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

Abstract

Peak vertical ground reaction force (Fz,max), contact time (tc), 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, tc, and tf. 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 Fz,max. The second aim of this study was to estimate tc and tf 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 tf 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 Fz,max, tc, and tf estimated using the IMUM to Fz,max, tc, and tf 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 tc and tf (8% and 18%, respectively). Moreover, even though small systematic biases of 0.07 BW for Fz,max and 13 ms for tc and tf were reported, the RMSEs were smaller than the smallest real differences [Fz,max: 0.28 BW (11%), tc: 32.0 ms (13%), and tf: 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 Fz,max, tc, and tf 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.

Wearables for Running Gait Analysis: A Systematic Review

“…Protocols to assess validity and reliability varied greatly. Overall, studies included between five [ 27 ] and 100 [ 95 , 96 ] participants, with the average number of participants being 22 (± 18). The mean age of participants was 26.8 (± 4.5) years.…”

“…Sixty-two articles stated that they used IMUs; however, 14 of these studies only used the accelerometer capabilities within the IMU [23,24,38,52,56,57,62,[72][73][74][75][76][77][78] and 20 studies stated they used the accelerometer and gyroscope components for the data analysis [26,33,43,49,50,54,59,61,[79][80][81][82][83][84][85][86][87][88][89][90]. The remaining 27 studies either did not comment on components used [70,[91][92][93][94][95][96][97][98] or implied they used all accelerometer, gyroscope and magnetometer components for the data analysis [19-21, 28, 29, 36, 42, 99-109]. One study used an IMU and a separate one-dimensional accelerometer [61].…”

“…Fifty-six studies focussed on the validation of wearables for running gait assessments, with 18 also examining the reliability of devices [47,98,99,103,104,110,117,120,130,134,136,140,144,149]. Eleven studies investigated between-day reliability [34,47,98,106,117,120,122,140,142,144,149], and three studies solely examined the reliability of wearable technology [87,134,138] Overall, studies included between five [27] and 100 [95,96] participants, with the average number of participants being 22 (± 18). The mean age of participants was 26.8 (± 4.5) years.…”

Inner-cycle Phases can be Estimated from a Single Inertial Sensor by Long Short-term Memory Neural Network in Roller Ski Skating

“…Therefore, methods focusing on a single point have been developed [8,15]. In running, an IMU placed on the sacrum was used to predict peak vertical ground reaction force, impulse and contact time [16], but these parameters could also be determined using a traditional approach [17]. In cross country roller skiing on a treadmill, IMUs placed on the skis and poles were used to detect temporal events in classical style [18] and in skating style with IMUs placed on skis and wrist [19].…”

Inner-Cycle Phases Can Be Estimated from a Single Inertial Sensor by Long Short-Term Memory Neural Network in Roller-Ski Skating

“…Therefore, methods focusing on a single point have been developed [ 8 , 15 ]. In running, an IMU placed on the sacrum was used to predict the peak vertical ground reaction force, impulse and contact time [ 16 ], but these parameters could also be determined using a traditional approach [ 17 ]. In cross-country roller skiing on a treadmill, IMUs placed on the skis and poles were used to detect temporal events in the classical style [ 18 ], and IMUs placed on skis and wrists were used in the skating style [ 19 ].…”