A Single Sacral-Mounted Inertial Measurement Unit to Estimate Peak Vertical Ground Reaction Force, Contact Time, and Flight Time in Running

Peak vertical ground reaction force (F-z,F-max), contact time (t(c)), and flight time (tf) are key variables of running biomechanics. The gold standard method (GSM) to measure these variables is a force plate. However, a force plate is not always at hand and not very portable overground. In such situation, the vertical acceleration signal recorded by an inertial measurement unit (IMU) might be used to estimate Fz,max, t(c), and t(f). Hence, the first purpose of this study was to propose a method that used data recorded by a single sacral-mounted IMU (IMU method: IMUM) to estimate F-z,F-max. The second aim of this study was to estimate t(c) and t(f) using the same IMU data. The vertical acceleration threshold of an already existing IMUM was modified to detect foot-strike and toe-off events instead of effective foot-strike and toe-off events. Thus, tc and t(f) estimations were obtained instead of effective contact and flight time estimations. One hundred runners ran at 9, 11, and 13 km/h. IMU data (208 Hz) and force data (200 Hz) were acquired by a sacral-mounted IMU and an instrumented treadmill, respectively. The errors obtained when comparing F-z,F-max, t(c), and t(f) estimated using the IMUM to F-z,F-max, t(c), and t(f) measured using the GSM were comparable to the errors obtained using previously published methods. In fact, a root mean square error (RMSE) of 0.15 BW (6%) was obtained for Fz,max while a RMSE of 20 ms was reported for both t(c) and t(f) (8% and 18%, respectively). Moreover, even though small systematic biases of 0.07 BW for F-z,F-max and 13 ms for t(c) and t(f) were reported, the RMSEs were smaller than the smallest real differences [F-z,F-max: 0.28 BW (11%), tc: 32.0 ms (13%), and t(f) 32.0 ms (30%)], indicating no clinically important difference between the GSM and IMUM. Therefore, these results support the use of the IMUM to estimate F-z,F-max, t(c), and t(f) for level treadmill runs at low running speeds, especially because an IMU has the advantage to be low-cost and portable and therefore seems very practical for coaches and healthcare professionals.