Comparison and evaluation of two common methods to measure center of mass displacement in three dimensions during gait Elena M. Gutierrez-Farewik a,b, * ,A ˚ sa Bartonek c , Helena Saraste a,d a Department of Surgical Sciences, Orthopaedics Section, Karolinska Institute, Stockholm, Sweden b KTH Mechanics, Royal Institute of Technology, Stockholm, Sweden c Department of Woman and Child Health, Karolinska Institute, Stockholm, Sweden d Department of Orthopaedics, Karolinska University Hospital, Stockholm, Sweden Available online 3 February 2006 Abstract Center of mass displacement during gait has frequently been used as an indicator of gait efficiency or as a complement to standard gait analysis. With technological advances, measuring the center of mass as the centroid of a multi-segment system is practical and feasible, but must first be compared to the well-established Newtonian computation of double-integrating the ground reaction force. This study aims to verify that the kinematic centroid obtained from a commonly-used model (Vicon PeakÒ Plug-In-Gait) provides at least as reliable measurements of center of mass displacement as those obtained from the ground reaction forces. Gait data was collected for able-bodied children and children with myelomeningocele who use larger lateral center of mass excursions during gait. Reasonable agreement between methods was found in fore-aft and vertical directions, where the methods’ excursions differed by an average of less than 10 mm in either direction, and the average RMS differences between methods’ computed curves were 6 and 13 mm. Particularly good agree- ment was observed in the lateral direction, where the calculated excursions differed by an average of less than 2 mm and the RMS difference was 5 mm. Error analyses in computing the center of mass displacement from ground reaction forces were performed. A 5% deviation in mass estimation increased the computed vertical excursion twofold, and a 5% deviation in the integration constant of initial velocity increased the computed fore-aft excursions by 10%. The suitability of calculating center of mass displacement using ground reaction forces in a patient population is questioned. The 0167-9457/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.humov.2005.11.001 * Corresponding author. Address: KTH Mechanics, Royal Institute of Technology, Stockholm, Sweden. Tel.: +46 8 790 7719; fax: +46 8 796 9850. E-mail address: Lanie@mech.kth.se (E.M. Gutierrez-Farewik). Human Movement Science 25 (2006) 238–256 www.elsevier.com/locate/humov