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This paper presents experimental results of a myoelectric controller designed for reciprocal stair ascent using a transfemoral prosthesis with an actively powered knee joint. The control architecture is derived from able-bodied gait data and estimates knee torque with a linear two-state (stance/swing) impedance control form that includes proportional myoelectric torque control combined with a state-determined knee impedance. The experimentally implemented control interface affords the amputee subject with direct control of knee torque using surface electromyogram (EMG) measurements of muscles in the residual thigh supplemented with a nominal knee impedance whose set-point switches based on the detection of ground contact at the foot. Preliminary clinical evaluations of the EMG-based control system with a single subject with unilateral transfemoral amputation show robust and repeatable performance for alternating stair ascent. The amputee subject effectively modulates power output at the knee using EMG commands during stance, while leveraging the knee's nominal swing-phase impedance and only modest EMG influence to achieve the desired knee trajectories during swing.