ACL Loading During Running

For Medical Students

The Principle

A structure can be loaded often without failing, if the load stays below what it tolerates and the direction stays predictable.

Repetition matters for fatigue and for tissue adaptation. It does not by itself cause a rupture.

So an activity's risk depends on the size of the load, its direction, and how predictable it is.

At the Knee

Running loads the anterior cruciate mainly in the sagittal plane. The knee is flexed at contact and flexes further, which is away from the highest strain range. 3.3.10 covers the kinematics.

Quadriceps activity raises strain and the flexion angle lowers it, so the two partly cancel during stance.

Rotation and valgus are small in straight line running, so the coupled loading that injures the ligament is largely absent. 3.2.9 covers coupling.

Conventional teaching therefore places running well below cutting and landing in anterior cruciate loading, which matches the injury epidemiology.

What Changes It

Speed, which raises the forces without changing the direction much.

Surface and footwear, which affect how predictable the foot's behaviour is.

Fatigue, which changes the muscle pattern and therefore the strain.

Any change of direction, which converts the activity into something else. 3.7.13 covers cutting.

Clinical Relevance

Returning to straight line running is an early milestone after reconstruction because its loading is comparatively benign. Section 6 covers management.

Returning to cutting and pivoting is a later and separate milestone, which follows from the loading rather than from time.

Non contact injuries during running are rare, and when they occur there is usually a direction change or an uneven surface in the account.

Explaining why running is safe makes the rest of the loading hierarchy easier to teach.

MRCS Level

The Principle

The quantity to compare between activities is peak strain rather than peak force, because failure is a strain phenomenon. 3.7.10 covers strain.

Direction matters as much as magnitude. A sagittal load at a flexed knee is tolerated far better than a combined load near extension.

Predictability matters because an unanticipated movement produces a different muscle pattern, with less protective co-contraction.

At the Knee

Running's knee flexion at contact of roughly 20 degrees is already past the range of highest strain, which is the main protective factor. 3.3.10 covers the kinematics.

Hamstring activity in late swing and early stance provides co-contraction that reduces strain in the range where it is highest.

Ground reaction forces in running are two to three times body weight, and that magnitude is not the limiting factor for the ligament. 3.4.2 covers the force.

Downhill running increases the eccentric demand and the quadriceps activity, which raises strain relative to level running.

What Changes It

Flexion angle at contact and through stance.

Quadriceps to hamstring balance and its timing.

Gradient, with downhill raising the demand.

Fatigue, which alters both the kinematics and the muscle pattern measurably.

How It Is Measured

In vivo strain measurement covered a limited set of activities and did not include running at speed.

Modelling estimates ligament loading across running conditions, from measured kinematics and kinetics.

Injury epidemiology is the indirect evidence that running's loading is tolerable.

Cadaveric simulation cannot reproduce running's muscle pattern, which is the protective element.

Clinical Relevance

Compare activities by peak strain and by direction rather than by force.

Expect downhill running to load the ligament more than level running.

Treat fatigue as a loading variable rather than only a performance one.

Read running loading estimates as modelled rather than measured.

FRCS Level

At the Knee

Treat running as the benchmark the other activities are read against, and as the milestone whose safety is mechanically defensible rather than conventional.

The graft in the first months is the variable rather than the activity. Running loading is modest and a remodelling graft is weak, so the staging is about the construct and not about the sport. 3.7.20 covers the construct.

Conventional teaching permits straight line running at around three months after reconstruction subject to criteria, and the criteria rather than the interval carry the argument.

What Changes It

Graft remodelling stage, which is the limiting factor early.

Quadriceps and hamstring capacity and symmetry.

Concurrent meniscal repair, which usually delays loading for its own reasons.

Surface and gradient, which change the demand.

How It Is Measured

Criterion based return to running assessment measures strength, symmetry and hop performance.

Running analysis measures the kinematics and kinetics achieved, in research settings.

Graft maturation is assessed on imaging, imperfectly, and does not give a mechanical figure.

No measurement of graft load in a running patient exists.

Clinical Relevance

Stage return to running by criteria rather than by interval.

Delay for a concurrent meniscal repair on its own grounds rather than the graft's.

Treat gradient and surface as part of the prescription.

State that graft load during running is not measured in patients.