Addressing Exoskeleton Architectural Diversity in Ergonomic and Biomechanical Studies

Abstract This article investigates the internal spinal forces experienced by the human body when assisted by either a rigid-body exoskeleton or a soft exosuit, both delivering equivalent supportive extension torques. Despite equivalent torque outputs, our analysis reveals that the spinal forces experienced by the user differ significantly between the two systems. This finding challenges the validity of an existing experimental equation that assumes identical extension torques from different assistive devices result in the same likelihood of spinal fatigue. We demonstrate that this assumption does not hold universally. Spinal fatigue failure, or structural tissue-level fatigue of the lumbar motion segments, depends on the forces acting on the spine. Therefore, variations in internal loading conditions (e.g., spinal compression and shear forces) should yield differing levels of risk for spinal fatigue, contradicting the equation's prediction. Furthermore, the study highlights the complexity of extrapolating ergonomic and biomechanical assessments from unassisted individuals to individuals using exoskeletons. Overlooking the specific architecture and dynamic interaction between the device and the user can lead to misleading conclusions about its safety and effectiveness in occupational settings. Finally, this article emphasizes the need for device-specific assessment tools, given the wide variation in exoskeleton geometries and mechanical behavior.