Exo devices allows users to decrease muscle effort by offloading it to the exo device. Compliance in the physical human-robot interface, originating from flexible cuffs and soft body tissue, can affect the forces transferred. The design of the cuff can alter the value of the pHEI compliance, but it is not yet known to what extent changes in pHEI compliance affect the muscle activation. Due to human variances and the difficulty of measuring objective performance metrics, it is difficult to experimentally pin point cause and effect. Therefore a novel model based method was used which combined experimentally obtained pHEI compliance data with musculoskeletal models to simulate the effect of pHEI compliance on muscle activation. A case study using this method was performed on a subject wearing a passive shoulder exotendon suit. Results indicate a large effect of cuff design on stiffness, damping and cuff migration values. Furthermore, optimising a rigid model and adding compliance afterwards resulted in an increase of total normalised muscle cost. Results from this compliant simulation let to results more closely representing EMG data from another study. Finally, optimising a compliant model results in a total normalised muscle cost equal to that of the rigid case, increased robustness of the exo to configuration errors and increased comfort. This indicates that compliance does not have a detrimental effect on optimised performance when taken into consideration during optimisation.

A substantial portion of workplace-related injuries stem from sprains, strains, and muscle tears, with a significant proportion occurring in the upper body, particularly the
shoulder. To address these issues, there is a growing interest in utilizing supportive devices like exoskeletons that often face challenges such as bulkiness, high costs, and parasitic forces, which have hindered their widespread adoption. Exosuits, an alternative to exoskeletons, offer potential solutions by primarily employing fabrics and leveraging the human body to transmit forces, eliminating the need for cumbersome and expensive external frames. A user study involving a passive-adaptive exosuit equipped with a controllable pretension spring showed a reduction in muscle effort while being limited to controlling only the system’s equilibrium position. It was theorized that the addition of a variable stiffness mechanism with controllable pretension and stiffness could eliminate observed issues and further increase muscle effort reduction, forming the need for
a variable stiffness mechanism. The vast majority of existing variable stiffness solutions are developed for use in joints as opposed to being designed for a linear motion and are thus not ideal for application in an exosuit. Additionally, the fusion of variable stiffness mechanisms and exosuits has not been studied extensively, with no studies using a variable stiffness mechanism capable of controlling both the stiffness and equilibrium position in an exosuit. This work presents the results of the user study, the development and testing of a variable linear stiffness mechanism for a linear motion capable of controlling the stiffness and equilibrium position, and the integration of said actuator in an
exosuit.