Center of pressure displacement due to graded controlled perturbations to the trunk in standing subjects: the force–impulse paradigm

Purpose: Many studies have investigated postural reactions (PR) to body-delivered perturbations. However, attention has been focused on the descriptive variables of the PR rather than on the characterization of the perturbation. This study aimed to test the hypothesis that the impulse rather than the force magnitude of the perturbation mostly affects the PR in terms of displacement of the center of foot pressure (ΔCoP). Methods: Fourteen healthy young adults (7 males and 7 females) received 2 series of 20 perturbations, delivered to the back in the anterior direction, at mid-scapular level, while standing on a force platform. In one series, the perturbations had the same force magnitude (40 N) but different impulse (range: 2–10 Ns). In the other series, the perturbations had the same impulse (5 Ns) but different force magnitude (20–100 N). A simple model of postural control restricted to the sagittal plane was also developed. Results: The results showed that ΔCoP and impulse were highly correlated (on average: r = 0.96), while the correlation ΔCoP–force magnitude was poor (r = 0.48) and not statistically significant in most subjects. The normalized response, ΔCoP_n = ΔCoP/I, was independent of the perturbation magnitude in a wide range of force amplitude and impulse and exhibited good repeatability across different sets of stimuli (on average: ICC = 0.88). These results were confirmed by simulations. Conclusion: The present findings support the concept that the magnitude of the applied force alone is a poor descriptor of trunk-delivered perturbations and suggest that the impulse should be considered instead.