While powered prosthetic feet with monoarticular actuation allow for the restoration of human ankle biomechanics, gait asymmetries and increased walking effort have been found for transtibial amputees. We hypothesize that this is because of the missing mechanical coupling between the foot and the thigh, which is realized in human legs by the biarticular gastrocnemius (GAS) muscle. This work investigated the effects of a powered prosthetic foot that includes a monoarticular powered prosthetic foot and a biarticular GAS actuator on nine non-impaired subjects while walking on a treadmill. While maintaining total ankle torque, we modulated the torque contribution ratio of the GAS actuator to the total ankle torque and recorded kinematics, kinetics and metabolic cost. We found a maximum reduction in metabolic cost by up to 8.8%±4.4% on average for a κ of 0.36 ± 0.09. We attribute this reduction to (a) reduced biological hip work due to energy transfer from the GAS actuator to assist hip flexion, (b) increased inter-leg symmetry of center of mass collision and propulsive work, knee push-off work and step length and (c) increased efficiency of the center of mass movement caused by a reduction in contralateral leg collision work. Our study demonstrates that the biarticular actuator provides an additional degree of freedom that allows for the modulation of human gait biomechanics.