Kinematics During Walking

For Medical Students

The Principle

A gait cycle is one stride, measured from one foot contacting the ground to the same foot contacting it again.

It divides into stance, when the foot is on the ground, and swing, when it is not. Conventional teaching gives stance about 60 per cent of the cycle and swing about 40.

Stance is closed chain and swing is open chain, so the knee changes working arrangement twice per stride. 3.3.3 and 3.3.4 cover the two chains.

At the Knee

The knee flexes twice in a cycle. The first wave is small, loaded and early in stance. The second is large, unloaded and in swing.

Conventional teaching gives the first wave as roughly 15 to 20 degrees of flexion during loading response, returning towards extension by mid stance.

The second wave reaches roughly 60 degrees in swing, which is the peak of the whole cycle and takes almost no muscle work to achieve.

Axial rotation through the cycle is small, a few degrees, and varus and valgus angulation smaller still. Walking is close to a single plane task for the knee.

What Changes It

Speed. Faster walking increases the stance flexion wave and shortens the time available for each phase.

Surface and footwear, which change the loading at contact.

Pain, which reduces the stance flexion wave because flexion under load is what hurts.

Range of motion, since a knee that cannot reach 60 degrees must find the clearance elsewhere in the limb.

Clinical Relevance

The small stance wave is the one that matters clinically. It is loaded, it is where pain shows, and it is the first thing a patient gives up.

A stiff knee gait, where the stance wave is lost, is a common and visible compensation. 3.14 covers gait pathology.

Swing flexion of 60 degrees is why a patient who cannot flex the knee to about that angle has to circumduct or vault to clear the foot.

Walking samples a small part of the knee's range, so a normal walk does not demonstrate a normal knee.

MRCS Level

The Principle

Describe gait by phase rather than by percentage alone, because the phases name what the limb is doing. Stance divides into initial contact, loading response, mid stance, terminal stance and pre-swing.

Each phase makes a different demand of the knee, and a kinematic abnormality is localised by naming the phase it appears in.

The knee's job in stance is shock absorption and then stable support. In swing it is clearance and then preparation for contact.

At the Knee

At initial contact the knee is near full extension, conventionally within about 5 degrees.

Loading response flexes the knee to roughly 15 to 20 degrees under body weight, which is the shock absorbing wave, controlled eccentrically by the quadriceps.

Mid stance returns the knee towards extension while the limb passes over the foot, and terminal stance holds it near extension.

Pre-swing and initial swing flex the knee rapidly towards its peak of roughly 60 degrees, largely by momentum rather than by hip flexor or hamstring work.

Terminal swing extends the knee again in preparation for contact, decelerated by the hamstrings.

What Changes It

Walking speed, which scales the stance flexion wave and the swing peak.

Quadriceps function, since the loading response wave is eccentric quadriceps work and is the first casualty of weakness.

Pain and effusion, both of which reduce the stance wave without any change in passive range.

Proximal and distal joint function, because a chain redistributes. 3.3.3 covers the chain.

How It Is Measured

Instrumented three dimensional gait analysis with optical markers and force plates is the reference, and it measures a laboratory walk rather than a daily one.

Inertial sensors measure sagittal knee angle outside a laboratory and the small rotations poorly.

Observational gait analysis detects a lost stance wave reliably and quantifies nothing, which is adequate for the common clinical question.

Soft tissue artefact is the main error source for the small transverse and coronal quantities, which is why those are reported with caution.

Clinical Relevance

Name the phase when describing a gait abnormality, since the phase localises the problem.

Attribute a lost stance flexion wave to pain, effusion or quadriceps function before concluding a structural cause.

Expect normal walking in a knee with significant pathology, because the task is undemanding.

Read coronal and transverse gait quantities with the measurement error in mind.

FRCS Level

At the Knee

Treat gait kinematics as the task most patients judge an operation by, and as a weak discriminator between constructs, since walking demands little of the knee.

The quantity that does discriminate is the adduction moment, which is kinetic rather than kinematic and is the measure alignment surgery changes. 3.4.6 and 3.13 cover it.

A stiff knee gait after arthroplasty or reconstruction is a kinematic sign with several causes, and separating weakness from pain from mechanical restriction is the clinical work.

What Changes It

Alignment, which sets the loading the knee meets in stance.

Extensor mechanism function and patellar height, which set what the loading response wave costs.

Implant or graft constraint, which can limit the swing peak or the terminal extension.

Habit and fear, which persist after the mechanical cause is corrected and are modifiable.

How It Is Measured

Three dimensional gait analysis with force plates gives both the kinematic and the kinetic account, in a laboratory and over a few strides.

Wearable monitoring measures real world walking volume and sagittal angle, which answers a different and often more relevant question.

Fluoroscopic gait studies give bone level kinematics over a short distance and are the method behind the replaced knee kinematics literature.

Patient reported outcome measures record whether walking is acceptable to the patient, which no kinematic measure substitutes for.

Clinical Relevance

Use gait as the patient's benchmark and not as the test that separates two constructs.

Measure or estimate the adduction moment when alignment is the question rather than relying on sagittal kinematics.

Separate weakness, pain and mechanical restriction before treating a stiff knee gait.

Expect habit to outlast the mechanical cause, and rehabilitate accordingly.