Optimizing skeletal muscle function is what endurance athletes strive for every day in training

Skeletal muscle is attached to the bones and is responsible for voluntary movements.   Optimizing skeletal muscle function is what endurance athletes strive for every day in training. Researchers and athletes have attempted to optimize the performance of skeletal muscle first by understanding its underlying physiology, and then looking at how we can enhance function and avoid dysfunction (cramps) in the various states of stress that we call exercise.

Electrolytes are positively and negatively charged ions found throughout the human body both inside and outside of cells. They are largely stored in skeletal muscles, bones, and soft tissues. Electrolytes are essential to muscle contraction and relaxation. We’ve previously written about the five electrolytes (sodium, potassium, calcium, magnesium, and chloride) that are important to athletes and what happens when electrolytes are out of balance in A Tale of Five Electrolytes and Energy and Electrolyte Considerations. Here we take a closer look at the role of electrolytes in normal skeletal muscle contraction on a cellular level.

Basic Skeletal Muscle Anatomy
Skeletal muscles are made up of groups of muscle cells also called muscle fibers or myofibers. Each fiber is covered in a cell membrane called the sarcolemma. Within each muscle fiber are electrolytes, motor proteins, and specialized cellular organs that all participate in muscle fiber relaxation and contraction. Motor proteins are shown as small black arrows in Figures 1 and 2. One of the special organs is called the sarcoplasmic reticulum (blue cylinder in Figures 1 and 2), which functions to store calcium ions.

The Role of Electrolytes in Skeletal Muscle Physiology
At the level of a skeletal muscle fiber, electrolytes are critical for allowing the muscle fibers to contract and relax. This process is immensely complicated and incompletely understood. Figures 1 and 2 show the role of electrolytes in skeletal muscle contractility. These are greatly simplified as there are many other proteins, receptors, hormones, ions, and channels that play significant roles in muscle fiber contraction. The role of each electrolyte is reviewed below.

Sodium (Na+) and Potassium (K+)
Both sodium and potassium are critical for nerve function. Nerves tell muscle cells to contract. At resting state, sodium is at higher concentrations outside of muscle cells than inside and potassium is higher inside than out (Figure 1). When a nerve signals a muscle fiber to contract, sodium rapidly flows into the cell, and simultaneously potassium trickles out of the cell (Figure 2). These steps reverse when a muscle relaxes (sodium moves out of the cell, and potassium back in).

 Calcium (Ca++)
In a relaxed state, calcium remains in highest concentrations in the sarcoplasmic reticulum ( in Figure 1 and 2). When the cell is excited by a nerve impulse, the influx of sodium into the muscle fiber then triggers release of calcium from the sarcoplasmic reticulum into the cell (Figure 2). The calcium works with specialized proteins within the cell to activate a process called the “Sliding Filament Theory” where the proteins crawl along each other causing muscle contraction (small black arrows in Figure 2).  When calcium moves back into the sarcoplasmic reticulum, the fiber lengthens again.

 Magnesium (Mg++)
Magnesium is a calcium antagonist (it works against calcium). Magnesium competes to interact with motor proteins in the muscle fiber. At rest, magnesium binds to the motor proteins within the cell and assists with the relaxed state (Figure 1). In the contracted state, after release of calcium from the sarcoplasmic reticulum, the calcium has a much higher affinity for the motor proteins than the magnesium, and therefore displaces it. In conditions of magnesium shortage, we see muscle cramping.

 Chloride (Cl-)
Chloride is often ignored because it is considered sodium’s other half in common table salt (NaCl). Over the last several years, research has shown an important role of chloride in maintaining the resting state of muscle cells. Further, chloride participates in the excitability and fatigue of a muscle fiber.   It is transported in and out of the cell both during contracted and relaxed states.

Why is this important?
Muscle fibers, and therefore large muscle groups, cannot fire properly without electrolytes. You can see that each of the five electrolytes play an integral role and are necessary for proper muscle contraction and relaxation. It’s not surprising when we look at the role electrolytes play in routine muscular function that when even a single electrolyte is out of balance, things go wrong. As heat and sweat increase, so does electrolyte loss from the body. Even mild electrolyte deficiency can result in performance decline. Severe electrolyte deficiency can have serious health consequences. High fluid intake, even with some sports drinks, can lead to deficiency. It is critical that we replace lost electrolytes to maintain athletic performance in training and race situations.

References

http://firstendurance.com/energy-and-electrolyte-drink-comparison/

http://firstendurance.com/a-tale-of-five-electrolytes/

https://www.uptodate.com/contents/exercise-associated-hyponatremia

de Baaij JH, Hoenderop JG, Bindels RJ. Magnesium in man: implications for health and disease. Physiol Rev. 2015 Jan;95(1):1-46.

Pedersen TH, Riisager A, de Paoli FV, Chen TY, Nielsen OB. Role of physiological ClC-1 Cl- ion channel regulation for the excitability and function of working skeletal muscle. J Gen Physiol. 2016 Apr;147(4):291-308.

Clausen T. Na+-K+ pump regulation and skeletal muscle contractility. Physiol Rev. 2003 Oct;83(4):1269-324.