ACL Loading During Landing

For Medical Students

The Principle

A load is most dangerous when it arrives in the position a structure is least able to resist it.

For the anterior cruciate that position is near extension, where it is already at its highest strain. 3.7.10 covers strain.

Landing delivers a large force in that position, which is why it is the archetypal loading event.

At the Knee

At the moment of landing the knee is only slightly flexed, and the peak force arrives within the first fraction of a second. 3.3.13 covers the kinematics.

The quadriceps is contracting hard to resist the collapse, and the quadriceps pulls the tibia forwards at that angle. 3.4.4 covers lever arms.

So the external force and the muscle force both load the ligament, in the position where it is least protected.

Landing with more flexion lowers the force and moves the knee out of the vulnerable range, which is why a soft landing is safer on two counts.

What Changes It

The flexion achieved and how quickly, which is technique and is trainable.

Landing height and body mass, which set the energy.

One leg or two, since a single leg landing doubles the limb's share.

Fatigue, which reduces the flexion achieved. 3.4.8 covers energy absorption.

Clinical Relevance

Teaching a softer landing reduces the loading by a mechanism, which is the basis of prevention programmes. 3.1.5 covers injury prevention.

A stiff landing is the pattern associated with injury, and it is modifiable.

Landing tasks appear in return to sport testing because they reproduce the loading. Section 6 covers management.

Most non contact injuries involve a landing or a deceleration, which is this loading condition. 3.7.14 covers mechanisms.

MRCS Level

The Principle

The loading is multiplanar even in a symmetrical landing. Sagittal shear from the quadriceps combines with any valgus moment and any rotation, and the combination loads the ligament more than the sum suggests. 3.2.9 covers coupling.

Timing matters because the muscle response has a latency. A load arriving faster than the reflex cannot be met by reflex activation.

Conventional teaching places the injury event within the first 50 milliseconds of ground contact, which is faster than a voluntary or reflex correction.

At the Knee

Peak vertical ground reaction force in landing is conventionally given as three to six times body weight, arriving at a knee flexed roughly 20 degrees or less. 3.4.2 covers the force.

The quadriceps acts through the patellar tendon, whose line produces anterior tibial shear at low flexion angles and reverses in flexion. 3.4.4 covers lever arms.

Dynamic knee valgus adds a coronal moment and an internal rotation, which is the pattern associated with injury. 3.3.13 covers landing kinematics.

Hamstring co-contraction reduces the strain and is the modifiable protective element.

What Changes It

Flexion angle at and after contact, which is the dominant modifiable variable.

Hip and trunk control, which set the coronal and rotational position.

Loading rate, through technique, surface and footwear.

Fatigue and anticipation, both of which worsen the pattern.

How It Is Measured

Drop vertical jump analysis with force plates and motion capture measures the kinematics and kinetics of a prescribed landing.

Modelling estimates ligament loading from those measurements, with assumptions about muscle force distribution.

Video analysis of actual injuries provides the mechanism evidence. 3.7.14 covers it.

No in vivo strain measurement of landing exists, so the loading is modelled.

Clinical Relevance

Train flexion at landing as the dominant modifiable variable.

Address hip and trunk control, which set the position the knee lands in.

Test under fatigue where the sport involves it.

Read landing ligament loading as modelled rather than measured.

FRCS Level

At the Knee

Treat landing mechanics as modifiable risk with modest predictive value for an individual, which is the honest position the prevention literature supports.

Prevention programmes reduce injury rates in populations, and screening does not identify the individual who will be injured. Both statements are supported and they are often conflated. 3.3.13 covers landing kinematics.

After reconstruction, asymmetrical landing persists in many patients with normal laxity, which is a loading risk that the graft does not address. 3.4.8 covers energy absorption.

What Changes It

Neuromuscular training, which is the modifiable element.

Graft choice and donor site, which affect the muscle available.

Bony morphology, slope and plateau shape, which change what a given landing does.

Fear and confidence, which alter movement strategy after the mechanics are restored.

How It Is Measured

Drop jump analysis measures the kinematics and the asymmetry in research cohorts.

Prospective screening studies report modest predictive value for individual injury.

Randomised prevention trials report population level reductions in injury rate.

Limb symmetry indices are the clinical surrogate and have a ceiling effect.

Clinical Relevance

Prescribe neuromuscular training on population evidence rather than individual prediction.

Expect landing asymmetry to persist after reconstruction with normal laxity.

Measure slope where loading risk seems disproportionate to the injury.

Keep population risk reduction and individual prediction apart when counselling.