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Balance

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Balance and Related Concepts

Relationship between line of gravity, centre of gravity, and base of support.

Balance refers to the body's ability to maintain its line of gravity within its base of support, whether stationary or moving. The line of gravity is an imaginary vertical line that passes through the body's centre of mass toward the earth's centre, while the base of support is the area beneath the body that includes all points of contact with the supporting surface. It is a complex neuromuscular skill that involves the integration of multiple sensory systems and motor responses. Balance allows an individual to safely navigate physical movements and participate in activities of daily living. In physical rehabilitation, balance can be divided into two categories:[1]

  1. Static balance: involves maintaining stability while stationary, such as standing still or holding a position.
  2. Dynamic balance: refers to maintaining control while moving, such as during walking, turning, or transitioning between positions.

Balance and equilibrium are closely related concepts. Though they are often used synonymously in clinical contexts, they are not entirely interchangeable. Equilibrium is the more precise mechanical term, defined as the state when all forces and moments acting on an object sum to zero, resulting in no net acceleration (Newton's First Law). This is a fundamental physics concept that applies to any object or system. Balance more commonly refers to the functional ability to maintain equilibrium, particularly in biological systems. Balance encompasses not just the mechanical state of equilibrium, but also the dynamic processes and control mechanisms that achieve and maintain that state.[1]

Balance and stability are also closely related but distinct concepts in biomechanics. Balance represents the state when an object maintains equilibrium. Stability, however, refers to the degree of resistance to losing that balance. The key connection is that stability determines how much disturbance an object can withstand before balance is lost. An object with greater stability can tolerate larger displacements of its line of gravity or greater external forces before becoming unbalanced and falling. Factors that enhance both balance and stability include:[1]

  • a larger base of support, which provides more area for the line of gravity to move within while maintaining balance
  • a lower centre of gravity, which reduces the leverage effect of displacing forces
  • a more centralised centre of gravity, which positions the line of gravity farther from the edges of the base of support

Balance and postural control are intimately connected, with postural control representing the active, dynamic process that enables humans to achieve and maintain balance. While inanimate objects are subject to the passive forces of gravity and will fall when their line of gravity moves outside their base of support, humans possess the unique ability to actively control their balance through postural control mechanisms. This control system allows the body to sense threats to stability and use muscular activity to counteract gravitational forces, preventing falls even when the line of gravity temporarily moves outside the base of support.[1] Postural control encompasses three broad categories of human activity:[2]

  1. Steady-state balance is the ability to control balance under fairly predictable and non-changing conditions, such as maintaining specified postures like sitting or standing.
  2. Proactive or anticipatory balance is the ability to maintain upright balance in advance of potentially destabilising voluntary movements, such as during expected transitional movements or anticipating postural changes during sports activities.
  3. Reactive balance control is the ability to recover postural control following an unexpected external perturbation, such as during a trip, fall, or after being pushed.

In each case, postural control works to maintain, achieve, or restore the line of gravity within the base of support. Human stability relies on the integration of sensory and motor systems in addition to the biomechanical relationships between the centre of gravity and the base of support.[1]

The Three Balance Systems

Balance and postural control depend on the complex integration of three primary sensory systems that continuously provide information about body position and movement in space.

The visual system provides information about the position and velocity of the surrounding environment and improves balance by reducing the sway evoked by external disturbances.[3]

The vestibular system, located in the inner ear, serves a wide variety of functions that contribute to our sense of proprioception and equilibrium, including the sensation of orientation and acceleration of the head in any direction with associated compensation in eye movement and posture.[3][4]

The somatosensory system, which includes proprioception, provides critical feedback about joint position, muscle length, and body orientation relative to the support surface.[5][3]

The central nervous system (CNS) integrates information from the visual, vestibular and somatosensory systems, and generates an appropriate motor response to prevent a loss of balance based on the sensory-derived estimate of body orientation.[5] To maintain postural balance, the proprioceptive, vestibular, and visual systems continuously provide body position and movement data to the central nervous system. The central nervous system also has the ability to "reweight" sensory inputs.

Sensory reweighting is the process by which the central nervous system dynamically adjusts the relative importance or "weight" given to information from different sensory systems based on their reliability in the current environment.[6] For example, when standing on an unstable surface where somatosensory information from the feet becomes less reliable, the brain automatically increases its reliance on visual and vestibular information to maintain balance. This reweighting also occurs in response to neurological injury, such as following a cerebrovascular accident (CVA), where there is a loss or change in visual ability or somatosensation.[7]

Please see the following articles for a more in-depth review of these sensory systems:

Strategies for Maintaining Balance

Humans use coordinated postural control strategies that involve multiple joints to maintain upright balance when faced with unexpected disturbances. These reactive balance strategies automatically adjust based on the type and size of the disturbance, as well as individual physical limitations.[8] The three primary postural control mechanisms include the ankle, hip, and stepping strategies.

Ankle strategy: this strategy is characterised by a specific muscle activation pattern that starts at the ankles and moves upward through the body (distal-to-proximal), with movement primarily occurring at the ankle joints. The ankle strategy is typically used during small disturbances to balance or when standing on a stable, firm surface. In this strategy, the body moves as a relatively rigid unit, pivoting mainly around the ankle joints to maintain the centre of mass over the base of support.[8][9]

Hip strategy: this strategy involves a muscle activation pattern that starts at the hips and moves downward through the body (proximal-to-distal), with movement primarily occurring at the hip joints. The hip strategy is employed during larger disturbances to balance or when standing on narrow or unstable surfaces, where ankle movements alone are insufficient. This strategy allows for faster and larger adjustments by using hip flexion and extension to quickly reposition the centre of mass within the base of support.[8][9]

Stepping strategy (also known as compensatory protective step strategy): this strategy is a coordinated motor response used to recover balance after a disturbance by changing the base of support through taking one or more steps. These strategies are employed when the body's centre of mass is displaced beyond the existing base of support and cannot be corrected through ankle or hip strategies alone. According to a 2024 scoping review by Melo-Alonso et al., there are multiple types of stepping strategies, including:[10]

  • lowering and elevating strategy: adjusting foot placement height
  • short- and long-step strategy: varying step length based on available reaction time
  • backward and forward stepping: directional responses to slip perturbations
  • single step vs. multiple steps: using one step versus several steps to regain balance
  • lateral sidesteps: stepping to the side
  • loaded leg sidestep: stepping with the weight-bearing leg
  • unloaded leg sidestep: stepping with the non-weight-bearing leg
  • crossover step (behind and front): crossing one leg over the other
  • medial sidestep: stepping toward the body's midline

The following optional 9-minute video overviews the differences between ankle, hip, and stepping reactive balance strategies.

[11]

Balance Assessments

Activities-Specific Balance Confidence (ABC) Scale

The Activities-Specific Balance Confidence (ABC) Scale is a 16-item self-report questionnaire that measures an individual's confidence in performing various ambulatory activities without falling. Each item is rated on a percentage scale from 0% (no confidence) to 100% (complete confidence), with the total score calculated as the average of all items. The scale assesses balance confidence across activities ranging from basic mobility tasks to more challenging activities like walking on icy surfaces.[12][13]

Intended population: the ABC Scale is designed for community-dwelling older adults (65+) at risk for falls and clinical populations with balance impairments. It has been validated across diverse conditions, including cerebrovascular accident, Parkinson's, multiple sclerosis, vestibular disorders, spinal cord injuries, COPD, and knee osteoarthritis.[13][14]

Reliability and validity: the ABC Scale demonstrates excellent psychometric properties with high internal consistency (Cronbach's alpha >0.90), good test-retest reliability, and strong construct validity. It effectively differentiates fallers from non-fallers, shows responsiveness to interventions, and has established minimal detectable change values for clinical interpretation.[13][14]

Link to the assessment form.

Romberg Test

The Romberg Test is a simple, non-technical neurological assessment that evaluates proprioceptive function and dorsal column integrity by removing visual compensation for balance. The patient is asked to stand with both feet together, first with eyes open and then with eyes closed, for one minute, while the examiner observes for increased body swaying, foot movement, or falling. A positive Romberg sign occurs when a patient can maintain balance with their eyes open but demonstrates significant postural instability or falls when their eyes are closed. This indicates impaired proprioception rather than cerebellar dysfunction.[15]

Intended population: the Romberg test is indicated for patients with imbalance, dizziness, falls, ataxia, or incoordination. It evaluates neurological conditions, including Parkinson's, Friedreich ataxia, vitamin B12 deficiency, tertiary syphilis, normal pressure hydrocephalus, and vestibular disorders. It is particularly useful for assessing proprioceptive deficits in myelopathies, sensory neuropathies, and dorsal column dysfunction.[15]

Reliability and validity: literature consensus on reliability and validity is lacking due to the test's qualitative nature.[16] However, it shows reliability in patients with Parkinson's and has high specificity—a positive Romberg sign strongly suggests dorsal column/medial lemniscus pathway deficits. Accuracy depends on proper administration and the examiner's skill in distinguishing normal sway from pathological instability.[15][16]

Link to the Romberg Test form and the Modified Romberg Test form.

The Single-Limb Stance (SLS) Test

The Single-Limb Stance (SLS) test (also known as the one-legged stance test or unipedal stance test) is a simple clinical assessment used to evaluate static postural and balance control. The client is timed as to the length of time (upper limit typically = 30 seconds) they are able to stand (preferably with shoes off) on one foot, first with eyes open, then eyes closed. It challenges the sensory integration system by reducing the base of support and can be performed with or without visual input to assess different aspects of balance function.[17]

Intended population: the SLS test can be used for healthy adults, older adults, and clinical populations, including Parkinson's, cerebrovascular accident, multiple sclerosis, and vestibular disorders. For patients with Parkinson's, a 10-second cut-off provides optimal sensitivity and specificity for fall history, with 75% of fallers scoring ≤10 seconds. It's particularly valuable for assessing fall risk in older adults and monitoring neurological balance changes.[17]

Reliability and validity: this test shows high reliability for group comparisons (high ICC values) but requires large individual performance differences to confidently detect real changes.[18] Studies demonstrate good intra-observer (0.896-0.949) and inter-observer (0.878-0.917) reliability using inertial sensors, making it valid for assessing postural control across various populations, including patients following a cerebrovascular accident.[17]

Link to testing instructions and normative data.

Perturbation tests

Perturbation tests are dynamic balance assessments that evaluate an individual's ability to recover postural control when exposed to unexpected external disturbances or challenges to stability. These tests involve applying controlled perturbations through various methods, including platform translations, surface rotations, cable releases, waist pulls, mechanical pushes, or sudden visual or vestibular stimuli to assess reactive balance responses. The tests measure parameters such as time to regain stability, centre of pressure displacement, muscle activation patterns, and stepping strategies used to regain balance. Unlike static balance tests, perturbation assessments provide insights into the dynamic postural control mechanisms that are essential for preventing falls during real-world activities.

Intended population: perturbation tests are used across healthy young adults (sports performance), older adults (fall risk assessment), and clinical populations with neurological conditions, including stroke, Parkinson's, multiple sclerosis, and vestibular disorders.

Reliability and validity: reliability has not been established across all protocols, though some show good to excellent reliability for specific measures.[19] Reliability varies by perturbation method, outcome variables, and population tested. The tests demonstrate validity by distinguishing fallers from non-fallers and correlating with functional mobility measures, but require more protocol standardisation and normative data for consistent clinical interpretation.[20][21]

Link to testing instructions.

Functional Reach Test (FRT)

The Functional Reach Test (FRT) assesses a person's stability by measuring the maximum distance they can reach forward while standing in a fixed position. The participant stands with their feet shoulder-width apart and one arm lifted to 90 degrees. They reach forward as far as possible without taking a step or losing balance. The distance reached is measured in centimeters using a yardstick. The test indicates the person's limits of stability and dynamic balance control. The pooled normative value for functional reach in older adults is 27.2 cm.[22]

Intended population: the FRT can be used with healthy older adults and individuals with neurological conditions, including cerebrovascular accident, Parkinson's, multiple sclerosis, spinal cord injury, and vestibular disorders. It's particularly valuable for fall risk assessment and monitoring balance changes in older adults in community and clinical settings.[23]

Reliability and validity: the FRT demonstrates good reliability and concurrent, predictive, and known groups validity among older adults. Test-retest reliability is good across populations (ICC: 0.81-0.98) with construct validity shown through distinguishing fallers from non-fallers. However, it should not be used alone for fall prediction and is best incorporated into comprehensive balance assessment batteries.[23][24]

Link to testing instructions and score sheet.

Timed Up and Go (TUG)Test

The Timed Up and Go (TUG) Test requires patients to rise from a standard chair, walk to a marker 3 metres away, turn, walk back, and sit down again. It is a simple, quick assessment that measures functional mobility, dynamic balance, and fall risk. The TUG test is a reliable, cost-effective, safe, and time-efficient way to evaluate overall functional mobility with high correlation with other proven tests that measure pure balance and gait performance.[25]

Intended population: the TUG is widely applicable across older adults for fall risk assessment and individuals with neurological conditions, including Parkinson's, stroke, spinal cord injuries, vestibular disorders, and traumatic brain injury. It has established normative values across age groups and is valuable for community-dwelling older persons, rehabilitation settings, and clinical research.[25]

Reliability and validity: the TUG is reliable and valid for quantifying functional mobility and tracking clinical change over time. It shows excellent associations with the Berg Balance Scale (ρ = -0.70) and strong relationships with walking tests.[18] Test-retest reliability is good to excellent (ICC: 0.95-0.99) across populations, with significant correlations to gait speed (r = .75), postural sway (r = -.48), step length (r = -.74), and Barthel Index (r = -.79), supporting its construct validity for functional mobility and fall risk assessment.[24][25]

Link to testing instructions and score sheet.

Five Times Sit-to-Stand (5TSTS) Test

The Five Times Sit-to-Stand (5TSTS) test measures functional lower limb muscle strength by requiring participants to stand up and sit down five times as quickly as possible from a standard chair without using their arms. The test is timed from the initial sitting position until the participant completes the fifth stand, providing an assessment of lower extremity power, balance, and functional mobility. The 5TSTS has been found to be reliable, safe and valid for measuring healthy adults' lower limb muscle strength and for determining balance control, fall risk, and exercise capacity.[26]

Intended population: the 5TSTS is a safe test for stratifying fall risk in moderate-risk and healthy populations. It has been extensively studied across community-dwelling older adults, neurological conditions (Parkinson's, cerebrovascular accident), COPD, multiple sclerosis, vestibular disorders, and musculoskeletal conditions, including knee osteoarthritis. It's highly valid and reliable for evaluating fall risk in adults ≥65 years.[26]

Reliability and validity: meta-analysis confirms the 5TSTS provides stable results across populations for assessing sit-to-stand performance, lower limb strength, and balance control. It shows excellent intra-rater reliability in older adults (ICC: 0.914-0.933) and moderate reliability in younger subjects (ICC: 0.644-0.879). The test demonstrates validity through strong correlation with TUG (r=0.64, p<0.001), supporting its use for functional performance and fall risk assessment.[24][26]

Link to testing instructions and normative data.

Additional Resources

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Pollock AS, Durward BR, Rowe PJ, Paul JP. What is balance?. Clinical rehabilitation. 2000 Aug;14(4):402-6.
  2. ↑ Shumway-Cook A, Wollacott M. Motor control: translating research to practice. Lippincott Williams and Wilkins, Philidelphia USA. 2016.
  3. ↑ 3.0 3.1 3.2 Pavlou M, Whitney SL. The principles of balance assessment and rehabilitation. Oxford Textbook of Vertigo and Imbalance. 2025 Feb 8:209.
  4. ↑ Casale J, Browne T, Murray I, Gupta G. Physiology, Vestibular System [Internet]. 2023 [cited 29 July 2025]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK532978/
  5. ↑ 5.0 5.1 Ketterer J, Gollhofer A, Ringhof S, Assländer L, Granacher U, Gehring D. Effects of balance training with visual input manipulations on balance performance and sensory integration in healthy young adults: a randomized controlled trial. Scientific Reports. 2024 Nov 19;14(1):28589.
  6. ↑ Assländer L, Peterka RJ. Sensory reweighting dynamics in human postural control. Journal of neurophysiology. 2014 May 1;111(9):1852-64.
  7. ↑ Missen KJ, Carpenter MG, Assländer L. Velocity dependence of sensory reweighting in human balance control. Journal of Neurophysiology. 2024 Aug 1;132(2):454-60.
  8. ↑ 8.0 8.1 8.2 Hong S, Park S. Biomechanical optimization and reinforcement learning provide insight into transition from ankle to hip strategy in human postural control. Scientific Reports. 2025 Apr 20;15(1):13640.
  9. ↑ 9.0 9.1 Morasso P. Integrating ankle and hip strategies for the stabilization of upright standing: An intermittent control model. Frontiers in Computational Neuroscience. 2022 Nov 17;16:956932.
  10. ↑ Melo-Alonso M, Murillo-Garcia A, Leon-Llamas JL, Villafaina S, Gomez-Alvaro MC, Morcillo-Parras FA, Gusi N. Classification and definitions of compensatory protective step strategies in older adults: a scoping review. Journal of Clinical Medicine. 2024 Jan 22;13(2):635.
  11. ↑ YouTube. Ankle, Hip, & Knee Strategy | SPTwithME. Available from: https://www.youtube.com/watch?v=NRLbmgJG1Q4 [last accessed 29/July/2025]
  12. ↑ Powell LE, Myers AM. The activities-specific balance confidence (ABC) scale. The journals of Gerontology Series A: Biological sciences and Medical sciences. 1995 Jan 1;50(1):M28-34.
  13. ↑ 13.0 13.1 13.2 Shirley Ryan Abilitylab. Activities-Specific Balance Confidence Scale. Available from: https://www.sralab.org/rehabilitation-measures/activities-specific-balance-confidence-scale (accessed 29 July 2025).
  14. ↑ 14.0 14.1 Montilla-Ibáñez A, Martínez-Amat A, Lomas-Vega R, Cruz-Díaz D, Torre-Cruz MJ, Casuso-Pérez R, Hita-Contreras F. The Activities-specific Balance Confidence scale: reliability and validity in Spanish patients with vestibular disorders. Disability and Rehabilitation. 2017 Mar 27;39(7):697-703.
  15. ↑ 15.0 15.1 15.2 Shirley Ryan Abilitylab. Romberg Test. Available from: https://www.sralab.org/rehabilitation-measures/romberg-test (accessed 29 July 2025).
  16. ↑ 16.0 16.1 Anagnostou E, Kouvli M, Karagianni E, Gamvroula A, Kalamatianos T, Stranjalis G, Skoularidou M. Romberg's test revisited: Changes in classical and advanced sway metrics in patients with pure sensory neuropathy. Neurophysiologie Clinique. 2024 Sep 1;54(5):102999.
  17. ↑ 17.0 17.1 17.2 Shirley Ryan Abilitylab. Single leg stance or "one legged-stance test". Available from: https://www.sralab.org/rehabilitation-measures/single-leg-stance-or-one-legged-stance-test (accessed 29 July 2025).
  18. ↑ 18.0 18.1 Alcock L, O’Brien TD, Vanicek N. Association between somatosensory, visual and vestibular contributions to postural control, reactive balance capacity and healthy ageing in older women. Health Care for Women International. 2018 Dec 2;39(12):1366-80.
  19. ↑ Missen KJ, Carpenter MG, Assländer L. Velocity dependence of sensory reweighting in human balance control. Journal of Neurophysiology. 2024 Aug 1;132(2):454-60.
  20. ↑ Robbins SM, Caplan RM, Aponte DI, St-Onge N. Test-retest reliability of a balance testing protocol with external perturbations in young healthy adults. Gait & Posture. 2017 Oct 1;58:433-9.
  21. ↑ Harburn KL, Hill KM, Kramer JF, Noh S, Vandervoort AA, Teasell R. Clinical applicability and test-retest reliability of an external perturbation test of balance in stroke subjects. Archives of physical medicine and rehabilitation. 1995 Apr 1;76(4):317-23.
  22. ↑ Bohannon RW, Wolfson LI, White WB. Functional reach of older adults: normative reference values based on new and published data. Physiotherapy. 2017 Dec;103(4):387-391.
  23. ↑ 23.0 23.1 Shirley Ryan Abilitylab. Functional Reach Test. Available from: https://www.sralab.org/rehabilitation-measures/functional-reach-test-modified-functional-reach-test (accessed 29 July 2025).
  24. ↑ 24.0 24.1 24.2 Grace Gaerlan M, Alpert PT, Cross C, Louis M, Kowalski S. Postural balance in young adults: the role of visual, vestibular and somatosensory systems. Journal of the American Academy of Nurse Practitioners. 2012 Jun;24(6):375-81.
  25. ↑ 25.0 25.1 25.2 Shirley Ryan Abilitylab. Timed Up and Go. Available from: https://www.sralab.org/rehabilitation-measures/timed-and-go (accessed 29 July 2025).
  26. ↑ 26.0 26.1 26.2 Shirley Ryan Abilitylab. Five Times Sit to Stand. Available from: https://www.sralab.org/rehabilitation-measures/five-times-sit-stand-test (accessed 29 July 2025).


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