Short communication A quantitative characterization of postural sway during human quiet standing using a thin pressure distribution measurement system Kunihiko Nomura a , Kei Fukada a,b , Teruaki Azuma c , Toshimitsu Hamasaki a,d , Saburo Sakoda a,b , Taishin Nomura a,e, * a The Center for Advanced Medical Engineering and Informatics, Osaka University, Japan b Department of Neurology, Graduate School of Medicine, Osaka University, Japan c Robotics and Effective Technology of Sensing, Nitta Corporation, Japan d Department of Biomedical Statistics, Graduate School of Medicine, Osaka University, Japan e Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Japan 1. Introduction The use of ultra-thin film pressure measurement systems has become popular in detecting pressure acting on human body. Static measurements of contact areas and forces using such sensor systems have been reported to be accurate [1–3], and have been applied to measuring tibiofemoral [4] and meniscofemoral [5] contact pressures. However, less literature has reported dynamic characteristics of those systems despite the fact that they are used for center of pressure (CoP) measurement during gait [6] and standing [7]. We aimed at examining the accuracy of CoP measurement during human quiet stance using the ultra- thin film pressure measurement system (I-Scan). To this end, CoP sway was measured using I-Scan and a force platform (FP) simultaneously to quantify the discrepancy between the two methods. The differences were analyzed to identify a filter transforming CoP measurements obtained by I-Scan close to those obtained by FP. 2. Methods 2.1. Subjects and protocols Following informed consent, six healthy subjects participated in this study. The subjects were asked to stand barefoot on the thin pressure sensor sheet. Each subject performed 20 trials of 70-s long quiet standing. Two visual conditions, eyes- open (EO) and eyes-closed (EC), were performed. Each subject performed 10 trials for each visual condition with random order. 2.2. Data acquisition and processing For CoP measurements using I-Scan (version 5.834I, Tekscan, Inc.), we used the pressure distribution sensor sheet (C-SCAN12S, Nitta Corporation, Japan) placed on the force platform (Model OR6-5-1000, AMTI, Inc.) with x, medial–lateral (ML) and y, anterior–posterior (AP) axes aligned for both systems. Plantar pressure distributions measurements with I-Scan and ground reaction force measurements with FP were recorded simultaneously at 50 Hz. The relative position of the sensor sheet to the FP was fixed throughout the measurements. I-Scan sensor included a matrix arrangement of 7744 transducers covering 36.1 cm  36.1 cm. Each transducer covered a square of 4.1 mm. For FP, the vertical force F z and moments M x and M y were recorded. CoP for I-Scan at time t is determined as CoP IScan ðtÞ¼ P i; j P i; j ðtÞx i; j P i; j P i; j ðtÞ  P i; j P i; j ðtÞx i; j P i; j P i; j ðtÞ * + ; P i; j P i; j ðtÞy i; j P i; j P i; j ðtÞ  P i; j P i; j ðtÞy i; j P i; j P i; j ðtÞ * +  ! where P i,j (t) is the pressure value of the transducer located at (x i,j , y i,j ). CoP for FP is determined as CoP FP ðtÞ¼ MyðtÞ Fz ðtÞ  My ðtÞ Fz ðtÞ   ; Mx ðtÞ Fz ðtÞ  Mx ðtÞ Fz ðtÞ     Gait & Posture 29 (2009) 654–657 ARTICLE INFO Article history: Received 7 January 2008 Received in revised form 9 December 2008 Accepted 4 February 2009 Keywords: Postural sway Center of pressure I-Scan Force platform ABSTRACT The accuracy of the center of pressure (CoP) measurement during human quiet standing using an ultra- thin film pressure measurement system (Tekscan, I-Scan) was examined. To this end, CoP sway was measured simultaneously by I-Scan and a force platform (FP) to quantify differences in CoP obtained by the two methods. The sway amplitudes of I-Scan were slightly smaller than those of FP. The differences were systematic, allowing us to identify a filter that could bring every CoP trajectory of I-Scan close to that of FP. We concluded that, with the use of the filter, I-Scan could be used for accurate measurement of CoP sway during quiet standing. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. Tel.: +81 6 6850 6532; fax: +81 6 6850 6557. E-mail address: taishin@bpe.es.osaka-u.ac.jp (T. Nomura). Contents lists available at ScienceDirect Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost 0966-6362/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2009.02.001