Abstract

Modern occupational tasks are often complex – consisting of multiple, discrete sub-tasks, each with its own unique combination of force, duration of exertion, posture, and frequency. Quantifying the physical stress from these complex tasks using simple, ‘mono-task’ assessment tools, such as the 1995 Strain Index (SI), can be difficult and assumes a large degree of ergonomics and MSD knowledge and training. Compared to the 1995 SI, the Revised Strain Index (RSI) minimises complicated measurement decisions and improves upon the 1995 SI by: (1) using percent maximum voluntary contraction (or Borg CR-10 equivalent) for applied hand force, (2) using duration per exertion (in seconds) rather than duty cycle, and (3) distinguishing between flexed and extended wrist postures. Thus the stress from each effort of a task (i.e., each sub task) can be individually quantified by the RSI and compared to other efforts in a cycle, or alternative efforts in the case of task intervention. By incorporating frequency of exertions (i.e., efforts per minute) and duration of task (i.e., hours per day), the RSI summarises stress associated with a simple task in a manner similar to the 1995 SI. For complex tasks, the RSI incorporates the Composite Strain Index (COSI) algorithm which integrates individual sub-tasks into an aggregate COSI score. The algorithm rank-orders subtasks from most to least stressful, thus ensuring that total task-stress is not lower than that of the highest stress sub-task alone. Sub-tasks are then incrementally added to one another by integrating over the frequency of exertion multiplier to produce the final COSI score (with all exertions explicitly represented). This presentation describes the RSI and COSI algorithm. A brief complex task example compares the RSI to the 1995 SI and showcases the RSI’s much improved utility as a tool for the design and evaluation of complex tasks.