Both a single sacral marker and the whole-body center of mass accurately estimate peak vertical ground reaction force in running

Background: While running, the human body absorbs repetitive shocks with every step. These shocks can be quantified by the peak vertical ground reaction force (Fv,max). To measure so, using a force plate is the gold standard method (GSM), but not always at hand. In this case, a motion capture system might be an alternative if it accurately estimates Fv,max. Research question: The purpose of this study was to estimate Fv,max based on motion capture data and validate the obtained estimates with force plate-based measures. Methods: One hundred and fifteen runners participated at this study and ran at 9, 11, and 13 km/h. Force data (1000 Hz) and whole-body kinematics (200 Hz) were acquired with an instrumented treadmill and an optoelectronic system, respectively. The vertical ground reaction force was reconstructed from either the whole-body center of mass (COM-M) or sacral marker (SACR-M) accelerations, calculated as the second derivative of their respective positions, and further low-pass filtered using several cutoff frequencies (2-20 Hz) and a fourth-order Butterworth filter. Results: The most accurate estimations of Fv,max were obtained using 5 and 4 Hz cutoff frequencies for the filtering of COM and sacral marker accelerations, respectively. GSM, COM-M, and SACR-M were not significantly different at 11 km/h but were at 9 and 13 km/h. The comparison between GSM and COM-M or SACR-M for each speed depicted root mean square error (RMSE) smaller or equal to 0.17BW (<6.5 %) and no systematic bias at 11 km/h but small systematic biases at 9 and 13 km/h (<0.09 BW). COM-M gave systematic biases three times smaller than SACR-M and two times smaller RMSE. Significance: The findings of this study support the use of either COM-M or SACR-M using data filtered at 5 and 4 Hz, respectively, to estimate Fv,max during level treadmill runs at endurance speeds.