Automatic postural responses are generated according to feet orientation and perturbation magnitude.

This investigation aimed to assess the effect of feet orientation angle in upright stance on automatic postural responses (APRs) to mechanical perturbations of different magnitudes. Perturbation was produced by releasing suddenly a load attached to the participant's trunk, leading to forward body sway. We evaluated APRs to loads corresponding to 5% (low) and 10% (high) of the participant's body weight, comparing the following feet orientations: parallel, preferred (M=10.46°), 15° and 30° for each foot regarding the body midline. Results showed that APRs were sensitive to perturbation magnitude, with the high load leading to increased amplitudes of center of pressure displacement and joints rotation, in addition to stronger and earlier muscular responses. Feet orientation at 30° led to a greater amplitude of center of pressure displacement than the other feet orientations. The low perturbation magnitude led to similar responses both at the hip and ankle across feet orientations, whereas the high load induced increased rotation amplitudes in both joints for feet orientation at 30°. Our results suggest that APRs are generated by the nervous system taking into consideration the biomechanical constraints in the response production. Relevant for standardization of feet placement in evaluations of balance recovery, our results indicated that a moderate range of outward feet orientation angles in stance lead to comparable APRs, while increased outward feet orientation angles lead to distinct postural responses.