Gait & Posture 25 (2007) 56–62 www.elsevier.com/locate/gaitpost
Age associated differences in postural equilibrium control: A comparison between EQscore and minimum time to contact (TTCmin) Katharine E. Forth a,b, E. Jeffrey Metter c, William H. Paloski d,* a
National Space Biomedical Research Institute, Baylor College of Medicine, Houston, TX, USA b Universities Space Research Association, Houston, TX, USA c NIA/Clinical Research Branch, Harbor Hospital, Baltimore, MD, USA d Human Adaptation & Countermeasures Office, Neurosciences Laboratory, Mail Code SK, NASA Johnson Space Center, Houston, TX 77058, USA
Received 22 April 2005; received in revised form 24 December 2005; accepted 30 December 2005
Abstract Increased postural instability and the subsequent elevation in fall incidence with increasing age are important contributors for hip fractures and developing frailty. When testing for such instability, most studies characterize balance in terms of center-of-mass (COM) deviation from a finite point, the ‘‘equilibrium point’’, located at the center of a subject’s stance. For example, the clinically accepted equilibrium score (EQscore) represents instability as the maximum peak-to-peak sway about the ‘‘equilibrium point’’. An alternative theory views balance as being controlled within a ‘‘stability margin’’ in which all corrective actions are based on the time to contact (TTC) of the body’s COM with that margin. This study examines the differences offered by evaluating balance control using the EQscore and TTC approach across several age groups and sessions. Consenting subjects from the Baltimore Longitudinal Study of Aging were recruited (N = 155) from each age decade (20s–80s) who were generally healthy and free from neurological diagnoses. Results showed TTC tests detected significant variations in eyes open versus eyes closed testing that were unpredictable by EQscore. Further, TTC produced differences in age-related stability threats not seen using EQscore. The TTC data also provided a discriminating difference between subjects who fell in the difficult tests and those who maintained posture. Overall, these data suggest EQscore might not sufficiently account for dynamic control components the body may be using to maintain balance. TTC may offer a more accurate estimate of postural stability (functional ability) than EQscore based on its inclusion of a velocity component to detect dynamic changes. Published by Elsevier B.V. Keywords: Aging; Stability boundary; Dynamic posturography
1. Introduction Human sensory-motor systems have evolved to optimize body movements and posture control in the terrestrial gravitational field. The body maintains its balance using sensory information from visual, vestibular, proprioceptive systems, and exteroreceptors. This afference provides the central nervous system (CNS) with near-real-time information that is used to assess the biomechanical state of the body * Corresponding author. Tel.: +1 281 244 5315; fax: +1 281 244 5734. E-mail address:
[email protected] (W.H. Paloski). 0966-6362/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.gaitpost.2005.12.008
and determine the appropriate direction and scaling for motor commands to correct biomechanical state errors [1–3]. Aging degrades these corrective responses and interactions between sensory-motor systems and is associated with greater susceptibility to physical disability and frailty. Indeed, many older adults exhibit poor balance control, which is an important contributor to falls [4]. Elderly individuals show increased body sway, less secure base-ofsupport, and greater dependence on sensory cues from vision [5], as vestibular and somatosensory systems decline in performance. Motor control also declines with age, as manifested by slower response times, lower accuracy of
K.E. Forth et al. / Gait & Posture 25 (2007) 56–62
movement, and loss of muscle strength [6]. Assessment of balance control performance degradation with aging is critical for successful prevention of falls in the elderly. The most commonly used balance control model suggests that under nominal conditions, the CNS selects a quasisteady equilibrium point that places the center-of-gravity near the center of the base-of-support. Subject to threshold limitations of the receptors, and the individual’s vigilance, the CNS continually analyzes the afferent sensory information to detect deviations of the center-of-mass position (COM) from this equilibrium point and adjusts the motor command signals to return the COM to the equilibrium point. This theoretical construct lends itself well to linear control theory modeling. Thus, COM sway is generally modeled as being symmetric about the equilibrium point, and postural stability is represented by the deviation of sway from the equilibrium point [7,8]. Perhaps the most popular stability estimator is peak-to-peak sway, which quantifies the maximum deviations from the equilibrium point, and is thought to vary inversely with postural stability. One of the most widely used clinical posturography systems (Equitest, NeuroCom International, Clackamas, OR) measures peakto-peak sway over independent 20 s sensory organization test (SOT) trials to compute its measure of balance control performance, the equilibrium score (EQscore) [9]. An alternative balance control model was suggested by Koozekanani et al. [10], who pointed out the objective function for posture control might be best represented by a stability margin, which is not the deviation of sway away from an equilibrium point, but rather the deviation of sway toward a stability limit. This insightful recommendation has been largely ignored, likely because when one assumes an equilibrium point centered in the base-of-support and a fixed stability limit, any sway excursions away from the equilibrium point are inversely proportional to the stability margin. However, sway excursions of the COM are rarely symmetric about the equilibrium point, so the relative stability is likely to be underestimated by peak-to-peak sway. Furthermore, impending postural instabilities cannot be predicted by position information alone, but requires velocity information. The relatively new approach of examining time to contact (TTC) with the stability boundary [11,12] may therefore improve the accuracy of the relative stability estimate by adding a velocity term to the position estimate proposed by Koozekanani et al. [10]. Postural TTC has been computed relative to the entire composite boundary defined by the feet [13], separate anterior–posterior and medial–lateral boundaries [14,15], and functional bound-
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aries [13]. Owing to the velocity term, TTC may better differentiate between movements that are threatening and non-threatening to future postural stability [16], and may better reflect the method used by the CNS to control postural stability [17]. Patton et al. [16] identified the minimum TTC (TTCmin) within a trial as being an important quantitative measure for assessing postural stability and compensatory strategies. TTCmin represents the least stable posture over the trial, and from that perspective is analogous to peak-topeak sway. The aim of this study was to compare TTCmin against EQscore in a putatively normal subject population of widely varying age performing conventional SOTs. It was expected that this population would afford comparison of EQscore and TTC over a wide range of normal stability variation, and thereby allow a broad comparison, as well as an assessment of age-related changes in balance control performance estimates.
2. Methods This study was conducted with 155 subjects (69 males, 86 females) recruited from the Baltimore Longitudinal Study of Aging. All subjects gave written consent to participate in this Johns Hopkins Bayview IRB approved study after all procedures and risks had been explained to them. Table 1 provides the demographic representation for each decade. Subjects selected were generally healthy, and reported no history of persistent vestibular problems, no diagnosis of benign positional vertigo, Meniere’s disease, or other medical conditions that could affect the vestibular system. Further exclusion criteria required no history of neurological diagnoses including stroke, Parkinson’s disease, polyneuropathy, muscle disease, or taking medications that affect the vestibular system. Balance control was evaluated using a computerized dynamic posturography system (Equitest, NeuroCom International). Subjects performed six sensory organization tests with SOT 1 (Romberg, eyes open) considered the simplest task to SOT 6 (sway-referenced support and vision) considered the most challenging. The remaining SOTs are defined as SOT 2 (Romberg, eyes closed), SOT 3 (swayreferenced vision only), SOT 4 (sway-referenced support only), and SOT 5 (absent vision and sway-referenced support). Sway referencing was achieved by rotating the force platform, and/or visual surround within the sagittal plane in direct proportion to the estimated instantaneous
Table 1 Demographics of the sample representing each age decade
Subjects Mean height (cm) Mean weight (kg) BMI
20–29
30–39
40–49
50–59
60–69
70–79
>80 years
10 (4
