Dominic Stone
Muscle fibres are responsible for causing movement in the body. Skeletal muscles, which are voluntary muscles, are the most common type of muscle in the body, comprising 30-40% of total body mass. They consist of flexible muscle fibres that contract to allow movement. These fibres can be classified into three types: Type I (slow oxidative) fibres, Type IIA (fast oxidative) fibres, and Type IIB (fast glycolytic) fibres. Each type has different characteristics, such as their speed of contraction, endurance, and energy generation mechanisms. For example, Type I fibres are slow-twitch fibres with high endurance, while Type IIB fibres are fast-twitch fibres that generate energy anaerobically for short bursts of movement. Muscle fibres also contain sensory receptors, such as muscle spindles and Golgi tendon organs, which sense muscle length, tension, and rate of contraction.
| Characteristics | Values |
|---|---|
| Muscle contraction | Caused by depolarization, a change in electric charge initiated by a nerve impulse or pacemaker cells |
| Muscle relaxation | Occurs when the muscle stops receiving a stimulatory input |
| Muscle movement | Muscle-shortening actions generate a force to move a resistance, while muscle-lengthening actions control and decelerate a force |
| Muscle types | Skeletal, cardiac, and smooth muscle |
| Skeletal muscle composition | Skeletal muscle comprises approximately 30-40% of the human body weight and contains 50-75% of all body proteins |
| Skeletal muscle function | Producing movement, sustaining body posture and position, maintaining body temperature, storing nutrients, and stabilizing joints |
| Skeletal muscle control | Skeletal muscles are voluntary muscles, meaning you control how and when they move and work |
| Cardiac muscle characteristics | Striated, branched and interconnected; found only in the heart |
| Smooth muscle characteristics | Non-striated, shorter than skeletal muscle fibers |
| Muscle fiber types | Type 1 (slow oxidative), Type 2A (fast oxidative), and Type 2B (fast glycolytic) |
| Type 1 characteristics | Slow-twitch, slow-contracting, high endurance, use oxygen to generate energy, high density of mitochondria, dark/red in appearance |
| Type 2A characteristics | Fast-twitch, fast-contracting, can use oxygen or switch to anaerobic respiration, may fatigue more quickly, contain mitochondria, can be involved in aerobic activities |
| Type 2B characteristics | Fast-twitch, do not use oxygen, store energy for short bursts of movement, contain little to no mitochondria, white in appearance, fatigue quickly |
| Muscle spindle receptors | Sense muscle length, tension, and rate of contraction |
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What You'll Learn
Muscle length
The ability to measure muscle length is crucial in comprehending muscle motion. Over the years, methods have been developed to measure the length of muscles and muscle fibres directly. One non-invasive approach involves measuring the change in angle at a joint, along with the moment arm of the muscle spanning that joint, to calculate muscle length. This method has been applied in studies involving both humans and animals. For example, Alexander (1974) analysed ankle muscle length changes during a dog's jumping motion, finding that the gastrocnemius muscle length changed by 23-35mm during takeoff.
However, it is important to note that joint motion does not always predict muscle fibre length change. Studies have shown that during stair descent in humans and landings in turkeys, the ankle extensors shortened in the early portion of the foot-contact period, even as the joint extended and the muscle-tendon unit lengthened. This highlights the complex nature of muscle and joint interactions.
The length of a muscle fibre can be influenced by strength training and mechanical tension. After strength training, muscles increase in volume and mass due to the growth of individual muscle fibres. Mechanical tension, whether generated by active contraction or passive resistance to stretch, can stimulate a muscle fibre to increase in length or diameter. This increase in length may be facilitated by the addition of sarcomeres in series.
Furthermore, muscle velocity, or speed of contraction, is influenced by muscle fibre length. Longer muscle fibres can contribute to faster contractions and vice versa. This relationship between muscle fibre length and contraction speed is essential in understanding muscle performance and movement.
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Tension
Muscle contraction is based on two variables: length and tension. Tension within a muscle can be produced without changes in the length of the muscle, such as when holding a dumbbell in the same position or holding a sleeping child in your arms. Upon termination of muscle contraction, muscle relaxation occurs, which is the return of muscle fibres to a low-tension state.
The force generated by the contraction of the muscle or the shortening of the sarcomeres is called muscle tension. Muscle tension is also generated when a muscle contracts against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions. In isotonic contractions, the tension in the muscle stays constant, and a load is moved as the length of the muscle changes. In eccentric isotonic contractions, the muscle lengthens as the tension diminishes. In concentric contractions, the muscle shortens to move a load.
Isometric contractions occur when the muscle produces tension without changing the angle of a skeletal joint. Isometric contractions involve sarcomere shortening and increasing muscle tension, but do not move a load, as the force produced cannot overcome the resistance provided by the load.
The stiffness of a muscle is determined by a number of factors under the regulation of complex neuromuscular control strategies involving the sensorimotor system, which integrates muscle control with the nervous system. The stiffness of the myofascial tissue is responsive, reactive, and predictive. The sensorimotor system also includes intrafusal fibres (muscle spindles) which are the mechanoreceptors set within muscles that provide information on tensions from within a muscle to the central nervous system (CNS). This allows the CNS to set the muscle stiffness before, during, and after locomotive or postural events.
Muscle tension is related to the amount of ATP available, which provides the energy for muscle contraction. The absence of ATP leads to muscle relaxation.
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Rate of contraction
Muscle fibres can sense their rate of contraction through muscle spindles, which are sensory receptors located in the muscle belly. These spindles are composed of intrafusal fibres, which are interspersed among extrafusal fibres. Intrafusal fibres are made up of two types of cells: nuclear bag fibres and nuclear chain fibres.
The rate of contraction is determined by how quickly the muscle acts on ATP. ATP is a molecule that releases energy when broken down. The speed of ATP breakdown is influenced by the presence or absence of oxygen. Type 1 muscle fibres, or slow oxidative fibres, use oxygen to generate energy and have a slow contractile speed. They are best suited for endurance activities such as marathon running. On the other hand, Type 2B fibres do not use oxygen and instead rely on stored energy for short bursts of movement. As a result, they have a faster rate of contraction and are suited for activities requiring bursts of strength, such as sprinting and weightlifting.
The rate of contraction is also influenced by the architectural organisation of the muscle. Muscle contraction is based on actin–myosin interactions within individual sarcomeres, the fundamental contractile units of a muscle fibre. The sliding filament model of muscle contraction describes how thin filaments slide past thick filaments within the sarcomeres, resulting in muscle shortening. The speed of this process depends on the availability of calcium ions and ATP. When calcium ions bind to troponin, they expose the binding sites on the actin filaments, allowing the myosin heads to attach and form cross-bridges. This initiates contraction, which is sustained by ATP. The muscle fibre continues to shorten until it reaches its anatomical limit or runs out of ATP.
The rate of contraction varies across different types of muscles, including skeletal, cardiac, and smooth muscles. Skeletal muscles are under voluntary control and are responsible for producing movement, maintaining posture, and stabilizing joints. Cardiac muscles, found only in the heart, have their own rhythm and can speed up or slow down as needed. Smooth muscles, which lack the striated appearance of skeletal and cardiac muscles, are found in blood vessels and organs such as the gastrointestinal tract and bladder. They are controlled involuntarily by reflexes and the body's ANS.
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Muscle tone
There are three types of vertebrate muscle tissue: skeletal, cardiac, and smooth muscle. Skeletal muscle is part of the voluntary muscular system and is attached to bones by tendons to give skeletal movement. The skeletal muscle cells are long and are also known as muscle fibers. These muscle fibers are classified into two types: type 1 and type 2. Type 1 muscle fibers contract slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They produce low-power contractions over long periods and are slow to fatigue. Type 2 is further divided into subtypes 2A and 2B. Type 2A fibers contract quickly and primarily use aerobic respiration but can switch to anaerobic respiration and fatigue more quickly. Type 2B fibers primarily use anaerobic glycolysis as their ATP source and have a large diameter and high amounts of glycogen. They are used to produce rapid, forceful contractions to make quick, powerful movements but fatigue quickly.
The tone of a muscle can change quickly in response to the demands made on the muscles. For example, when standing, the muscles of the legs, torso, and neck must increase their level of tone to maintain the body in an upright position against gravity. This change in tone is mediated by sensory input from the eyes, ears, muscles, joints, and skin, which travels along sensory neurons to the spinal cord. The inhibitory or excitatory signals carried by these neurons lead to responses in alpha motor neurons, which are nerve fibers directly responsible for muscle contraction.
The gamma motor neuron system also plays a vital role in controlling muscle tone. When active, gamma motor neurons cause the intrafusal fibers to contract, and the central, elastic portion of the intrafusal fibers is placed under a slight stretch. This makes the annulospiral receptor more sensitive and reactive to stretching of the extrafusal fibers. The annulospiral receptor is a sensory ending wrapped around the central section of the intrafusal fibers. When stretched sufficiently, it triggers an impulse along a sensory neuron to the spinal cord, exciting the alpha motor neuron and stimulating the muscle fibers associated with the spindle to contract.
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Muscle growth
The three types of skeletal muscle fibres are slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). SO fibres produce low-power contractions over long periods and are slow to fatigue. They use aerobic respiration to produce ATP, which provides energy for muscle contraction. FO fibres can also use aerobic respiration but may switch to anaerobic respiration, and so fatigue more quickly. FG fibres primarily use anaerobic glycolysis, which allows them to produce rapid, powerful contractions but only for short periods.
The speed of muscle contraction is determined by how quickly the muscle can act on ATP. FT fibres, such as Type 2A and 2B, break down ATP twice as fast as ST fibres, such as Type 1. This means that FT fibres are good for explosive bursts of energy, like sprinting or weightlifting, while ST fibres are better for endurance activities like running or cycling.
Muscle fibres can experience two types of hypertrophy: myofibrillar and sarcoplasmic. Myofibrillar hypertrophy increases the thickness of individual actin and myosin protein filaments, which improves the force-production capacity of the fibres. Sarcoplasmic hypertrophy increases the volume of the fluid surrounding the muscle fibre, which contains proteins that promote tissue repair and growth.
To stimulate muscle growth, Type II muscle fibres must be activated. Type II fibres are fast-twitch fibres that are responsible for muscle growth and definition. Using heavier weights for fewer repetitions is an effective way to stimulate growth in Type II fibres.
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Frequently asked questions
Muscle fibers are the contractile parts of a muscle that allow for movement in the body. They are made up of repeating thick and thin filaments, giving them a striped appearance.
Muscle fibers can be classified into three groups: Type I (slow oxidative), Type IIA (fast oxidative), and Type IIB (fast glycolytic). Type I fibers are slow-twitch fibers, while Type IIA and Type IIB are considered fast-twitch.
Slow-twitch (Type I) fibers contract slowly and use oxygen and glucose to produce energy (ATP). They produce low-power contractions over long periods and are resistant to fatigue. Fast-twitch (Type IIA and IIB) fibers have fast contractions and can use both aerobic and anaerobic respiration. They fatigue more quickly and are used for short bursts of energy.
Slow-twitch muscle fibers are used for endurance activities such as marathon running, cycling, or swimming. They help maintain body posture and stabilize joints.
Fast-twitch muscle fibers are used for short, explosive movements that require a high amount of strength or power. Examples include sprinting, weightlifting, and other activities involving bursts of energy.
