Estimating effective contact and flight times using a sacral-mounted inertial measurement unit

Effective ground contact (t_ce) and flight (t_fe) times were proven to be more appropriate to decipher the landing-take-off asymmetry of running than usual ground contact (t_c) and flight (t_f) times. To measure these effective timings, force plate is the gold standard method (GSM), though not very portable overground. In such situation, alternatives could be to use portable tools such as inertial measurement unit (IMU). Therefore, the purpose of this study was to propose a method that uses the vertical acceleration recorded using a sacral-mounted IMU to estimate t_ce and t_fe and to compare these estimations to those from GSM. Besides, t_ce and t_fe were used to evaluate the landing-take-off asymmetry, which was further compared to GSM. One hundred runners ran at 9, 11, and 13 km/h. Force data (200 Hz) and IMU data (208 Hz) were acquired by an instrumented treadmill and a sacral-mounted IMU, respectively. The comparison between GSM and IMU method depicted root mean square error ≤22 ms (≤14%) for t_ce and t_fe along with small systematic biases (≤20 ms) for each tested speed. These errors are similar to previously published methods that estimated usual t_c and t_f. The systematic biases on t_ce and t_fe were subtracted before calculating the landing-take-off asymmetry, which permitted to correctly evaluate it at a group level. Therefore, the findings of this study support the use of this method based on vertical acceleration recorded using a sacral-mounted IMU to estimate t_ce and t_fe for level treadmill runs and to evaluate the landing-take-off asymmetry but only after subtraction of systematic biases and at a group level.