It is the goal of this paper to introduce an analytical model that allows predicting and interpreting the characteristics of constraint forces generated by misaligned joint axes between human operators and wearable robots during physical human-robot interaction (pHRI). The pHRI model is based on geometric parameters that describe the combined human-robot system. It is applied in this paper to measured constraint forces from a pHRI experiment. The model is validated with the experimental data. The geometrical model parameters are identified from force and position measurements by non-linear parameter optimization. The attachment stiffness and the actual offsets between the exoskeleton and the human joints are estimated. For the tested subject, the stiffness reaches 222 N/m and constraint forces are shown to be in the order of plusmn 10 N. It is shown in this paper how an ergonomically designed wearable robot with passive compensation joints can reduce such interaction forces.