Foot placement is not controlled based on angular momentum

This study examined how mediolateral foot placement is controlled following mechanical perturbations that affected either whole-body linear or angular momentum. Predictive foot placement models based on center of mass state alone were compared with models that additionally included whole-body angular momentum to determine whether whole-body angular momentum contributes to foot placement control beyond linear momentum. Ten healthy adults walked on a treadmill at 2 km/h and 5 km/h while being exposed to two perturbation types: (1) a pull to the pelvis that primarily altered linear momentum (translation perturbation) and (2) simultaneous pulls to the pelvis and shoulder in opposite directions that primarily altered angular momentum (rotation perturbation). Perturbations were applied at heel strike, with a magnitude of [~]120 N and a duration of 300 ms. Whole-body kinematics were recorded using 3D motion capture and processed in OpenSim to compute linear and angular momentum. Translation perturbations caused large deviations in whole-body linear momentum with minimal changes in whole-body angular momentum, whereas rotation perturbations induced strong whole-body angular momentum deviations with smaller changes in linear momentum. Including whole-body angular momentum minimally improved foot placement predictions during early swing after rotation perturbations. These findings indicate that mediolateral foot placement is primarily governed by linear momentum dynamics.