Quantifying Tibial Bone Load During An Outdoor Run Using Only Three Inertial Measurement Units

Peak tibial acceleration (PTA) is used as an impact load estimate in running research, due to its potential injury relation and easy assessment with an accelerometer or inertial measurement unit (IMU). However, PTA is based solely on reaction forces and is not related to tibial bone load (TBL), which includes reaction and muscle forces. Still, muscle force estimates often require optical motion capturing in a lab or pressure insoles. PURPOSE: To estimate TBL at the ankle using only three IMUs during outdoor running, without optical motion capture or pressure insoles, providing insight into tibial bone load. METHODS: 7 rear-foot strikers (3F/4M, running >10 km p/w for >6 months; uninjured <6 months) ran one lap on a 400 m asphalt athletics track at 9, 11, and 13 kph. Data were obtained on the straights from a pelvis and two tibia IMUs (240 Hz). TBL was estimated as ankle reaction + muscle forces during stance. Ankle reaction force was assumed similar as the vertical ground reaction force (vGRF), estimated as the product of body mass and vertical IMU accelerations. Ankle moment was obtained by the product of vGRF and the Center of Pressure (CoP). CoP was estimated as a projection of the anteroposterior position of the tibia IMU relative to the ankle joint during stance. This position is estimated by the global IMU orientation and IMU distance to the ankle. Ankle moment was divided by an Achilles tendon arm of 0.05 m to obtain ankle muscle forces. The vGRF and TBL across the three speeds were tested with a Friedman test. RESULTS: vGRF increased from 2.18 ± 0.20 BW, 2.32 ± 0.21 BW, to 2.46 ± 0.23 BW across the three speeds (p < 0.001). TBL increased from 6.93 ± 0.95 BW, 7.50 ± 0.95 BW, to 8.24 ± 0.96 BW (p < 0.001). CONCLUSION: TBL was estimated using only three IMUs in a sports-specific setting, where the importance of the inclusion of estimated muscle forces is shown. This method could also be applied in fatiguing runs to assess if TBL indicates potential tibia overloading in a fatigued state. This research was funded by an internal grant from the University of Twente