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Quantifying the Biomechanical Costs Used to Make Behavioural Decisions When Choosing Between Action Plans

Environmental obstacles can constrain the placement of a person’s body in relation to their task. This often results in the adoption of awkward hand, limb, and body postures, as the degrees of freedom that can be used by the system when selecting a posture for task performance are reduced. In a manufacturing setting (e.g., an automotive assembly plant), workers often have to complete their task around an existing framework, navigating through an encumbered, constrained environment to complete their job. How workers plan these movements, and which biomechanical and/or cognitive factors may influence these action plans, are of great interest from a Human Factors perspective. For example, Rosenbaum et al. (2008, 2011) investigated the trade-off between walking and reaching when retrieving an object. The probability of proceeding along a certain path decreased as the functional distance (i.e., the summation of the distance travelled by walking and the distance reached by the hand) along that path increased (Rosenbaum, 2008, Rosenbaum et al., 2011). This result suggests that reaching is “costlier” than walking by a factor of ~11, which leads to the avoidance of this behaviour wherever possible (Rosenbaum et al., 2008, 2011, 2012). Although these results serve to catalogue certain aspects of the task environment in terms of movement process and provides putative evidence for how actions would be performed in these scenarios, it does not include any mechanical explanation as to why individuals respond in such a manner, or what is driving these decisions. Simply put, there is no explicit or testable definition of “cost”. The purpose of this study is to address this issue by expanding upon this work to include kinematic and biomechanical analyses of motion. Our study further extends this earlier work by presenting participants with each trial condition twice, in a forced-choice fashion (see Cappelletto & Potvin, 2014), allowing the demands of the chosen vs. forced choice path to be directly compared. We hypothesized that, consistent with Rosenbaum et al. (2008; 2011; 2012), participants would choose movement paths that minimize reach distance. We also hypothesized that the chosen paths will primarily minimize shoulder loading, thus having a lower total cost than the unchosen posture. These predictions are based on psychophysical research suggesting that postures are chosen in such a way to minimize the total minimum perceived cost (Cruse et al., 1990).