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.
I’ve been taking risperidone (risperidal) for three years now. I just found an article the other day examining how the effects of this drug can negatively affect your kidneys and in general your adh (anti-diuretic hormone). There a couple of weeks ago, I was convinced I needed more salt, and almost died of dehydration from dosing too much. Then tonight, I was laying in bed watching a movie, and I swear it felt like the muscle in my left arm was detaching from my bone. As soon as the event happened a few weeks ago of dehydration I had given up on salt. Now tonight and here I am drinking a quarter teaspoon in a bottle of water to help with unusual severe cramping (probably due to high intake of water, 9-11 bottles today). I am amazed at all the effects of these chemicals on my body, but also question my sanity as I start to truly believe that maybe I should have never started messing with vitamins and minerals. Then there are days where I truly believe life could not be better because I feel so good. Thank you for this article, very informative. Hope to read more soon. Try branching out what into vitamin chemistry. It may surprise you what you find.
Josiah,
I took my time to answer your comments, because we sell dietary supplements, and have severe restrictions about mentioning anything about drugs, thanks to the DSHEA regulations.
Finally figured out how to respond.
You illustrate the complexities of drug effects on nutrition, and even though it’s all I can say to consult your physician, they typically do not know anything about drug-nutrient interactions or effects.
However, we also strive to provide supplements that add to your overall health and well-being, not just make you perform better. So your comments are very much appreciated, and we will continue to educate you and everyone as much as possible about how nutrition and supplements can maintain and improve your health.
Thank you!
Luke
I awake throughout the night with terrible leg cramps that travel my whole leg up to the crotch area and I can’t move from it. Any attempt is brutal. I’m afraid to sleep because it happens usually in my sleep. There has been the odd time when I’m awake and sitting. I’ve been told when I bend my knee that it causes my muscles to shrink and when I go stretch them the spasms occur. I mostly sleep with my legs straight out in fear if I bend them I sill suffer. I’m tired I need to sleep. I am a diabetic and so when this happens I drink G Zero to help. Should I buy electrolyte drinks instead. They are expensive and this seems the cheaper way. I do drink plenty of water every day, probably too much maybe rather than not enough.
My grandson, age 27, has been incapacitated for almost a year, with a self diagnosis of a defective psoas muscle. He’s been examined time and time again, with no diagnosis. He’s been scheduled to be examined by specialized doctors in a city outside of his own. This seems to be the last resort. I understand, that this muscle might be repaired by balancing unbalanced electrolytes. How likely is it, that this might be the problem, an imbalanced of electrolytes?
which is most important among them?
No one is more important than the others. Being deficient in any will cause muscle contractions to weaken and potentially cramp.
Will this help with (cramp fasciculation syndrome) I have problems with my quads fatiguing fast when I do a long bike ride.
Yes Shawn, it should.