Dominic Stone
Skeletal muscle atrophy and hypertrophy are opposing conditions that refer to the decrease and increase, respectively, of skeletal muscle mass. The causes of these conditions vary and can include factors such as exercise, diet, genetics, nerve problems, disease, and ageing. Understanding the mechanisms behind skeletal muscle atrophy and hypertrophy is essential for developing novel treatments and therapies for muscle-related conditions.
| Characteristics | Values |
|---|---|
| Muscle hypertrophy | Increase in muscle mass |
| Causes of muscle hypertrophy | Strength training, high-intensity interval training (HIIT), strenuous anaerobic activity, microtrauma from weight training |
| Muscle atrophy | Loss of muscle mass |
| Causes of muscle atrophy | Malnutrition, age, genetics, lack of physical activity, neurogenic conditions, nerve problems, disuse atrophy, certain medications |
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Muscle disuse
Muscle atrophy or muscle wasting is the loss of muscle mass. It can be caused by muscle disuse or neurogenic conditions. Physiologic atrophy or muscle disuse atrophy occurs when muscles are not used enough. When muscles are not used, the body stops wasting energy on them and starts breaking them down, leading to a decrease in muscle size and strength.
Disuse atrophy is more common in people who are malnourished or do not get enough exercise. It can severely limit a person's mobility, leading to a cycle of muscle disuse and atrophy. Certain viral infections or autoimmune conditions can cause myositis, which is the inflammation of muscles, resulting in muscle weakness and pain. Polio, an infectious disease that attacks the nervous system, can also lead to muscle atrophy.
The amount of time it takes for muscles to atrophy depends on age, fitness level, and the cause of atrophy. Disuse atrophy can be reversed with exercise and a healthy diet. Healthcare providers can recommend a plan to help rebuild muscle mass and strength.
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Neurogenic conditions
Neurogenic atrophy occurs due to nerve problems or diseases. It is caused by an injury or disease affecting nerves that connect to the muscles. When these nerves are damaged, they can't trigger the muscle contractions necessary to stimulate muscle activity. Neurogenic atrophy can be caused by a variety of diseases and conditions, including:
- Amyotrophic lateral sclerosis (ALS)
- Guillain-Barre syndrome
- Carpal tunnel syndrome
- Spinal cord injury
- Multiple sclerosis
- Mitochondrial dysfunction
Neurogenic atrophy can also be caused by certain neurogenic disorders, such as neuropathies, radiculopathies, spinal muscular atrophy, and post-polio syndrome. Radiculopathy is a neurogenic disorder with symptoms such as pain, paresthesia, numbness, and hypotonia.
Neurogenic hypertrophy is a rare condition that can be caused by neurogenic disorders. Radiculopathy is one of the neurogenic disorders that can cause neurogenic hypertrophy. In this case, the patient experiences pain and calf hypertrophy.
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Ageing
Skeletal muscle atrophy is an inevitable part of ageing. Age-related muscle atrophy, also known as sarcopenia, is characterised by a loss of muscle mass and strength. Sarcopenia is a serious problem that can greatly impact one's quality of life, making it difficult to perform basic daily activities such as getting out of chairs, opening jars, or carrying groceries. The likelihood of developing sarcopenia increases with age, with studies suggesting that somewhere between 11% and 50% of people aged 80 and older have the condition.
The process of losing muscle mass typically begins in one's 30s or 40s, with a loss of about 3-5% of muscle mass every decade. This muscle loss occurs due to a decrease in both the number and size of muscle fibres. As we age, our bodies undergo certain changes that contribute to the development of sarcopenia. One such change is a reduction in the production of proteins essential for muscle growth, leading to a decrease in muscle cell size. Ageing also brings about changes in hormone levels, particularly testosterone and insulin-like growth factor, which can affect muscle fibres and lead to sarcopenia.
In addition to hormonal changes, a decline in nerve cells responsible for transmitting signals from the brain to the muscles can also contribute to age-related muscle atrophy. This reduction in nerve cells impairs the body's ability to initiate movement, further exacerbating muscle loss. Furthermore, oxidative stress and a decrease in mitochondrial content and function during ageing may also play a role in skeletal muscle atrophy.
While ageing is the dominant factor in the development of sarcopenia, physical inactivity and diet can also contribute to the condition. Leading a sedentary lifestyle or not exercising enough can accelerate muscle loss. Additionally, a poor-quality diet, particularly one low in protein, is suspected to be a contributing factor. This is because the body's ability to convert protein into energy decreases with age, and insufficient protein intake can further exacerbate this issue.
Although sarcopenia is a common condition in older adults, it is not an inevitable consequence of ageing. Staying active and maintaining a healthy diet can significantly reduce the risk of developing sarcopenia. Resistance training and increased protein intake are often recommended as part of the management and treatment for sarcopenia, helping to improve muscle mass, strength, and functionality.
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Genetic disorders
Muscle atrophy is the wasting or thinning of muscle mass. It can be caused by disuse of muscles, neurogenic conditions, malnutrition, age, genetics, or certain medical conditions. Disuse atrophy occurs when muscles are not used enough, leading to a decrease in size and strength. Neurogenic atrophy, on the other hand, is caused by nerve problems or diseases that affect the nerves connecting to the muscles, impairing their ability to contract and stimulate muscle activity.
Another example of a genetic disorder causing muscle atrophy is Charcot-Marie-Tooth disease, which can lead to disuse atrophy by impairing an individual's ability to use their muscles. This condition affects the nerves outside of the brain and spinal cord, known as peripheral nerves, causing muscle weakness and atrophy in the limbs.
In contrast, myostatin-related muscle hypertrophy is a rare genetic disorder characterised by significantly increased muscle mass and decreased body fat. It is caused by mutations in the MSTN gene, which provides instructions for producing the myostatin protein that prevents muscles from growing too big. People with this condition may have enlarged muscles identified at birth or during infancy, particularly in the thighs, calves, and upper arms. However, it is important to note that myostatin-related muscle hypertrophy does not cause any known medical or health problems and does not require treatment.
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Malnutrition
There are several mechanisms through which malnutrition causes muscle atrophy. Firstly, malnutrition can disrupt the balance between protein synthesis and protein degradation, leading to a net loss of muscle tissue. Skeletal muscles, in particular, serve as a storage site for amino acids, which are essential for muscle growth and repair. When the body is deprived of adequate protein intake, it can break down skeletal muscle to release amino acids, resulting in muscle atrophy.
Additionally, malnutrition can impair the body's ability to regulate skeletal muscle growth and atrophy. This regulation is influenced by mechanical, oxidative, nutritional, and energy stresses, as well as growth factors or cytokines. Malnutrition can disrupt these signalling pathways, leading to a decline in muscle mass. For example, specific ablation of the Smad4 protein in skeletal muscle has been shown to cause atrophy and weakness in mice.
Moreover, malnutrition can exacerbate the effects of inactivity or disuse of muscles. When muscles are not used regularly, the body may perceive this as a need to conserve energy and will start breaking down muscle tissue, leading to atrophy. Malnutrition can accelerate this process as the body will have fewer resources to maintain muscle mass.
Lastly, malnutrition can be a consequence or symptom of certain medical conditions, which in turn can cause muscle atrophy. For example, cachexia is a metabolic condition characterised by extreme weight loss and muscle atrophy, often associated with underlying diseases such as cancer, HIV, or multiple sclerosis. In such cases, malnutrition may contribute to muscle atrophy by impairing the body's ability to absorb nutrients effectively.
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Frequently asked questions
Skeletal muscle atrophy is the loss of skeletal muscle mass. This can be caused by immobility, aging, malnutrition, medications, or a wide range of injuries or diseases that impact the musculoskeletal or nervous system.
Symptoms of skeletal muscle atrophy include muscle weakness, difficulty performing physical tasks, and in some cases, difficulty swallowing or breathing. In certain cases, atrophy may cause a decrease in muscle mass, with one limb being smaller than the other.
Skeletal muscle hypertrophy is an increase in skeletal muscle mass. This can be caused by strength training, short-duration high-intensity anaerobic exercises, and progressive overload training. Certain neuromuscular diseases can also result in skeletal muscle hypertrophy.
Yes, getting good quality sleep is important for muscle recovery and growth. Additionally, consuming adequate protein is essential for building muscle, although the exact amount required for optimal muscle growth is still unclear.
In many cases, skeletal muscle atrophy due to inactivity or disuse can be reversed with regular exercise, physical therapy, and a healthy diet. However, in cases of atrophy caused by certain diseases or medical conditions, the reversibility depends on the specific condition and the individual's health status.
