Limits of Stability (LoS)

What the Limits of Stability test measures

The Limits of Stability (LoS) test measures how far a person can deliberately lean — shifting their centre of gravity towards a target — without stepping, reaching, or falling. Where the SOT asks how well a patient stays still under sensory challenge, the LoS asks how confidently they can move to the edge of their base of support and back.

The patient stands on the force plate and watches a screen showing their real-time centre-of-gravity cursor and a set of targets arranged around them, typically at eight compass points. On a cue, they lean to move the cursor onto each target as quickly and accurately as they can, then hold.

Because it probes volitional postural control rather than reactive or sensory-organisation control, the LoS complements the SOT, MCT and ADT. It is especially informative in cautious, deconditioned, and fear-of-falling patients whose quiet-stance scores look deceptively normal.

The five LoS parameters

Reaction time — the delay between the go cue and the first movement of the centre of gravity. Prolonged in central disorders, Parkinsonism, and with cautious set.

Movement velocity — the average speed of the intentional lean towards the target. Slow movement velocity is one of the most sensitive markers of reduced postural confidence.

Endpoint excursion — how far the centre of gravity travels on the first attempt toward the target, expressed as a percentage of the theoretical maximum. Maximal excursion is the farthest reached during the whole trial.

Directional control — how much of the movement is on-axis toward the target versus wasted off-axis sway. Low directional control suggests poor coordination or a tremor-laden trajectory.

Reading the LoS — constriction patterns

The hallmark abnormality is a constricted stability envelope — endpoint and maximal excursions fall short of the target ring in some or all directions, so the leaning area the patient is willing to use is smaller than normal.

Direction matters. A symmetric global constriction is typical of generalised cautious set, deconditioning, and fear of falling. A directionally asymmetric constriction — short excursions only toward one side — points to a lateralised deficit, hemiparesis, or a painful or unstable limb.

Reading the temporal parameters alongside the spatial ones separates mechanisms: slow reaction time with preserved excursion suggests a central initiation problem, whereas normal reaction time with poor directional control suggests an execution or coordination problem.

Fall risk and rehabilitation targets

LoS metrics correlate with fall risk in older adults and in neurological populations, and a constricted, slow envelope is a practical, objective marker of the fear-of-falling cycle that perpetuates disability after a first fall.

Because the parameters are intuitive and visual, the LoS doubles as a rehabilitation tool: patients can see their cursor reach further and faster over a course of therapy, which is motivating and provides an objective progress record that pairs well with serial SOT composites.

The test is not diagnostic of any single disease. Its value is functional — it quantifies the volitional-control component of balance that quiet-stance testing misses, and it gives rehabilitation a concrete, trainable target.