Interpreting common standing postural sway measures

Background There are numerous measures of standing balance using force plates presented in the literature. Two of the most common measures are the root-mean-square (RMS) and mean velocity (MV) of the Center of Pressure (CoP). The purpose of this short communication is to promote a greater understanding of the implications of these two common metrics. Research Question What aspects of postural control do the RMS and MV of the CoP measure? Methods CoP time series measured with a force plate and CoM calculated from motion capture during quiet standing in the AP and ML directions were analyzed. The RMS and MV of the CoP, Center of Mass (CoM), and the difference between the CoP and the CoM (CoP-CoM) were calculated. The relationships among these measures are presented. Results The CoP RMS was highly correlated with the CoM RMS (r>.96), indicating that CoP RMS measures the amount of sway. The CoP MV was highly correlated with ( CoP-CoM) RMS (r>.90). The ( CoP-CoM) RMS is related to the torque generation used to maintain stability; therefore CoP MV is related to the control effort used. The AP and ML measures do have some different characteristics due to the mechanism by which stability is maintained. Significance The RMS and MV of CoP are effective at capturing two different fundamental aspects of stability: the amount of sway and control effort. This highlights the importance of reporting both CoP RMS and CoP MV to allow for a better understanding of standing balance.