Normal Gait

For Patients

The Principle

Walking looks simple and is not. Each leg repeats the same sequence, and the two legs take turns.

The knee bends and straightens twice in every step, which surprises most people.

Because the sequence repeats, any change to it can be seen and measured.

At the Knee

A single cycle runs from one heel strike to the next heel strike on the same foot.

When the foot lands, the knee bends a little to absorb the impact, then straightens again as you pass over it.

It then bends much further to swing the leg through without catching the toes on the ground.

The muscles at the front of the thigh control the first bend and the muscles at the back slow the leg down at the end of the swing.

Clinical Relevance

Pain changes walking before a person notices it. Shortening the time spent on the sore leg is automatic.

That is why someone watching you walk can learn things you cannot describe.

Most knee problems show up as a change in this pattern rather than as a limp you would call a limp.

See a clinician if your knee gives way while walking, locks, swells quickly, or will not take your weight.

For Medical Students

The Principle

A cyclical movement can be described once and then compared against. That is what makes gait measurable where most human movement is not.

The cycle is divided into stance, when the foot is down, and swing, when it is not.

Each phase has its own mechanical problem to solve, so each is described separately. 3.14.2 covers stance and 3.14.3 covers swing.

At the Knee

One gait cycle runs heel strike to heel strike on the same limb. Stance occupies roughly 60 per cent of it and swing roughly 40.

The knee flexes to around 15 to 20 degrees in early stance, extends through midstance, then flexes to around 60 degrees in swing.

That double flexion pattern is the knee's signature in gait, and it is the first thing lost in a painful or stiff knee.

The quadriceps act eccentrically in loading response and the hamstrings act eccentrically in terminal swing. Both are braking rather than driving. 3.4.8 covers energy absorption.

Both feet are on the ground together for about 10 per cent of the cycle at each changeover, which is what distinguishes walking from running. 3.14.4 covers running.

What Changes It

Walking speed, which changes every force and most angles. 3.4.2 covers ground reaction forces.

Pain, which shortens stance time on the affected side automatically.

Joint range, which limits the swing flexion before it limits anything else.

Muscle strength and control, particularly the quadriceps. Section 8 covers the training.

Clinical Relevance

Measure gait as a cycle rather than as a posture, because the abnormality is usually a timing change. 4.4.1 covers the examination.

Expect the swing flexion to be lost early in a stiff knee, since it is the largest excursion the cycle asks for.

Observe walking speed, since it is the single most informative free measurement in clinic. 4.3.2 covers gait observation.

Describe a change by the phase it occurs in rather than calling it a limp.

MRCS Level

The Principle

Gait analysis reports three things: kinematics, which is motion, kinetics, which is force and moment, and muscle activity.

Kinematics can be seen. Kinetics must be computed from a force plate and a model, and that computation is where most of the error lives. 3.4.3 covers joint reaction forces.

Knowing which of the three a figure came from is the difference between reading a gait report and quoting it.

At the Knee

The knee flexion curve has two peaks, the stance peak near 15 to 20 degrees and the swing peak near 60, and the shape between them is as informative as the values.

The external knee adduction moment peaks twice in stance and is the quantity most strongly linked to medial compartment load. 3.13.7 covers it.

The external flexion moment peaks in loading response, which is what the quadriceps resist.

Ground reaction force has the classical two humped shape in walking, with a trough at midstance. 3.4.2 covers it.

Chapter 3.3 covers the joint angles through walking as kinematics. This chapter covers the cycle and what measuring it reveals. 3.3.9 covers the kinematics.

What Changes It

Speed, which raises the ground reaction force and the moments, and alters the timing of every peak.

Cadence and step length independently, which is why speed alone is an incomplete description.

Trunk and pelvic motion, which change the moment arm without changing the knee. 3.13.7 covers that.

Footwear and surface, which change the loading rate at heel strike. 3.4.10 covers shock absorption.

Pain and effusion, both of which inhibit the quadriceps and flatten the stance flexion wave.

How It Is Measured

Optical motion capture with reflective markers and force plates, which is the reference and is laboratory bound. 3.16.7 covers it.

Instrumented walkways and pressure mats, which give temporal and spatial parameters without kinetics.

Inertial sensors and instrumented insoles, which measure outside a laboratory with less accuracy and many more steps. 3.16.8 covers them.

Observational gait analysis in clinic, which detects gross abnormality reliably and grades it poorly. 4.3.2 covers it.

Marker placement error is the dominant source of variation in optical capture, particularly for rotation.

Clinical Relevance

State which of kinematics, kinetics or muscle activity a quoted figure came from.

Record walking speed with any gait measurement, since most parameters are speed dependent.

Expect pain and effusion to flatten the stance flexion wave through quadriceps inhibition. 6.71 covers the arthritic knee.

Treat observational analysis as a screen rather than a measurement. 4.4 covers the examination.

FRCS Level

At the Knee

The defensible claim for gait analysis is that it is the only measurement of the knee doing its actual job under its actual loads, and the honest qualification is that it is almost never available to the clinician who would use it.

Its real strength is detecting adaptation. A patient cannot report a change in stance time or a reduced flexion wave, and both are measurable and both precede the complaint.

Its real weakness is that kinetic outputs are modelled rather than measured, so an error chain runs through marker placement, segment assumptions and inverse dynamics. 3.16.4 covers modelling.

Instrumented implants are the only direct in vivo load measurement at the knee, and they exist only in replaced joints. 3.14.12 covers that gait.

Conventional teaching treats gait analysis as a research tool, and the rise of wearables is the thing most likely to change that. 3.16.8 covers them.

What Changes It

Speed, which must be controlled or reported before any comparison is meaningful.

Marker set and model, which differ between laboratories and make absolute values poorly comparable.

The task chosen, since a laboratory walk is not a day of walking. 3.16.8 covers the field alternative.

Pain on the day, which changes the pattern more than most pathology does.

Footwear, surface and whether the patient knows they are being watched.

How It Is Measured

Optical capture with force plates remains the reference, with reported joint angle errors of a few degrees and larger errors in rotation. 3.16.7 covers it.

Inverse dynamics converts measured motion and force into joint moments, inheriting every assumption in the segment model. 3.4.3 covers the forces.

Electromyography adds muscle timing and does not give force.

Wearables measure far more steps far less precisely, which is the opposite trade and arguably the more clinically useful one.

No method measures knee contact force in a native joint, so every figure of that kind is estimated or comes from an implant.

Clinical Relevance

Compare gait within a laboratory and a protocol rather than between them, and say so when quoting figures.

Read an adaptation as information the patient cannot give you, which is where this measurement earns its place.

Control or report speed in any comparison, since it confounds nearly every parameter.

State that kinetic outputs are modelled rather than measured.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into any decision a gait finding informs, because a pattern describes a limb and not a person.