1. A method for determining ground reaction forces and zero moment pivot imparted by an underlying surface onto a foot member of a lower extremity prosthetic apparatus worn by a wearer, the apparatus comprising a foot member, a lower leg member, an ankle joint for connecting the foot member to the lower leg member and a first actuator for applying torque to the ankle joint to rotate the foot member with respect to the lower leg member, the method comprising: calculating the ground reaction force based on an inertial pose of the lower leg member, the torque applied to the lower leg member by the actuator, axial force applied to the lower leg member, and an angle between the foot member and lower leg member.