LIMITS OF STABILITY (LOS)
Description
LOS Report
Functional Implications
back to Motor Impairment Assessments
Description
The LOS quantifies the maximum distance a person can intentionally displace their Center of Gravity (COG), i.e. lean their body in a given direction without losing balance, stepping, or reaching for assistance. The measured parameters are reaction time, COG movement velocity, directional control, end point excursion, and maximum excursion.
For each of eight trials, the patient maintains their COG centered over the base of support as indicated by a cursor display of the COG position relative to a center target. On command, the patient moves the COG cursor as quickly and accurately as possible towards a second target located on the LOS perimeter (100% of theoretical limits of stability) and then holds a position as close to the target as possible. The patient is allowed up to 8 seconds to complete each trial.
[back to top]
LOS Comprehensive Report
- The COG traces for each trial are shown at the top left of the report.
- Reaction Time (RT) is the time in seconds between the command to move and the patient's first movement.
- Movement Velocity (MVL) is the average speed of COG movement in degrees per second.
- Endpoint Excursion (EPE) is the distance of the first movement toward the designated target, expressed as a percentage of maximum LOS distance. The endpoint is considered to be the point at which the initial movement toward the target ceases.
- Maximum Excursion (MXE) is the maximum distance achieved during the trial.
- Directional Control (DCL) is a comparison of the amount of movement in the intended direction (towards the target) to the amount of extraneous movement (away from the target).
- The shaded area on each graphic represents performance outside of the normative data range. Green bars indicate performance within the normal range; red bars indicate performance outside the normal range. A numerical value is given at the top of each bar.
[back to top]
Functional Implications
Ability to voluntarily move the COG to positions within the LOS is fundamental to mobility tasks such as reaching for objects, transitioning from a seated to standing position (or standing to seated), and walking.
Reaction time delays are commonly associated with difficulties in cognitive processing and/or motor diseases. Reduced movement velocities are indicative of high-level central nervous system deficits such as Parkinson's disease and age-related disorders. Inability to reach targets in single movements (reduced endpoint excursions or excessively larger maximum excursions) and poor directional control are indicators of motor control abnormalities. Excursions may be restricted by biomechanical (i.e. range) limitations. Dizzy and/or unsteady patients and those fearful of falling may artificially restrict their excursions, while the strength of those with lower extremity weakness may be insufficient to attain and/or maintain stable target positions.
Limitations in a patient's LOS may correlate to risk for fall or instability during weight shifting activities such as leaning forward to take objects from a shelf, leaning back for hair washing in the shower, opening the refrigerator door, etc. Patients with reduced stability limits in the AP direction tend to take smaller steps during gait, while laterally reduced limits can lead to broad-based gaits.
[back to top]