Reliability of System Identification Techniques to Assess Standing Balance in Healthy Elderly

OBJECTIVES: System identification techniques have the potential to assess the contribution of the underlying systems involved in standing balance by applying well-known disturbances. We investigated the reliability of standing balance parameters obtained with multivariate closed loop system identification techniques. METHODS: In twelve healthy elderly balance tests were performed twice a day during three days. Body sway was measured during two minutes of standing with eyes closed and the Balance test Room (BalRoom) was used to apply four disturbances simultaneously: two sensory disturbances, to the proprioceptive and the visual system, and two mechanical disturbances applied at the leg and trunk segment. Using system identification techniques, sensitivity functions of the sensory disturbances and the neuromuscular controller were estimated. Based on the generalizability theory (G theory), systematic errors and sources of variability were assessed using linear mixed models and reliability was assessed by computing indexes of dependability (ID), standard error of measurement (SEM) and minimal detectable change (MDC). RESULTS: A systematic error was found between the first and second trial in the sensitivity functions. No systematic error was found in the neuromuscular controller and body sway. The reliability of 15 of 25 parameters and body sway were moderate to excellent when the results of two trials on three days were averaged. To reach an excellent reliability on one day in 7 out of 25 parameters, it was predicted that at least seven trials must be averaged. CONCLUSION: This study shows that system identification techniques are a promising method to assess the underlying systems involved in standing balance in elderly. However, most of the parameters do not appear to be reliable unless a large number of trials are collected across multiple days. To reach an excellent reliability in one third of the parameters, a training session for participants is needed and at least seven trials of two minutes must be performed on one day

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Diagnosing orthostatic hypotension with continuous and interval blood pressure measurement devices

Orthostatic hypotension (OH) is defined as a drop in systolic blood pressure (SBP) of >= 20 mm Hg and/or a drop in diastolic blood pressure (DBP) of >= 10 mm Hg within 3 min of standing. The international guidelines recommend ideally diagnosing OH with a continuous blood pressure (BP) measurement device, although in daily practice interval BP measurement devices are used more often. We aimed to investigate the difference in observed prevalence of OH between an interval and a continuous BP measurement device. A total of 104 patients with a mean age of 69 years were included. The prevalence of OH was 35.6% (95% CI: 26.4-44.8) with the interval BP measurement and 45.2% (95% CI: 35.6-54.8) with the continuous BP measurement device (P = .121). Lin's coefficient of concordance ranged from 0.47 to 0.59 for the drop in systolic blood pressure and from 0.33 to 0.42 for the drop in diastolic blood pressure. The positive proportion of agreement in diagnosis of OH between the interval and continuous measure was 59.5% and the negative proportion of agreement was 72.5%. Although the prevalence of OH was not significantly different between the continuous and the interval BP measurement devices using a similar amount of measurement, the concordance between interval and continuous measure is low resulting in low positive and negative proportions of agreement in the diagnosis of OH. We conclude that continuous BP measurement cannot be substituted by an interval BP measurement to diagnose OH