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Muscle fascicles are groups of bundled muscle fibres, enclosed in a layer of connective tissue called perimysium. The length of these fascicles is an important topic in sports science and physical therapy, as it is believed to play a role in the power versus economy performance trade-off. Long fascicles, for example, are associated with superior power-based performance among athletes, but they are likely disadvantageous for economical force production during walking and running. The length of muscle fibres has also been found to influence the shortening velocity of the muscle fibres. While the cross-sectional area of a muscle increases with resistance training, it is controversial whether the length of the muscle fibre increases as well.
| Characteristics | Values |
|---|---|
| Definition | Muscle fascicles are enclosed in the perimysium, comprised of concentric strands of connective tissue enclosing the fascicle. |
| Muscle fiber composition | Myofibers are bundled into muscle fascicles and surrounded by the perimysium, a layer of dense irregular connective tissue. |
| Types of fascicles | Parallel, circular, convergent, pennate (uni, bi, and multi-), fusiform, or triangular. |
| Fascicle length and muscle force | Long fascicles produce more relative force than short fascicles. |
| Fascicle length and muscle velocity | Long fascicles have a faster shortening velocity than short fascicles. |
| Fascicle length and muscle economy | Longer fascicles decrease economy. |
| Fascicle length and muscle power | Fascicle length plays a role in the power vs. economy performance trade-off. |
| Fascicle length and resistance training | The effect of resistance training on fascicle length is inconclusive. |
| Fascicle length and muscle function | Fascicle arrangement is correlated with the force generated by a muscle and affects its range of motion. |
| Fascicle length and muscle architecture | Fascicle length is believed to reflect increased myofiber lengths. |
| Fascicle length and intramuscular collagen | Without a concurrent increase in intramuscular collagen, longitudinal muscle fascicle growth reduces passive tension at long muscle lengths. |
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What You'll Learn
Resistance training and muscle fascicle length
Muscle fascicles are enclosed in the perimysium, which is made up of concentric strands of connective tissue. They are separated from each other by the epimysium, which contains adipose tissue, the arterial and venous supply to the fascicle, and larger peripheral nerve branches. The muscle fascicle structure is a useful diagnostic tool for dermatomyositis. Myocytes towards the edges of the muscle fascicle are typically narrower, while those at the centre are of normal thickness.
Muscle plasticity is an important topic in the fields of sport science and physical therapy. The purpose of some studies has been to examine whether muscle fascicle length increases with resistance training through a comparison between resistance-trained and untrained individuals. If the hypothesis that fascicle length increases with resistance training is true, fascicle length should be longer in the resistance-trained individuals than in the untrained individuals.
In one study, 16 individuals who were either bodybuilders or rugby players were recruited as the training group, and 11 individuals without regular resistance training experience were recruited as the control group. Fascicle length, pennation angle, and muscle thickness of the vastus lateralis and medial gastrocnemius were measured from ultrasonographic images. The muscle thickness and pennation angles in the training group were significantly larger than those in the control group. However, fascicle length did not significantly differ between the two groups. These results indicate that fascicle length is not associated with muscle size, suggesting that fascicle length may not increase with resistance training.
However, some studies have reported increases in fascicle length after resistance training. For example, Potier et al. (2009) found that the fascicle length of the biceps femoris increased after 8 weeks of resistance training, and Seynnes et al. (2007) reported that only 10 days of training increased fascicle length in vastus lateralis. On the other hand, studies by Erskine et al. (2010) and Fukutani and Kurihara (2015) could not confirm an increase in fascicle length after resistance training.
The conflicting results may be due to the relatively short duration of the resistance training interventions. It is thought that it takes about 12 weeks for significant architectural adaptation of the muscle to resistance training. Therefore, longer intervention periods or comparisons between highly trained and untrained individuals may be necessary to determine whether fascicle length increases with resistance training.
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Muscle fascicle length and power vs economy
Muscle fascicles are enclosed in the perimysium, which is made up of concentric strands of connective tissue. They are separated from each other by the epimysium, which contains adipose tissue, the arterial and venous supply to the fascicle, and larger peripheral nerve branches. The arrangement of fascicles, or muscle fibre groups, varies among muscles. The fasciculi may be parallel to the long axis of the muscle, may spiral around the long axis, or may be at an angle to the long axis.
Longer fascicles composed of more serial sarcomeres can achieve faster shortening velocities, allowing for greater power production. Long fascicles likely reduce economy, however, because more energy-consuming contractile units are activated for a given force production. In other words, longer fascicles are associated with both increased power production and locomotor cost. This relationship between muscle fascicle length and power versus economy has been referred to as a "trade-off".
Longitudinal muscle fascicle growth can increase the optimal muscle length for active force. Increased fascicle length is believed to reflect increased myofiber lengths, which can either run the entire fascicle or present intrafascicular terminations. It is believed that longitudinal fascicle growth occurs via an increase in serial sarcomere number (SSN) — a process termed "sarcomerogenesis".
Studies have found that longer gastrocnemius muscle fascicles were correlated with greater lower-body power production and cost of transport. Multiple regression analyses revealed that variability in maximal power was explained by fiber type (46% for cycling, 24% for jumping) and average fascicle length (20% for cycling, 13% for jumping), while average fascicle length accounted for 15% of the variation in cost of transport.
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Longitudinal muscle fascicle growth
Muscle fascicles are enclosed in the perimysium, which is made up of concentric strands of connective tissue. They are separated from each other by the epimysium, which contains adipose tissue, the arterial and venous supply to the fascicle, and larger peripheral nerve branches. Fascicles are bundles of myofibers surrounded by the perimysium, a layer of dense irregular connective tissue.
As myofibers, on the smallest scale, comprise serially aligned sarcomeres, it is believed that longitudinal fascicle growth occurs via an increase in serial sarcomere number (SSN)—a process termed “sarcomerogenesis” that has been corroborated by numerous studies on animals. The traditional hypothesis for sarcomerogenesis is dictated by sarcomere force-length properties: active force production is believed to be optimal within a certain sarcomere length range.
Longitudinal fascicle growth has been advocated in clinical settings for hamstring strain injury prevention in athletes, and as therapy for sarcopenia and spastic muscle. Various interventions indicate that longitudinal muscle fascicle growth can increase the optimal muscle length for active force. Without a concurrent increase in intramuscular collagen, longitudinal muscle fascicle growth also reduces passive tension at long muscle lengths. Some evidence suggests that longitudinal fascicle growth can increase maximum shortening velocity and peak isotonic power.
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Muscle fascicle structure and dermatomyositis
Dermatomyositis is a rare condition that causes muscle inflammation, characterised by muscle weakness, skin rash, and extramacular manifestations such as esophageal dysfunction and interstitial lung disease. It is classified as an idiopathic inflammatory myopathy. The muscle fascicle structure is a useful diagnostic tool for dermatomyositis.
A muscle fascicle is a bundle of skeletal muscle fibres surrounded by perimysium, a type of connective tissue. Muscle cells are grouped into muscle fascicles by enveloping perimysium connective tissue. Fascicles are then bundled together by epimysium connective tissue. Myocytes towards the edges of the muscle fascicle are typically narrower, while those at the centre are of normal thickness. In the heart, specialised cardiac muscle cells transmit electrical impulses from the atrioventricular node (AV node) to the Purkinje fibres.
The onset of dermatomyositis may be insidious or acute with a waxing and waning course. The weakness usually has a subacute onset with the development of gradually progressive symmetric proximal muscle weakness. Patients may report difficulty in carrying out activities such as climbing stairs, getting up from a seated position, lifting objects, combing hair, and raising their head from a pillow.
Dermatomyositis presents with characteristic skin findings and symmetric proximal skeletal muscle weakness. It can affect other organ systems such as the pulmonary, cardiovascular, and gastrointestinal systems. A significant proportion of patients with dermatomyositis have an underlying malignancy, which can alter the prognosis of the condition. Perifascicular atrophy is a hallmark histopathological feature of dermatomyositis. Degenerating and regenerating muscle fibres may be observed in the perifascicular region.
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Muscle fascicle arrangement
Muscle fascicles are bundles of muscle fibres enveloped by perimysium, a layer of dense irregular connective tissue. The arrangement of these fascicles varies among muscles and can be categorised into four primary types: parallel, convergent, pennate, and circular.
Parallel muscles have fascicles that run parallel to the long axis of the muscle. When these muscles contract, they shorten in length and increase in diameter, enabling movement. Examples include the biceps brachii and the rectus abdominis.
Convergent muscles have fascicles that extend over a broad area but converge on a common attachment site, such as a tendon, aponeurosis, or raphe. The pectoralis major is an example of a convergent muscle.
Pennate muscles have fascicles that attach obliquely to a central tendon that runs the length of the muscle, resembling the pattern of feathers along a quill. This arrangement allows for a higher density of muscle fibres and greater force production but with a reduced range of motion and speed. Examples of pennate muscles include unipennate, bipennate, and multipennate muscles.
Circular muscles, also known as sphincters, have fascicles arranged concentrically around an opening. These muscles act as valves or gates, controlling the passage of substances through the body's various pathways, such as the orbicularis oris muscle surrounding the mouth.
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Frequently asked questions
A muscle fascicle is a bundle of muscle fibres enclosed in a layer of connective tissue called perimysium.
Muscle fascicle length is the length of a muscle fascicle. Fascicle length is believed to play an important role in the power versus economy performance trade-off.
It is controversial whether the length of a muscle fibre increases with resistance training. Some studies have found that fascicle length increases after resistance training, while others have found no significant difference.
The optimal muscle fascicle length varies depending on the muscle's function. For example, short muscle fascicles are ideal for maintaining isometric tension, while long muscle fascicles augment maximal power production.
