Skeletal Muscle
Original Editor - Lucinda hampton
Top Contributors - Ewa Jaraczewska, Lucinda hampton, Nupur Smit Shah and Jess Bell
Introduction
Skeletal muscle is a highly versatile tissue. It enables voluntary movement and is involved in metabolic, immune, and endocrine functions. Understanding the complex architecture and multiple roles of skeletal muscle is essential for rehabilitation professionals to optimise human movement and function.
There are three types of muscle tissue, each with unique structural and functional properties. Cardiac muscle powers the heart's rhythmic contractions. Smooth muscle facilitates involuntary movements in organs and vessels. Skeletal (or striated) muscle operates under voluntary control and attaches to bones via tendons to produce movement and provide stability.
This article discusses the architecture of skeletal muscle, its functional contractile units, the classification of skeletal muscle fibres, and its role as a metabolic, endocrine, and immune organ.
Skeletal Muscle Organisation
Skeletal muscle is organised into distinct units.
At the organ level, the entire skeletal muscle is wrapped in a dense connective tissue sheath called the epimysium, which provides structural integrity and facilitates force transmission. Within this outer covering, muscle fibres are organised into bundles called fascicles. Each fascicle is wrapped in a connective tissue layer, called the perimysium. This intermediate layer contains blood vessels and nerves that supply the enclosed muscle fibres. This structure allows each fascicle to function independently.[1]
Elongated muscle cells or muscle fibres are the cellular components of skeletal muscles. Each muscle fibre (myocyte) is surrounded by a connective tissue layer called the endomysium. The endomysium contains capillaries and nerve endings that are essential for muscle fibre function. These muscle fibres often extend the entire length of the muscle and contain hundreds of nuclei to support their extensive protein synthesis requirements.[1]
Each muscle fibre contains long cylindrical structures, called myofibrils, that run parallel to the fibre's length. These are the contractile elements of the skeletal muscle. They contain contractile proteins and are responsible for generating the force that produces movement. The regular arrangement of contractile proteins within myofibrils creates the characteristic striated appearance of skeletal muscle; hence, it is also known as striated muscle.[1]
Sarcomere
Each myofibril contains thousands of sarcomeres arranged in series. The sarcomere is the functional unit of skeletal muscle. It consists of two primary protein filaments: actin and myosin. Thin actin filaments are anchored at the sarcomere's boundaries, called Z-lines, and extend toward the centre. Thick myosin filaments occupy the central region and contain specialised heads that can bind to actin filaments to form cross-bridges.[2]
During muscle contraction, myosin heads bind to actin filaments and undergo a conformational change that pulls the actin filaments toward the sarcomere's centre. This sliding filament mechanism results in greater overlap between actin and myosin filaments, causing the sarcomere to shorten while maintaining constant filament lengths. The coordinated shortening of millions of sarcomeres throughout the muscle produces macroscopic muscle contraction. During muscle relaxation, the cross-bridges between actin and myosin are broken, allowing the filaments to slide apart. This reduces the amount of overlap and lengthens the sarcomeres, returning the muscle to its resting length.[2]
Classification of Skeletal Muscle Fibres
Muscle fibres are typically classified based on two key factors: their functions and innervation patterns, and their metabolic and contractile properties.
1. Classification Based on Function and Innervation Patterns
This approach to classification differentiates between extrafusal and intrafusal muscle fibres.
Extrafusal fibres make up the majority of muscle tissue and are responsible for generating the contractile force that moves joints. Extrafusal fibres are innervated by alpha motor neurons. All fibres supplied by a single alpha motor neuron form a motor unit, which represents "the smallest unit of force that can be activated to produce movement."[3]
Intrafusal fibres are less numerous and serve a sensory function within specialised structures called muscle spindles. They are innervated by gamma motor neurons. Intrafusal fibres respond to changes in muscle length and provide the central nervous system with continuous feedback about muscle stretch and position.[4]
2. Classification Based on Metabolic and Contractile Properties
Skeletal muscle fibres can also be classified into three types based on their metabolic and contractile properties: Type I fibres, Type IIa fibres, and Type IIb fibres.[5]
Type I fibres are slow-oxidative or slow-twitch fibres. These fibres contract slowly, produce moderate force levels, and resist fatigue due to their high oxidative capacity. Type I fibres contain numerous large mitochondria that support aerobic energy production. They also contain high concentrations of oxidative enzymes and myoglobin that facilitate oxygen utilisation. The rich capillary supply to Type I fibres ensures adequate oxygen delivery for sustained activity.[5]
Type IIa fibres are fast-oxidative or fast-twitch fibres. These fibres contract rapidly and generate high force levels but fatigue more quickly than Type I fibres. They possess numerous mitochondria for aerobic metabolism but can switch to anaerobic pathways when needed. This metabolic flexibility allows Type IIa fibres to support both endurance and power activities.[5]
Type IIb fibres are fast-glycolytic fibres. They are ideal for brief, high-intensity activities. These fibres contract rapidly and produce the highest force levels, but fatigue quickly due to their reliance on anaerobic metabolism. They contain fewer mitochondria but have high concentrations of glycolytic enzymes that enable rapid energy production without oxygen, making them ideal for explosive movements.[5]
Fibre Type Distribution and Muscle Function
Individual genetic variation influences fibre type distribution. However, training can induce adaptations within fibre types in response to training.[6]
Most skeletal muscles contain a mix of different fibre types. Their distribution varies based on the muscle's functional demands and individual genetic factors, and can be adapted to specific training stimuli and repeated tasks. For example, postural muscles contain higher proportions of Type I fibres to support the sustained, low-level contractions required for postural maintenance during daily activities.[7] The fatigue resistance of Type I fibres allows these muscles to function continuously without experiencing significant fatigue.
Endurance athletes typically have higher proportions of Type I and Type IIa fibres, while strength and power athletes tend to have more Type IIb fibres.
Metabolic, Endocrine and Immune Roles of Skeletal Muscle
While the primary role of skeletal muscle is to produce movement, it also serves as a metabolic organ, an active endocrine organ, and plays an important role in immune system function.
Metabolic Organ
Skeletal muscle acts as the body's primary protein reservoir, storing amino acids that can be mobilised during periods of increased demand or inadequate dietary intake. During catabolic states, muscle proteins can be broken down to provide amino acids for the essential functions of other organs. Additionally, these amino acids serve as gluconeogenic precursors for glucose production. This metabolic flexibility helps maintain stable blood glucose levels and supports essential physiological processes during periods of stress or fasting.[8]
Skeletal muscle is also involved in glucose metabolism. Skeletal muscle accounts for more than 75% of all insulin-mediated glucose uptake in the body. Muscle contraction enhances glucose uptake through insulin-dependent and insulin-independent mechanisms, highlighting the importance of physical activity in metabolic health.[9]
Mitochondria in skeletal muscle produce adenosine triphosphate (ATP) for muscle contraction and play a crucial role in maintaining cellular health and longevity. Exercise training increases mitochondrial content and improves respiratory function, thereby contributing to enhanced quality of life and potentially extending life expectancy.[10]
Endocrine Organ
Skeletal muscles produce and release signalling molecules called myokines. Myokines are responsible for regulating various physiological processes, and they contribute to the health benefits associated with regular physical activity.[11]
Myokines released during muscle contraction can influence metabolism in distant tissues, modulate inflammatory responses, and affect organ function throughout the body. This endocrine function helps explain many of the systemic health benefits associated with regular exercise beyond the local adaptations within the exercised muscles themselves.[11]
However, disrupted production or function of myokines contributes to the pathogenesis of various metabolic diseases, including obesity and type 2 diabetes.[8]
Immune Function
Skeletal muscle plays an important role in immune system function by producing glutamine, a non-essential amino acid that serves as the primary energy source for immune cells, including lymphocytes and monocytes/macrophages. Glutamine production supports immune cell function and enhances the body's ability to fight infections. Regular, moderate exercise and physical activity can increase glutamine production, thereby enhancing immune function and contributing to overall health and disease resistance. However, excessive exercise can temporarily suppress the immune system due to glutamine depletion.[12]
Conditions Affecting Skeletal Muscles
Skeletal muscle can be affected by a wide range of conditions. Genetic disorders, such as muscular dystrophy, cause progressive muscle degeneration.[13][14] Systemic conditions, like heart failure and diabetes, alter muscle metabolism and fibre composition.[15] Neuromuscular conditions, such as myasthenia gravis, disrupt nerve-muscle communication. Acute injuries, such as muscle or tendon injuries, can have a sudden impact on function. Understanding normal muscle physiology is essential for recognising and managing these diverse conditions.[4]
References
- ↑ 1.0 1.1 1.2 Dave HD, Shook M, Varacallo MA. Anatomy, Skeletal Muscle. [Updated 2023 Aug 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from https://www.ncbi.nlm.nih.gov/books/NBK537236/ [last access 28.5.2025]
- ↑ 2.0 2.1 Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997. Section II, The Organization, Structure, and Function of Muscle. Available from https://www.ncbi.nlm.nih.gov/books/NBK20168/ [last access 28.5.2025]
- ↑ Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. The Motor Unit.Available from: https://www.ncbi.nlm.nih.gov/books/NBK10874/ [last access 28.5.2025]
- ↑ 4.0 4.1 Frontera WR, Ochala J. Skeletal muscle: a brief review of structure and function. Calcif Tissue Int. 2015 Mar;96(3):183-95.
- ↑ 5.0 5.1 5.2 5.3 Zierath JR, Hawley JA. Skeletal muscle fibre type: influence on contractile and metabolic properties. PLoS Biol. 2004 Oct;2(10):e348.
- ↑ Hostrup M, Deshmukh AS. Fiber Type-Specific Adaptations to Exercise Training in Human Skeletal Muscle: Lessons From Proteome Analyses and Future Directions. Scand J Med Sci Sports. 2025 May;35(5):e70059.
- ↑ Demoulin C, Crielaard JM, Vanderthommen M. Spinal muscle evaluation in healthy individuals and low-back-pain patients: a literature review. Joint Bone Spine. 2007 Jan;74(1):9-13.
- ↑ 8.0 8.1 Feraco A, Gorini S, Armani A, Camajani E, Rizzo M, Caprio M. Exploring the Role of Skeletal Muscle in Insulin Resistance: Lessons from Cultured Cells to Animal Models. Int J Mol Sci. 2021 Aug 28;22(17):9327.
- ↑ Merz KE, Thurmond DC. Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake. Compr Physiol. 2020 Jul 8;10(3):785-809.
- ↑ Dong H, Tsai SY. Mitochondrial Properties in Skeletal Muscle Fiber. Cells. 2023 Aug 30;12(17):2183.
- ↑ 11.0 11.1 Hoffmann C, Weigert C. Skeletal Muscle as an Endocrine Organ: The Role of Myokines in Exercise Adaptations. Cold Spring Harb Perspect Med. 2017 Nov 1;7(11):a029793.
- ↑ Rogeri PS, Gasparini SO, Martins GL, Costa LKF, Araujo CC, Lugaresi R, Kopfler M, Lancha AH Jr. Crosstalk Between Skeletal Muscle and Immune System: Which Roles Do IL-6 and Glutamine Play? Front Physiol. 2020 Oct 16;11:582258.
- ↑ Nigro V, Piluso G. Spectrum of muscular dystrophies associated with sarcolemmal-protein genetic defects. Biochim Biophys Acta. 2015 Apr;1852(4):585-93.
- ↑ Dubuisson N, Versele R, Planchon C, Selvais CM, Noel L, Abou-Samra M, Davis-López de Carrizosa MA. Histological Methods to Assess Skeletal Muscle Degeneration and Regeneration in Duchenne Muscular Dystrophy. Int J Mol Sci. 2022 Dec 16;23(24):16080.
- ↑ Kinugawa S, Takada S, Matsushima S, Okita K, Tsutsui H. Skeletal Muscle Abnormalities in Heart Failure. Int Heart J. 2015;56(5):475-84.