Among steering methods of sedentary mobility, foothold with joystick-like control system is designed and springs are installed to front and rear foothold to enable free force control. Pushing the leg forward or pulling back with 1-axis drive foothold determines direction like forward or reverse. The speed of mobility is decided by displacement of foothold from the reference point. Displacement is set by spring force, but spring forces that human feels comfortable can vary from person to person. Maximum push-pull force to maintain comfort may be obtained through maximum displacement, which will reflect geometrical characteristics of human leg. Maximum front and rear force in pushing and pulling the foothold is obtained from multiple linear regression setting upper leg length, lower leg length, angle between two legs and angle between lower leg and foothold surface as an independent variable. Spring length is restricted and average value of independent variables can be used to help design the optimal spring spec.