2 Original article Annals of Military & Health Sciences Research • Vol 12, No 1, Winter 2014 Complex feature analysis of center of pressure signal for age-related subject classification Omid Khayat 1 MS, Fereidoun Nowshiravan-Rahatabad 2 PhD 1 Young Researchers and Elite Club, South Tehran Branch, Islamic Azad University, Tehran, Iran. 2 Department of Biomedical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. ABSTRACT Purpose: The aim of this study was to characterize prolonged standing and its effect on postural control in elderly individuals in comparison to adults. Materials and Methods: The elderly individuals’ behavior during standing and how demanding such a task is for them, is still unknown. We recorded the center of pressure (COP) position of 12 elder and 15 young participants while they were standing for 30 seconds. Then an analysis was performed to find the most appropriate and discriminative features for the elderly and young posture signals discrimination. Features were selected in frequency and time domain. Largest Lyapunov exponents of the COP signals were also computed to show the impact of chaotic behavior in static balance characterization relative to age. Results: Working in frequency domain is preferred to time domain analysis and largest Lyapunov exponent of the posture signal can be representatively used for COP signal discrimination between the two classes of subjects. Conclusion: In investigation and analysis of static balance for elders and unhealthy participants the signal of COP can be studied in chaotic domain beside frequency domain. Extraction of features from both chaotic and frequency domains significantly improves the discrimination rate of balance signals in age-related classes. Keywords: static balance; center of pressure; age relation; largest Lyapunov exponent; feature extraction. INTRODUCTION Postural balance is the ability to stabilize the center of pressure (COP) for the body during a prolonged standing or walking. In our everyday life, we frequently stand for a prolonged period (more than a few minutes) while chatting to somebody, waiting in a line, or standing in a work environment, i.e. we stand in order to perform another task which in this context may be referred to as a suprapostural task. 1,2 In such natural standing, continuous low amplitude and slow swaying of the body is commonly interrupted by postural changes characterized by fast and gross body movements. 3-5 These postural changes are thought to be performed in order to diminish the discomfort caused by psychological factors (including increase of tension, mental stress, and reduction of motivation and concentration) and physiological factors (including increase of venous pooling in the lower extremities, occlusion of blood flow, vertigo, muscular fatigue and increased joint pressure). 4-9 Many aspects of postural control decline with age and then the postural deficits are a contributing factor to an increased likelihood for falls in many older adults. 10 One third to one half of all people over the age of 65 years old fall at least once per year 11 and a prolonged fear of falling as a result decreases their activity levels. 12 Subsequently, decreased mobility resulting from fear or injury can cause a decline in independence. 11 Many studies have reported improvements in postural stability after visual biofeedback-based training of balance in elderly. 16-18 However, the extent to which biofeedback information can improve balance has not been determined yet. Using a magnetic-based force platform allows the AMHSR 2014;12:2-7 www.journals.ajaums.ac.ir