Analyzing the kinematic and kinetic contributions of the human upper body's joints for ergonomics assessment

During an eight-hour shift, an industrial worker will inevitably cycle through specific postures. Those postures can cause
microtrauma on the musculoskeletal system that accumulates, which in turn can lead to chronic injury. To assess how
problematic a posture is, the rapid upper limb assessment (RULA) scoring system is widely employed by the industry.
Even though it is a very quick and efficient method of assessment, RULA is not a biomechanics-based measurement that is
anchored in a physical parameter of the human body. As such RULA does not give a detailed description of the impact each
posture has on the human joints but rather, an overarching, simplified assessment of a posture. To address this issue, this
paper proposes the use of joint angles and torques as an alternative way of ergonomics evaluation. The cumulative motion
and torque throughout a trial is compared with the average motions and torques for the same task. This allows the evaluation of each joint’s kinematic and kinetic performance while still be able to assess a task“at-a-glance”. To do this, an upper
human body model was created and the mass of each segment were assigned. The joint torques and the RULA scores were
calculated for simple range of motion (ROM) tasks, as well as actual tasks from a TV assembly line. The joint angles and
torques series were integrated and then normalized to give the kinematic and kinetic contribution of each joint during a
task as a percentage. This made possible to examine each joint’s strain during each task as well as highlight joints that need
to be more closely examined. Results show how the joint angles and torques can identify which joint is moving more and
which one is under the most strain during a task. It was also possible to compare the performance of a task with the average
performance and identify deviations that may imply improper execution. Even though the RULA is a very fast and concise
assessment tool, it leaves little room for further analyses. However, the proposed work suggests a richer alternative without
sacrificing the benefit of a quick evaluation. The biggest limitation of this work is that a pool of proper executions needs to
be recorded for each task before individual comparisons can be done.