Muscle

What it is

The body has three kinds of muscle tissue: skeletal, cardiac and smooth¹. This page covers skeletal muscle, which depends completely on signals from the nervous system¹. Skeletal muscles make movement and also stop it¹. They hold posture against gravity, keep joints stable and make heat¹.

Fibres and their wrappings

Skeletal muscle cells are called muscle fibres¹. Each is a long cylinder with many nuclei, formed when young cells fuse before birth¹. Three layers of connective tissue (tissue packed with collagen, a strong, rope-like protein) wrap each muscle¹. One surrounds the whole muscle, one each fascicle (bundle of fibres) and one each single fibre¹.

In muscles that pull on bone through a tendon (a tough cord joining muscle to bone), the layers’ collagen weaves into the tendon’s collagen¹. So the pull the fibres make passes through the layers to the tendon, then on to the bone¹.

Slow and fast fibres

Fibres fall into three main types, sorted by how fast they contract and how they make energy¹. The three are slow oxidative, fast oxidative and fast glycolytic, and most human muscles hold all three¹. Slow oxidative fibres are thin, resist fatigue and make little tension¹. Fast glycolytic fibres are thick, make high tension for quick moves, and tire quickly¹. A muscle’s main job sets its main fibre type; posture muscles, for example, hold many slow fibres¹.

A motor unit is one nerve cell and the fibres it switches on¹. Small units fire first, and larger ones join as more force is needed¹.

How it responds to load

Bigger fibres, and the debate over new ones

A meta-analysis (a study that pools the results of several earlier studies) looked at fibre number⁵. It states that resistance exercise grows human muscle mainly by hypertrophy, the enlarging of fibres⁵. It notes some debate over whether hyperplasia (new fibres) also occurs⁵. Pooling 11 human studies, it found no significant change in fibre number after training of up to 6 months⁵. The authors add that a change may be hard to detect, as trials were short and fibre number is hard to estimate⁵.

On the other side, a 1996 meta-analysis of 17 animal studies found that overload raised fibre number by about 15% on average⁶. A 2020 review states that whether new fibres can form in adult muscle is debated, with no firm conclusion yet⁷.

A review names faster protein building as the key route, though not the only one, by which resistance exercise makes muscle grow⁸. Satellite cells (muscle stem cells) sit just outside the fibre’s membrane and join it under some kinds of stress, mainly to repair damage¹.

Progressive overload

In a trial in 55 untrained young women, each arm was randomly given rising loads, the same load throughout, or no exercise⁹. After 8 weeks of elbow extension, the triceps thickened in both trained arms⁹. The rising-load arms grew about twice as much, about 23% against 12%⁹. The authors note that the same load still grew these women’s muscle, and that whether it keeps working longer is an open question⁹.

In 39 untrained young men and women, legs were randomly set to add either load or repetitions for 10 weeks¹⁰. Both legs gained strength and muscle size, with no difference between the two¹⁰. In 43 people with at least a year of lower-body training, the two ways gave similar changes over 8 weeks¹¹.

Damage and soreness

A review states that eccentric (lengthening) exercise damages muscle unused to it more than other exercise does¹². A 2019 review states that such damage is probably not essential for growth, and that whether it adds to growth is still unclear⁸.

A review states that delayed-onset muscle soreness (soreness that starts hours to days after exercise) usually appears 12 to 24 hours after unaccustomed eccentric exercise¹². It peaks between 24 and 72 hours and is gone within 5 to 7 days¹². How sore a muscle feels matches other damage markers poorly, so soreness seems a weak gauge of damage¹².

How it gets injured

Naming muscle injuries

A survey behind the 2013 Munich consensus found wide variation in muscle injury terms, especially for “strain”¹³. The consensus sorts injuries into functional disorders with no visible tear, and structural injuries with a tear¹³.

How common and where

In one cohort study of 2,299 male professional footballers, muscle injuries were 31% of all injuries and 27% of time lost³. Of these, 92% hit four leg muscle groups: hamstrings, adductors, quadriceps and calf³. In the same players, 16% were reinjuries, which kept players out significantly longer than first injuries³.

A review of 21 video studies covered 728 non-contact and indirect muscle injuries in sport¹⁴. Most happened while running or in sport moves that change muscle length while it is contracting¹⁴. Studies of athletes found older age and a previous strain the strongest risk factors for hamstring and calf strains^15,16.

Training load and injury

A systematic review of 22 studies found evidence linking the acute:chronic workload ratio (recent training load compared with longer-term load) with non-contact injuries¹⁷. A 2020 critique argues that no study has properly estimated whether the ratio causes injury¹⁸. It finds no evidence for using it to lower injury risk¹⁸. The ratio evidence covers non-contact injuries in general, not muscle injuries alone¹⁷.

How muscle heals

A review describes three phases of repair¹⁹. First comes breakdown with inflammation, then satellite cells wake and multiply, then the new fibres mature¹⁹. The same review says regeneration usually starts within 4 to 5 days, peaks at about 2 weeks and fades by 3 to 4 weeks¹⁹.

By the same review’s account, early scarring helps, steadying the tissue as a scaffold for new fibres¹⁹. Too much collagen after injury, though, often enlarges the scar and can stop the muscle working normally¹⁹. A 2020 review states that muscle can regenerate fully after minor injuries²⁰. After severe ones, scar tissue keeps it from recovering to a functional level²⁰.

How training builds it

Size and strength

In one network meta-analysis of trials in healthy adults, every resistance training plan tested beat no exercise for muscle growth². Every plan also beat no exercise for strength; loads over 80% of the one-repetition maximum (heaviest single lift) gave the most². The resistance training page covers doses in detail.

A review of heavy strength training finds the main structural change is a larger cross-section of the whole muscle and of each fibre²¹. It says this comes from more and larger myofibrils (contracting strands inside a muscle fibre), and that satellite cells switch on very early²¹. A 2020 review states that whether fibres grow through larger myofibrils, more of them, or both is still unresolved⁷.

The first weeks

A review concludes that strength gains come from both nerve and structural changes²¹. Nerve changes may count most early on, but growth starts at the outset too²¹. A meta-analysis of 30 trials found subtle training changes in the nervous system, in and below the brain’s outer layer, that likely add to strength²². A review puts forward that size gains in the first sessions are mostly swelling, with true growth not seen until about 18 sessions²³.

How much, and who gains most

An analysis of 67 studies in mostly young men found size and strength gains rose with training volume, with diminishing returns, more so for strength²⁴. Across 28 studies in 747 healthy adults, light, moderate and heavy loads taken to failure did not differ in growth²⁵. Strength gains were greater with heavy and moderate loads than light ones²⁵. After 12 weeks of one-arm training in 585 adults, biceps size changes ranged from −2% to +59%²⁶.

Endurance training changes muscle in other ways. A review pooled training studies in 5,973 people²⁷. Continuous endurance training raised mitochondria (the small parts inside a cell that release energy from food) by about 23%²⁷. Capillaries (tiny, thin-walled blood vessels) per fibre rose about 15%, mostly in the first 4 weeks and only in untrained to moderately trained people²⁷. The review concludes that less fit people change more, and that the ability to adapt lasts throughout life²⁷.

Across the lifespan

Children and teenagers

A pooled analysis of trials in healthy under-18s found resistance training raised strength²⁸. The capacity to gain strength seemed to grow with age and maturity, with no clear jump at puberty²⁸.

A 2008 Canadian position paper states that children’s strength gains have been put down mainly to nerve changes, though scans suggest growth may occur too²⁹. A 2023 systematic review of 22 studies in 604 prepubertal children (before puberty) found strength rose in every case³⁰. In its authors’ view, gains over 8 to 12 weeks come mainly from nerve changes, and small structural changes probably matter little for strength³⁰. A 2021 review found that a few such studies reported trivial to moderate changes in muscle size and motor unit activation³¹.

A meta-analysis of 23 studies in 1,443 under-18s found a small rise in muscle size against inactive controls³². A literature review states there is no evidence that resistance training harms growth in height³³.

Adults and older age

A review drew on studies following people aged 75 over time⁴. Muscle mass fell 0.64–0.70% a year in women and 0.80–0.98% in men⁴. Strength fell faster: about 3–4% a year in men and 2.5–3% in women⁴. The same review finds that losing strength is more consistently linked to disability and death than losing mass⁴.

A review states that nerve studies confirm fewer motor units with age, and some remaining units grow larger⁴. Another review states that, fibre by fibre, age-related loss is mainly shrinking of type II fibres³⁴. A meta-analysis of 55 studies of master athletes, training for 20 years or more with mean ages over 59, looked at long-term training³⁵. It suggests such training keeps strength and physical function at levels like those of young, healthy people, in ways that depend on the type of exercise³⁵.

A 2019 European consensus describes sarcopenia as a muscle disease built up from harmful muscle changes over a lifetime³⁶. It is common in older adults but can start earlier, and counts low strength as a key feature³⁶. Across 151 studies of people with a mean age of 68.5, sarcopenia affected 10% to 27%, depending on how it was defined³⁷.

A Cochrane review (a systematic review to Cochrane’s published standards) pooled 121 trials in 6,700 older people³⁸. Progressive resistance training had a large effect on strength³⁸. In 25 trials of healthy adults with a mean age of 65 or over, strength rose substantially; in the nine that measured it, muscle size rose little³⁹. A review of 14 studies in 528 older adults with sarcopenia found that supervised resistance training gave a small but significant gain in muscle mass⁴⁰.

Connections

In one cohort of male professional footballers, 92% of muscle injuries hit the hamstrings, adductors, quadriceps and calf³. The library has entries on the hamstrings, hip adductors, quadriceps, gastrocnemius and soleus. Other entries cover the gluteus maximus, gluteus medius, gluteus minimus, rectus abdominis, external oblique, internal oblique and transversus abdominis.

Postural muscles tend to hold many slow fibres¹. Back and upper-body entries cover the erector spinae, multifidus, deltoid, pectoralis major, latissimus dorsi, trapezius and serratus anterior. Arm entries cover the biceps brachii and triceps brachii.

Where muscles pull on bone, they do it through a tendon, and skeletal muscle depends on nerve signals to work¹. Training methods have their own pages: resistance training, eccentric training, isometric training and blood flow restriction training.

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Every source the library cites

Changes

• September 30, 2026 First published.