Evidence-based physiotherapy and rehabilitation resources
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Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Anatomy · General anatomy
Muscle is the only tissue that can shorten on command, and it is the tissue your profession is built around. This chapter is its structure — from the whole muscle down to the unit that actually generates force, and the nerve arrangement that decides how finely you can control it.
All muscle shortens, but the body builds it three different ways depending on what the job demands.
Figure 1 · The three types of muscle
Skeletal muscle is what this chapter is mostly about, because it is what you assess, train and rehabilitate. Note in passing that it is the only one of the three under voluntary control, and the only one attached to the skeleton.
One structural detail is worth pausing on. A skeletal muscle fibre has many nuclei, pushed out to the edge of the cell, because it forms by many small cells fusing together during development. Cardiac and smooth muscle cells each keep a single central nucleus. This is a favourite examination question and it is easy marks.
A muscle is not a single lump of contractile tissue. It is a bundle of bundles, and connective tissue wraps every level.
Figure 2 · Levels of skeletal muscle structure
| Level | What it is | Wrapped by |
|---|---|---|
| Muscle | The whole organ, from one attachment to the other | Epimysium — a dense sheath around the whole muscle |
| Fascicle | A visible bundle of fibres. What you see as the grain of a piece of meat. | Perimysium — carries the larger vessels and nerves |
| Muscle fibre | One long multinucleate cell, running up to the whole length of the muscle | Endomysium — a delicate layer carrying capillaries and the nerve endings |
| Myofibril | A rod-like strand filling the fibre, made of sarcomeres end to end | — |
| Sarcomere | The contractile unit itself, between two Z lines | — |
The detail that explains tendon injury
The three connective tissue layers do not stop at the end of the muscle. They run on and merge to become the tendon. Muscle and tendon are therefore one continuous structure, not two things joined. The weakest point in that chain is usually the musculotendinous junction, which is exactly where strains most often occur.
Everything else exists to serve this. A sarcomere is the stretch of myofibril between two Z lines, and it contains two sets of overlapping filaments.
| Feature | What it contains | What happens on contraction |
|---|---|---|
| Z line | The boundary; thin filaments anchor here | Z lines are pulled towards each other |
| I band | Thin (actin) filaments only | Gets shorter |
| A band | The full length of the thick (myosin) filaments, including where they overlap actin | Does not change length |
| H zone | Thick filaments only, no overlap | Gets shorter, and can disappear |
| M line | The centre, holding thick filaments in place | Stays central |
The filaments themselves do not shorten. They slide past one another, drawing the Z lines together, and the whole sarcomere shortens as a result. Because the A band is defined by the length of the thick filament, it cannot change — which is why "the A band stays the same" is the answer examiners are looking for.
Figure 3 · Skeletal muscle fibre types
Not all skeletal muscle fibres behave alike. Every muscle contains a mixture, and the proportions differ according to what that muscle does.
Postural muscles such as soleus and the deep spinal muscles are rich in type I fibres, because they must hold you upright all day without tiring. Muscles used for bursts of power carry more type II. Training shifts the properties of fibres considerably, and this is one reason endurance and strength training produce such different results.
Why this matters when someone has been unwell
Type II fibres are lost faster than type I during bed rest, illness and ageing. That is why an older adult who has been in hospital often struggles first with the quick tasks — standing up in a hurry, recovering from a stumble — rather than with slow ones. Rehabilitation that only ever moves slowly will not restore what was lost.
Figure 4 · Fascicular arrangements
Illustration to be added
Seven muscle shapes drawn as clean silhouettes with the fascicle direction shown inside each, and a real example named beneath. Parallel or strap (sartorius), fusiform (biceps brachii), unipennate (extensor digitorum longus), bipennate (rectus femoris), multipennate (deltoid), convergent or triangular (pectoralis major), circular (orbicularis oris). Draw the internal tendon clearly in the pennate forms so the angle of pull is visible. Muscle in brick, tendon in pale ivory, navy outlines.
The direction the fascicles run inside a muscle decides what that muscle is good at. There is a straightforward trade-off: fascicles arranged along the length of the muscle give range, and fascicles arranged obliquely give force.
| Arrangement | Fascicles run | Good for | Example |
|---|---|---|---|
| Parallel or strap | Along the length of the muscle | Range of movement and speed of shortening | Sartorius |
| Fusiform | Along the length, with a bulging belly | Range, with a little more force | Biceps brachii |
| Unipennate | Obliquely into one side of a tendon | Force rather than range | Extensor digitorum longus |
| Bipennate | Obliquely into both sides of a central tendon | More force again | Rectus femoris |
| Multipennate | Into several tendons within the muscle | The greatest force for the size | Deltoid |
| Convergent | From a broad origin to a narrow insertion | Force that can be directed, since different parts can act separately | Pectoralis major |
| Circular | In rings around an opening | Closing an orifice | Orbicularis oris and oculi |
The reason pennate muscles are strong is simple once seen: angling the fascicles lets far more of them be packed into the same volume. You lose some range because each fascicle is shorter and pulls at an angle, and you gain a great deal of force.
Muscle is heavily vascular, as it must be. Arteries enter with the nerve, branch through the perimysium, and end in a dense capillary bed running alongside every fibre in the endomysium.
Two consequences follow. First, muscle bleeds substantially when torn, which is why a significant strain produces visible bruising a day or two later, often lower down the limb than the injury. Second, blood flow through muscle is squeezed during strong sustained contraction — which is part of why an isometric hold at high effort tires so quickly.
Figure 5 · The motor unit
A muscle is supplied by a nerve carrying both motor and sensory fibres. The motor axons end at the neuromuscular junction, where the nerve signal is passed to the muscle fibre.
A single motor neuron does not supply a single fibre. It branches and supplies a group of them, and that neuron together with all the fibres it reaches is a motor unit — the smallest amount of muscle the nervous system can switch on. You cannot contract half a motor unit.
The innervation ratio is the number of fibres per neuron, and it decides how finely a muscle can be graded. Muscles needing precision have small units; muscles needing power have large ones.
How force is graded, and why it matters to you
Muscle force is increased in two ways. Recruitment brings in more motor units, and they are recruited in order — small, fatigue-resistant units first, large powerful ones last. Rate coding increases the firing frequency of units already active.
The consequence for practice is direct: light effort never reaches the largest units at all. If a patient needs their type II capacity back, the exercise has to be heavy enough or fast enough to recruit it. Gentle repetitions will not get there.
The muscle also reports back. Muscle spindles lie among the fibres and signal length and the speed of length change; Golgi tendon organs sit at the musculotendinous junction and signal tension. Together they are the basis of the stretch reflex and of your sense of where your limbs are.
| Situation | What happens to the muscle | Recovery |
|---|---|---|
| Disuse — bed rest, immobilisation | Fibres shrink. The number of fibres does not fall. Type II shrinks fastest. Losses begin within days. | Good, with loading. The machinery is intact. |
| Denervation — the nerve supply is lost | Rapid and severe wasting, with fibrillation. The fibres lose their signal entirely. | Depends wholly on whether the nerve recovers. Time-critical. |
| Immobilisation in a shortened position | The muscle loses sarcomeres from the ends of its fibres and becomes functionally shorter. | Position matters. Where possible, immobilise a muscle long rather than short. |
| Training | Fibres enlarge; the muscle grows by hypertrophy. Early strength gains come from better recruitment before any size change. | — |
Two points that change what you do
Strength returns before size. The first few weeks of gain come from the nervous system recruiting better, not from bigger fibres. A patient who says nothing looks different is not failing.
Position during immobilisation matters. A muscle held short adapts by becoming short. Where a choice exists, splinting in a lengthened position saves a great deal of later work.
They slide. Actin and myosin keep their length; the sarcomere shortens because they overlap more. This is why the A band never changes.
The connective tissue of the muscle becomes the tendon. They are continuous, and the junction between them is the usual site of strain.
Arrangement matters as much as size. A multipennate deltoid packs far more fascicles into its volume than a strap muscle of the same weight.
Motor units are recruited smallest first. Low effort never reaches the large, fast, fatigable units, so it cannot restore them.
Disuse shrinks fibres that are otherwise intact and reverses with loading. Denervation removes the nerve signal altogether, wastes faster, and its outcome depends on nerve recovery.
Many nuclei, at the periphery, because the fibre formed by fusion. Cardiac and smooth muscle keep one central nucleus each.
Ten questions on this chapter. Tap one to see the answer and the reasoning.
Answer: (B) The fibre forms when many myoblasts fuse, so it retains all their nuclei, pushed to the periphery by the myofibrils. Cardiac and smooth muscle cells keep one central nucleus.
Answer: (C) Endomysium, a delicate layer carrying the capillaries and the nerve endings. Perimysium wraps fascicles and epimysium the whole muscle.
Answer: (C) The A band is defined by the length of the thick filaments, and the filaments themselves do not shorten. They slide, so the I band and H zone narrow while the A band is unchanged.
Answer: (B) A small ratio, with few fibres per neuron, so force can be adjusted in very small steps. Powerful muscles such as gastrocnemius have very large ratios.
Answer: (C) Smallest first. This is why low-effort exercise never reaches the large, fast, fatigable units, and cannot restore them.
Answer: (C) Pennation packs more fascicles into a given volume. The cost is range, since each fascicle is shorter and pulls at an angle.
Answer: (B) The musculotendinous junction is the weakest link in a chain that runs continuously from muscle connective tissue into tendon.
Answer: (B) Type II. This is why an older adult after hospital admission struggles first with quick tasks, such as standing up in a hurry or recovering from a stumble.
Answer: (B) Muscle adapts its number of sarcomeres to the length it is held at. Where a choice exists, immobilise in a lengthened position.
Answer: (C) Neural adaptation comes first. Visible size change follows later, which is worth telling a patient who thinks nothing is happening.
Everything on this page, in one screen
| Book | What it adds here |
|---|---|
| Anatomy and Human Movement: Structure and Function Palastanga, Field and Soames |
The best account for a physiotherapist of how fascicular arrangement translates into force and range. |
| B D Chaurasia's Handbook of General Anatomy Chaurasia and Garg |
The chapter on muscles, closely matched to Indian examination questions. |
| BRS Cell Biology and Histology Gartner |
The sarcomere, the neuromuscular junction and the three muscle types in microscopic detail. |
| The Concise Book of Muscles Jarmey |
A muscle-by-muscle reference for when you begin the regional chapters. |
Reviewed by the Physiotherapist India Team. · Human Anatomy contents