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Muscle hypertrophy and atrophy are opposing conditions that refer to the increase and decrease in muscle mass, respectively. Hypertrophy is a common phenomenon in males during puberty, resulting from increased testosterone, a major growth hormone. It can be induced by strength training or anaerobic training, and is characterised by an increase in the size of muscle cells. Conversely, atrophy is caused by disuse, malnutrition, neurogenic conditions, genetics, or certain medical conditions, and leads to a decrease in muscle mass and strength. Understanding the mechanisms of these conditions can aid in developing novel treatments for muscle disorders.
| Characteristics | Values |
|---|---|
| Muscle Hypertrophy | Increase in muscle fiber cross-sectional area, volume, and mass |
| Increase in the size of a muscle through an increase in the size of its component cells | |
| Occurs in response to a higher load on the muscle | |
| Occurs at an increased rate during puberty in males | |
| Occurs more commonly in fast-twitch than in slow-twitch muscles | |
| Type 2A fibers exhibit the greatest growth | |
| Occurs after 6 to 7 weeks of resistance training | |
| Can be induced by strength training or anaerobic training | |
| Can be induced by testosterone or anabolic steroids | |
| Muscle Atrophy | Wasting or thinning of muscle mass |
| Loss or thinning of muscle tissue | |
| Decrease in muscle mass and strength | |
| Occurs due to disuse of muscles, neurogenic conditions, malnutrition, age, genetics, lack of physical activity, or certain medical conditions | |
| Can be caused by nerve problems or diseases | |
| Can be caused by systemic disease, prolonged immobilization, or microgravity |
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What You'll Learn
Muscle atrophy causes
Muscle atrophy refers to the wasting or thinning of muscle mass. It is characterised by a decrease in muscle mass, weakness, and numbness or tingling in the limbs. There are three types of muscle atrophy: physiologic, pathologic, and neurogenic. Each type has distinct causes that contribute to the deterioration of muscle tissue.
Physiologic atrophy, also known as disuse atrophy, is caused by insufficient muscle activity. This can be the result of a sedentary lifestyle, malnutrition, or health problems that limit movement. For example, individuals with seated jobs or those who are bedridden may be prone to physiologic atrophy. The condition can develop within two to three weeks of muscle disuse and is often reversible with exercise and improved nutrition.
Pathologic atrophy is associated with aging, starvation, and certain diseases. Aging-related atrophy, known as sarcopenia, leads to a loss of skeletal muscle mass and strength, often accompanied by osteoporosis. Starvation or malnutrition can also contribute to pathologic atrophy by depriving the body of the nutrients necessary for muscle maintenance. Additionally, specific diseases, such as Cushing disease, can cause pathologic atrophy due to the overconsumption of corticosteroids or overactive adrenal glands.
Neurogenic atrophy is the most severe form of muscle atrophy and is caused by injuries or diseases affecting the nerves that connect to the muscles. When these nerves are damaged, they cannot trigger the muscle contractions required for muscle activity. As a result, the muscles begin to break down, leading to a decrease in size and strength. Examples of conditions associated with neurogenic atrophy include amyotrophic lateral sclerosis (ALS), carpal tunnel syndrome, and muscular dystrophy.
In summary, muscle atrophy can be caused by a combination of inactivity, malnutrition, aging, and underlying health conditions. Depending on the type of atrophy and its severity, interventions such as exercise, nutritional improvements, and medical treatments may help slow or reverse the atrophy.
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Muscle hypertrophy vs hyperplasia
Muscle atrophy refers to the wasting or thinning of muscle mass. It can be caused by muscle disuse, neurogenic conditions, malnutrition, age, genetics, or certain medical conditions. On the other hand, muscle hypertrophy is the process of increasing muscle mass through the enlargement of existing muscle cells. Hypertrophy can be induced by strength training or anaerobic training, and biological factors such as age and nutrition can influence this process.
Hypertrophy specifically refers to the increase in the size or diameter of individual muscle fibres, resulting in thicker and more powerful muscle fibres. This can be achieved by increasing the size of contractile proteins or the fluid and enzyme content within the muscle cell. The most prolific stimulus for muscle hypertrophy is a well-designed resistance exercise program with sufficient volume and intensity. Additionally, an adequate supply of amino acids is essential for muscle growth, as they provide the building blocks for new protein synthesis.
Muscle hyperplasia, on the other hand, is a controversial process that refers to the formation of new muscle cells, increasing the number of muscle fibres. It is important to distinguish skeletal muscle hyperplasia from other types, as uncontrolled cellular proliferation in other tissues is often associated with tumour growth. While hyperplasia has been observed in animals, there is limited evidence of its occurrence in humans. Some researchers argue that muscle growth in humans is primarily achieved through hypertrophy rather than hyperplasia.
In summary, muscle hypertrophy and hyperplasia represent two distinct mechanisms of muscle growth. Hypertrophy involves increasing the size of individual muscle fibres, while hyperplasia involves the formation of new muscle fibres. While hypertrophy is a well-established process in humans, the existence and significance of hyperplasia in human muscle growth remain a subject of ongoing debate and further research.
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Factors affecting hypertrophy
Muscle hypertrophy is an increase in muscle size through an increase in the size of its component cells. This phenomenon is commonly known as being "pumped up" or getting "a pump". It is important to note that muscle hypertrophy is distinct from muscle hyperplasia, which involves the formation of new muscle cells.
Training Variables
Training variables such as frequency, intensity, and total volume directly influence the development of muscle hypertrophy. A gradual increase in these variables contributes to muscular hypertrophy. Range of motion (ROM) is another factor, with training through a full ROM, particularly at elongated muscle lengths, enhancing hypertrophy compared to partial ROM. For instance, exercises like deep squats and full-ROM deadlifts increase mechanical tension on muscle fibers, stimulating greater muscle growth.
Type of Training
The type of training can induce hypertrophy through increased sarcoplasmic volume or increased contractile proteins. Strength training or resistance training causes neural and muscular adaptations, increasing the capacity to exert force through voluntary muscular contraction. After an initial period of adaptation, the muscle tissue expands by creating sarcomeres (contractile elements) and increasing non-contractile elements like sarcoplasmic fluid. Blood flow restriction training (BFR) is another form of training that induces hypertrophy by partially restricting blood flow to the working muscles during low-load resistance exercises. This method is beneficial for individuals who cannot handle high mechanical loads, such as those recovering from injuries.
Biological Factors
Biological factors such as age, sex, and nutrition can influence muscle hypertrophy. For example, testosterone, a major growth hormone, makes it easier for men to achieve hypertrophy than women. Additionally, an adequate supply of amino acids is essential for muscle hypertrophy, with protein intakes of 1.2-1.8 g per kilogram of body weight recommended for athletes.
Genetic Factors
Genetics play a role in muscle hypertrophy. The activin/myostatin and BMP pathways are involved in regulating muscle growth and counteracting atrophy. The inhibition of myostatin/activins and the consequent reduction of phosphorylated Smad2/3 release Smad4, which interacts with phosphorylated Smad1/5/8 to sustain growth.
Metabolic Stress and Mechanical Tension
Metabolic stress and mechanical tension are also factors in exercise-induced muscle growth. Bodybuilders typically train with moderate loads and short rest intervals, inducing high metabolic stress. In contrast, powerlifters train with high-intensity loads and longer rest periods. While both groups exhibit muscularity, the optimal protocol for maximizing muscle growth is still under investigation.
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Reversing muscle atrophy
Muscle atrophy refers to the wasting or thinning of muscle mass. It can be caused by muscle disuse, malnutrition, age, genetics, neurogenic conditions, or other medical conditions. The good news is that physiologic atrophy, or muscle atrophy caused by physical inactivity, can be reversed.
To support muscle growth, it is crucial to consume a sufficient amount of protein. A high-protein diet of 25 to 40 grams of protein per meal is recommended to help maintain muscle mass. This can be complemented by an adequate intake of other essential nutrients to promote overall health and muscle development.
In addition to exercise and nutrition, passive movements can also aid in reversing muscle atrophy. These passive movements can include assisted walking or even bathing, which may seem exhausting for individuals experiencing muscle weakness but are crucial for regaining strength and rebuilding muscle.
It is important to remember that reversing muscle atrophy requires a consistent and disciplined approach. The process may be challenging, but with perseverance and a combination of the aforementioned strategies, it is possible to regain muscle strength and size.
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Types of muscle atrophy
Muscle atrophy is the wasting or thinning of muscle mass. It can be caused by various factors, including disuse of muscles, neurogenic conditions, malnutrition, age, genetics, and medical conditions. The specific causes and treatments depend on the type of muscle atrophy. Here are the four main types of muscle atrophy:
Disuse (Physiologic) Atrophy
Disuse atrophy occurs when muscles are not used enough, leading to a decrease in size and strength. This can be caused by leading a sedentary lifestyle, malnutrition, lack of exercise, desk jobs, or certain genetic disorders. Treatment for disuse atrophy involves regular exercise and improved nutrition. Physiologic atrophy is usually reversible but requires consistent effort and time to see improvements.
Neurogenic Atrophy
Neurogenic atrophy is caused by injuries or diseases that affect the nerves connecting to the muscles. When these nerves are damaged, they cannot trigger the muscle contractions necessary for muscle activity. Neurogenic atrophy typically cannot be reversed due to the physical damage to the nerves. However, it can be treated with a specific type of physical therapy called electrical stimulation.
Pathologic Atrophy
Pathologic atrophy occurs as a direct result of various diseases, cancers, severe infections, and malnutrition. The treatment for pathologic atrophy depends on the underlying cause and may involve addressing the disease or condition causing the atrophy.
Age-Related Atrophy (Sarcopenia)
Sarcopenia is the age-related loss of skeletal muscle, resulting in frailty. It is often associated with osteoporosis, a loss of bone that also occurs during the aging process. Treatment for age-related atrophy may include exercise programs, physical therapy, and nutritional interventions to slow or manage the progression of muscle loss.
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Frequently asked questions
Muscle atrophy is the wasting or thinning of muscle mass.
Muscle atrophy can be caused by disuse of muscles, neurogenic conditions, malnutrition, age, genetics, or certain medical conditions.
Symptoms of muscle atrophy include a decrease in muscle mass, weakness, numbness, and tingling in the limbs.
Yes, disuse atrophy can be reversed with exercise and a healthy diet.
Muscle hypertrophy is the increase in muscle size through an increase in the size of its component cells.
