Stance Phase

For Medical Students

The Principle

A limb carrying weight has to do two opposite things: absorb a landing and then push the body past.

Absorbing means giving way in a controlled manner. Pushing means becoming rigid.

A joint that must do both within half a second is doing the hardest thing in walking.

At the Knee

Stance begins at heel strike and ends at toe off, and it occupies about 60 per cent of the cycle. 3.14.1 covers the whole cycle.

In loading response the knee flexes to around 15 to 20 degrees while the quadriceps lengthen under tension, braking the collapse.

Through midstance the knee extends again as the body passes over the foot, and loading is at its most efficient.

In terminal stance the knee begins to flex again in preparation for swing, while the ankle is still pushing. 3.14.3 covers swing.

Ground reaction force has two peaks across stance, one after landing and one before push off. 3.4.2 covers it.

What Changes It

Walking speed, which raises both force peaks.

Quadriceps strength, which decides whether the loading response flexion happens at all.

Pain, which shortens the time spent on that limb. 3.14.11 covers the arthritic pattern.

Footwear and surface, which change how quickly the force rises at heel strike. 3.4.10 covers shock absorption.

Clinical Relevance

A knee that stays straight through loading response has usually lost quadriceps function rather than movement. 4.4.3 covers the gait.

Shortened stance time on one side is the commonest sign of pain in that limb. 4.4.2 covers the antalgic pattern.

Most knee loading in daily life happens in this phase, repeated thousands of times a day.

Section 8 covers the rehabilitation that targets it.

MRCS Level

The Principle

Stance subdivides into loading response, midstance, terminal stance and preswing, and each has a different mechanical demand.

Naming the subphase an abnormality occupies is more useful than naming the limp, because the subphase identifies the structure under load at that moment.

The knee's moments reverse within stance, so a single average figure for the phase conceals the thing worth knowing.

At the Knee

Loading response: external flexion moment peaks, quadriceps work eccentrically, and the stance flexion wave reaches 15 to 20 degrees.

Midstance: the knee approaches extension, the external moment swings towards extension, and quadriceps demand falls.

Terminal stance: the moment arm is anterior, the knee is near full extension and the posterior structures are tensioned.

Preswing: the knee flexes rapidly while still loaded, which is the transition most often disrupted by stiffness.

The external adduction moment is double peaked across stance and tracks medial compartment load throughout. 3.13.7 covers it.

What Changes It

Speed and cadence, which scale the moments and compress the timing.

Quadriceps inhibition from pain or effusion, which flattens the stance flexion wave.

Coronal alignment, which sets the adduction moment arm. 3.13.3 covers varus.

Trunk lean, which reduces the adduction moment without altering the knee. 3.13.7 covers it.

Ankle and hip function, since a stiff ankle transfers demand upward.

How It Is Measured

Force plate and motion capture together, which is the only way to obtain the moments. 3.16.7 covers motion capture.

Temporal parameters alone, from a pressure mat or instrumented walkway, which give stance time without kinetics.

Electromyography for the timing of quadriceps activity, which distinguishes weakness from inhibition.

Observation, which reliably identifies a missing stance flexion wave and little else. 4.3.2 covers it.

Phase boundaries are defined by events that themselves must be detected, so subphase timings differ between laboratories.

Clinical Relevance

Name the subphase rather than the limp, because the subphase names the structure under load.

Read a flattened stance flexion wave as quadriceps inhibition until proved otherwise. 6.71 covers the arthritic knee.

Measure stance time symmetry, which is the cheapest reliable asymmetry marker available.

Avoid averaging a moment across stance, since the knee's moments reverse within it.

FRCS Level

At the Knee

Stance is where the clinically relevant mechanics almost entirely live, and the stance flexion wave is its most informative single feature.

Its amplitude falls with pain, with effusion, with quadriceps weakness and with arthrofibrosis, and the four are distinguishable by what else accompanies them rather than by the wave itself.

Loss of the wave raises peak loading rate, because the limb no longer has a compliant phase to spread the impact. 3.4.10 covers shock absorption.

The adduction moment's first peak is the parameter with the strongest reported link to medial compartment progression, and it is a stance parameter. 3.13.8 covers the link.

Conventional teaching discusses gait abnormality in terms of named limps; the measurable literature is in terms of subphase parameters, and the two map onto each other poorly.

What Changes It

Quadriceps capacity and its inhibition, which are different problems with the same appearance.

Joint range, since a knee that cannot reach extension cannot complete midstance.

Alignment and the adduction moment arm. 3.13.3 covers varus.

Speed, which must be matched before any two stance measurements are compared.

Compensations at the hip and ankle, which can conceal a knee abnormality entirely.

How It Is Measured

Three dimensional capture with force plates, reported as a moment curve rather than a peak, because the shape carries the information.

Loading rate from the force trace, which is sensitive to heel strike detection.

Electromyography to separate weakness from inhibition, which imaging cannot do.

Stance time symmetry from a simple instrumented walkway, which is the most deployable measurement here.

There is no clinical measurement of the stance flexion wave, which is the thing the literature says matters most.

Clinical Relevance

Assess the stance flexion wave by observation and treat its absence as significant, since nothing in clinic measures it. 4.4.1 covers the examination.

Separate quadriceps weakness from inhibition before prescribing, because the treatments differ. Section 8 covers them.

Match walking speed before comparing any two stance measurements.

Report a moment curve rather than a peak when one is available.

Expect a lost flexion wave to raise loading rate, which is a mechanical argument for restoring it rather than working around it.