The Muscular System

The muscular system is composed of different types of very specialized fibrous cells, all with the ability to contract. Muscle contraction is responsible for nearly all movement in the human body. The muscular system also plays an important role in circulation, digestion, posture, balance, heat production, and many other functions. There are three main types of muscles—skeletal, cardiac, and smooth—each with its own distinct set of functions.1

Skeletal Muscle

Skeletal muscle is probably what comes to mind first when thinking about muscles in the body. That’s because these are the only muscles over which we have voluntary control, meaning we can move these muscles when we want to, so of course, we are very aware of them.1 As their name suggests, skeletal muscles are the muscles attached to our bones that allow us to move our bodies and appendages. In addition to movement, skeletal muscles provide structural support to the body, help maintain body posture, act as a storage source for amino acids, play a central role in maintaining body heat, and can serve as an energy source during starvation.2 With exercise, we can make our skeletal muscles bigger and stronger, and with inactivity, we can make our skeletal muscles smaller and weaker.

Skeletal muscles are composed of multinucleic, striated (i.e., crossed with repeating bands of the proteins actin and myosin) fibers, each of which acts independently from its neighboring fibers.1 There are two main classifications of skeletal muscle—Type I (slow twitch) and Type II (a and b) (fast-twitch)—which allow for vast variations in our skeletal muscles’ speed of movement and length of contraction, depending on their specific function.3 

Type I muscle fibers are referred to as slow twitch because they contract slowly. They are rich in mitochondria (organelles in cells that convert oxygen to energy) and myoglobin (the protein that supplies the oxygen to the cells), which allows them to be very efficient in using oxygen to produce adenosine triphosphate (ATP), which hydrolyses and releases its energy for cellular needs. For this reason, muscles with Type 1 fibers are able to resist fatigue for a long time. These would be the muscles we would rely on, for example, if running a marathon.4 

Type II muscle fibers are classified as either IIa or IIb. Type IIa fibers are also known as intermediate fibers because they fall somewhere in between slow twitch and fast twitch. Type IIa are rich in mitochondria and are able to produce ATP, but they lack myoglobin. Muscles with this fiber type are able to produce more forceful contractions than slow-twitch muscles, without fatiguing as quickly as fast twitch muscles, but they are not as fatigue-resistant as Type I muscle fibers nor as powerful as Type IIb muscle fibers.5 These would be the muscles we would rely on, for example, if going for a walk around the neighborhood. 

Type IIb muscle fibers, on the other hand, are referred to as fast twitch muscles because they contract quickly. These muscle fibers use anaerobic respiration to produce energy, meaning they convert sugar into energy instead of oxygen.4 Because they do not use oxygen to produce energy, they contain little mitochondria or myoglobin. Type IIb fibers use glucose to fuel very rapid, forceful contractions, but once the glucose is burned off, there is nothing to left to fuel the energy required for muscle contraction, which means Type IIb muscle fibers will fatigue quickly and can only be used for short periods of time. These are the muscles we would rely on, for example, when sprinting.4,5 

Cardiac Muscle

The cardiac muscle, or myocardium, comprises the thick middle layer of the heart, and is composed of cardiomyocytes, which are striated, single-nucleus cells that govern the myocardium functioning. These cells are interconnected via intercalated disks that allow the cells to receive rapid electrical transmissions and contract as a single unit. The cardiac muscle also contains cardiac pacemaker cells, highly specialized cells that spontaneously fire to trigger each heartbeat. The cardiac muscle is involuntary, meaning it fulfills its function without the conscious effort of the person.3  

Smooth Muscle

Smooth muscles are involuntary muscles comprising spindle-shaped, smooth (nonstriated), narrow cells with a single, centrally located nucleus. Using ATP as energy, smooth muscle serves multiple specialized functions in the body through its ability to contract and hold that force for long periods of time. Smooth muscle has been adapted for use throughout the body, such as in the cardiovascular and digestive systems, serving a critical role in nearly every autonomic function in the body. For example, smooth muscle lines every vein and artery, contracting and relaxing to help regulate the flow of blood. Smooth muscle also lines the gastrointestinal tract, moving food through the intestines; contracts the irises; moves fluids through organs; raises the hairs on your arm; and much more.6 

Nutrients that Benefit the Muscular System

Protein. It should come as no surprise that protein is an essential nutrient for maintaining a healthy muscular system. Protein is one of the building blocks of body tissue, including muscle. Muscle makes up about 40 percent of a healthy human body’s weight, of which about 20 percent is muscle protein.7 Protein can also serve as an energy source. During periods of fasting or stress, the body uses skeletal muscle proteins to supply essential amino acids to other bodily systems. There are nine essential amino acids the human body cannot produce on its own: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. If an individual fails to take in the daily requirements of dietary protein, a negative balance will occur in the muscular system, resulting in muscular atrophy, impaired muscle growth, and overall functional decline.8 A protein that contains all nine of the essential amino acids is called a complete protein. Dietary sources of complete proteins include fish, meat, dairy products, quinoa, hemp seeds, chia seeds, and soy. While most whole, plant-based foods contain some protein, very few plant-based foods contain all nine of the essential amino acids. However, different plant foods can be combined to make a complete protein. For example, kidney beans, which have leucine, lysine, and valine, can be combined with brown rice, which has a fair amount of all essential amino acids except lysine, to make a complete protein. Peanut butter on whole grain toast, hummus with whole grain pita bread, and lentil bean and barley soup are a few other examples of plant-based food combinations that make complete proteins.8

Calcium. Calcium is one of the most important minerals in the human body. We know that calcium is necessary for healthy bones and teeth. But calcium is also essential to the muscular system due to its critical role in the body’s ability to send and receive nerve signals to and from the brain, as well as in regulating muscle contractions, including the cardiac muscle.9 Calcium triggers the heart muscle to contract. When the body does not receive enough dietary calcium, it will pull it from the bones. A chronic state of calcium deficiency can result in bone loss and irregular heartbeat. Calcium can be found in many different types of food sources, including dairy (e.g., milk, cheese, yogurt), fortified plant-based milks, fortified orange juice, winter squash, edamame, canned sardines with bones, almonds, and leafy greens (collard, mustard, turnip, kale, bok choy, spinach).9

Magnesium. Like calcium, magnesium is an essential mineral in the human body. It is required for over 300 enzymatic reactions in the human body that are needed to regulate protein and energy production, muscle contraction and relaxation, and mitochondria activity. Calcium and magnesium compete against each other. Calcium triggers the heart to contract, whereas magnesium triggers the heart to relax. A deficiency in magnesium can result in overstimulation of the heart by unchecked calcium. Magnesium helps regulate other muscles in the body as well. A deficiency in magnesium can result in muscle cramping and spasms. Dietary magnesium can be found in pumpkin seeds, boiled spinach, boiled Swiss chard, boiled beet greens, cooked black beans, almonds, cashews, dark chocolate, avocado, tofu, and salmon.10,11

Potassium. Potassium is vital to maintaining electrochemical balance across cell membranes and neurotransmission. It also is needed for electrolyte and pH homeostasis. Potassium relays signals to the brain to stimulate skeletal, cardiac, and smooth muscle contraction and relaxation.12 Like calcium and magnesium, potassium is needed to regulate the heartbeat. Potassium deficiency can result in prolonged muscle contractions (cramping) and an irregular heartbeat.13 Dietary potassium is widely available in many foods, particularly fruits and vegetables, such as leafy greens, beans and lentils, nuts, dairy, and winter squash. Other sources include dried apricots and raisins, potatoes, avocado, bananas, cantaloupe, oranges, coconut water, tomatoes, chicken, and salmon.14

Editor’s note: Consult with a qualified healthcare professional or dietitian/nutritionist to determine a diet that best meets your nutritional needs. 

Sources

1. NIH National Cancer Institute website. SEER training modules. Muscle types. https://training.seer.cancer.gov/anatomy/muscular/types.html. Accessed 4 Oct 2022.

2. Dave HD, Shook M, Varacallo M. Anatomy, skeletal muscle. 5 Sep 2021. StatPearls [internet]. NIH National Library of Medicine website. https://www.ncbi.nlm.nih.gov/books/NBK537236/. Accessed 4 Oct 2022.

3. Noto RE, Leavitt L, Edens MA. Physiology, muscle. 8 May 2022. StatPearls [internet]. NIH National Library of Medicine website. https://www.ncbi.nlm.nih.gov/books/NBK532258/. Accessed 4 Oct 2022.

4. Difference Between website. Difference between Type I and Type II muscle fibers. 23 Jul 2018. https://www.differencebetween.com/difference-between-type-1-and-type-2-muscle-fibers/. Accessed 4 Oct 2022.

5. Physiopedia website. Muscle fibre types. https://www.physio-pedia.com/Muscle_Fibre_Types. Accessed 4 Oct 2022.

6. Biology Dictionary editors. Smooth muscle. 4 Oct 2019. Biology Dictionary website. https://biologydictionary.net/smooth-muscle/. Accessed 4 Oct 2022.

7. Britannica website. The muscle proteins. https://www.britannica.com/science/protein/The-muscle-proteins. Accessed 5 Oct 2022.

8. Physiopedia website. Protein intake and muscle function in older adults. https://www.physio-pedia.com/Muscle_Function_and_Protein. Accessed 5 Oct 2022.

9. Harvard TH Chan School of Public Health website. Calcium. https://www.hsph.harvard.edu/nutritionsource/calcium/#:~:text=Calcium%20is%20a%20mineral%20most,heart%20rhythms%20and%20nerve%20functions. Accessed 6 Oct 2022.

10. Gröber U, Schmidt J, Kisters K. Magnesium in prevention and therapy. Nutrients. 2015;7(9):8199–8226.

11. Raman R. What does magnesium do for your body? 9 Jun 2018. Healthline website. https://www.healthline.com/nutrition/what-does-magnesium-do#other-benefits. Accessed 6 Oct 2022.

12. University of Rochester Medical Center website. Health encyclopedia. Potassium. https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid= 19&contentid=potassium. Accessed 6 Oct 2022.

13. Davidson K. Symptoms of low potassium (hypokalemia). Last updated 25 Jan 2022. Healthline website. https://www.healthline.com/nutrition/potassium-deficiency-symptoms. Accessed 6 Oct 2022.

14. Harvard TH Chan School of Public Health website. The nutrition source. Potassium. https://www.hsph.harvard.edu/ nutritionsource/potassium/. Accessed 6 Oct 2022.   

Written by NHR Staff

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