Kinematics During Running

For Medical Students

The Principle

Walking always has one foot on the ground and has two periods of double support. Running has neither, and it has a flight phase when neither foot is down.

That single difference changes the task. Every step is a landing, and the limb has to absorb the whole body's descent on its own.

Stance is shorter and the motion faster, so the same range has to be covered in less time.

At the Knee

The knee contacts the ground already flexed. Conventional teaching gives roughly 20 degrees at initial contact in running, against near extension in walking.

Stance flexion then increases to roughly 40 to 45 degrees, which is more than double the walking wave and under a larger load.

Swing flexion is much greater. Conventional teaching gives roughly 90 to 130 degrees depending on speed, since the heel rises towards the buttock as speed increases.

Stance occupies a smaller share of the cycle as speed rises, so the absorbing work is compressed into less time.

What Changes It

Speed, which increases both the swing flexion peak and the rate at which stance flexion occurs.

Footstrike pattern, which changes the knee's share of the absorbing work relative to the ankle.

Fatigue, which reduces the stance flexion and therefore the absorption.

Surface and gradient, since downhill running increases the absorbing demand.

Clinical Relevance

Running loads the knee in flexion repeatedly, which is why a patient can walk without pain and run with it.

Returning to running is a separate milestone from returning to walking, because it is a separate task rather than a faster one.

A patient who cannot reach the swing flexion range will change their stride rather than stop, which moves the demand elsewhere.

Sport specific application belongs to 3.15, and running mechanics as a cause of injury to 3.1.5.

MRCS Level

The Principle

Treat running as a repeated absorb and return cycle. The limb absorbs kinetic energy in early stance and returns part of it in late stance, and the knee contributes to both.

Absorption is eccentric work and return is partly elastic, stored in tendon rather than produced by muscle. 3.4.8 covers energy absorption.

The kinematic signature of absorption is flexion under load, so a knee that flexes less in stance is absorbing less and passing more elsewhere.

At the Knee

Initial contact flexion of roughly 20 degrees means the knee is already off its locking position and depends on muscle and ligament rather than on congruence.

Peak stance flexion of roughly 40 to 45 degrees places the knee in mid flexion, where rotation and translation are greatest. 3.3.8 covers the zones.

Axial rotation and coronal motion remain small in the sagittal plane dominated task, and they increase when running is combined with direction change. 3.3.14 covers cutting.

The patellofemoral joint sees its load rise sharply with stance flexion, since the compressive force scales with both quadriceps force and flexion angle. 3.12 covers it.

What Changes It

Speed and stride length, which set the flexion peak and the absorbing rate.

Footstrike pattern, which shifts work between the ankle and the knee.

Quadriceps and gluteal function, which decide whether the absorbing flexion is controlled.

Fatigue, which is measurable as reduced flexion and altered coronal motion late in a run.

How It Is Measured

Treadmill based optical capture with an instrumented belt is the usual method and measures treadmill running, which differs slightly from overground.

Overground running with embedded force plates gives fewer strides and more fidelity.

High speed video with two dimensional analysis is used in practice and measures sagittal angles adequately while confounding the coronal ones with rotation.

Soft tissue artefact grows with speed, so the fast transverse plane quantities are the least reliable in the whole kinematic literature.

Clinical Relevance

Assess running separately from walking when a patient's symptoms are activity specific.

Expect patellofemoral symptoms to appear in running before walking, on the loading argument rather than on a structural one.

Read two dimensional video coronal measurements cautiously, since rotation confounds them.

Consider fatigue when symptoms appear late in an activity rather than at the start.

FRCS Level

At the Knee

Treat return to running as a mechanical decision rather than a time based one. The task requires flexion under load in mid flexion, which is where a reconstruction is least protected by congruence.

Repeated loading is the variable that distinguishes running from most clinical testing. A construct adequate for one cycle may not be adequate for several thousand per session.

Patellofemoral and cartilage consequences follow from the loading rather than from the kinematics alone, and they are the reason running advice differs after a cartilage procedure. 3.5 covers contact mechanics.

What Changes It

Graft maturity and fixation, which change what repeated mid flexion loading costs.

Cartilage and meniscal status, which set the surface's tolerance of the load.

Alignment, which concentrates the repeated load in one compartment.

Body mass and training volume, which scale the total number of loading cycles.

How It Is Measured

Criterion based return to sport testing measures hop, strength and symmetry, which are performance proxies rather than joint kinematics.

Laboratory running analysis measures the kinematics in a few strides and cannot measure cumulative load.

Wearable load monitoring measures cycle counts and is the closest available measure of cumulative exposure.

No measurement predicts an individual's tolerance, which is the honest position in counselling.

Clinical Relevance

Stage return to running by criteria rather than by time after surgery.

Account for cumulative cycles rather than peak load when advising after a cartilage procedure.

Correct alignment where repeated single compartment loading is the problem.

State the uncertainty about individual tolerance rather than offering a figure.