How Muscles Work Together

Part 1 · General anatomy

Agonists and fixators, contraction types, levers, moment arms, and the two insufficiencies

The problem with learning muscles one at a time

A muscle table tells you that biceps brachii flexes the elbow. It does not tell you that biceps is a weak elbow flexor when the forearm is pronated, that it is a powerful supinator, that it cannot flex the elbow effectively without the scapula and shoulder being stabilised, that it becomes actively insufficient when the shoulder is flexed and the elbow fully flexed, or that in a pull-up its “insertion” is fixed and its “origin” moves.

No muscle acts alone, and no muscle has a single action. This chapter is the grammar that turns a vocabulary of muscles into an account of movement.

Learning outcomes

• Define and identify agonist, antagonist, synergist, fixator and neutraliser in a described movement.
• Explain reciprocal inhibition and co-contraction, and when each is appropriate.
• Distinguish isometric, concentric and eccentric contraction, and identify which is occurring in a given task.
• Distinguish open- and closed-chain movement and explain the mechanical differences.
• Define torque, moment arm and lever, classify levers, and calculate simple mechanical advantage.
• Explain why most human levers are third-class and what that costs.
• Explain active and passive insufficiency and demonstrate them at named joints.
• Explain the tenodesis effect and its clinical use.
• Explain force couples, with the scapula and pelvis as worked examples.
• Explain the difference between local stabilisers and global mobilisers, and evaluate the concept critically.

The roles a muscle can play

Figure 1 · The four roles a muscle can play

Figure 2 · Origin, insertion and reversed action

A muscle’s role is defined by the task, not by the muscle. The same muscle takes different roles in different movements.

Worked example — making a fist

The agonists are flexor digitorum superficialis and profundus. But those muscles cross the wrist, and if the wrist were free it would flex, shortening the flexors into active insufficiency and destroying grip. So the wrist extensors contract as synergists, holding the wrist in ~30° extension where the finger flexors are at their optimal length.

This is not theory. Denervate or inhibit the wrist extensors — as in a radial nerve palsy — and grip strength falls by roughly 70%, despite the finger flexors and their median/ulnar nerve supply being entirely intact. A patient with wrist drop is not primarily complaining of an inability to extend the wrist; they are complaining that they cannot hold anything. Splinting the wrist in extension restores most of the grip immediately, which is one of the most satisfying interventions in hand rehabilitation.

Reciprocal inhibition versus co-contraction

• Reciprocal inhibition — Ia afferents from the contracting agonist’s spindles inhibit the antagonist’s motor neuron pool through an inhibitory interneuron. This is efficient and is the default for fast, unopposed movement.
• Co-contraction — simultaneous activation of agonist and antagonist. It increases joint stiffness and stability at a metabolic cost and with a loss of speed. It is appropriate and normal in unstable, unfamiliar or high-precision tasks, in the early stage of motor learning, and around a joint whose passive restraints are compromised.

Excessive persistent co-contraction is, however, a common maladaptive pattern — after knee injury, in chronic low back pain, in anxiety-related bracing, and in the neurologically impaired patient. Distinguishing protective co-contraction (appropriate) from habitual co-contraction (a treatment target) is a core clinical judgement.

Types of muscle work

Figure 3 · The three kinds of contraction

Three properties of eccentric work matter clinically:

• Highest force capacity (~1.2–1.8 × isometric maximum), so it permits loading beyond concentric capacity.
• Lowest metabolic cost for a given force, which is why eccentric-biased training is feasible in cardiac and respiratory populations.
• Greatest mechanical stress, hence the most damage, the most DOMS, the strongest hypertrophic and tendon-remodelling stimulus, and the highest strain-injury risk.

Eccentric loading is the single best-evidenced intervention in tendinopathy (Alfredson’s protocol for the Achilles; heavy slow resistance as the modern alternative), and eccentric hamstring work at long muscle length (Nordic curl) has the strongest evidence of any single intervention for reducing hamstring strain incidence.

Naming the work, not the muscle

Ask “which muscle is working, and in which direction is gravity acting?” Walking downstairs, the knee flexes while the quadriceps works — eccentrically, controlling the descent. Students routinely name the hamstrings because the knee is flexing. The joint movement does not tell you which muscle is active; the direction of the resisting force does.

Open and closed kinetic chain

Neither is superior. The modern position is that both are used, sequenced by tissue tolerance: closed chain early for safety and function, open chain added deliberately because it is the only reliable way to correct an isolated quadriceps deficit — a deficit that persists for years after ACL reconstruction if never specifically addressed.

Torque, moment arm and mechanical advantage

Figure 4 · The three classes of lever

Torque

Torque (moment) = Force × Moment arm

The moment arm is the perpendicular distance from the line of action of the force to the joint’s axis of rotation. Because it is perpendicular, it changes continuously through range as the joint angle changes.

This one equation explains a great deal of clinical reasoning:

• A patient’s strength curve through range reflects the changing moment arm as much as the length–tension relationship.
• Resistance is angle-dependent. A dumbbell held out at 90° of shoulder abduction imposes far more torque than the same dumbbell at 20°, because the external moment arm has lengthened, even though the load is unchanged.
• Modifying the external moment arm is the simplest way to grade an exercise: bend the elbow during a straight-arm raise; hold a weight closer to the body; shorten a lever by holding at the knee rather than the ankle.
• Surgical procedures manipulate internal moment arms deliberately — a tendon transfer changes the line of action; a high tibial osteotomy changes the ground reaction force’s moment arm at the knee.

Levers

A lever is a rigid bar (bone) rotating about a fulcrum (joint), acted on by an effort (muscle) and a resistance (load, including segment weight).

Why the body chose the inefficient option. A third-class lever means the muscle must generate several times the external load — biceps typically produces around 6–8 times the weight held in the hand. In exchange, a small amount of muscle shortening produces a large, fast movement of the hand. Muscle is compact and can generate large force in a small space; long, fast limb movement cannot be generated any other way. The body trades force, which it has, for range and speed, which it needs.

The clinical corollary is that joint compressive loads are large. Hip joint reaction force in single-leg stance is approximately 2.5–3 × body weight, most of it generated by the abductors to counterbalance a body-weight moment arm roughly 2.5 times longer than their own. It follows that:

• A stick held in the contralateral hand lengthens the counterbalancing moment arm enormously, reducing hip joint reaction force by up to 20–30% — far more than the small force applied through the stick would suggest.
• A Trendelenburg gait (lateral trunk lean toward the stance side) is the patient’s own solution: it shifts the centre of mass over the hip, shortening the body-weight moment arm and reducing the abductor force required.
• Losing 5 kg reduces peak knee joint load during walking by considerably more than 5 kg, because of the multiplier effect of the moment arms.

Active and passive insufficiency

Figure 5 · What stops a joint moving further

These phenomena occur only in multi-joint (biarticular or polyarticular) muscles, and they are the reason multi-joint muscles cannot be understood one joint at a time.

Clinical uses

• Muscle testing. To test gastrocnemius, test plantarflexion with the knee extended; to test soleus in isolation, test it with the knee flexed (gastrocnemius is actively insufficient there). To differentiate flexor digitorum superficialis from profundus, block the adjacent digits’ profundus tendons.
• Length testing. Every classic muscle length test is an application of passive insufficiency: straight leg raise (hamstrings), Thomas test (iliopsoas and, with the knee flexed, rectus femoris), Ober’s test (ITB and tensor fasciae latae), Ely’s test (rectus femoris), Silfverskiöld test (differentiating gastrocnemius from soleus contracture by testing ankle dorsiflexion with the knee extended and then flexed).
• Stretch prescription. To stretch a biarticular muscle you must position both joints; hamstring stretching with the lumbar spine flexed loads the spine rather than the muscle.
• Positioning. A patient positioned so that a biarticular muscle is shortened across both joints will develop contracture faster — hence the vigilance about hip and knee flexion positioning in the bed-bound patient, and about ankle plantarflexion in the patient with the foot unsupported.

The tenodesis effect

The passive-insufficiency phenomenon converted into function. Passively extending the wrist shortens the distance the long finger flexors must travel, causing the fingers to close passively into a functional grip; passively flexing the wrist opens the hand.

For a patient with a C6 spinal cord injury — who has active wrist extension (extensor carpi radialis longus, C6) but no active finger flexion (C8) — this is the entire basis of functional hand use. Two clinical implications follow, and they are critical:

• Tenodesis grasp must be trained and is a primary rehabilitation goal after C6 tetraplegia.
• The long finger flexors must be allowed to shorten slightly. Aggressive stretching of the finger flexors into full extension with the wrist extended destroys the tenodesis effect permanently and removes the patient’s grip. This is one of the few situations in physiotherapy where deliberate mild contracture is the correct goal, and it is a well-known cause of avoidable harm when a clinician unaware of it “corrects” the tightness.

Force couples

A force couple is two or more forces acting on the same segment in different directions, producing rotation.

Force couples are how the body produces rotation of a segment that has no fixed bony axis, and they are among the most useful concepts in musculoskeletal rehabilitation.

The scapular upward rotation couple

Full arm elevation requires roughly 60° of scapular upward rotation, contributed by three muscles pulling in three different directions:

• Upper trapezius — elevates and upwardly rotates via the clavicle and acromion.
• Lower trapezius — depresses the medial scapular spine, contributing an upward-rotation moment and stabilising against the upper fibres’ elevation.
• Serratus anterior — protracts and upwardly rotates, and critically holds the medial border against the thorax.

Together they rotate the glenoid upwards; individually none of them can. This is why scapular dyskinesis is a coordination problem as much as a strength problem, why serratus anterior weakness produces medial border winging (long thoracic nerve, C5–7), and why trapezius palsy (spinal accessory nerve, often iatrogenic after posterior triangle lymph node surgery) produces lateral winging with a drooping shoulder and inability to elevate above shoulder height.

Other force couples worth knowing

Stabiliser and mobiliser systems — with a caveat

A widely taught model divides muscles into two systems:

The observation underpinning this — that transversus abdominis and multifidus normally activate before limb movement, and that this feedforward timing is delayed in people with low back pain, and that multifidus atrophies segmentally and does not spontaneously recover — is real and well replicated (Hodges, Richardson, Hides).

But the clinical inference drawn from it has not held up. Trials and meta-analyses consistently find that specific “core stability” or motor control training is no more effective than general exercise for chronic low back pain, and that the assumed mechanism (restoring transversus abdominis timing) does not explain the improvement observed. Contemporary practice therefore uses motor control training as one option among several, selected for particular patients (those with clear movement control impairment, or in the early painful phase where general loading is not tolerated), rather than as a universal prescription.

Teach the anatomy as anatomy and the model as a model. The deep muscles do behave differently from the superficial ones; the therapeutic conclusion once drawn from that fact was too strong.

Putting it together: analysing a movement

Use this sequence for any movement, in any examination or clinic:

• Name the joint(s), and the movement occurring at each — with plane and axis (Chapter 1).
• Identify the direction of the resistance, usually gravity plus any external load.
• Decide which muscle group is working: the one on the side opposite to the direction the resistance is trying to move the segment.
• Decide the type of contraction: shortening against resistance = concentric; lengthening under control = eccentric; no change = isometric.
• Identify the fixators — what must be stabilised for this to work?
• Identify the neutralisers — what unwanted actions must be cancelled?
• Check for insufficiency — is any biarticular muscle shortened or lengthened across both joints?
• Consider the lever and moment arm — how does the demand change through range?

Worked example — lowering slowly into a chair

• Joints and movements: hip flexion, knee flexion, ankle dorsiflexion — all in the sagittal plane about frontal axes.
• Resistance: gravity, acting to collapse the body downwards.
• Muscles working: hip extensors (gluteus maximus, hamstrings), knee extensors (quadriceps), ankle plantarflexors (soleus) — the anti-gravity groups.
• Contraction type: eccentric throughout; they are lengthening while resisting.
• Fixators: trunk extensors and abdominals stabilising the spine; foot intrinsics and ankle everters/inverters stabilising the base.
• Moment arm: the external moment arm at the knee increases as the knee flexes and the trunk moves back, so quadriceps demand rises through the descent — which is exactly why a patient with quadriceps weakness drops into the chair over the last 20–30°.
• The rehabilitation conclusion writes itself: train eccentric quadriceps control through the outer range of knee flexion, and grade it by raising the seat height (shortening the range and the moment arm).

Where students consistently go wrong

• Naming the muscle from the joint movement. The knee flexing downstairs does not mean the hamstrings are working. Look at gravity.
• Assuming the origin is always fixed. In closed chain the insertion is fixed and the origin moves.
• Ignoring fixators. A prime mover cannot function without a stabilised proximal base — this is the whole rationale for proximal-to-distal rehabilitation.
• Forgetting insufficiency in muscle testing. Testing gastrocnemius with the knee flexed will underestimate it.
• Calling every muscle-lengthening a stretch. Passive insufficiency is a normal mechanical limit, not tightness.
• Stretching the finger flexors in a C6 tetraplegic patient. This destroys tenodesis grip.
• Treating levers as trivia. Mechanical advantage explains gait aids, joint reaction forces, exercise grading and surgical decisions.
• Treating a force couple’s components as independent muscles. Strengthening serratus without addressing lower trapezius rarely restores scapular rotation.
• Prescribing core stability universally. The evidence does not support it as superior to general exercise.
• Testing strength at only one joint angle. Torque varies with moment arm; a deficit may only appear in outer or inner range.

Check yourself

15 questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Chapter 6 of 24 · Human Anatomy · Physiotherapist India Next: Chapter 7 — Connective Tissue, Ligaments and Fascia: one recipe behind tendon, ligament and fascia — and why compartments are dangerous.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents