Swing Phase

For Medical Students

The Principle

A limb swinging freely has to clear the ground and then stop before it lands.

Clearing needs bending. Stopping needs a brake at the end of the swing.

Neither job involves carrying weight, so this phase is about control rather than strength.

At the Knee

Swing runs from toe off to the next heel strike and occupies about 40 per cent of the cycle. 3.14.1 covers the whole cycle.

The knee flexes rapidly to around 60 degrees in early swing, which is the largest excursion the cycle requires.

That flexion is what lifts the foot clear of the ground. Without it the toes catch.

In terminal swing the hamstrings work eccentrically to slow the shin before heel strike. 3.4.8 covers energy absorption.

Loads here are small compared with stance, because the limb is carrying only itself.

What Changes It

Available knee flexion, which is the first thing a stiff knee loses.

Walking speed, which raises the flexion needed and the braking required.

Hip flexor strength, which drives the limb forward.

Quadriceps tone, since a tight quadriceps resists the flexion.

Clinical Relevance

A stiff knee shows up here first, because this phase asks for the most movement. 6.72 covers early arthritis.

People compensate by swinging the leg out to the side or lifting the hip, which is easier to see than the knee itself.

Catching the toes is a trip risk and a reason to measure knee flexion rather than ask about it.

4.4 covers the gait examination.

MRCS Level

The Principle

Swing is largely passive. Much of the knee flexion is a consequence of hip flexion and momentum rather than of knee flexor activity.

A phase driven by momentum is cheap in energy and vulnerable to anything that changes timing.

So the abnormalities here are usually about range and control rather than about power.

At the Knee

Initial swing: rapid flexion to peak, driven largely by hip flexion and the momentum of preswing. Knee flexor activity is modest.

Midswing: the knee begins to extend passively as the shin swings forward under its own momentum.

Terminal swing: the hamstrings decelerate the shin eccentrically, and the quadriceps activate to prepare for heel strike.

Peak swing flexion near 60 degrees is required for foot clearance with normal ankle and hip function, and less is needed if the hip hitches.

Chapter 3.3 covers the joint angles through walking as kinematics. 3.3.9 covers them.

What Changes It

Passive knee range, which caps the peak directly.

Rectus femoris tone and timing, which resists early swing flexion when it fires late.

Walking speed, which raises both the required flexion and the braking demand.

Hip and ankle function, which supply the compensations when knee flexion is short.

Footwear mass, which changes the limb's moment of inertia.

How It Is Measured

Motion capture gives the swing flexion curve directly and is the reference. 3.16.7 covers it.

Toe clearance is measurable and is a better description of the functional problem than peak flexion alone.

Electromyography distinguishes a rectus femoris timing problem from a range problem, which look identical.

Observation identifies circumduction and hip hitching reliably, which are the compensations rather than the fault.

Swing parameters are less studied than stance parameters, because the loads are smaller and the clinical interest has followed the loads.

Clinical Relevance

Measure passive knee flexion before attributing a swing abnormality to control. 4.4 covers the examination.

Read circumduction and hip hitching as compensations and look at the knee and ankle for the cause.

Consider rectus femoris timing where range is adequate and swing flexion is not.

Expect this phase to deteriorate first in a stiffening knee, and ask about catching the toes specifically.

FRCS Level

At the Knee

Swing is the phase where a stiff knee declares itself, and it is the phase a clinic assessment is least likely to interrogate.

Peak swing flexion is a sensitive marker of early stiffness, because it is the cycle's largest demand and the first to be rationed.

After arthroplasty, swing flexion often recovers less completely than stance parameters do, which patients notice as awkwardness rather than as restriction. 3.14.12 covers that gait.

Rectus femoris dysfunction produces a stiff knee gait with normal passive range, and it is missed whenever range alone is checked.

Conventional teaching concentrates on stance because that is where the load is, and the patient's complaint of catching the toes is a swing complaint.

What Changes It

Passive range, which is the hard ceiling.

Muscle timing, particularly rectus femoris, which lowers the ceiling without changing it.

Prior surgery and arthrofibrosis, which restrict flexion preferentially. 3.7.20 covers reconstruction.

Speed, which raises the demand and can convert an adequate range into an inadequate one.

Hip and ankle compensations, which conceal the deficit and cost energy.

How It Is Measured

Peak swing flexion and minimum toe clearance from motion capture, which together describe the functional problem. 3.16.7 covers it.

Dynamic electromyography for rectus femoris timing, which is the measurement that changes management and is rarely obtained.

Passive range in clinic, which gives the ceiling and not the use made of it.

Energy cost measurement, which quantifies what the compensations are costing.

No clinical measurement distinguishes a range problem from a timing problem, which is why both get treated as stiffness.

Clinical Relevance

Ask about catching the toes, which is a swing complaint patients volunteer and examiners forget.

Compare passive range against peak swing flexion where both are available, since the gap is the control deficit.

Consider rectus femoris timing in a stiff knee gait with adequate passive range.

Expect swing flexion to recover less completely than stance after arthroplasty, and counsel on that. 3.14.12 covers it.

Take the patient's age, physiology, comorbidities, knee demand, psychological state and their own ideas, concerns and expectations into any intervention here, because the deficit is often tolerated rather than treated.