Predicting center of mass position in non-cyclic activities: The influence of acceleration, prediction horizon, and ground reaction forces.

The whole-body center of mass (CoM) plays an important role in quantifying human movement. Prediction of future CoM trajectory, modeled as a point mass under influence of external forces, can be a surrogate for inferring intent. Given the current CoM position and velocity, predicting the future CoM position by forward integration requires a forecast of CoM accelerations during the prediction horizon. However, it is unclear how assumptions about the acceleration, prediction horizon length, and information from ground reaction forces (GRFs), which provide the instantaneous acceleration, affect the prediction. We study these factors by analyzing data of 10 healthy young adults performing 14 non-cyclic activities. We assume that the acceleration over a horizon will be (1) zero, (2) remain constant, or (3) converge to zero as a cubic trajectory, and perform predictions for horizons of 125 to 625 ms. We quantify the prediction performance by comparing the position error and accuracy of identifying the main direction of displacement against trajectories obtained from a whole-body marker set. For all the assumed accelerations profiles, position errors grow quadratically with horizon length (R>0.930) while the accuracy of the predicted direction decreases linearly (R>0.615). Post-hoc tests reveal that the constant and cubic profiles, which utilize the GRFs, outperform the zero-acceleration assumption in position error (p<0.001, Cohen's d>3.23) and accuracy (p<0.034, Cohen's d>1.44) at horizons of 125 and 250ms. The results provide evidence for benefits of incorporating GRFs into predictions and point to 250ms as a threshold for horizon length in predictive applications.