Soft tissue deformation at the human exoskeleton interface can deform under load to absorb, return, and dissipate the mechanical energy generated by the exoskeleton. These soft tissue effects are often not accounted for and may mislead researchers on the actual joint assistance an exoskeleton provides. We assessed the effects of soft tissue by quantifying the performance and energy distribution of a knee exoskeleton under different assistance strategies using a mechanical lower limb phantom. The phantom emulated knee kinematics and soft tissue deformation at the exoskeleton interface. We loaded the exoskeleton on the phantom under six different spring stiffness conditions. Motion capture marker and load cell data from the phantom-exoskeleton assembly allowed us to estimate the moments, stiffness, and energy contributions of the exoskeleton and physical interface to the total knee power. We found that soft tissue caused interface power to increase and exoskeleton power to decrease with increasing spring stiffness. Additionally, increases in exoskeleton peak moments were not proportional to the change in spring stiffness despite consistent phantom joint motion under all conditions. Our methodology improves the exoskeleton design process by estimating energy distribution and transfer for exoskeletons while accounting for the effects of soft tissue deformation before human testing.