A joint motion simulator to simulate biomechanical motion comprising: a lower mount to which a prosthetic device is mounted; an upper mount; actuators (206, 208, 210, 302) coupled to the lower mount to drive the lower mount; and a programmable controller to drive the actuators to translate the lower mount and to rotate the lower mount; the simulator characterised in that: the actuators comprise at least three linear actuators to translate the lower mount and to rotate the lower mount with a center of rotation controllable independent of translation, the center of rotation being an instant center of rotation, wherein the at least three actuators form a 3RPR actuator; the controller is programmed to rotate the lower mount about an axis, to translate the lower mount in lateral directions relative to the axis of rotation and to laterally translate the axis of rotation independent of translation of the lower mount, the actuators further comprise at least one linear actuator translating the lower mount in a linear direction substantially parallel to the axis of rotation; wherein the controller controls position of the center of rotation r 0, angle of rotation Φ, and translation r p of the lower mount; and wherein the controller is programmed to vary the center of rotation as the lower mount is translated and rotated. a lower mount to which a prosthetic device is mounted; an upper mount; actuators (206, 208, 210, 302) coupled to the lower mount to drive the lower mount; and a programmable controller to drive the actuators to translate the lower mount and to rotate the lower mount; the simulator characterised in that: the actuators comprise at least three linear actuators to translate the lower mount and to rotate the lower mount with a center of rotation controllable independent of translation, the center of rotation being an instant center of rotation, wherein the at least three actuators form a 3RPR actuator; the controller is programmed to rotate the lower mount about an axis, to translate the lower mount in lateral directions relative to the axis of rotation and to laterally translate the axis of rotation independent of translation of the lower mount, the actuators further comprise at least one linear actuator translating the lower mount in a linear direction substantially parallel to the axis of rotation; wherein the controller controls position of the center of rotation r 0, angle of rotation Φ, and translation r p of the lower mount; and wherein the controller is programmed to vary the center of rotation as the lower mount is translated and rotated. the actuators comprise at least three linear actuators to translate the lower mount and to rotate the lower mount with a center of rotation controllable independent of translation, the center of rotation being an instant center of rotation, wherein the at least three actuators form a 3RPR actuator; the controller is programmed to rotate the lower mount about an axis, to translate the lower mount in lateral directions relative to the axis of rotation and to laterally translate the axis of rotation independent of translation of the lower mount, the actuators further comprise at least one linear actuator translating the lower mount in a linear direction substantially parallel to the axis of rotation; wherein the controller controls position of the center of rotation r 0, angle of rotation Φ, and translation r p of the lower mount; and wherein the controller is programmed to vary the center of rotation as the lower mount is translated and rotated.