The joint torque tracking performance of the two SEAs with sinusoidal desired torque trajectory: (a) output joint torque trajectory comparison for LC-SEA and (b) output joint torque trajectory comparison for HT-SEA. The identified stiffness value used for evaluating the estimated torque for LC-SEA and HT-SEA was k = 1103 N/m and kj = 0.265 N·m/rad, respectively. The torque measured using the load cell, desired torque trajectory as available to the real-time controller and the torque trajectory as estimated by the real-time controller using SEA are referred to as measured, desired, and estimated torques, respectively.