The dynamic balance of the children with cerebral palsy and typical developing during gait. Part I: Spatial relationship between COM and COP trajectories Bih-Jen Hsue a , Freeman Miller b , Fong-Chin Su a, * a Institute of Biomedical Engineering, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan b Department of Orthopedics, Alfred I. duPont Hospital for Children, 1600 Rockland Road, Wilmington, DE 19803-3607, USA 1. Introduction COM is a point derived from the weighted average of each body segment, and moves when the body segments change position. COP acquired from force plates is a combination of inertial forces one uses to restore equilibrium. Quantification of the displace- ment of COM and COP has been suggested to be a useful measurement to provide valuable insight into balance and postural control in healthy or balance-challenged adults and elderly. However, there is little information relative to children with balance deficits. It is well known that the children with CP have varying levels of deficits in balance and postural control, which is a major component of the gait disorder. Previous studies have suggested that three-dimensional COM displacement successfully distin- guished toe walkers from plantigrade walkers with CP [1,2]. The COM trajectory characterized during gait initiation can possibly differentiate hemiplegic CP and age-matched TD children [3,4]. By identifying the trajectory, amplitude, velocity or sway area of COP, it has been proved that CP children have larger postural sway than the TD children in static stance in different foot positions (e.g. tandem stance vs. side-by-side) or altered sensory conditions (e.g. eyes open vs. eyes close) [5–8]. Such sway to restore balance may be developed through coordination of proximal (e.g. trunk or hip) and distal (e.g. ankle or foot) areas [8–10], though the CP children may prefer one strategy over another or they may adopt their own pattern. Consequently, the shifts of COM would be observed along specific postural sway patterns. However, the information is usually limited to static stance. Examining dynamic stability in gait would provide a more complete understanding of the impairments on postural control in CP. Unlike static standing, higher values of COP parameters during walking do not necessarily indicate instability. Neither do the larger COM movements. To overcome these limitations, it was proposed that joint interpretation of COM and COP can give more complete assessment than examining COP and COM separately [9–17]. If the COP and COM distance is larger, the moment arm for the ground reaction forces to create momentum generation would be greater. In other words, leaning too far resulting in excessive COM and COP separation requires greater active postural control and more energy to counteract the increased moment. Failure to do so may lead to a loss of balance. Gait & Posture 29 (2009) 465–470 ARTICLE INFO Keywords: Cerebral palsy Gait Dynamic stability Center of mass Center of pressure ABSTRACT Analysis of the COM or COP movement has been a simplified method to illustrate the balance disorders in static stance and gait, but has its limitation when examined alone. Dynamic stability of 32 children with cerebral palsy (CP) was examined and compared with 10 typically developing (TD) children by measuring the displacement of center of mass (COM) and center of pressure (COP) and their spatial relationship. The children with CP were further divided into two groups based on topographical involvement, hemiplegia (Hemi) and diplegia (Di). The participants walked with their preferred speed at least 5 successful trials on a walkway with two force plates mounted in the middle. An eight-camera motion analysis was used to capture 26 reflective markers secured at the bony landmarks of the participant. The data obtained from motion analysis and force plates was used to calculate COM and COP. The results showed either of two CP groups demonstrated significantly greater peak-to-peak COM and COP displacement in medio-lateral (ML) and lower peak-to-peak COM and COP displacement in anterio- posterior (AP) direction than TD group. The root mean square (RMS) of COM–COP divergence of Hemi and Di groups were higher than that of TD group in AP and ML direction, but only the difference in ML direction was significant. Present study demonstrates that COM–COP divergence can characterize the dynamic balance of the CP children in walking, and thus assist in comparing and differentiating balance patterns. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +886 6 276 0665; fax: +886 6 234 3270. E-mail address: fcsu@mail.ncku.edu.tw (F.C. Su). Contents lists available at ScienceDirect Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost 0966-6362/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2008.11.007