Limits of Stability are a concept defined as the points at which the center of gravity approaches the limits of the base of support. In other words, it is defined as the maximum distance a person can displace the center of pressure (CoP) by leaning the body within the base of support without having to take a step, without losing balance (Tomita et al., 2021). The range of displacement in the anterior directions is around 6º to 8º, in a mediolateral displacement as center to left and center to right is around 8º, and in backward direction is around 4,5º. This stable swaying area is often referred to as the 'Cone of Stability’. Our central nervous system has an internal representation of this stability area that allows the producing of appropriate postural responses to the internal perturbations that accompany daily motor actions, determining how to move to maintain balance.
Assessing the Limits of Stability enables an evaluation of stability and voluntary motor control in dynamic states. It can be used as a screening test for fall risk, particularly in elderly populations. In fact, studies have shown a reduced capacity to control CoP displacement within the borders of the base of support may increase the possibility of a fall, especially when performing daily activities that challenge the limits of stability, such as leaning to reach an object (Tomita et al., 2021).
How to perform:
This protocol measures several movement characteristics in 8 different directions – front, forward/right, right, backward/right, backward, backward/left, left, and forward/left. The patient controls the blue point in real-time with its center of pressure .
Resultant Parameters:
The reaction time (RT) is the time between the indication to move (2 seconds after clicking the Start button and the color of the target changes to orange) and the first movement of the patient, in seconds.
The endpoint excursion (EPE) is the distance from the first attempt to reach the orange target, expressed as a percentage of LOS. The end of the first attempt is considered the point at which the initial movement towards the goal ceases.
The maximum excursion (MXE) is the maximum distance reached during the 8 seconds trial, expressed as a percentage of LOS.
The movement velocity (MVL) is the distance travelled by the center of pressure, between 5% and 95% of the first attempt, divided by the elapsed time (°/s).
The direction control (DCL) is the percentage of movement in the intended direction minus off-axis movement during the first attempt.
5 Reasons for Limits of Stability reduction
1. Impaired cognitive processing
In recent years, research has enlightened the fact that postural skills of the elderly are dependent on both the biomechanical complexities as well as the specific sensory-attentional processing requirements of the postural task (Vermette et al., 2023). The decrease in sensory information reliability, frequently observed in the elderly, has been thought to reduce the efficiency of automatic processing and reweighting of visual, vestibular, and proprioceptive inputs, increasing the controlled processing and the cognitive-attentional load associated with postural control (Goble et al., 2011; Henry & Baudry, 2019). Therefore, it is expected that impaired cognition may have a big impact on postural control, reducing the limits of stability and increasing the risk of fall. This becomes more evident when a second task is added.
2. Neuromuscular impairments
The inability to efficiently coordinate muscular activity and joint torque, especially at the ankle joint, resultant from changes to the neuromuscular system may be responsible for a reduction in limits of stability. This can be seen on the endpoint excursion, maximum excursion, and movement velocity parameters.
Another very important requisite for maintaining balance during dynamic activities and achieving acceptable levels of stability limits is the ability to prepare for the movement and adjust to it to respond to external and internal perturbations, in other words, the ability to produce correct and efficient postural adjustments (Vermette et al., 2023). One parameter that can elucidate us on neuromuscular control is the direction control.
3. Musculoskeletal impairments
Reduced muscle strength on the plantar flexors, resulting in difficulties in supporting of the leaning of the body to the front, limited range of motion of ankle joint, presence of pain, or a lower limb pathology can strongly influence limits of stability, especially the endpoint excursion and maximum excursion parameters (Melzer et al., 2008).
4. Emotional Overlay
There is evidence that fear of falling is associated with a reduction in the limits of stability, namely, for the forward – backward and left-right direction (Tomita et al., 2021). It is thought that fear of falling reduces the amount of attention resources available for balance control, this effect is more apparent when subjects perform a second task during activities that require balance control, like most daily live activities do (Uemura et al., 2012).
An increase in reaction time and a decrease in movement velocity are often observed during the performance of Limits of Stability Protocol in people with fear of falling.
5. Imbalance due to altered sensory input reception and integration
Expectably, changes in proprioceptive and vestibular functions are linked to increased postural imbalance. If the sensory input available is unreliable for postural orientation it is expected that the time needed to sense alterations in body position and adjust to it in matters of muscle activation will increase (Henry & Baudry, 2019). Therefore, we can presume that both velocity of movement and directional control might be deeply affected. Again, specific changes in environmental conditions and dual tasking can make difficulties in performing Limits of Stability protocol more evident (Tomita et al., 2021; Vermette et al., 2023).
Balance assessment using the limits of stability protocol allows an objective measurement of the ability to perform a type of movement which is fundamental for safely conducting daily life activities, counting on reproducible, practical, and reliable parameters.
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Ana Souto
Meet Ana, a physiotherapist with a master's degree in human physiology, currently
specializing in neurobiology. Her professional journey has led her to gain extensive expertise in both neurology and sports physiotherapy.
Ana currently serves as the clinical specialist at PhysioSensing, a cutting-edge Balance Assessment and training device. Leveraging her strong foundation in scientific research and evidence-based practices, Ana creates customized assessment and training plans. Her approach is firmly rooted in the latest scientific findings, ensuring that PhysioSensing users receive the most effective and up-to-date care.
In addition to her role in designing tailored programs, Ana plays a pivotal role in guiding new clients through the learning process of using PhysioSensing. She also provides advanced training and support to existing customers seeking to further deepen their clinical practice knowledge and stay on top of the latest scientific advancements.
References:
Goble, D. J., Coxon, J. P., Van Impe, A., Geurts, M., Doumas, M., Wenderoth, N., & Swinnen, S. P. (2011). Brain Activity during Ankle Proprioceptive Stimulation Predicts Balance Performance in Young and Older Adults. The Journal of Neuroscience, 31(45), 16344–16352. https://doi.org/10.1523/JNEUROSCI.4159-11.2011
Henry, M., & Baudry, S. (2019). Age-related changes in leg proprioception: Implications for postural control. Journal of Neurophysiology, 122(2), 525–538. https://doi.org/10.1152/jn.00067.2019
Melzer, I., Benjuya, N., Kaplanski, J., & Alexander, N. (2008). Association between ankle muscle strength and limit of stability in older adults. Age and Ageing, 38(1), 119–123. https://doi.org/10.1093/ageing/afn249
Tomita, H., Kuno, S., Kawaguchi, D., & Nojima, O. (2021). Limits of Stability and Functional Base of Support While Standing in Community-Dwelling Older Adults. Journal of Motor Behavior, 53(1), 83–91. https://doi.org/10.1080/00222895.2020.1723484
Uemura, K., Yamada, M., Nagai, K., Tanaka, B., Mori, S., & Ichihashi, N. (2012). Fear of falling is associated with prolonged anticipatory postural adjustment during gait initiation under dual-task conditions in older adults. Gait & Posture, 35(2), 282–286. https://doi.org/10.1016/j.gaitpost.2011.09.100
Vermette, M. J., Prince, F., Bherer, L., & Messier, J. (2023). Concentrating to avoid falling: Interaction between peripheral sensory and central attentional demands during a postural stability limit task in sedentary seniors. GeroScience, 46(1), 1181–1200. https://doi.org/10.1007/s11357-023-00860-z