A Comparison of Platform Motion Waveforms During Constrained and Unconstrained Standing in Moving Environments

OCCUPATIONAL APPLICATIONS Using a principal component waveform analysis technique to examine the platform kinematics during motion-induced interruptions and motion-induced corrections found that these events significantly differ in perturbation magnitude at the time of stepping initiation. These results suggest that differences between the motion-induced interruptions and motion-induced corrections are more than just theoretical, and therefore, when examining these events in offshore environments, they cannot be considered one and the same. Since these change-in-support reactions may be an alternative instead of a last resource response, they may also not be a clear indicator of ship operability. Therefore, when evaluating the risks associated with performing tasks in moving environments, the resultant effects of the change-in-support reactions, such as motion-induced interruptions or motion-induced corrections and their potential effect on injury mechanisms, must also considered. TECHNICAL ABSTRACT Background: Postural response research suggests that change-in-support reactions involving the movement of feet may occur long before the physics-based stability limits have been reached. As such, the current definition of a motion-induced interruption used to describe change-in-support reactions in offshore environments may not be valid, and the alternative motion-induced correction definition may be more appropriate. However, differences between motion-induced interruptions and motion-induced corrections are currently unknown. Purpose: The purpose of this study was to determine the differences in platform motion waveforms between motion-induced interruption and motion-induced correction occurrences when standing on a 6 degree of freedom motion platform. Methods: Twenty participants (ten male, ten female) with little or no experience working in marine environments performed a constrained and unconstrained stationary standing task while being exposed to a continuous multi-directional wave-like perturbation. The constrained standing task was representative of motion-induced interruptions, while the unconstrained standing task was representative of motion-induced corrections. Principal component analysis was incorporated, permitting the preservation of temporal characteristics unique to each motion curve in the analysis. Students’ t-tests were performed on the derived significant principal component scores to determine if these components were statistically significantly different between constrained and unconstrained standing. Results: Analysis of pitch and roll axes suggest that most of the variability of platform motions between motion-induced interruptions and motion-induced corrections can be described by two principal components. The first component, accounting for 80%–90% of all variability, was a magnitude modifier and suggests that there are quantifiable differences in the platform motions that cause motion-induced interruptions and motion-induced corrections. Conclusions: The results of this research suggest that it is likely that the motions to induce these events are distinctly different; therefore, motion-induced interruptions and motion-induced corrections should be considered as different events when examining the human response to wave-induced ship motions and ship operability.