forth the new theory that abnormal spinal reflex activity is the real culprit behind muscle cramps. In other words, muscle fatigue leading to abnormal functioning at the spinal level of the muscle contraction mechanism is the root cause of muscle cramping during activity.
Receptors called muscle spindles cause muscles to contract when they are stretched. Other receptors called Golgi tendon organs (GTO) cause muscles to relax when they are contracted. Both types of receptors are needed to help protect muscles from over-stretching and over-contracting, respectively. These receptors act on muscles by sending an electric signal to the appropriate motor neuron, which is located in the spine. During a normal contraction, signals from both receptors are in balance. According to the theory, when a muscle fatigues the activity of the muscle spindles increases (causing a contraction) and the Golgi tendon organ activity is inhibited (no relaxing) leading to a muscle cramp.
They also cite another factor that contributes to cramping. Golgi tendon organ activity is also limited when a muscle contracts in its shortest position (also called the inner range). Muscles that are the most prone to cramps are those that cross two joints. Examples of such muscles are the hamstrings, gastrocnemius (one of the calf muscles) and rectus femoris (the longest of the quadriceps muscles). The hamstrings span the hip and knee, the gastrocnemius spans the knee and ankle and the rectus femoris crosses the hip and knee. During exercise these two-joint muscles are often contracted in their shortened position leading to less tension in their tendons as well as less activity of their GTO.
Further Evidence
In their study, Schwellnus et al had runners list conditions they associated with cramps, which were as follows:
* High intensity running (racing)
* Long duration of running
* Subjective muscle fatigue
* Hill running
* Poor performance in the race
From this list it's obvious that conditions leading to premature muscle fatigue are linked to cramping. They also found that poor stretching habits seem to increase risk of cramping. The reasoning behind this is that irregular or not stretching the muscles "could lead to an exaggerated myotonic reflex, thereby increasing spindle activity."(1)
Other Theories
Some other explanations have also been offered as to what causes muscle cramps. Poor posture and inefficient movement patterns may cause the Golgi tendon organs to malfunction in a similar way as explained above.(2) The GTO cannot get the muscle to relax and a cramp begins.
Another theory offers that carbohydrate depletion may also cause muscle cramps.(2) This ties in with the abnormal spinal reflex theory as muscle fatigue is thought to play an important role in developing cramps. Carbohydrates are stored in the muscles as glycogen and used for energy during activity. Fully "topped up", a human being has enough glycogen stores to last for about 2 - 2.5 hours. If you run out then your muscles fatigue, your nervous system begins to malfunction and you get fuzzy, light headed and unable to think clearly; a condition athletes typically refer to as "bonking". That is why it is so important to take in adequate food during long distance/duration events.
Finally there is the theory of electrolyte imbalance. Dr. Bill Misner offers a thorough explanation in his article, "Muscle Cramps: Dealing with Heat Stress During Endurance Exercise".(3) While there is certainly more debate as to the role electrolyte imbalances may play in muscle cramps a proper level is still needed to perform well during events.
How to Prevent Muscle Cramps
Having some idea of why and how muscle cramps occur the $64,000 question is how to prevent them. Based on the major theories discussed take the following precautions:
* Train adequately for the conditions (pace, terrain, temperatures, duration, etc.) you plan to compete in
* Follow a regular stretching program (read my article on stretching for recommendations on how to stretch properly)
* Work on correcting any muscle imbalances or incorrect movement patterns; develop an efficient technique required for your sport
* Take in enough carbohydrates before, during and after your event; the amount will vary among individuals but aim for 250 - 400 calories per hour during the event
* Hydrate properly during the event, especially events lasting longer than 3 hours; using a sports drink and not just water will give you the electrolytes you need; again this will vary among individuals and conditions but aim for 125 - 250 ml every 10 - 20 minutes
What to Do If You Cramp
If you do cramp there are a few things you can try:
* Slow down and lower the intensity of the activity
* Stretch and try to relax the affected muscle(s)
* Apply pressure to the affected muscle group(s) to get the muscles to "release"
None of these are guaranteed to relieve a cramp and in these cases all you can do is grin and bear it.
Stay fit, stay healthy!
Curb Ivanic, CSCS
References
1. Schwellnus, Marti P., "Skeletal Muscle Cramps During Exercise", Physician and Sportsmedicine, 1999; 27 (12)
2. Friel, Joe; "Muscle Cramps", 2000
3. Misner, Bill, "Muscle Cramps: Dealing with Heat Stress During Endurance Exercise"
4. Noakes, Tim, Lore of Running (Third Edition); Leisure Press; Champaign, Illinois; 1991
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