Stimulus parameters and inertial load: Effects on the incidence of protective stepping responses in healthy human subjects

Objective: To test the hypotheses that the incidence of protective stepping in response to sudden translations of the support would (1) increase as a function of both the magnitude of surface displacement and velocity of platform movement, and (2) decrease in association with an increase in external loading applied to the body. Design: A log-linear approach was used to analyze the incidence of stepping by testing several models incorporating different platform stimulus parameters (direction, displacement, velocity) and external loading (0% and 20% body weight). Setting: Institutional-based research laboratory. Participants: Eight healthy younger adult (21 to 28 years) volunteers. Main Outcome Measures: The incidence and number of protective steps served as the primary planned outcome variables. Results: Steps occurred more frequently for anterior (83 steps) versus posterior (45 steps) translations. Step occurrence was generally proportional to platform velocity, and increased with displacements up to 15cm, but then plateaued. External loading was associated with a reduction in the number of steps for lower magnitudes of platform motion but had little effect at higher magnitudes. Conclusion: The tendency to step in response to externally applied disturbances to stance appears to be a complex function of direction, velocity, displacement, and inertial load.