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Muscle Cramps

A muscle cramp is defined as a painful, involuntary, spasmodic contraction of a muscle. The muscle remains contracted and may last for a few seconds to several minutes. The muscles most prone to EAMCs are those that cross two joints – for example the calf muscle called the gastocnemius (crosses the ankle and knee joint) and the hamstrings (cross the knee and hip joint).

There are many theories surrounding the cause of muscle cramps. Some proposed causes are fluid loss and dehydration, electrolyte imbalances (sodium, potassium, magnesium), heat and congenital/inherited conditions. Recent evidence collected by Professor Martin Schwellnus at the Sports Science Institute of South Africa indicates no strong relationship between these causes and exercise cramps. After completing several studies and studying the results of other experiments using electromyography or EMG (measures muscle nerve electric activity), Schwellnus has proposed a novel model of the cause of EAMCs.

Dr. Schwellnus identifies two possible factors that may affect nerve activity – causing excessive muscle stimulation to contract and resulting in a cramp. The first suspected factor is fatigue; since motor nerve firing patterns have been demonstrated to be irregular during conditions of fatigue. The second factor is proposed as resulting from the muscle working too much on its “inner range” or “on slack”.

To explain this concept it must first be understood that a muscle cannot work efficiently if it is not at its optimal length – a muscle works progressively less efficiently when overly stretched or overly loose/on slack. The protein filaments (actin and myosin) that make up muscle fibers require an optimal “overlap” to be able to generate force.

The position of the body’s joints determine muscle length, so it follows that muscles that cross two joints like the gastroc and hamstrings might be more likely to operate in the slackened position and experience a cramp. For example, consider a free-style, swimmer who performs flutter kicks at the ankle with a slight knee bend. The flutter kick involves the ankle flexing and extending in a small range very near the plantarflexed (toes pointed) position. Couple this with a slight knee bend, and it makes the gastrocnemius muscle even more “passively insufficient”.

Muscle physiology plays crucial role in the understanding of EAMC’s. Most significantly, the small cellular bodies of the muscle spindle and the Golgi Tendon Organ (GTO). The muscle spindle is a tiny cellular structure usually located in the middle portion of each muscle fiber. Very basically its role is to “switch on” a muscle and determine the amount of activation and the strength and speed of the contraction. The GTO is a small structure located in the tendon that joins the muscle to a bone. This structure senses muscle tension and performs the opposite role of “switching off ” the muscle in order to protect it from generating so much force as to rip right off the bone.

Dr. Schwellnus suggests that when a muscle works within its inner range and/or when fatigued, muscle nerve activity shifts progressively toward muscle spindle activity (contraction) and less toward GTO activity (relaxation). More specifically, the nerves that control the muscle spindle (Type IA and type II nerves) becomes overly active while the nerves that controls the GTO (Type Ib nerves) become under active or inhibited. The result of this nerve activity imbalance is an uncontrolled, painful cramp.