Abstract
Peak vertical ground reaction force (), contact time (), and flight time () 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 , , and . 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 . The second aim of this study was to estimate and 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, and 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 , , and estimated using the IMUM to , , and 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 while a RMSE of 20 ms was reported for both and (8% and 18%, respectively). Moreover, even though small systematic biases of 0.07 BW for and 13 ms for and were reported, the RMSEs were smaller than the smallest real differences [: 0.28 BW (11%), : 32.0 ms (13%), and : 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 , , and 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.
Highlights
Fz,max, even if the IMUM is a simple method which does not rely on machine learning, as was the 3D kinematic method [26], and (ii) tc and t f should have an root mean square error (RMSE) smaller than or equal to that we previously reported for effective contact and flight times (i.e., 0.22 ms) [39]
The raw forward acceleration recorded by the inertial measurement unit (IMU) and the filtered vertical acceleration recorded by the IMU, as well as the vertical acceleration recorded by the force plate during a running stride for three representative participants running at 11 km/h, are depicted in Systematic biases were obtained for Fz,max (0.07 BW)
13 ms) and the RMSEs were smaller than the smallest real difference (SRD), indicating no clinically important difference between the gold standard method (GSM) and IMUM
Summary
Running is defined as a cyclic alternance of support and flight phases, where at most one limb is in contact with the ground. Novacheck [1] postulated that the presence of this flight phase (t f ) marks the distinction between walking and running gaits. The duty factor, i.e., the ratio of ground contact time (tc ) over stride duration, should be under 50% to observe a running gait [2,3]. Though running provides many health benefits, it is associated with lower limb injuries [4,5], with a yearly incidence of running related injuries of up to 85% across novice to competitive runners [6,7]
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