Internal Moments

For Medical Students

The Principle

An internal moment is produced by the body itself: by a muscle pulling, or by a ligament or capsule being stretched.

In steady movement the internal moment matches the external one. If it were smaller the joint would collapse, and if larger it would accelerate.

Muscles are the adjustable part and ligaments are the backstop. A ligament produces a moment only once it is tight, so it acts at the end of range rather than throughout.

At the Knee

The quadriceps produces the internal extension moment and is the largest contributor in most loaded activities.

The hamstrings produce the internal flexion moment, and they also cross the hip, so their capacity depends on hip position.

The collateral and capsular structures produce internal coronal moments, and they do so passively once the joint has angulated enough to tighten them.

Internal moments are produced at short lever arms, so the forces involved are much larger than the external loads they resist. 3.4.4 covers lever arms.

What Changes It

Muscle strength, which sets the maximum internal moment available.

Flexion angle, which changes both the lever arms and the muscles' length.

Pain and swelling, which reduce voluntary muscle activation without changing the muscle.

Ligament integrity, which decides whether the backstop is there.

Clinical Relevance

A weak quadriceps cannot produce the internal moment that stairs demand, which is why stairs fail before level walking.

When muscle cannot supply the demand, the passive structures take it, which is one route to ligament and cartilage overload.

Strengthening increases the internal moment available and is therefore a mechanical treatment. Section 6 covers management.

Co-contraction produces a large joint force for a small net moment, so a guarded knee is a loaded knee.

MRCS Level

The Principle

The muscle redundancy problem is the central difficulty. More muscles cross the knee than are needed to produce any given moment, so the net moment does not determine the individual forces.

Inverse dynamics therefore reports a net internal moment, and musculoskeletal modelling resolves the redundancy by an assumption, usually that the body minimises some cost.

Co-contraction is the clinically important case the net moment hides, because it raises joint reaction force while leaving the net moment unchanged. 3.4.3 covers joint force.

At the Knee

Quadriceps force in loaded tasks is several times body weight, because it works at a short arm against a long one.

The hamstrings act as an agonist to the posterior cruciate and an antagonist to anterior translation, so their internal moment has a stabilising role as well as a flexing one. 3.7 and 3.8 cover the cruciates.

Gastrocnemius crosses the knee and the ankle, so ankle position alters the internal moment available at the knee.

Passive internal moments from the collaterals and capsule rise steeply at the end of range, which is what a stability test measures. Section 4 covers the tests.

What Changes It

Strength and activation, which are separable: a muscle can be strong and poorly activated after injury.

Joint angle, through length tension and through the moment arm.

Pain and effusion, which inhibit activation measurably.

Graft or repair state, which changes the passive contribution at the end of range.

How It Is Measured

Isokinetic dynamometry measures the maximum internal moment through range and is the usual clinical strength measurement.

Electromyography measures activation timing and amplitude, and converting amplitude to force requires assumptions.

Modelling estimates individual muscle forces, with the cost function as its main limitation.

Co-contraction is estimated from paired electromyography rather than measured directly, which is why its contribution to joint load is a modelled quantity.

Clinical Relevance

Measure strength through range rather than at one angle, since both the arm and the muscle change.

Separate weakness from inhibition, because the treatments differ.

Treat effusion as a cause of reduced internal moment before concluding muscle failure.

Expect co-contraction to raise joint load in painful and unstable knees, and read modelled loads accordingly.

FRCS Level

At the Knee

Treat the internal moment as the half of the equation surgery usually cannot fix. An operation changes geometry and restraint; it does not supply a quadriceps.

That is the mechanical reason prehabilitation and postoperative strength matter to outcome independently of the construct, and why a technically good reconstruction in a weak limb underperforms.

Passive internal moments are what a reconstruction restores, and they act at the end of range only. A graft cannot substitute for the mid range control a muscle provides, which is the mechanical account of functional instability with a tight graft.

What Changes It

Graft choice and donor site, which remove part of the muscular contribution to supply the passive one.

Rehabilitation quality and duration, which set the muscular contribution available.

Nerve injury, which removes it altogether and changes the whole problem. 2.10.6 covers the peroneal nerve.

Pain and kinesiophobia, which reduce voluntary activation after the mechanics are corrected.

How It Is Measured

Isokinetic testing and limb symmetry indices are the usual measures, with a ceiling effect when both limbs deconditioned.

Electromyography during function identifies co-contraction patterns in research settings.

Modelling estimates the joint load consequence of a given co-contraction pattern, by assumption.

No clinical measurement separates a construct's passive contribution from the muscle's, which is why outcome studies struggle to attribute a result to either.

Clinical Relevance

Optimise strength before reconstruction where the timing allows, on mechanical grounds.

Expect a hamstring graft to remove part of the internal flexion moment, and account for it in rehabilitation.

Explain functional instability with a tight graft by the absence of mid range muscular control.

Treat a quadriceps deficit as a mechanical problem rather than a motivational one.