Magnethoreological-fluid (MR-fluid) actuators are consider as as suitable instruments in rehabilitation trainings so that they are passive actuators. Passive actuators will not generate force by themselves. Only when external force acts on them, they generate reaction force. Considering these properties, passive actuators are stable actuators and appropriate for the human society because they would be little risk for humans when they are loss of control. In past, the author has proposed robust velocity constraint control, in which nonlinear term of MR fluids are compensated. This research uses the control system and measures frequency responses. In the constructed system, bandwidth of velocity constraint control can be investigated, and quantitative performance evaluation of rehabilitation trainings using MR-fluid actuators is expected in the future.