Human body-sway steady-state responses to small amplitude tilts and translations of the support surface – Effects of superposition of the two stimuli

•Upright stance tested with a superposition of support surface tilt and translation.•Steady state response is characterized by frequency response function.•Interaction between two stimuli absent in most of the cases.•Larger stimuli may create interaction.•Simulations suggest that the observed effects can be due to joint stiffness modulation.