The human body has well over 600 distinct muscles that control just about every function we perform, from our motor skills to blinking to breathing. Each muscle contains a unique ratio of muscle fiber types that defines its use and purpose. These muscle fiber types change during human growth and development, and also in cases of neuromuscular disease and dysfunction. Let's review the various types of muscle fibers in the body, what weakens them, what strengthens them, and what you can do to influence and benefit your own muscle growth.
What Are Muscles? The Basics
The word muscle comes from the Latin word musculus, which translates to "little mouse." Muscle cells are made up of protein filaments (myosin and actin) that contract by sliding past one another, causing the cell itself to bunch and then lengthen back and forth. Muscles pump our blood, transport our food through the digestive tract (peristalsis), and allow us to bend, walk, reach, and move throughout the world.
There are three types of muscles: cardiac, smooth, and skeletal (or striated). These three can be classified as either voluntary or involuntary muscles. Skeletal muscle fibers can be further classified into fast-twitch and slow-twitch muscle fibers. For further specifics, read on.
Muscle Fiber Types
Here are the types of muscle fibers currently at work in your body.
1. Cardiac Muscle Fiber
The most specialized of the bunch, cardiac muscle fibers control cardiac contractions (your heartbeat).
Cardiac muscle (myocardium) is an "involuntary" muscle that is found exclusively in the heart. It contracts on its own, usually regularly, except in cases of skipped or extra beats (palpitations), which are interruptions to the heart's rhythm that are common among people of all ages and all health conditions, though sometimes they are caused by outside influences such as fatigue, stress, or caffeine or alcohol consumption.
Cardiac muscle is also known as a "striated" muscle because it is made up of sarcomeres, the basic structural unit of the muscle. Sarcomeres are divided into bands of filaments made of myosin or actin (otherwise known as myofibrils, the elongated contractile threads in muscle cells). These parallel bundles appear as stripes under a microscope, hence the word striated, which can also refer to anything marked with striations like scratches, grooves, or channels in a parallel series, such as the furrows and lines in rock walls.
Striated muscles contract and relax in quick bursts, while smooth muscles maintain prolonged and nearly permanent contractions.
2. Smooth Muscle Fiber
The other "involuntary" muscle type is smooth muscle. Smooth muscle fibers are those that apply pressure in the blood vessels and organs such as the bladder, uterus, intestines, esophagus, stomach, bronchi, urethra, and the arrector pili (small muscles that contract to make each of your hairs stand on end).
Just as with your heart muscle, you cannot consciously control these muscles. They act on a deeper physiological authority.
3. Skeletal Muscle Fiber
Skeletal muscles are attached to your skeleton and are "voluntary" muscles because under normal circumstances you can control them. These muscles are also striated and are enveloped in three layers of connective tissue (mysia) that wrap around and enclose entire muscles and also compartmentalize the fibers within.
The outermost layer of the mysia is called the "epimysium," which allows for contractions, provides structure, and isolates the skeletal muscles from surrounding organs. The innermost layer is the "endomysium," which surrounds each muscle fiber. The middling layer is the "perimysium," which wraps specific groups of muscle fibers to form a "fascicle" or bundle. These divisions allow for precision of movement, especially in the limbs, so that the nervous system can trigger isolated muscle fiber subsets.
A person's body is made up of between 36% (women) and 42% (men) skeletal muscle mass. Each of these muscles is connected to blood vessels that supply oxygen, nourishment, and waste removal. However, there is further distinction among skeletal muscle fiber types.
- Type I, slow-twitch: Also known as "red" muscle, this type of muscle fiber is full of capillaries and myoglobin (heme protein that stores and carries oxygen). Because they can hold more oxygen, slow-twitch muscle fibers contract for longer periods of time, and can be further developed through high-intensity endurance training.
Type II, fast-twitch: Fast-twitch muscles contract powerfully but in shorter bursts (think of the difference between a marathon runner and a sprinter). There are three subtypes of fast-twitch muscle fibers.
- Type IIa fibers: Quick-firing and more powerful than type I fibers, these fibers are used in heavy weight lifting and sprinting. They also produce lactic acid (more on this in the exercise section below).
- Type IIx fibers: More powerful than type IIa fibers, type IIx fibers are nevertheless more energy inefficient, and only useful in instances of very short duration. They are sometimes called "couch potato" muscle fibers, available even to those who are sedentary for use in an emergency (as in the need to run away from a threat or lift something critical to save a loved one from being crushed).
- Type IIb fibers: This is the "white" muscle that has the least density of myoglobin and therefore relies principally on bloodless anaerobic metabolism.
The fast-twitch muscle fibers have the most potential to grow and develop as you work out and exercise, but before we discuss how to better build muscle, let's quickly cover what can afflict the various muscle fiber types.
Muscle Fiber Types and Neuromuscular Diseases
There are certain afflictions of the musculature that are caused by functional defects either through an indirect disruption of the nervous system's communications or a form of muscle pathology at the cellular level. What follows is not a complete list of neuromuscular diseases, but it is an indication of the types of diseases that are largely debilitating, as they can lead to wasting and muscle atrophy.
- Muscular dystrophies: These diseases (including Duchenne muscular dystrophy, congenital muscular dystrophies, limb-girdle muscular dystrophies and more) bring about muscle wasting due to muscle fiber necrosis. Certain genetic mutations cause a change to both the quality and the force production of type I and type II muscle fibers. The result of this dysfunction is apoptosis (cell death) and necrosis (organ death).
- Sarcopenia: Skeletal muscle loss due to aging is known as sarcopenia, and often leads to difficulty walking and increased skeletal weakness. This is mostly characterized by a loss of cells related to fast type II muscle fibers, but slow muscle fibers (type I) may experience a loss of strength as well.
- Amyotrophic lateral sclerosis (ALS): Also known as Lou Gehrig's disease, this neurodegenerative condition is fatal. It involves the degeneration and loss of motor neurons in the brain stem and spinal cord resulting in a rapid decrease of motor function, muscle atrophy, and eventual death. The exact cause of this condition is unknown but believed to stem from multiple factors including aging, lifestyle, environmental exposures, and genetic predisposition.
Muscle Fiber Types and Exercise
Diseases of the musculature are often incurable, but that is all the more reason to value and prioritize healthy, functional muscles as long as you have them. Exercise can increase the growth (not the number) of fast-twitch fibers and help strengthen fatigue-resistant fast fibers.
So without further ado, here's a breakdown of what workouts target which muscle fiber types and tips on what you can do to better support muscle function.
Aerobic Exercise and Slow-Twitch Fibers
It's widely known that exercise helps increase muscle and bone strength and can improve one's balance, motor skills, and fitness. Aerobic exercise entails lower levels of exertion over longer periods of time. Examples include the classic marathon, but also activities like walking, hiking, jogging, running, spinning, swimming, dancing, kickboxing, cross-country skiing, and the use of cardio machines.
From the Greek aēr meaning "air" and bios meaning "life," aerobic indicates exercises intended to utilize free oxygen and improve the body's efficiency at absorbing and transporting that oxygen. These athletic events use a far higher amount of slow-twitch muscle fibers, which fuel themselves with a mix of carbs, protein, and fat, but with a low-glycolytic and high-oxidative capacity (lower use of quickly depleted sugar energy from carbs, higher use of longer-term fuel from fat stores for muscle contractions).
Aerobic workouts utilizing slow-twitch muscle fibers consume large amounts of oxygen but produce very little lactic acid (which contributes to delayed onset muscle soreness).
Bodybuilding, Hypertrophy, and Fast-Twitch Fibers
One of the most apparent effects of anaerobic exercise (activity that is either absent of or not targeted towards oxygen use) is muscle hypertrophy, meaning muscle increase and bodybuilding. Fast-twitch muscle fibers are largely used for short, intense bursts of energy, like sprinting, jumping, and weight lifting. These actions rely on glucose (sugar) or ATP (adenosine triphosphate) for energy, and consume very little fat, protein, or oxygen, but produce a lot of lactic acid as a result.
Lactic acid becomes an issue not only in later muscle soreness, but also in inhibiting ATP generation. Intense anaerobic activity also redistributes potassium ions within the muscle, which interrupts water and nutrient transport as potassium is an electrolyte. The lack of sufficient potassium can lead to muscle cramps if levels are off-balance.
Furthermore, anaerobic strength training exercises cause microtears in the muscle, tears which do lead to increased muscle mass when they are repaired, but also cause pain and sometimes a delay in training if the repair takes too long.
As a general rule, men find muscle-building easier to achieve than women thanks to the growth hormone advantage of having more testosterone, but every bodybuilder experiences the same struggles when it comes to increasing and maintaining type II fiber growth in their muscles. There are ways to boost your advantage.
Amino Acids, Lactic Acid, and Muscle Support
Not all physical activity exists in a binary world between aerobic and anaerobic, some (like rock climbing or soccer) require both. How can you naturally support both types of muscle fiber in each individual muscle for comprehensive coverage during your workout? Here's how.
1. Amino Acids
Improving amino acid utilization has been found to help attenuate sarcopenia, in a sense reverse the effects of aging and atrophy on human muscles. Moreover, when it comes to building new muscle cells, essential amino acid support (including the branched-chain amino acids known well among bodybuilders) is crucial—if you lack any one of the necessary nine aminos, your body may catabolize (cannibalize) healthy muscle tissue to make up for what it needs.
The essential amino acids required to repair and build new muscle include:
- Histidine: Important in reducing inflammation and protecting the myelin sheathing that keeps the central nervous system operational.
- Isoleucine: One of the three branched-chain amino acids (BCAAs), isoleucine is needed in wound-healing, energy utilization in the muscles, and recovery.
- Leucine: Another BCAA, leucine assists directly in protein synthesis and tissue repair when it comes to muscle building.
- Lysine: Important in protein creation and in calcium absorption for strong bones to match your strong skeletal muscles.
- Methionine: A precursor for protein synthesis, methionine also has anti-inflammatory, antioxidant, and analgesic abilities.
- Phenylalanine: Necessary as a precursor to the amino acid tyrosine, which helps moderate thyroid hormones and chemicals including epinephrine, norepinephrine, and dopamine.
- Threonine: An agent of the cardiovascular and central nervous systems, threonine is also a precursor of both serine and glycine, needed for the muscle tissue, collagen, and elastin creation necessary for supple connective tissues like our blood vessels and tendons.
- Tryptophan: A precursor to serotonin, the "happy hormone" that is also needed for proper mood regulation, digestion, and the balance between protein breakdown and synthesis in the body.
- Valine: The final BCAA, valine is needed for muscle metabolism, tissue repair, and blood sugar control.
When supplementing with amino acids, don't just stop at BCAAs, because an imbalance of the nine essential amino acids (EAAs) can be counterproductive if your body has to rip those missing aminos from other cells.
Here are a few proven helpers for cutting down on lactic acid buildup and increasing your workout potential.
- Potassium: Necessary for transmitting nerve signals and regulating muscle contractions, potassium helps you avoid muscle cramps.
- Magnesium: In clinical studies, magnesium supplementation has been shown to increase athletic performance by way of lowering lactic acid levels.
- Calcium: Research reveals that calcium taken before workouts can help prolong endurance and increase bone strength.
Drinking orange juice before a workout has also been shown to help lower levels of lactic acid and improve physical performance, but more on that below.
3. Essential Vitamins
Don't miss out on these key vitamins for workouts and muscle fiber support.
- Vitamin C: Found easily in orange juice and citrus fruits and a valuable antioxidant for the immune system, vitamin C helps reduce lactic acid and supports collagen production necessary for the repair of connective tissue.
- Vitamin D: The most relevant uses of vitamin D for muscle health is first that it helps boost calcium absorption, and second that it plays an independent role in muscle protein synthesis.
- Vitamin E: Often found in beauty products to enhance the suppleness of skin, vitamin E is also an antioxidant that cuts down on free radical damage and inflammation due to oxidative stress. Both skills are useful for workout recovery and muscle building.
Fuel That Fiber
When it comes to muscle fibers, there are many types at work in your body all the time in an automatic capacity. The muscle fiber types you have control over are the voluntary ones, your skeletal muscles that can be built up and improved by how you exercise and how you supplement. We encourage you to take advantage of the miracle that is a healthy and functioning musculature and treat your muscles well by giving them the fuel and purpose they need.