Dominic Stone
The length of muscle fibres is an important factor in muscle force generation. Muscle fibres can be categorized into three types: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). SO fibres produce low-power contractions over long periods and are slow to fatigue, while FG fibres produce rapid, forceful contractions for quick, powerful movements but fatigue quickly. The length of muscle fibres affects their ability to generate force during human locomotion, with shorter fibres producing more force than longer fibres in certain cases. People with shorter muscle fibres may have an advantage in strength relative to their body weight, as longer muscles weigh more. However, the difference in strength between individuals with short and long muscle fibres is not significant, and both types can be trained to increase strength.
| Characteristics | Values |
|---|---|
| Muscle fiber length | The length of a muscle fiber is determined by the distance between its attachments to bones. |
| Muscle fiber velocity | The velocity of a muscle fiber refers to its shortening or lengthening speed, which can affect force generation during movement. |
| Muscle appearance | Shorter muscle fibers can result in a higher peak or mound, leading to a more defined appearance when flexed. |
| Muscle function | Muscle-shortening actions generate force, while muscle-lengthening actions control and decelerate force. |
| Muscle strength | Shorter muscle fibers can provide higher relative strength due to lower body weight, but muscle strength is primarily determined by the type of muscle fibers and the work put in through training. |
| Muscle power | Longer muscle tendons are associated with increased power, while shorter muscle bellies can result in longer tendons, contributing to power. |
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What You'll Learn
Shorter muscle fibres can be stronger due to higher peak
Muscle fibres are activated by motor neurons, which act as a connection between the central nervous system and the specific muscle required to perform a particular activity. A muscle motor unit is the motor neuron and the attached muscle fibres. When a motor unit is signalled to contract, it activates all of its attached muscle fibres.
The shortening of muscle fibres can generate force to move a resistance. For example, when moving from a seated to a standing position, the quadriceps and gluteus maximus shorten to help the body stand up against gravity.
The length of a muscle is determined by where the bone attachments are. For instance, if the distance from one bone attachment to another is ten inches, the entire muscle plus tendon structure will be ten inches. Muscles can become shorter or longer for short periods of time, such as when stretching or sitting at a desk all day.
Shorter muscle fibres can be stronger due to their higher peak. This means that shorter muscles can handle more weight and do bodyweight movements more easily. Shorter muscle bellies also have longer tendons, which make them more powerful.
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Muscle fibres can be lengthened or shortened temporarily
Muscles are like elastic bands, and they stretch and contract depending on the movement being performed. This movement comes from the joints, which pull the muscles in various directions, causing them to lengthen and shorten. For example, when moving from a seated to a standing position, the quadriceps and gluteus maximus shorten to help the body stand up against gravity. On the other hand, when returning to a seated position, the quadriceps and glutes lengthen to control the motion of the body.
A study on the medial gastrocnemius muscle in cats found that during isometric contractions, the muscle fibres shortened by stretching the compliant tendons until the muscle fibres could no longer produce enough force to stretch the tendons further. At optimal muscle length, the maximal shortening of muscle fibres was 28%. Additionally, when slow to medium-speed stretches were applied shortly after the onset of contraction, the muscle fibres shortened during the stretch.
In the context of human movement, countermovement jumps have been studied to understand the relationship between muscle length and force development. Countermovement jumps involve muscle fibre lengthening prior to shortening, which may enhance the total work done in a cycle. For example, studies on fish muscles performing oscillatory work and bird pectoralis powering flight have shown that active lengthening of muscle fibres prior to shortening can result in increased force and work during the shortening phase.
Furthermore, studies have shown that a stretch of muscle fibres prior to shortening can result in potentiation of muscle contractile elements, leading to increased work in subsequent contractions. This effect has been attributed to the role of the giant protein titin in force potentiation following stretch.
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Muscle-shortening actions generate force to move resistance
The process of muscle contraction involves the activation of tension-generating sites within muscle cells. Muscle contraction is described by two variables: length and tension. Muscle shortening and contraction are not synonymous, as tension can be produced without changes in muscle length, such as when holding a heavy object in the same position. Muscle relaxation occurs when the contraction terminates, and the muscle fibres return to a low-tension state.
The three types of 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. FG fibres produce rapid, forceful contractions for quick, powerful movements but fatigue quickly. Type IIB fibres, for example, are used for short, explosive, high-intensity activities and are involved in strength and power activities requiring high force over a short period.
The speed of contraction depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP twice as rapidly as slow fibres, resulting in quicker cross-bridge cycling. Muscle fibres can adapt to changing demands by altering size or fibre type composition, which serves as the basis for physical therapy interventions designed to increase force development or endurance.
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Slow-twitch (type I) vs fast-twitch (type II) muscle fibres
The length of a muscle is determined by the distance between its attachments to bones. While some people are simply born with longer or shorter muscles, muscles can become shorter or longer for short periods of time due to stretching or sitting.
Now, onto the topic of slow-twitch (type I) vs fast-twitch (type II) muscle fibres:
Slow-twitch (type I) muscle fibres are responsible for endurance movements and activities such as distance running, swimming, cycling, hiking, low-to-moderate intensity dancing, and walking. They have slower twitch speeds and are resistant to fatigue. Type I fibres use aerobic respiration (oxygen delivery) to produce ATP in muscle cells and have a higher density of mitochondria, which are efficient at aerobic metabolism. The mitochondria give the cell a darker colour, which is why these are known as red muscle fibres.
Fast-twitch (type II) muscle fibres are responsible for short, powerful movements. They can produce a lot more force and power for a short time but get fatigued quickly. Type II muscle fibres are further broken down into type IIx and IIa. Type IIx fibres produce force that is much greater than type I fibres, but they use anaerobic (without oxygen) metabolic pathways to get their energy. Type IIa muscle fibres are like a hybrid of type I and type IIx, using both aerobic and anaerobic pathways and producing a medium amount of power for a medium amount of time.
It is important to note that everyone has a mix of type I and type II muscle fibres, but the distribution of these fibres depends on the individual and their activity levels and types.
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Muscle fibres can adapt to the type of exercise stimulus
Muscle fibres are activated by motor neurons, which act as a connection between the central nervous system and the specific muscle required to perform a particular activity. Motor units, which are made up of a motor neuron and its attached muscle fibres, can be thought of as light switches for muscles. When a muscle is required to generate force, the motor units "light up" to stimulate the fibres to shorten and produce that force.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles contain all three types, but in varying proportions. SO fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. FO and FG fibres, on the other hand, are capable of producing rapid, forceful contractions to make quick, powerful movements. However, these fibres fatigue quickly and can only be used for short periods.
In addition, muscles can change length for short periods of time due to stretching or inactivity. For example, sitting at a desk all day can cause muscles to become shorter, while stretching can lengthen them. These changes in muscle length are temporary and do not affect how the muscles function.
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Frequently asked questions
The length of a muscle is determined by the distance between bone attachments. Shorter muscles have a higher peak, which may be desirable for some. Shorter muscles do not make you stronger, but they can give the appearance of more muscle.
Shorter muscle fibers can generate force to move resistance. For example, when standing up, the quadriceps and gluteus maximus shorten to help the body stand against gravity.
Shorter muscle fibers can be beneficial for those wanting to increase muscle definition, as they can create the appearance of a more peaked muscle.
Muscle length can be altered by stretching to make them longer, or sitting at a desk all day can shorten them. However, these changes are not permanent.
