About the structure of posturography: Sampling duration, parametrization, focus of attention (part I)
Affiliation(s)
Institute of Sport Sciences, Johann Wolfgang Goethe University, Frankfurt am Main, Germany.
Institute of Sport Sciences, Johann Wolfgang Goethe University, Frankfurt am Main, Germany.
ABSTRACT
This study investigates the choice of posturographic parameter sets with respect to the influence of different sampling durations (30 s, 60 s, 300 s). Center of pressure (COP) data are derived from 16 healthy subjects standing quietly on a force plate. They were advised to focus on the postural control process ( i.e. internal focus of attention). 33 common linear and 10 nonlinear parameters are calculated and grouped into five classes. Component structure in each group is obtained via exploratory factor analysis. We demonstrate that COP evaluation—irrespective of sampling duration—necessitates a set of diverse parameters to explain more variance of the data. Further more, parameter sets are uniformly invariant towards sampling durations and display a consistent factor loading pattern. These findings pose a structure for COP parametrization. Hence, specific recommendations are preserved in order to avoid redundancy or misleading basis for inter-study comparisons. The choice of 11 parameters from the groups is recommended as a framework for future research in posturography.
This study investigates the choice of posturographic parameter sets with respect to the influence of different sampling durations (30 s, 60 s, 300 s). Center of pressure (COP) data are derived from 16 healthy subjects standing quietly on a force plate. They were advised to focus on the postural control process ( i.e. internal focus of attention). 33 common linear and 10 nonlinear parameters are calculated and grouped into five classes. Component structure in each group is obtained via exploratory factor analysis. We demonstrate that COP evaluation—irrespective of sampling duration—necessitates a set of diverse parameters to explain more variance of the data. Further more, parameter sets are uniformly invariant towards sampling durations and display a consistent factor loading pattern. These findings pose a structure for COP parametrization. Hence, specific recommendations are preserved in order to avoid redundancy or misleading basis for inter-study comparisons. The choice of 11 parameters from the groups is recommended as a framework for future research in posturography.
Cite this paper
Schubert, P. , Kirchner, M. , Schmidtbleicher, D. and Haas, C. (2012) About the structure of posturography: Sampling duration, parametrization, focus of attention (part I). Journal of Biomedical Science and Engineering, 5, 496-507. doi: 10.4236/jbise.2012.59062.
Schubert, P. , Kirchner, M. , Schmidtbleicher, D. and Haas, C. (2012) About the structure of posturography: Sampling duration, parametrization, focus of attention (part I). Journal of Biomedical Science and Engineering, 5, 496-507. doi: 10.4236/jbise.2012.59062.
References
[1] Winter, D. (2009) Biomechanics and motor control of human movement. 3rd Edition, John Wiley & Sons, Hoboken. [2] Latash, M. (2008) Neurophysiological basis of movement. 2nd Edition, Human Kinetics, Champaign. [3] Chagdes, J., Rietdyk, S., Haddad, J., Zelaznik, H., Raman, A., Rhea, C. and Silver, T. (2009) Multiple timescales in postural dynamics associated with vision and a secondary task are revealed by wavelet analysis. Experimental Brain Research, 197, 297-310. doi:10.1007/s00221-009-1915-1 [4] Deliagina, T.G., Orlovsky, G.N., Zelenin, P.V. and Beloozerova I.N. (2006) Neural bases of postural control. Physiology, 21, 216-225. doi:10.1152/physiol.00001.2006 [5] Collins, J., and De Luca, C. (1993) Open-loop and closedloop control of posture: A random-walk analysis of center-of-pressure trajectories. Experimental Brain Research, 95, 308-318. doi:10.1007/BF00229788 [6] Visser, J.E., Carpenter, M.G., van der Kooij, H. and Bloem, B.R. (2008) The clinical utility of posturography. Clinical Neurophysiology, 119, 2424-2436. doi:10.1016/j.clinph.2008.07.220 [7] van der Kooij, H., Campbell, A.D. and Carpenter, M.G. (2011) Sampling duration effects on centre of pressure descriptive measures. Gait & Posture, 34, 19-24. doi:10.1016/j.gaitpost.2011.02.025 [8] Vuillerme, N. and Nafati, G. (2007) How attentional focus on body sway affects postural control during quiet standing. Psychological Research, 71, 192-200. doi:10.1007/s00426-005-0018-2 [9] Fraizer, E. and Mitra, S. (2008) Methodological and interpretive issues in posture-cognition dual-tasking in up-right stance. Gait Posture, 27, 271-279. doi:10.1016/j.gaitpost.2007.04.002 [10] Wulf, G., Mercer, J., McNevin, N. and Guadagnoli, M.A. (2004) Reciprocal influences of attentional focus on postural and suprapostural task performance. Journal of Motor Behavior, 36, 189-199. doi:10.3200/JMBR.36.2.189-199 [11] Laughton, C.A., Slavin, M., Katdare, K., Nolan, L., Bean, J.F., Kerrigan, D.C., Phillips, E., Lipsitz, L.A. and Collins, J.J. (2003) Aging, muscle activity, and balance control: Physiologic changes associated with balance impairment. Gait & Posture, 18, 101-108. doi:10.1016/S0966-6362(02)00200-X [12] Le Clair, K. and Riach, C. (1993) Postural stability measures: What to measure and for how long. Clinical Biomechanics, 11, 176-178. doi:10.1016/0268-0033(95)00027-5 [13] Duarte, M. and Sternard, D. (2008) Complexity of human postural control in young and older adults during prolonged standing. Experimental Brain Research, 191, 265-276. doi:10.1007/s00221-008-1521-7 [14] Freitas, S.M.S.F., Wieczorek, S.A., Marchetti, P.H. and Duarte, M. (2005) Age-related changes in human postural control of prolonged standing. Gait & Posture, 22, 322- 330. doi: 10.1016/j.gaitpost.2004.11.001 [15] Duarte, M., Harvey, W. and Zatsiorsky, V.M. (2000) Stabilographic analysis of unconstrained standing. Ergonomics, 43, 1824-1839. doi:10.1080/00140130050174491 [16] Carpenter, M.G., Frank, J.S., Winter, D.A. and Peysar, G.W. (2001) Sampling duration effects on centre of pressure summary measures. Gait and Posture, 13, 35-40. doi:10.1016/S0966-6362(00)00093-X [17] Duarte, M., Freitas, S.M.S.F. and Zatsiorsky, V. (2011) Control of equilibrium in humans—Sway over sway. In: Latash, M. and Danion, F. Eds., Motor control. Oxford University Press, Oxford, 219-242. [18] Duarte, M. and Zatsiorsky, V.M. (1999) Patterns of center of pressure migration during prolonged unconstrained standing. Motor Control, 3, 12-27. [19] Carrol, J.P. and Freedman, W. (1993) Nonstationary properties of postural sway. Journal of Biomechanics, 26, 409-416. doi:10.1016/0021-9290(93)90004-X [20] Riley, M.A., Balasubramaniam, R. and Turvey, M.T. (1999) Recurrence quantification analysis of postural fluctuations. Gait and Posture, 9, 65-78. doi:10.1016/S0966-6362(98)00044-7 [21] Rocchi, L., Chiari, L. and Capello, A. (2004) Feature selection of stabilometricparameters based on principal component analysis. Medical & Biological Engineering & Computing, 42, 71-79. doi:10.1007/BF02351013 [22] Pavol, M.J. (2005) Detecting and understanding differences in postural sway. Focus on “a new interpretation of spontaneous sway measures based on a simple model of human postural control”. Journal of Neurophysiology, 93, 20-21. doi:10.1152/jn.00864.2004 [23] Kitibayashi, T., Demura, S. and Noda, M. (2003) Examination of the factor structure of center of foot pressure movement and cross-validity. Journal of Physiological Anthropology and Applied Human Science, 22, 265-272. doi:10.2114/jpa.22.265 [24] Kirchner, M., Schubert, P., Schmidtbleicher, D. and Haas, C.T. (2012) Evaluation of the temporal structure of postural sway fluctuations based on a comprehensive set of analysis tools. Physica A, 391, 4692-4703. doi:10.1016/j.physa.2012.05.034 [25] Bolbecker, A.R., Lee Hong, S., Kent, J.S., Klaunig, M.J., O’Donnell, B.F., Hetrick, W.P. (2011). Postural control in bipolar disorder: Increased sway area and decreased dynamical complexity. Plos ONE, 6, e19824 doi:10.1371/journal.pone.0019824 [26] Prado, J., Stoffregen, T., and Duarte, M. (2007) Postural sway during dual tasks in young and elderly adults. Gerontology, 53, 274-281. doi:10.1159/000102938 [27] Ruhe, A., Fejer, R. and Walker, B. (2010) The test-retest reliability of centre of pressure measures in bipedal static task conditions—A systematic review of the literature. Gait & Posture, 32, 436-445. doi:10.1016/j.gaitpost.2010.09.012 [28] Maurer, C., and Peterka, R. (2005) A new interpretation of spontaneous sway measures based on a simple model of human postural control. Journal of Neurophysiology, 93, 189-200. doi:10.1152/jn.00221.2004 [29] Duarte, M., and Freitas, S. (2010) Revision of posturo-graphy based on force plate for balance evaluation. Revista Brasileira de Fisioterapia, 14, 183-192. doi:10.1590/S1413-35552010000300003 [30] Prieto, T., Myklebust, J., Hoffmann, R., Lovett, E., and Myklebust, B. (1996) Measures of postural steadiness: Differences between healthy young and elderly adults. IEEE Transactions on Biomedical Engineering, 43, 956- 966. doi:10.1109/10.532130 [31] Raymakers, J.A., Samson, M.M. and Verhaar, H.J.J. (2005) The assessment of body sway and the choice of the stability parameter(s). Gait & Posture, 21, 48-58. doi:10.1016/j.gaitpost.2003.11.006 [32] Stergiou, N. and Decker, L.M. (2011) Human movement variability, nonlinear dynamics, and pathology: Is there a connection? Human Movement Science, 30, 869-888. doi:10.1016/j.humov.2011.06.002 [33] Harbourne, R. and Stergiou, N. (2009) Movement variability and the use of nonlinear tools: Principles to guide physical therapist practice. Physical Therapy, 89, 267-283. doi:10.2522/ptj.20080130 [34] Demura, S., Kitibayashi, T. and Noda, M. (2006) Selection of useful parameters to evaluate center-of-foot pressure movement. Perceptual and Motor Skills, 103, 959- 973. doi:10.2466/PMS.103.7.959-973 [35] Rocchi, M.B.L., Sisti, D., Ditroilo, M., Calavalle, A. and Panebianco, R. (2005) The misuse of the confidence ellipse in evaluating statokinesigram. International Journal of Sports Science, 12, 169-171. [36] Donker, S.F., Roerdink, M., Greven, A.J. and Beek, P.J. (2007) Regularity of center-of-pressure trajectories depends on the amount of attention invested in postural control. Experimental Brain Research, 181, 1-11. doi:10.1007/s00221-007-0905-4 [37] Stergiou, N., Harbourne, R., and Cavanaugh, J. (2006) Optimal movement variability: A new theoretical perspective for neurologic physical therapy. Journal of Neurologic Physical Therapy, 30, 120-130. [38] Cavanaugh, J.T., Mercer, V.S. and Stergiou, N. (2007) Approximate entropy detects the effect of a secondary cognitive task on postural control in healthy young adults: A methodological report. Journal of NeuroEngineering and Rehabilitation, 4, 1-7. doi:10.1186/1743-0003-4-42 [39] Richman, J., and Moorman, J. (2000) Physiological time-series analysis using approximate entropy and sample entropy. American Journal of Physiology—Heart and Circulatory Physiology, 278, H2039-H2049. [40] Costa, M., Goldberger, A., and Peng, C. (2002) Multiscale entropy analysis of complex phyiologic time series. Physical Review Letters, 89, 068102. doi:10.1103/PhysRevLett.89.068102 [41] Blatter, C. (2003) Wavelets—An introduction (German title: Wavelets—Eine Einführung). 2nd Edition, Vieweg, Brunswick. [42] Kaiser, H.F. (1974) An index of factorial simplicity. Psychometrica, 39, 31-36. doi:10.1007/BF02291575 [43] Kaiser, H.F. (1960) The application of electronic computers to factor analysis. Educational and Psychological Measurement, 20, 141-151. doi:10.1177/001316446002000116 [44] MacCallum, R. C., Widaman, K. F., Zhang, S. and Hong, S. (1999) Sample size in factor analysis. Psychological Methods, 4, 84-99. doi:10.1037/1082-989X.4.1.84 [45] Costello, A. B. and Osborne, J. W. (2005) Best practices in exploratory factor analysis: Four recommendations for getting the most from your analysis. Practical Assessment, Research & Evaluation, 10, 1-9. [46] Harbourne, R.T., Deffeyes, J.E., Kyvelidou, A. and Stergiou, N. (2009) Complexity of postural control in infants: linear and nonlinear features revealed by principal component analysis. Nonlinear Dynamics, Psychology, and Life Sciences, 13, 123-144. [47] Baratto, L., Morasso, P.G., Re, C. and Spada, G. (2002) A new look at posturographic analysis in the clinical context: Sway-density vs. other parameterization techniques. Motor Control, 6, 246-270. [48] Cornilleau-Pérès, V., Shabana, N., Droulez, J., Goh, J.C.H., Lee, G.S.M. and Chew, P.T.K. (2005) Measurement of the visual contribution to postural steadiness from the COP movement: Methodology and reliability. Gait & Posture, 22, 96-106. doi:10.1016/j.gaitpost.2004.07.009 [49] Lafond, D., Corriveau, H., Hébert, R. and Prince, F. (2004) Intrasession reliability of center of pressure measures of postural steadiness in healthy elderly people. Archives of Physical Medicine and Rehabilitation, 85, 896-901. doi:10.1016/j.apmr.2003.08.089
[1] Winter, D. (2009) Biomechanics and motor control of human movement. 3rd Edition, John Wiley & Sons, Hoboken. [2] Latash, M. (2008) Neurophysiological basis of movement. 2nd Edition, Human Kinetics, Champaign. [3] Chagdes, J., Rietdyk, S., Haddad, J., Zelaznik, H., Raman, A., Rhea, C. and Silver, T. (2009) Multiple timescales in postural dynamics associated with vision and a secondary task are revealed by wavelet analysis. Experimental Brain Research, 197, 297-310. doi:10.1007/s00221-009-1915-1 [4] Deliagina, T.G., Orlovsky, G.N., Zelenin, P.V. and Beloozerova I.N. (2006) Neural bases of postural control. Physiology, 21, 216-225. doi:10.1152/physiol.00001.2006 [5] Collins, J., and De Luca, C. (1993) Open-loop and closedloop control of posture: A random-walk analysis of center-of-pressure trajectories. Experimental Brain Research, 95, 308-318. doi:10.1007/BF00229788 [6] Visser, J.E., Carpenter, M.G., van der Kooij, H. and Bloem, B.R. (2008) The clinical utility of posturography. Clinical Neurophysiology, 119, 2424-2436. doi:10.1016/j.clinph.2008.07.220 [7] van der Kooij, H., Campbell, A.D. and Carpenter, M.G. (2011) Sampling duration effects on centre of pressure descriptive measures. Gait & Posture, 34, 19-24. doi:10.1016/j.gaitpost.2011.02.025 [8] Vuillerme, N. and Nafati, G. (2007) How attentional focus on body sway affects postural control during quiet standing. Psychological Research, 71, 192-200. doi:10.1007/s00426-005-0018-2 [9] Fraizer, E. and Mitra, S. (2008) Methodological and interpretive issues in posture-cognition dual-tasking in up-right stance. Gait Posture, 27, 271-279. doi:10.1016/j.gaitpost.2007.04.002 [10] Wulf, G., Mercer, J., McNevin, N. and Guadagnoli, M.A. (2004) Reciprocal influences of attentional focus on postural and suprapostural task performance. Journal of Motor Behavior, 36, 189-199. doi:10.3200/JMBR.36.2.189-199 [11] Laughton, C.A., Slavin, M., Katdare, K., Nolan, L., Bean, J.F., Kerrigan, D.C., Phillips, E., Lipsitz, L.A. and Collins, J.J. (2003) Aging, muscle activity, and balance control: Physiologic changes associated with balance impairment. Gait & Posture, 18, 101-108. doi:10.1016/S0966-6362(02)00200-X [12] Le Clair, K. and Riach, C. (1993) Postural stability measures: What to measure and for how long. Clinical Biomechanics, 11, 176-178. doi:10.1016/0268-0033(95)00027-5 [13] Duarte, M. and Sternard, D. (2008) Complexity of human postural control in young and older adults during prolonged standing. Experimental Brain Research, 191, 265-276. doi:10.1007/s00221-008-1521-7 [14] Freitas, S.M.S.F., Wieczorek, S.A., Marchetti, P.H. and Duarte, M. (2005) Age-related changes in human postural control of prolonged standing. Gait & Posture, 22, 322- 330. doi: 10.1016/j.gaitpost.2004.11.001 [15] Duarte, M., Harvey, W. and Zatsiorsky, V.M. (2000) Stabilographic analysis of unconstrained standing. Ergonomics, 43, 1824-1839. doi:10.1080/00140130050174491 [16] Carpenter, M.G., Frank, J.S., Winter, D.A. and Peysar, G.W. (2001) Sampling duration effects on centre of pressure summary measures. Gait and Posture, 13, 35-40. doi:10.1016/S0966-6362(00)00093-X [17] Duarte, M., Freitas, S.M.S.F. and Zatsiorsky, V. (2011) Control of equilibrium in humans—Sway over sway. In: Latash, M. and Danion, F. Eds., Motor control. Oxford University Press, Oxford, 219-242. [18] Duarte, M. and Zatsiorsky, V.M. (1999) Patterns of center of pressure migration during prolonged unconstrained standing. Motor Control, 3, 12-27. [19] Carrol, J.P. and Freedman, W. (1993) Nonstationary properties of postural sway. Journal of Biomechanics, 26, 409-416. doi:10.1016/0021-9290(93)90004-X [20] Riley, M.A., Balasubramaniam, R. and Turvey, M.T. (1999) Recurrence quantification analysis of postural fluctuations. Gait and Posture, 9, 65-78. doi:10.1016/S0966-6362(98)00044-7 [21] Rocchi, L., Chiari, L. and Capello, A. (2004) Feature selection of stabilometricparameters based on principal component analysis. Medical & Biological Engineering & Computing, 42, 71-79. doi:10.1007/BF02351013 [22] Pavol, M.J. (2005) Detecting and understanding differences in postural sway. Focus on “a new interpretation of spontaneous sway measures based on a simple model of human postural control”. Journal of Neurophysiology, 93, 20-21. doi:10.1152/jn.00864.2004 [23] Kitibayashi, T., Demura, S. and Noda, M. (2003) Examination of the factor structure of center of foot pressure movement and cross-validity. Journal of Physiological Anthropology and Applied Human Science, 22, 265-272. doi:10.2114/jpa.22.265 [24] Kirchner, M., Schubert, P., Schmidtbleicher, D. and Haas, C.T. (2012) Evaluation of the temporal structure of postural sway fluctuations based on a comprehensive set of analysis tools. Physica A, 391, 4692-4703. doi:10.1016/j.physa.2012.05.034 [25] Bolbecker, A.R., Lee Hong, S., Kent, J.S., Klaunig, M.J., O’Donnell, B.F., Hetrick, W.P. (2011). Postural control in bipolar disorder: Increased sway area and decreased dynamical complexity. Plos ONE, 6, e19824 doi:10.1371/journal.pone.0019824 [26] Prado, J., Stoffregen, T., and Duarte, M. (2007) Postural sway during dual tasks in young and elderly adults. Gerontology, 53, 274-281. doi:10.1159/000102938 [27] Ruhe, A., Fejer, R. and Walker, B. (2010) The test-retest reliability of centre of pressure measures in bipedal static task conditions—A systematic review of the literature. Gait & Posture, 32, 436-445. doi:10.1016/j.gaitpost.2010.09.012 [28] Maurer, C., and Peterka, R. (2005) A new interpretation of spontaneous sway measures based on a simple model of human postural control. Journal of Neurophysiology, 93, 189-200. doi:10.1152/jn.00221.2004 [29] Duarte, M., and Freitas, S. (2010) Revision of posturo-graphy based on force plate for balance evaluation. Revista Brasileira de Fisioterapia, 14, 183-192. doi:10.1590/S1413-35552010000300003 [30] Prieto, T., Myklebust, J., Hoffmann, R., Lovett, E., and Myklebust, B. (1996) Measures of postural steadiness: Differences between healthy young and elderly adults. IEEE Transactions on Biomedical Engineering, 43, 956- 966. doi:10.1109/10.532130 [31] Raymakers, J.A., Samson, M.M. and Verhaar, H.J.J. (2005) The assessment of body sway and the choice of the stability parameter(s). Gait & Posture, 21, 48-58. doi:10.1016/j.gaitpost.2003.11.006 [32] Stergiou, N. and Decker, L.M. (2011) Human movement variability, nonlinear dynamics, and pathology: Is there a connection? Human Movement Science, 30, 869-888. doi:10.1016/j.humov.2011.06.002 [33] Harbourne, R. and Stergiou, N. (2009) Movement variability and the use of nonlinear tools: Principles to guide physical therapist practice. Physical Therapy, 89, 267-283. doi:10.2522/ptj.20080130 [34] Demura, S., Kitibayashi, T. and Noda, M. (2006) Selection of useful parameters to evaluate center-of-foot pressure movement. Perceptual and Motor Skills, 103, 959- 973. doi:10.2466/PMS.103.7.959-973 [35] Rocchi, M.B.L., Sisti, D., Ditroilo, M., Calavalle, A. and Panebianco, R. (2005) The misuse of the confidence ellipse in evaluating statokinesigram. International Journal of Sports Science, 12, 169-171. [36] Donker, S.F., Roerdink, M., Greven, A.J. and Beek, P.J. (2007) Regularity of center-of-pressure trajectories depends on the amount of attention invested in postural control. Experimental Brain Research, 181, 1-11. doi:10.1007/s00221-007-0905-4 [37] Stergiou, N., Harbourne, R., and Cavanaugh, J. (2006) Optimal movement variability: A new theoretical perspective for neurologic physical therapy. Journal of Neurologic Physical Therapy, 30, 120-130. [38] Cavanaugh, J.T., Mercer, V.S. and Stergiou, N. (2007) Approximate entropy detects the effect of a secondary cognitive task on postural control in healthy young adults: A methodological report. Journal of NeuroEngineering and Rehabilitation, 4, 1-7. doi:10.1186/1743-0003-4-42 [39] Richman, J., and Moorman, J. (2000) Physiological time-series analysis using approximate entropy and sample entropy. American Journal of Physiology—Heart and Circulatory Physiology, 278, H2039-H2049. [40] Costa, M., Goldberger, A., and Peng, C. (2002) Multiscale entropy analysis of complex phyiologic time series. Physical Review Letters, 89, 068102. doi:10.1103/PhysRevLett.89.068102 [41] Blatter, C. (2003) Wavelets—An introduction (German title: Wavelets—Eine Einführung). 2nd Edition, Vieweg, Brunswick. [42] Kaiser, H.F. (1974) An index of factorial simplicity. Psychometrica, 39, 31-36. doi:10.1007/BF02291575 [43] Kaiser, H.F. (1960) The application of electronic computers to factor analysis. Educational and Psychological Measurement, 20, 141-151. doi:10.1177/001316446002000116 [44] MacCallum, R. C., Widaman, K. F., Zhang, S. and Hong, S. (1999) Sample size in factor analysis. Psychological Methods, 4, 84-99. doi:10.1037/1082-989X.4.1.84 [45] Costello, A. B. and Osborne, J. W. (2005) Best practices in exploratory factor analysis: Four recommendations for getting the most from your analysis. Practical Assessment, Research & Evaluation, 10, 1-9. [46] Harbourne, R.T., Deffeyes, J.E., Kyvelidou, A. and Stergiou, N. (2009) Complexity of postural control in infants: linear and nonlinear features revealed by principal component analysis. Nonlinear Dynamics, Psychology, and Life Sciences, 13, 123-144. [47] Baratto, L., Morasso, P.G., Re, C. and Spada, G. (2002) A new look at posturographic analysis in the clinical context: Sway-density vs. other parameterization techniques. Motor Control, 6, 246-270. [48] Cornilleau-Pérès, V., Shabana, N., Droulez, J., Goh, J.C.H., Lee, G.S.M. and Chew, P.T.K. (2005) Measurement of the visual contribution to postural steadiness from the COP movement: Methodology and reliability. Gait & Posture, 22, 96-106. doi:10.1016/j.gaitpost.2004.07.009 [49] Lafond, D., Corriveau, H., Hébert, R. and Prince, F. (2004) Intrasession reliability of center of pressure measures of postural steadiness in healthy elderly people. Archives of Physical Medicine and Rehabilitation, 85, 896-901. doi:10.1016/j.apmr.2003.08.089