Limits of stability

Limits of Stability (LoS) are a concept in balance and stability, defined as the points at which the center of gravity (CoG) approaches the limits of the base of support (BoS) and requires a corrective strategy to bring the center of mass (CoM) back within the BoS. In simpler terms, LoS represents the maximum distance an individual can intentionally sway in any direction without losing balance or needing to take a step. The typical range of stable swaying is approximately 12.5° in the front-back (antero-posterior) direction and 16° in the side-to-side (medio-lateral) direction. This stable swaying area is often referred to as the 'Cone of Stability', which varies depending on the specific task being performed.

When the CoG moves beyond the BoS, the individual must take a step or hold onto an external support to maintain balance and prevent a fall.

It is essential to note that these stability limits are perceived rather than solely physiological; they represent the subject's readiness to adjust their CoG position.

Clinical significance
Limits of Stability (LoS) is a significant variable in assessing stability and voluntary motor control in dynamic states. It provides valuable information by tracking the instantaneous change in the center of mass (COM) velocity and position. LoS is a useful measure for evaluating postural instability and identifying individuals at higher risk of falling, making it a valuable screening tool.

Individuals with decreased LoS are more susceptible to falling when shifting their bodyweight forward, backward, or sideways, thus increasing their risk of injuries. A restricted LoS can significantly affect an individual's ability to respond to balance control tests and react to perturbations. This reduction in LoS may be attributed to various factors, including weakness in ankle and foot muscles, musculoskeletal issues in the lower limbs, or an internal perception to resist larger displacements. These impairments can be correlated with medical examination findings and serve as an essential outcome measure for rehabilitation of specific underlying body impairments.

From a clinical perspective, individuals with better LoS can perform complex mobility tasks without support and are more capable of tolerating environmental challenges.

Possible causes of LoS impairment

 * 1) Impaired cognitive processing: This is often associated with aging and can result in attention deficits.
 * 2) Neuromuscular impairments: Conditions such as bradykinesia (slowness of movement), ataxia (lack of muscle coordination), and poor motor control can affect attention and cognitive functions.
 * 3) Musculoskeletal impairments: Weakness, limited range of motion (ROM), and pain in the musculoskeletal system can also impact attention and cognitive abilities.
 * 4) Emotional Overlay: Emotions like fear or anxiety can influence cognitive processing and attention.
 * 5) Aphysiology: This refers to exaggeration or poor effort in performing tasks, which can affect attention and cognitive performance.

Limits of Stability Testing
Various tools such as the Functional Reach Test (FRT) and Limits Of Stability (LOS) test have been used to assess LoS.
 * 1) Functional Reach Test (FRT): This test is commonly used to assess balance and LoS in the forward direction. It is cost-effective and easy to administer. However, it only measures LoS in the forward direction and is performed in a standing posture with the feet in a static position.
 * 2) Limits Of Stability (LOS) Test: This is a more advanced tool compared to FRT and is used to measure balance under multi-directional conditions. In this test, the subject stands on force plates and intentionally shifts their body weight in the cued direction.

Parameters measured in LOS test

 * 1) Reaction Time (RT): The time taken by an individual to start shifting their center of gravity (COG) from the static position after receiving a cue, measured in seconds.
 * 2) Movement Velocity (MVL): The average speed at which the COG shifts.
 * 3) EndPoint Excursions (EPE): The distance willingly covered by the subject in their very first attempt towards the target, expressed as a percentage.
 * 4) Maximum Excursions (MXE): The amount of distance the subject actually covered or moved their COG.
 * 5) Directional Control (DCL): A comparison between the amount of movement demonstrated in the desired direction (towards the target) to the amount of external movement in the opposite direction of the target, expressed as a percentage.

Interpretation of LOS Results
The ability to move around without falling is essential for performing activities of daily living (ADLs). Patients who exhibit delays in reaction time, decreased movement velocity, restricted Limits of Stability (LoS) boundary or cone of stability, or uncontrolled center of gravity (CoG) movement are at a higher risk of falling. A delayed reaction time may indicate cognitive processing issues, while reduced movement velocities may indicate higher-level central nervous system deficits. Reduced endpoint excursions, excessively larger maximum excursions, and poor directional control are all indicative of motor control abnormalities.

A LoS score close to 100 represents minimal sway and hence a reduced risk of falling, while scores close to 0 imply a higher risk of falling.

Validity and reliability of LOS
The LOS test has been validated for use across multiple patient populations, including community-dwelling elderly individuals, those with neurological disorders, and those with back and knee injuries. A study conducted by Wernick-Robinson and collaborators in 1999 on the test-retest reliability suggests that using the amount of distance covered in the functional reach test alone may not be an adequate measure of dynamic balance. The study also highlights that for a better evaluation of postural control, additional assessment of movement strategies is indispensable.

Similarly, another study conducted by Brouwer et al. also claims that the Limits of Stability (LOS) test is a reliable measure for balance testing in healthy populations.

Functional Impact and Implications of LOS
The capability of moving around without falling is necessary for activities of daily living (ADLs).Instability during weight-shifting activities or the inability to perform certain weight transfer tasks, such as bending forward to take objects from a shelf or leaning backward to rinse hair in the shower, can result from a restricted LoS boundary. The ability to voluntarily move the COG to positions within the Limits of Stability (LOS) with control is fundamental to independence and safety in mobility tasks, such as reaching for objects, transitioning from seated to standing positions (or standing to seated), and walking.

The LoS can be indicative of fall risks in various populations, including the elderly, individuals with movement disorders, and those with neurological impairments. Ensuring adequate control and movement within the LoS is crucial for maintaining independence and safety in everyday mobility tasks.