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.