Muscle fatigue
Encyclopedia
Muscle fatigue is the decline in ability of a muscle
to generate force
. It can be a result of vigorous exercise
but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction
. There are two main causes of muscle fatigue - limitations of nerve
’s ability to generate a sustained signal
and the reduced ability of calcium
to stimulate contraction.
of electrical impulses from the brain
which signals them to contract
through the release of calcium
by the sarcoplasmic reticulum. Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves.
s are responsible for controlling the contraction of muscles, determining the number, sequence and force of muscular contraction. Most movements require a force far below what a muscle could in potential generate, and barring pathology
nervous fatigue is seldom an issue. For extremely powerful contractions that are close to the upper limit of a muscle's ability to generate force, nervous fatigue can be a limiting factor in untrained individuals. In novice strength trainers
, the muscle's ability to generate force is most strongly limited by nerve’s ability to sustain a high-frequency signal. After a period of maximum contraction, the nerve’s signal reduces in frequency and the force generated by the contraction diminishes. There is no sensation of pain or discomfort, the muscle appears to simply ‘stop listening’ and gradually cease to move, often going backwards. As there is insufficient stress on the muscles and tendons, there will often be no delayed onset muscle soreness
following the workout. Part of the process of strength training is increasing the nerve's ability to generate sustained, high frequency signals which allow a muscle to contract with its greatest force. It is this neural training that causes several weeks worth of rapid gains in strength, which level off once the nerve is generating maximum contractions and the muscle reaches its physiological limit. Past this point, training effects increase muscular strength through myofibrilar or sarcoplasmic hypertrophy and metabolic fatigue becomes the factor limiting contractile force.
and myosin
to contract.
within the muscle generally serve to power muscular contractions. They include molecules such as adenosine triphosphate
(ATP), glycogen
and creatine phosphate
. ATP binds to the myosin
head and causes the ‘ratchetting’ that results in contraction according to the sliding filament model
. Creatine phosphate stores energy so ATP can be rapidly regenerated within the muscle cells from adenosine diphosphate
(ADP) and inorganic phosphate ions, allowing for sustained powerful contractions that last between 5–7 seconds. Glycogen is the intramuscular storage form of glucose
, used to generate energy quickly once intramuscular creatine stores are exhausted, producing lactic acid
as a metabolic byproduct.
Substrates produce metabolic fatigue by being depleted during exercise, resulting in a lack of intracellular energy sources to fuel contractions. In essence, the muscle stops contracting because it lacks the energy to do so.
, reactive oxygen species
and inorganic phosphate. Accumulation of metabolites can directly or indirectly produce metabolic fatigue within muscle fibers through interference with the release of calcium from the sarcoplasmic reticulum or reduction of the sensitivity of contractile molecules actin
and myosin
to calcium.
inhibits the contraction of muscles, preventing them from contracting due to "false alarms", small stimuli which may cause them to contract (akin to myoclonus
). This natural brake helps muscles respond solely to the conscious control or spinal reflexes
but also has the effect of reducing the force of conscious contractions.
also causes the muscle cells to decrease in efficiency, causing cramping and fatigue. Potassium builds up in the t-tubule
system and around the muscle fiber in general. This has the effect of depolarizing the muscle fiber, preventing the sodium-potassium pump
from moving Na+
out of the cell. This reduces the amplitude of action potential
s, or stops them entirely, resulting in neurological fatigue.
build-up was the cause of muscle fatigue. The assumption was lactic acid had a "pickling" effect on muscles, inhibiting their ability to contract. The impact of lactic acid on performance is now uncertain, it may assist or hinder muscle fatigue.
Produced as a by-product of fermentation
, lactic acid can increase intracellular acidity of muscles. This can lower the sensitivity of contractile apparatus to Ca2+ but also has the effect of increasing cytoplasm
ic Ca2+ concentration through an inhibition of the chemical pump
that actively transports
calcium out of the cell. This counters inhibiting effects of K+ on muscular action potentials. Lactic acid also has a negating effect on the chloride ions in the muscles, reducing their inhibition of contraction and leaving potassium ions as the only restricting influence on muscle contractions, though the effects of potassium are much less than if there were no lactic acid to remove the chloride ions. Ultimately, it is uncertain if lactic acid reduces fatigue through increased intracellular calcium or increases fatigue through reduced sensitivity of contractile proteins to Ca2+.
such as myasthenia gravis
) or problems with muscle itself. The latter category includes polymyositis
and other muscle disorders
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
to generate force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
. It can be a result of vigorous exercise
Physical exercise
Physical exercise is any bodily activity that enhances or maintains physical fitness and overall health and wellness. It is performed for various reasons including strengthening muscles and the cardiovascular system, honing athletic skills, weight loss or maintenance, as well as for the purpose of...
but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
. There are two main causes of muscle fatigue - limitations of nerve
Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
’s ability to generate a sustained signal
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
and the reduced ability of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
to stimulate contraction.
Muscle contraction
Muscle cells work by detecting a flowAction potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
of electrical impulses from the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
which signals them to contract
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
through the release of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
by the sarcoplasmic reticulum. Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves.
Nervous fatigue
NerveNerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
s are responsible for controlling the contraction of muscles, determining the number, sequence and force of muscular contraction. Most movements require a force far below what a muscle could in potential generate, and barring pathology
Pathology
Pathology is the precise study and diagnosis of disease. The word pathology is from Ancient Greek , pathos, "feeling, suffering"; and , -logia, "the study of". Pathologization, to pathologize, refers to the process of defining a condition or behavior as pathological, e.g. pathological gambling....
nervous fatigue is seldom an issue. For extremely powerful contractions that are close to the upper limit of a muscle's ability to generate force, nervous fatigue can be a limiting factor in untrained individuals. In novice strength trainers
Strength training
Strength training is the use of resistance to muscular contraction to build the strength, anaerobic endurance, and size of skeletal muscles. There are many different methods of strength training, the most common being the use of gravity or elastic/hydraulic forces to oppose muscle contraction...
, the muscle's ability to generate force is most strongly limited by nerve’s ability to sustain a high-frequency signal. After a period of maximum contraction, the nerve’s signal reduces in frequency and the force generated by the contraction diminishes. There is no sensation of pain or discomfort, the muscle appears to simply ‘stop listening’ and gradually cease to move, often going backwards. As there is insufficient stress on the muscles and tendons, there will often be no delayed onset muscle soreness
Delayed onset muscle soreness
Delayed onset muscle soreness , also called muscle fever, is the pain and stiffness felt in muscles several hours to days after unaccustomed or strenuous exercise. The soreness is felt most strongly 24 to 72 hours after the exercise. It is caused by eccentric exercise...
following the workout. Part of the process of strength training is increasing the nerve's ability to generate sustained, high frequency signals which allow a muscle to contract with its greatest force. It is this neural training that causes several weeks worth of rapid gains in strength, which level off once the nerve is generating maximum contractions and the muscle reaches its physiological limit. Past this point, training effects increase muscular strength through myofibrilar or sarcoplasmic hypertrophy and metabolic fatigue becomes the factor limiting contractile force.
Metabolic fatigue
Though not universally used, ‘metabolic fatigue’ is a common term for the reduction in contractile force due to the direct or indirect effects of the reduction of substrates or accumulation of metabolites within the muscle fiber. This can occur through a simple lack of energy to fuel contraction, or interference with the ability of Ca2+ to stimulate actinActin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
and myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
to contract.
Substrates
SubstratesSubstrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
within the muscle generally serve to power muscular contractions. They include molecules such as adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(ATP), glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
and creatine phosphate
Phosphocreatine
Phosphocreatine, also known as creatine phosphate or PCr , is a phosphorylated creatine molecule that serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle and brain.-Chemistry:...
. ATP binds to the myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
head and causes the ‘ratchetting’ that results in contraction according to the sliding filament model
Sliding filament mechanism
The sliding filament theory describes a process used by muscles to contract. It was independently developed by Andrew F. Huxley and Rolf Niedergerke and by Hugh Huxley and Jean Hanson in 1954.-Process of movement:...
. Creatine phosphate stores energy so ATP can be rapidly regenerated within the muscle cells from adenosine diphosphate
Adenosine diphosphate
Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....
(ADP) and inorganic phosphate ions, allowing for sustained powerful contractions that last between 5–7 seconds. Glycogen is the intramuscular storage form of glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, used to generate energy quickly once intramuscular creatine stores are exhausted, producing lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
as a metabolic byproduct.
Substrates produce metabolic fatigue by being depleted during exercise, resulting in a lack of intracellular energy sources to fuel contractions. In essence, the muscle stops contracting because it lacks the energy to do so.
Metabolites
Metabolites are the substances (generally waste products) produced as a result of muscular contraction. They include ADP, Mg2+Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
, reactive oxygen species
Reactive oxygen species
Reactive oxygen species are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. Reactive oxygen species are highly reactive due to the presence of unpaired valence shell electrons....
and inorganic phosphate. Accumulation of metabolites can directly or indirectly produce metabolic fatigue within muscle fibers through interference with the release of calcium from the sarcoplasmic reticulum or reduction of the sensitivity of contractile molecules actin
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
and myosin
Myosin
Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...
to calcium.
Chloride
Intracellular chlorideChloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
inhibits the contraction of muscles, preventing them from contracting due to "false alarms", small stimuli which may cause them to contract (akin to myoclonus
Myoclonus
Myoclonus is brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign and, generally, is not a diagnosis of a disease. Brief twitches are perfectly normal. The myoclonic twitches are usually caused by sudden muscle contractions; they also can result from brief...
). This natural brake helps muscles respond solely to the conscious control or spinal reflexes
Stretch reflex
The stretch reflex is a muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length....
but also has the effect of reducing the force of conscious contractions.
Potassium
High concentrations of potassiumPotassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
also causes the muscle cells to decrease in efficiency, causing cramping and fatigue. Potassium builds up in the t-tubule
T-tubule
A T-tubule is a deep invagination of the sarcolemma, which is the plasma membrane, only found in skeletal and cardiac muscle cells...
system and around the muscle fiber in general. This has the effect of depolarizing the muscle fiber, preventing the sodium-potassium pump
Na+/K+-ATPase
Na+/K+-ATPase is an enzyme located in the plasma membrane in all animals.- Sodium-potassium pumps :Active transport is responsible for cells containing relatively high...
from moving Na+
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
out of the cell. This reduces the amplitude of action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
s, or stops them entirely, resulting in neurological fatigue.
Lactic acid
It was once believed that lactic acidLactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
build-up was the cause of muscle fatigue. The assumption was lactic acid had a "pickling" effect on muscles, inhibiting their ability to contract. The impact of lactic acid on performance is now uncertain, it may assist or hinder muscle fatigue.
Produced as a by-product of fermentation
Lactic acid fermentation
Lactic acid fermentation is a biological process by which sugars such as glucose, fructose, and sucrose, are converted into cellular energy and the metabolic byproduct lactate. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells, in the...
, lactic acid can increase intracellular acidity of muscles. This can lower the sensitivity of contractile apparatus to Ca2+ but also has the effect of increasing cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
ic Ca2+ concentration through an inhibition of the chemical pump
Sodium-calcium exchanger
The sodium-calcium exchanger is an antiporter membrane protein that removes calcium from cells. It uses the energy that is stored in the electrochemical gradient of sodium by allowing Na+ to flow down its gradient across the plasma membrane in exchange for the countertransport of calcium ions...
that actively transports
Active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
calcium out of the cell. This counters inhibiting effects of K+ on muscular action potentials. Lactic acid also has a negating effect on the chloride ions in the muscles, reducing their inhibition of contraction and leaving potassium ions as the only restricting influence on muscle contractions, though the effects of potassium are much less than if there were no lactic acid to remove the chloride ions. Ultimately, it is uncertain if lactic acid reduces fatigue through increased intracellular calcium or increases fatigue through reduced sensitivity of contractile proteins to Ca2+.
Pathology
Muscle weakness may be due to problems with the nerve supply, neuromuscular diseaseNeuromuscular disease
Neuromuscular disease is a very broad term that encompasses many diseases and ailments that either directly, via intrinsic muscle pathology, or indirectly, via nerve pathology, impair the functioning of the muscles....
such as myasthenia gravis
Myasthenia gravis
Myasthenia gravis is an autoimmune neuromuscular disease leading to fluctuating muscle weakness and fatiguability...
) or problems with muscle itself. The latter category includes polymyositis
Polymyositis
Polymyositis is a type of chronic inflammation of the muscles related to dermatomyositis and inclusion body myositis.-Signs and symptoms:...
and other muscle disorders
Myopathy
In medicine, a myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness. "Myopathy" simply means muscle disease...
See also
- MalaiseMalaiseMalaise is a feeling of general discomfort or uneasiness, of being "out of sorts", often the first indication of an infection or other disease. Malaise is often defined in medicinal research as a "general feeling of being unwell"...
- Asthenia
- ParesisParesisParesis is a condition typified by partial loss of voluntary movement or by impaired movement. When used without qualifiers, it usually refers to the limbs, but it also can be used to describe the muscles of the eyes , the stomach , and also the vocal cords...
- Debility
- Muscle weaknessMuscle weaknessMuscle weakness or myasthenia is a lack of muscle strength. The causes are many and can be divided into conditions that have true or perceived muscle weakness...
- Fatigue (physical)Fatigue (physical)Fatigue is a state of awareness describing a range of afflictions, usually associated with physical and/or mental weakness, though varying from a general state of lethargy to a specific work-induced burning sensation within one's muscles...