Muscle Physiology 101: Understanding How Muscles Work

When people inquire about muscle function, they are typically referring to skeletal muscle. While skeletal muscle is just one of the three main categories of muscle, it is the one that athletes try to optimize in terms of function and performance. Understanding how skeletal muscles work can make it easier to make nutrition and training decisions.

What are the three types of muscle?

The three types of muscle tissue are skeletal, cardiac, and smooth muscle.

Characteristics of the three types of muscle:

Skeletal muscle has four properties:

• Excitability: Skeletal muscle can receive and respond to nerve stimuli. The brain sends impulses to skeletal muscle, which responds by contracting or lengthening.
• Extensibility: Skeletal muscles can be stretched or extended.
• Elasticity: Skeletal muscle can return to its original length once it is relaxed.
• Contractility: Skeletal muscle can forcibly shorten when it is stimulated by a nerve impulse.

How is skeletal muscle structured?

Skeletal muscle cells are long and thin, extending from one end of the muscle to the other. Each muscle fiber is wrapped in connective tissue. This connective tissue surrounds the muscle fiber and the extracellular fluid that provides nutrients to the muscle fibers.

Muscle fibers are bundled together into fascicles. Another layer of connective tissue surrounds each fascicle. The fascicles are bundled together into a muscle. A final thick layer of connective tissue surrounds the muscle. The connective tissue surrounding the muscle intertwines with the connective tissue in tendons, which allows the muscle to contract and pull the bone in a smooth motion.

Blood vessels supply muscle tissue with nutrients and oxygen. Nerve fibers stimulate the muscle cells to contract, and muscle spindles are distributed throughout the muscle to provide feedback to the nervous system. The sarcoplasmic reticulum is a network of tubules inside the muscle cell that stores and releases calcium ions. Calcium is essential for muscle contraction.¹

Inside a Muscle Cell

Skeletal muscle has a striated appearance due to the orderly arrangement of actin and myosin in the muscle cell.

A sarcomere is a unit of muscle cells. It extends from one Z disc to the next. The Z disc bisects the actin filaments (thin filaments). The myosin filaments extend across the sarcomere but do not reach the Z disc. There is overlap between the actin and myosin fibers, so the myosin can grab onto the actin and pull the Z discs toward the center of the sarcomere. The muscle contracts when thousands of sarcomeres shorten as the actin slides past the myosin.

Besides actin and myosin, two other filaments are important in muscle contraction. Tropomyosin stiffens actin and blocks the myosin binding site when the muscle is relaxed. This keeps the myosin head from attaching to actin.

Troponin is a protein with three binding sites: one for actin, one for tropomyosin, and the last for calcium. When calcium binds to troponin, it changes its configuration. This pulls tropomyosin off the myosin binding site.

The Sliding Filament Model of Contraction

Muscle contraction is an orderly process that involves the following steps:

1. Cross bridge attachment: The myosin head binds to actin.
2. Myosin head pivots: The myosin head uses energy from adenosine triphosphate (ATP) to pivot and pull the actin toward the center of the sarcomere.
3. Myosin head detaches: Another ATP binds, which provides the energy to detach the myosin head from the actin.
4. Cocking the myosin head: The myosin head pivots again to get ready to bind to actin again.

The movement of actin and myosin in a sarcomere is very similar to the game of tug of war. The main difference is that in a muscle sarcomere, myosin pulls actin toward the center of the sarcomere.

How do the nervous system and muscles work together to contract the muscle?

The neuromuscular junction is the point where the nerve that stimulates muscle contraction meets the muscle fiber.

When a change in the electrical gradient along its membrane excites a motor neuron or nerve cell, it carries a nerve impulse down its axon. At the end of the axon are vesicles that contain acetylcholine.

When a nerve impulse is carried down the axon of the motor neuron, and acetylcholine is released, it opens sodium channels on the muscle cell’s membrane. This depolarizes or changes the electrical potential across the muscle cell membrane.

The depolarization wave spreads along the muscle cell membrane and into deep indentations called T-tubules in the muscle cell membrane. This causes gates to open in the sarcoplasmic reticulum and release calcium into the muscle cell.

Once calcium is free in the muscle cell, it binds to troponin. Tropomyosin is pulled off the myosin binding site. The myosin heads bind to actin and pull the actin myofilaments toward the center of the sarcomere, and the muscle shortens.

ATP-dependent calcium pumps pump the calcium back into the sarcoplasmic reticulum to end the contraction. Tropomyosin moves back over the myosin binding site, and the contraction ends.

Sources of ATP

Without calcium and ATP, a muscle cell cannot contract. ATP provides the energy needed for many steps in the nerve-muscular contraction process. Muscle cells store enough ATP for about 4 to 6 seconds of energy.

There are three ways ATP can be regenerated in muscle cells:

• Creatine phosphate: Creatine phosphate can hold the energy in a high-energy phosphate bond and transfer the energy to ADP to make ATP. Creatine phosphate provides a small amount of energy to power a muscle cell for about 15 seconds.
• Anaerobic metabolism: This process is faster than aerobic metabolism but incompletely breaks down glucose, resulting in an oxygen debt that must be paid back. Muscles using anaerobic metabolism will fatigue within one to two minutes.
• Aerobic metabolism: This process is the complete breakdown of glucose to provide ATP, carbon dioxide, and water. It is a relatively slow process but provides the most ATP per glucose molecule. Aerobic endurance is the amount of time muscles can contract using aerobic pathways. Low-level, long-term exercise will stay below the aerobic threshold. Under this threshold, muscle cells can access enough oxygen to fully metabolize glucose.

Muscles fatigue when there is a buildup of potassium ions outside the muscle cells and an increase in hydrogen ions, ADP, and inorganic phosphate. Fuel stores begin to decline. Lactate levels increase, but they are not a cause of muscle fatigue. Lactate is a fuel source that is processed in your liver and kidneys. Once you stop exercising, your body will metabolize lactate.

Type of Muscle Contractions and Fibers

Muscles can contract in several ways, including:

• Isometric contraction: Tension in the muscle increases, but the muscle does not shorten.
• Isotonic contraction: The muscle changes in length, but the muscle tension remains constant.
  • Concentric contraction: Muscle contraction while shortening.
  • Eccentric contraction: Muscle contractions while lengthening. These contractions are more likely to cause muscle tears and delayed-onset muscle soreness (DOMS).^2,3

When muscle fibers contract, they do not act alone. A motor unit contains one motor neuron, and many muscle fibers spread throughout a muscle.

In addition to different contraction types, there are different types of muscle fibers based on their preferred energy source.

• Type 1: Slow oxidative or slow-twitch fibers that are rich in capillaries, myoglobin, and mitochondria can sustain prolonged aerobic exercise. These fibers contract slowly and are fatigue-resistant.
• Type II: Fast-twitch fibers:
  • IIa fast-oxidative fibers are rich in mitochondria and capillaries. These fibers contract quickly and are moderately fatigue-resistant.
  •  IIx fast-glycolytic fibers are fast and can sustain short, anaerobic bursts of activity. They are low in mitochondria and myoglobin and fatigue quickly.

Muscle cells recruit slow oxidative fibers first, followed by fast oxidative and glycolytic fibers.

Resting muscle tone makes it easier for muscle cells to shorten because they do not have to fully take up slack in a muscle with each contraction. Muscle contractions build on each other, gradually increasing the amount of calcium available in muscle tissue. This increases the strength of muscle contractions.

Muscle Response to Exercise

Physical training stresses muscle cells, causing them to hypertrophy by thickening myofilaments and increasing structural proteins in muscle fibers.

Endurance/Aerobic Exercise

Endurance exercise relies more heavily on slow fibers that use aerobic metabolism. Muscles respond to endurance exercise by producing more mitochondria to produce ATP, increasing myoglobin in muscle cells to store oxygen, and increasing the capillary network around muscle fibers.

Regular endurance exercise can improve physical and mental stamina, improve cardiovascular fitness, reduce fatigue, and improve mood.

After exercise, some people develop muscle cramps. While the cause is unknown, restoring electrolytes and fluids during and after exercise may reduce their occurrence. Recovery is an important aspect of muscle training.  

Resistance Exercise

Resistance exercise requires fast glycolytic fibers to produce short, powerful contractions. Repeated resistance exercise triggers muscle cells to produce more myofibrils and increase in thickness. Resistance exercise also triggers the production of more connective tissue around muscle cells.

Muscle hypertrophy is an increase in the size and volume of skeletal muscle fibers. Each individual muscle fiber can enlarge as long as sufficient protein is available. Muscle cells can also increase glycogen storage.

Protein consumption and strength training are essential to achieve muscle hypertrophy. Muscle protein synthesis must exceed muscle protein breakdown in order to build muscle. Many factors influence how quickly strength and muscle hypertrophy are gained, including muscle actions, intensity, exercise volume, exercise selection and order, rest periods between sets, and frequency.⁴

Force generation depends on motor unit activation. Slower, lower-force-producing motor units are usually activated before faster, higher-force-producing ones. Resistance training activates more motor units.

Muscle units (nerves and corresponding muscle fibers) are recruited based on their size and characteristics, according to the Henneman size principle. Smaller units are recruited before medium and larger units to ensure a range of forces and a smooth gradation of force output.⁵

Muscle strength and hypertrophy gains become more difficult as growth hormone levels decrease with age. Sermorelin aids in releasing natural growth hormones from your pituitary gland, working alongside your body’s natural feedback mechanisms. Incorporating Sermorelin into your muscle strength and hypertrophy program may make an enormous difference in your potential gains. Sermorelin has many potential benefits for both men and women.

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Disclaimer

While we strive to always provide accurate, current, and safe advice in all of our articles and guides, it’s important to stress that they are no substitute for medical advice from a doctor or healthcare provider. You should always consult a practicing professional who can diagnose your specific case. The content we’ve included in this guide is merely meant to be informational and does not constitute medical advice.

References

1. McCuller C, Jessu R, Callahan AL. Physiology, Skeletal Muscle. [Updated 2023 Jul 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537139/

2.Cheung K, Hume P, Maxwell L. Delayed onset muscle soreness : treatment strategies and performance factors. Sports Med. 2003;33(2):145-64. doi:10.2165/00007256-200333020-00005

3. Hody S, Croisier J-L, Bury T, Rogister B, Leprince P. Eccentric Muscle Contractions: Risks and Benefits. Review. Frontiers in Physiology. 2019-May-03 2019;10doi:10.3389/fphys.2019.00536

4. American College of Sports Medicine. American College of Sports Medicine position stand. Progression models in resistance training for healthy adults. Med Sci Sports Exerc. 2009 Mar;41(3):687-708. doi: 10.1249/MSS.0b013e3181915670. PMID: 19204579.

5.  Foundations of Fitness Programming. National Strength and Conditioning Association. 2015

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