A biomimetic mechanical joint for generating a variable torque between support members of a biomimetic robotic device, including a base support member, a rotary support member rotatably coupled to the base support member, and a variable-radius pulley operably coupled between the base support member and rotary support member. The variable-radius pulley comprises a sheave body having a variable radius and one or more tendon grooves formed in the circumferential outer surface. The mechanical joint further includes one or more flexible tendons and antagonistic actuator pairs, with each actuator pair being coupled to one or more tendons and configured to operate the tendon around the variable-radius pulley in either direction to create a variable torque between the base and rotary support members.