The use of a single sacral marker method to approximate the centre of mass trajectory during treadmill running

The three-dimensional trajectory of the body's centre of mass (COM) is useful to determine a number of biomechanical outcomes in running research. Previous studies have used the COM to calculate measures such as overstriding, vertical stiffness, and vertical oscillation. The COM is traditionally computed using the segmental analysis method, though this is expensive and time-consuming owing to the need for a full-body marker set. The purpose of this study was to determine whether the COM trajectory can be approximated by a single sacral marker during running. Seventy-one participants underwent a biomechanical running analysis on a treadmill utilizing a full-body marker set. Marker trajectories from the sacral marker and from the COM calculated using the segmental analysis method were compared over the entire gait cycle by computing intraclass correlation coefficient (ICC) and root-mean-square error. Paired t-tests were used to determine if the positions differed in mediolateral, anteroposterior, and vertical directions at three gait events (initial contact, midstance, and toe-off). The trajectories from the two methods exhibited a similar pattern in vertical and anteroposterior directions throughout the gait cycle, displaying strong correlations in these directions (ICC = 0.98 ± 0.01 and 0.83 ± 0.07). Our results suggest that a single sacral marker is a valid proxy for COM trajectory in vertical and anteroposterior directions at key events during the stance phase of running in a female recreational population. Researchers can therefore use a single sacral marker to estimate COM trajectory, rather than a full-body marker set, saving on both time and supplies.