Predicting continuous ground reaction forces from accelerometers during uphill and downhill running: a recurrent neural network solution

Background. Ground reaction forces (GRFs) are important for understanding human movement, but their measurement is generally limited to a laboratory environment. Previous studies have used neural networks to predict GRF waveforms during running from wearable device data, but these predictions are limited to the stance phase of level-ground running. A method of predicting the normal (perpendicular to running surface) GRF waveform using wearable devices across a range of running speeds and slopes could allow researchers and clinicians to predict kinetic and kinematic variables outside the laboratory environment. Purpose. We sought to develop a recurrent neural network capable of predicting continuous normal (perpendicular to surface) GRFs across a range of running speeds and slopes from accelerometer data. Methods. Nineteen subjects ran on a force-measuring treadmill at five slopes (0 degrees, +/- 5 degrees, +/- 10 degrees) and three speeds (2.5, 3.33, 4.17 m/s) per slope with sacral-and shoe mounted accelerometers. We then trained a recurrent neural network to predict normal GRF waveforms frame-by-frame. The predicted versus measured GRF waveforms had an average +/- SD RMSE of 0.16 +/- 0.04 BW and relative RMSE of 6.4 +/- 1.5% across all conditions and subjects. Results. The recurrent neural network predicted continuous normal GRF waveforms across a range of running speeds and slopes with greater accuracy than neural networks implemented in previous studies. This approach may facilitate predictions of biomechanical variables outside the laboratory in near real-time and improves the accuracy of quantifying and monitoring external forces experienced by the body when running.