Evidence-based physiotherapy and rehabilitation resources
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Evidence-based physiotherapy and rehabilitation resources
Editorial & review policyHuman Anatomy · General anatomy
No muscle ever acts alone. Every movement you will ever analyse is a group of muscles working in defined roles, pulling on levers, at lengths that change what they can produce. This chapter is how that actually works.
A muscle is conventionally described as running from an origin, the more fixed attachment, to an insertion, the more mobile one. When the muscle shortens, the insertion is pulled towards the origin.
That is a useful convention and it is not a law. Which end moves depends entirely on which end is free.
Figure 1 · Origin, insertion and reversed action
Illustration to be added
Two paired panels. Left pair: psoas major shown from the lumbar spine to the lesser trochanter. Panel A, a person lying supine lifting the leg, with the spine fixed and the femur moving, arrows showing the pull. Panel B, the same person performing a sit-up, femur fixed by the weight of the legs, lumbar spine moving towards it. Right pair: the same principle at the elbow, biceps brachii lifting the forearm to the arm, then a pull-up where the arm is drawn to the fixed forearm. Label origin and insertion in each, and mark clearly which end is fixed. Muscle in brick, bone in warm ivory, movement arrows in gold.
Reversed origin and insertion
Think about psoas major, running from the lumbar spine to the femur. Lying down and lifting your leg, the femur moves towards the spine — the conventional action.
Now sit up from lying. The femur is fixed by the weight of your legs, so the same muscle pulls the lumbar spine towards the femur instead. Same muscle, same contraction, opposite end moving. Miss this and you will misread half the movements you observe.
The practical rule: ask which end is fixed at that moment, then work out what the pull must do. Whether a textbook calls that end the origin is beside the point.
Figure 2 · The four roles a muscle can play
In any given movement, the muscles crossing a joint fall into roles. The same muscle takes a different role in a different movement, so these are jobs rather than labels.
Two of them deserve extra attention because students routinely skip them.
A synergist often exists to cancel an unwanted action. The long finger flexors cross the wrist as well as the fingers, so gripping hard would flex your wrist and put the fingers in a useless position. The wrist extensors fire to prevent it. That is why grip strength falls sharply when the wrist is flexed, and why a wrist problem can present as a weak hand.
A fixator gives the agonist something firm to pull against. Raising your arm requires the scapula to be held, otherwise the deltoid would simply drag the scapula towards the humerus. This is the entire anatomical basis of scapular stability work.
Figure 3 · The three kinds of contraction
| Type | Length | What it is used for | Notes |
|---|---|---|---|
| Isometric | Unchanged | Holding a position; stabilising a joint while another moves | No joint movement, so it can be started very early after injury. Strength gains are largely specific to the angle trained. |
| Concentric | Shortens | Producing movement against a resistance | The muscle overcomes the load. |
| Eccentric | Lengthens under tension | Controlling and braking movement | Produces the greatest force of the three, uses less energy, and causes most of the soreness felt a day or two later. |
How much of the day is eccentric
Walking downstairs, sitting into a chair, putting a cup down, absorbing each step in running: all eccentric. The quadriceps do more braking than lifting on a normal day. Any rehabilitation that only trains the shortening direction has left out most of what the muscle is actually asked to do.
You will also meet two terms describing conditions rather than contraction types. Isotonic means the load stays constant while the muscle changes length, which is what happens lifting a fixed weight. Isokinetic means the speed of movement is held constant by a machine, so resistance varies to match the effort.
Figure 4 · The three classes of lever
Every joint is a fulcrum, every muscle supplies effort, and every body part or object supplies load. The arrangement of those three decides how efficiently force is transmitted.
| Class | Order | Body example | Mechanical result |
|---|---|---|---|
| First | Effort, fulcrum, load | The head balanced on the atlas; triceps extending the elbow | Can favour either force or range, depending on where the fulcrum sits |
| Second | Fulcrum, load, effort | Rising on to tiptoe, with the toes as fulcrum | Favours force. Rare in the body. |
| Third | Fulcrum, effort, load | Biceps bending the elbow; almost every muscle in the body | Favours range and speed at the cost of force. The muscle must produce more force than the load it moves. |
The consequence nobody spells out
The body is built almost entirely from third class levers, which are mechanically inefficient for force. Holding a small weight in your hand requires your elbow flexors to generate many times that force, because the muscle attaches close to the joint while the load sits far away. The body accepted that trade because it buys speed and range — a small muscle shortening slightly moves the hand a long way, quickly. That is a good bargain for a creature that needs to move, and a bad one for a joint carrying the resulting load.
This also explains why holding a load further from the body is so much harder. Moving the weight away lengthens the load arm, and the muscle must answer with proportionally more force. It is the reason you teach patients to carry close to the trunk, and the reason a long lever position is used deliberately when you want an exercise to be harder.
A muscle does not produce the same force at every length. Force depends on how much the thick and thin filaments overlap.
| Muscle length | Filament overlap | Force produced |
|---|---|---|
| Very short | Filaments crowd and interfere | Low |
| Slightly longer than resting | Optimal — the greatest number of cross-bridges can form | Greatest |
| Very long | Too little overlap to form cross-bridges | Low from the contractile part, though passive tension in the connective tissue rises |
This is why muscle testing position matters so much. Test a muscle at the extreme of its range and it will appear weak even when it is normal. Standardising the position is not pedantry; it is the only way two measurements can be compared.
A muscle crossing two joints cannot be at the optimum length for both at once, and this produces two named problems that examiners like.
Figure 5 · What stops a joint moving further
The muscle is too short to produce useful force, because it is shortened over both joints at once.
Try making a tight fist with your wrist fully flexed. The grip is feeble — the long finger flexors are shortened at wrist and fingers together, and have run out of overlap.
The muscle is too long to allow full range, because it is stretched over both joints at once, and it now limits the movement itself.
Straighten your knee and try to flex your hip fully. The hamstrings, stretched at both ends, stop you long before the hip joint does.
The clinical use of this is immediate. If range changes when you move a neighbouring joint, the limit is muscle length, not the joint. If strength changes when you move a neighbouring joint, you have found a two-joint muscle working at a poor length. Both are tested in seconds and both change what you treat.
Muscle names are descriptions, not arbitrary labels. Once you can read them, hundreds of names stop needing to be memorised.
| Named after | Clue in the name | Example |
|---|---|---|
| Shape | deltoid, trapezius, rhomboid, quadratus | Deltoid — shaped like a triangle |
| Size | maximus, minimus, longus, brevis, major, minor | Gluteus maximus — the largest of the three |
| Location | brachii, femoris, oris, oculi, dorsi, abdominis | Biceps brachii — in the arm |
| Direction of fibres | rectus, oblique, transversus | Rectus abdominis — running straight up and down |
| Number of heads | biceps, triceps, quadriceps | Triceps — three heads of origin |
| Action | flexor, extensor, adductor, levator, supinator | Extensor digitorum — extends the fingers |
| Attachments | the two ends, named in order | Sternocleidomastoid — sternum and clavicle to mastoid process |
Most names combine several. Flexor digitorum profundus tells you the action, what it acts on, and that it is the deep one of two — before you have read a word about it.
Either end can move. Ask which end is fixed in the movement you are watching, then work out the consequence.
An answer that names the prime mover and stops is half an answer. The synergists and fixators are usually where the clinical problem is.
Most daily muscle work is eccentric braking. A programme built only on lifting has missed the majority of what the muscle does.
A muscle tested at the end of its range will look weak when it is not. Standardise the position or the measurement means nothing.
Active is too short to pull; passive is too long to allow movement. The words describe what the muscle is doing, not what the joint is.
It is a deliberate trade. The body gave up mechanical advantage to buy speed and range, which is what a moving animal needs.
Ten questions on this chapter. Tap one to see the answer and the reasoning.
Answer: (B) Which end moves depends on which end is fixed. With the legs weighted down, the femur becomes the fixed point and the spine moves instead.
Answer: (C) Synergist, cancelling the unwanted wrist flexion the long finger flexors would otherwise produce. It is why grip weakens sharply with the wrist flexed.
Answer: (C) Eccentrically. The muscle produces tension while lengthening, braking the movement. Most everyday muscle work is of this kind.
Answer: (C) Eccentric. It also uses less energy for the same force, and it accounts for most of the soreness felt a day or two after unaccustomed exercise.
Answer: (B) Second class, with the toes as fulcrum, body weight as load and the calf as effort. It is one of the few second class levers in the body.
Answer: (B) A small shortening of a muscle attached near the joint moves the far end of the limb a long way, quickly. The cost is that the muscle must generate more force than the load.
Answer: (B) At the length giving maximum overlap between thick and thin filaments, which is around or just beyond resting length. This is why testing position must be standardised.
Answer: (B) Active insufficiency. The long finger flexors are shortened across the wrist and the fingers at once, and have too little filament overlap left to pull effectively.
Answer: (B) Passive insufficiency. The hamstrings are stretched over hip and knee together and run out of length, limiting the movement themselves.
Answer: (C) Its attachments, named in order: sternum and clavicle to the mastoid process. Many muscle names are descriptions that remove the need to memorise them.
Everything on this page, in one screen
| Book | What it adds here |
|---|---|
| Anatomy and Human Movement: Structure and Function Palastanga, Field and Soames |
The core text for this chapter. Group action, levers and insufficiency are treated as the main subject rather than an aside. |
| B D Chaurasia's Handbook of General Anatomy Chaurasia and Garg |
Concise coverage of muscle roles and naming, in examination language. |
| The Concise Book of Muscles Jarmey |
Muscle by muscle, with actions listed. Useful once you begin the regional chapters. |
| Clinical Anatomy by Regions Snell |
For seeing these principles applied to each region in turn. |
Reviewed by the Physiotherapist India Team. · Human Anatomy contents