Ground Reaction Forces

For Medical Students

The Principle

Standing on the ground, the body pushes down and the ground pushes back with an equal and opposite force. That is the ground reaction force.

In still standing on two legs it equals body weight, shared between the feet. In movement it is larger, because the body is being accelerated as well as supported.

It has a direction as well as a size. Its line relative to the knee is what decides whether it tends to bend, straighten or angulate the joint.

At the Knee

The force itself acts at the foot. What reaches the knee is the moment it produces, which depends on how far its line passes from the joint.

Conventional teaching gives peak vertical ground reaction force as roughly 1.0 to 1.2 times body weight in walking, 2 to 3 times in running, and 3 to 6 times in landing from a jump.

It has three components. The vertical one is the largest, with smaller anteroposterior and mediolateral components that produce braking, push off and side to side control.

The point the force acts through on the foot moves during stance, which moves its line relative to the knee through the step.

What Changes It

Speed, which raises the peak in every activity.

Landing height, which sets the energy to be absorbed.

Technique, since flexing more and over a longer time lowers the peak for the same energy. 3.3.13 covers landing.

Surface and footwear, which change the rate at which the force rises rather than its total.

Clinical Relevance

Running and landing load the limb several times more than walking, which is why returning to each is a separate milestone.

A patient who can walk comfortably and cannot run is reporting a difference in external load, rather than a difference in confidence.

The loading rate matters as well as the peak, which is why a softer landing helps even when body weight has not changed.

This is the measured quantity in every gait laboratory, so a kinetic figure without a force plate behind it is an estimate.

MRCS Level

The Principle

Treat the ground reaction force as a vector with a point of application, not as a number. Its magnitude, its direction and the centre of pressure together decide the moment at every joint in the limb.

The vertical component in walking is conventionally described as two peaks with a trough between them, the first at loading response and the second at terminal stance.

Running loses that shape. A single larger peak replaces it, because there is no double support and no mid stance unloading.

At the Knee

The perpendicular distance from the force line to the knee is the moment arm, so the same force produces different knee moments at different parts of stance. 3.4.4 covers moment arms.

A force line passing medial to the knee produces an external adduction moment, which loads the medial compartment. That is the quantity alignment surgery changes.

A force line passing behind the knee produces an external flexion moment, resisted by the quadriceps, and the magnitude of that demand is what makes stairs harder than level walking.

Loading rate, the slope of the force curve rather than its peak, is the quantity associated with impact related symptoms and is altered by technique and footwear.

What Changes It

Walking or running speed, which scales both the peak and the rate.

Foot progression angle and stance width, which move the force line relative to the knee without changing its magnitude.

Trunk position, which moves the centre of mass and therefore the force line.

A walking aid, which removes a measurable share of the force from the limb.

How It Is Measured

Force plates embedded in a walkway measure all three components and the centre of pressure, over the steps that land on the plate.

Instrumented treadmills measure many consecutive steps and constrain the task to treadmill locomotion.

Pressure insoles measure the vertical component outside a laboratory and the shear components poorly.

Accelerometers measure impact related acceleration rather than force, which is a related quantity and not the same one.

Clinical Relevance

Use a walking aid to reduce limb load when the aim is mechanical rather than only balance.

Consider foot progression angle and stance width, since they change the knee moment without changing the force.

Separate peak force from loading rate when discussing impact, because interventions differ.

Read an insole or accelerometer figure as a proxy rather than as a ground reaction force.

FRCS Level

At the Knee

Treat the ground reaction force as the input that alignment surgery works on. The operation does not change the force; it changes where the force line falls relative to the knee. 3.13 covers alignment.

That is also why gait retraining and a lateral wedge have a mechanical rationale: both move the line rather than the load. Their effect sizes are small and the mechanism is sound.

The clinically important derived quantity is the external knee adduction moment, which is associated with medial compartment progression. 3.4.6 covers external moments.

What Changes It

Coronal alignment, which is the dominant determinant of where the line falls.

Body mass, which scales the force and therefore every moment derived from it.

Gait modification, trunk lean and foot rotation among them, which move the line.

Bracing, which applies an additional external moment rather than reducing the ground reaction force.

How It Is Measured

Force plate gait analysis is the reference and gives the moment as well as the force.

Long leg standing radiography gives the static alignment that the dynamic moment is often inferred from, and the two correlate imperfectly.

Wearable systems estimate loading outside the laboratory with a model between the sensor and the number.

No measurement available in a clinic gives the adduction moment, which is why alignment stands in for it.

Clinical Relevance

Plan an osteotomy by where the force line should fall rather than by a target angle alone.

Expect a lateral wedge or a gait modification to produce a small mechanical change, and describe it as such.

Treat static alignment as a proxy for the dynamic moment, since they correlate imperfectly.

Account for body mass when estimating what an alignment correction will achieve.