Dominic Stone
Muscle contractions are a complex process that involves the nervous system, motor neurons, and muscle fibers. When a muscle contracts, it is due to a signal from the nervous system, which travels through motor neurons to the muscle, triggering chemical reactions that lead to the reorganisation of muscle fibres and subsequent contraction. This process is influenced by factors such as oxygen levels, load or force applied to the muscle, and the size of motor neurons. Understanding muscle contractions is important in various contexts, from athletic performance and muscle growth to medical conditions like insulin resistance and muscle twitching.
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What You'll Learn
Motor neurons and muscle fibres
Motor neurons are clustered in columnar, spinal nuclei called motor neuron pools (or motor nuclei). Each individual muscle fibre in a muscle is innervated by one, and only one, motor neuron. However, a single motor neuron can innervate many muscle fibres. The combination of an individual motor neuron and all of the muscle fibres it innervates is called a motor unit. The number of fibres innervated by a motor unit is called its innervation ratio.
Motor neurons use a rate code to signal the amount of force to be exerted by a muscle. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the motor unit generates. This phenomenon is called wave summation. Eventually, the frequency of action potentials would be so high that there would be no time for the muscle to relax, and it would remain totally contracted, a condition called tetanus.
Motor units vary in size. Small motor units innervate small "red" muscle fibres that contract slowly and generate relatively small forces. Because of their rich myoglobin content, plentiful mitochondria, and rich capillary beds, such small red fibres are resistant to fatigue. These small units are called slow (S) motor units and are especially important for activities that require sustained muscular contraction, such as the maintenance of an upright posture.
Larger motor units innervate larger, pale muscle fibres that generate more force. However, these fibres have sparse mitochondria and are therefore easily fatigued. These units are called fast fatigable (FF) motor units and are especially important for brief exertions that require large forces, such as running or jumping.
Fast-twitch, fatigue-resistant fibres are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons. These fibres generate more force than slow-twitch fibres, but they are not able to maintain the force as long as the slow-twitch fibres. Finally, fast-twitch, fatigable fibres are recruited when the largest motor neurons are activated. These fibres produce large amounts of force, but they fatigue very quickly. They are used when the organism must generate a burst of large amounts of force, such as in an escape mechanism.
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Muscle contractions
Firstly, a message is sent from the nervous system to the muscular system, triggering chemical reactions. This message is an electrical impulse known as an action potential, which travels through motor neurons, a type of nerve cell. As the alpha motor neuron enters a muscle, it branches out, with each branch innervating a muscle fibre. This combination of one alpha motor neuron and the muscle fibres it innervates is called a motor unit. The size of the motor unit varies depending on the function of the muscle. Muscles requiring fine, coordinated control, such as those in the eyes and hands, have smaller motor units, while muscles involved in powerful but less coordinated actions, like leg and back muscles, have larger motor units.
Secondly, the chemical reactions initiated by the nervous system signal cause the muscle fibres to reorganise themselves, resulting in a shortening of the muscle, which is the contraction. This process involves the release of acetylcholine, which binds to receptors on the muscle fibre membrane. As a result, membrane channels open, allowing sodium ions to enter the cytoplasm of the muscle fibre. This sodium influx triggers the release of stored calcium ions, which then diffuse into the muscle fibre. The presence of these ions alters the relationship between the chains of proteins within the muscle cells, leading to the contraction.
Finally, when the nervous system signal ceases, the chemical reactions reverse, and the muscle fibres return to their original arrangement, causing the muscle to relax. This relaxation phase is crucial to prevent muscle fatigue. Additionally, the repair of small tears in the muscle fibres during contraction can lead to muscle enlargement and increased connective tissue, a phenomenon known as hypertrophy, commonly observed in weight training.
It is worth noting that the force generated by a muscle contraction can be influenced by the number of motor units firing simultaneously and the frequency of action potentials. Increasing the number of recruited motor units and the frequency of stimulation can lead to a condition called tetanus, where the muscle remains completely contracted without relaxation.
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Muscle twitching
Some common triggers for muscle twitching include stress, anxiety, and lack of sleep. Additionally, consuming too much caffeine, amphetamines, or other stimulants can also lead to muscle twitching. Certain medications, such as antidepressants, diuretics, corticosteroids, and estrogens, may also contribute to this issue. In some cases, muscle twitching can occur after exercise or as a result of nutrient deficiencies, particularly in vitamins D, B, and calcium. Dehydration and electrolyte imbalances, often caused by excessive sweating, intense exercise, vomiting, or diarrhea, can also lead to muscle twitching.
While muscle twitching is usually benign and goes unnoticed, it can sometimes indicate more serious health conditions, particularly those affecting the nervous system. For example, twitching in the hands and feet could be an early symptom of amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease. Neuromyotonia (Isaac's syndrome) and chronic kidney disease (CKD) are two other conditions that can cause frequent muscle twitching. In rare cases, eye twitches can be a sign of brain or nerve disorders such as Bell's palsy, multiple sclerosis, or Tourette's syndrome.
If you are experiencing persistent or frequent muscle twitching, it is important to consult a healthcare professional, especially if it is accompanied by weakness or loss of muscle. They will be able to assess your medical history, perform a physical examination, and determine if any underlying conditions require treatment.
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Muscle soreness and hypertrophy
DOMS typically lasts from 3 to 5 days but can persist for longer, depending on the intensity of the exercise and individual factors. It is characterised by microscopic tears in muscle fibres, inflammation, and metabolite accumulation within skeletal muscles and connective tissues. These tears are a natural response to the physical stress of exercise, and the repair process leads to muscle growth and hypertrophy.
The tearing and repair process in muscles results in an increase in muscle fibre size and the amount of connective tissue. This contributes to the phenomenon of "bulking up" from weight training, where a significant portion of the increase in muscle size is due to the growth of connective tissue. However, endurance training does not lead to significant muscle size increases but improves the muscle's ability to produce ATP aerobically.
To minimise the effects of DOMS, proper warm-up and cool-down routines are essential. A warm-up should elevate the heart rate and induce perspiration, increasing blood flow and heat to the muscles, making them more pliable and resilient. A cool-down routine may include stretching and using a foam roller, which can help alleviate muscle soreness. Additionally, a light workout, such as a long walk, after an intense session can reduce the duration of DOMS.
The experience of muscle soreness and the pursuit of hypertrophy are complex topics with ongoing research. While DOMS is a common occurrence, it is not a definitive indicator of muscle damage or workout effectiveness. The pursuit of hypertrophy involves a combination of factors, including muscle damage, mechanical tension, and metabolic stress. Understanding these processes can help guide exercise programming and recovery strategies to optimise muscle growth and performance while minimising discomfort.
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Insulin resistance and oxygen
A muscle contracts when the alpha motor neurons that innervate skeletal muscle fibres are stimulated. The muscle fibres then shorten, leading to a contraction.
Insulin resistance is a condition in which the body's cells become resistant to insulin, a hormone produced by the pancreas. Insulin is responsible for regulating blood sugar levels by promoting the absorption of glucose from the bloodstream into the body's cells. When the body becomes resistant to insulin, it is unable to effectively use glucose, leading to high blood sugar levels.
There is evidence to suggest that oxygen intake may play a role in insulin resistance. A study conducted by researchers from the University of Southampton and University College London found that adults who were taken up Mount Everest and exposed to low oxygen levels developed insulin resistance. Similarly, individuals with sleep apnea, a condition that disrupts breathing during sleep, are also at an increased risk of developing insulin resistance and type 2 diabetes. Interestingly, providing sleep apnea patients with breathing treatment, such as continuous positive airway pressure (CPAP), improved insulin sensitivity and blood glucose levels.
In addition, a study on mice by researchers from the University of California, San Diego, found that reduced oxygen availability to fat cells led to increased inflammation, which had a knock-on effect on the liver and resulted in insulin resistance. Higher intensity exercise has been shown to improve glucose tolerance, which may be due to the increased oxygen intake that occurs during these activities. This evidence suggests that oxygen intake may indeed be a factor in insulin resistance, and increasing oxygen intake through physical activity may be a potential strategy to improve insulin sensitivity.
Furthermore, diabetes, which is characterized by insulin resistance, is associated with increased oxidative stress and reactive oxygen species (ROS). These ROS are believed to contribute to the development of diabetic complications such as renal injury and atherosclerosis. Thus, while oxygen intake may play a role in insulin resistance, the relationship is complex and involves multiple interconnected factors.
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Frequently asked questions
Muscles fire when a nerve fires and an electrical impulse starts in the nerve, moving out toward the muscle. This triggers the release of acetylcholine, a chemical that binds to receptors on the muscle, causing it to contract.
For muscles to fire optimally, oxygen is required. Oxygen allows for oxidative phosphorylation (OXPHOS) to occur, which results in better glycemic control and improved insulin sensitivity.
Muscle firing may play a role in curing insulin resistance. By increasing oxygen levels, German researchers were able to demonstrate that muscles of type 2 diabetics were able to absorb and burn sugar more efficiently, improving glycemic control.
Muscle twitching is very common and can be caused by various factors such as stress, fatigue, anxiety, dehydration, or even a pinched nerve in the spine.
