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 ste pping in response to sudden translations of the support would (1) incr ease as a function of both the magnitude of surface displacement and v elocity 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 sev eral models incorporating different platform stimulus parameters (dire ction, displacement, velocity) and external loading (0% and 20% body w eight). Setting: Institutional-based research laboratory. Participants : Eight healthy younger adult (21 to 28 years) volunteers. Main Outcom e Measures: The incidence and number of protective steps served as the primary planned outcome variables. Results: Steps occurred more frequ ently 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 lo wer magnitudes of platform motion but had little effect at higher magn itudes. Conclusion: The tendency to step in response to externally app lied disturbances to stance appears to be a complex function of direct ion, velocity, displacement, and inertial load. (C) 1996 by the Americ an Congress of Rehabilitation Medicine and the American Academy of Phy sical Medicine and Rehabilitation