Alec Titan
Muscle tension physiology is a fundamental process in muscle physiology that involves the conversion of an electrical stimulus into a mechanical response, known as excitation-contraction coupling. This process was first discovered by Luigi Galvani in 1780 and has been the subject of ongoing research since. Muscle tension refers to the force generated by the contraction of muscles or the shortening of sarcomeres within the muscle fibres. This tension is created by the sliding of thin filaments over thick filaments, resulting in sarcomere shortening and subsequent muscle contraction. The tension generated in skeletal muscles depends on the magnitude of overlap between the actin and myosin myofilaments, with maximal tension occurring when there is optimal overlap. Muscle tension can be classified into two main types of skeletal muscle contractions: isotonic and isometric contractions, each with their unique characteristics and functions.
| Characteristics | Values |
|---|---|
| Definition of Muscle Tension | The force generated by the contraction of the muscle (or shortening of the sarcomeres) is called muscle tension. |
| Types of Muscle Contractions | Isotonic contractions, isometric contractions, eccentric contractions, and concentric contractions. |
| Isotonic Contractions | The tension in the muscle stays constant, and a load is moved as the length of the muscle changes (shortens). |
| Isotonic Contractions Types | Concentric and eccentric. |
| Concentric Contraction | The muscle shortens to move a load. |
| Eccentric Contraction | The muscle tension diminishes and the muscle lengthens. |
| Isometric Contraction | The muscle produces tension without changing the angle of a skeletal joint. |
| Sarcomeres | Sarcomeres produce maximal tension when thick and thin filaments overlap between about 80% to 120%. |
| Sliding Filament Theory | A thin filament slides over a thick filament and generates tension in the muscle. |
| Excitation-Contraction Coupling | The physiological process of converting an electrical stimulus to a mechanical response. |
| Summation | The force exerted by the skeletal muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers. |
| Muscle Tone | Muscle fibers are at least partially contracted even when at rest, possessing a small degree of tension called muscle tone or tonus. |
| Muscle Tone Regulation | The main regulator of muscle tone is the muscle spindle, a small sensory unit closely associated with the muscle. |
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What You'll Learn
Isotonic contractions
Muscle tension is the force generated by the contraction of a muscle or the shortening of sarcomeres. Sarcomeres are the muscle fibres of the skeletal muscle that must shorten to move an object, or load. When a muscle contracts against a load that does not move, two types of skeletal muscle contractions occur: isotonic and isometric contractions.
Concentric contractions
Concentric contractions involve the shortening of the muscle to move a load. An example of this is the biceps brachii muscle contracting when a hand weight is brought upward with increasing muscle tension. As the biceps brachii contract, the angle of the elbow joint decreases as the forearm is brought toward the body.
Eccentric contractions
Eccentric contractions occur when the muscle lengthens as tension is released. In this case, the hand weight is lowered in a slow and controlled manner as the amount of cross-bridge being activated by nervous system stimulation decreases. As tension is released from the biceps brachii, the angle of the elbow joint increases. Eccentric contractions are also used for movement and balance of the body.
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Isometric contractions
Muscle tension is the force generated by the contraction of a muscle or the shortening of sarcomeres. There are two main types of skeletal muscle contractions: isotonic and isometric.
Isometric exercises are ideal for those with limited workout space, people recovering from an injury, or anyone needing a change in their fitness routine. They can be used as a complement to a more dynamic exercise regimen. However, isometric exercises are controversial for their strength and muscle-building benefits, and they may cause or worsen existing injuries if performed with poor form.
During an isometric contraction, the muscle produces tension without changing the angle of a skeletal joint. Sarcomeres shorten and muscle tension increases, but the load is not moved as the force produced cannot overcome the resistance provided by the load. For example, attempting to lift a hand weight that is too heavy will result in sarcomere activation and shortening, with increasing muscle tension, but no change in the angle of the elbow joint.
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Muscle tone
Disorders of muscle tone can result in abnormal levels of tone, known as hypertonia (high) or hypotonia (low). Hypertonia can manifest as spasticity or rigidity, while hypotonia can present as muscle flaccidity with decreased stretch reflex responses and resistance to passive movement. These disorders arise from dysfunction in neural circuits involving the brain, spinal cord, and muscle spindle.
The maintenance of muscle tone involves a complex hierarchy of motor control, including the cortex, basal ganglia, cerebellum, brainstem reticular system, spinal cord, and muscle spindle. This intricate system regulates muscle tone and its role in movement and postural tasks.
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Excitation-contraction coupling
The excitation-contraction coupling (ECC) phenomenon was first defined by Alexander Sandow in 1952 as the series of events that occur from the generation of the action potential (AP) in skeletal muscle fibres to the beginning of muscle tension. ECC refers to the physiological process of converting an electrical stimulus (action potential) to a mechanical response (muscle contraction). This process is fundamental to muscle physiology and can be categorized into three phases.
The first phase of ECC involves the depolarization and spread of an action potential along the sarcolemma and the propagation of the action potential into the T tubules. The second phase involves the release of calcium ions (Ca++) by the sarcoplasmic reticulum, which initiates the energy needed for various cardiac systems to function. Calcium sparks occur at the level of the triadic junctional region, and the presence of RyR3 in amphibian skeletal muscle fibres increases the fraction of Ca2+ release channels. In the third phase, when calcium is released from the sarcoplasmic reticulum, it binds to the troponin molecules on the thin filament. This binding causes troponin to undergo a configurational change, removing tropomyosin from its blocking position on the actin filament.
The time course of ECC can be observed in a mammalian myocyte, with the central role of the Ca transient linking cardiac excitation and contraction. ECC can be dysregulated in many diseases and is still an active area of biomedical research.
The mechanism for muscle contraction involves the sliding filament theory, developed in 1954, which describes a cycle of repetitive events causing a thin filament to slide over a thick filament and generate tension in the muscle. This process is not uniform across the sarcomere, and cross-bridge cycling, a sequence of molecular events, underlies the sliding filament theory. A cross-bridge is a myosin projection consisting of two heads that extends from the thick filaments. Sarcomeres produce maximal tension when thick and thin filaments overlap between 80-120%muscle contracts against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic and isometric contractions.
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Sarcomere activation
Muscle tension is the force generated by the contraction of a muscle or the shortening of sarcomeres. Sarcomeres are the functional units of skeletal muscle and are essential for proper contraction.
Calcium ions play a crucial role in sarcomere activation by regulating the availability of binding sites on actin and the availability of myosin molecules needed to bind these sites and generate mechanical forces. When calcium enters the sarcomere, it binds to a thin filament regulatory protein, unblocking initial binding sites and allowing myosin molecules to bind actin. This process is known as excitation-contraction coupling, which involves converting an electrical stimulus to a mechanical response.
The TnC biosensor transient in intact FDB myofibers at 37 °C showed that the timing of peak activation of the biosensor ratio was not significantly different from the time to peak of the sarcomere length. This indicates that the biosensor is reporting dynamic sarcomere activation/inactivation due to the ensemble effects of multiple activating ligands within the myofilaments.
Furthermore, the force exerted by the skeletal muscle is controlled by varying the frequency at which action potentials are sent to muscle fibres. During a typical contraction, only a fraction of the fibres in the muscle will be firing at any given time.
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Frequently asked questions
Muscle tension is the force generated by the contraction of a muscle or the shortening of sarcomeres, which are the muscle fibres of the skeletal muscle.
Muscle tension is caused by the sliding filament theory, where a thin filament slides over a thick filament and generates tension in the muscle. This process is also known as cross-bridge cycling, where myosin projections pull actin.
There are two main types of skeletal muscle contractions: isotonic contractions and isometric contractions.
In isotonic contractions, the tension in the muscle stays constant, and the load is moved as the length of the muscle changes. There are two types of isotonic contractions: concentric and eccentric.
Isometric contractions occur when the muscle produces tension without changing the angle of a skeletal joint. The muscle length does not change because the load exceeds the tension the muscle can generate.
