A torque transmission apparatus comprising: a differential gear system including an input element , a transmission output element , and a measurement output element , wherein the transmission output element rotates in response to rotation of the input element and the measurement output element reacts an output torque transmitted by the transmission output element ; a rotatable input shaft arranged to drive rotation of the input element ; a rotatable output shaft driven to rotate by rotation of the transmission output element , wherein the rotatable output shaft transmits the output torque; a measurement member connected to the measurement output element and to a structural ground (G); and a stationary sensor characterized in that either a.) the measurement member is a torsionally compliant member arranged to undergo torsional deformation due to angular displacement of the measurement output element , and in that the apparatus further comprises a measurement gear train associated with the measurement output element , and in that the measurement gear train amplifies the angular displacement of the measurement output element and in that the sensor is a sensor measuring an angular displacement of the measurement output element ; or in that b.) the measurement member is a rigid member arranged to transmit torque from the measurement output element , and in that the measurement member includes a pivotally-mounted bell crank having a first arm engaged by the measurement output element and a second arm arranged to apply force to the stationary sensor and in that the stationary sensor measures a force applied by the measurement member . a differential gear system including an input element , a transmission output element , and a measurement output element , wherein the transmission output element rotates in response to rotation of the input element and the measurement output element reacts an output torque transmitted by the transmission output element ; a rotatable input shaft arranged to drive rotation of the input element ; a rotatable output shaft driven to rotate by rotation of the transmission output element , wherein the rotatable output shaft transmits the output torque; a measurement member connected to the measurement output element and to a structural ground (G); and a stationary sensor characterized in that either a.) the measurement member is a torsionally compliant member arranged to undergo torsional deformation due to angular displacement of the measurement output element , and in that the apparatus further comprises a measurement gear train associated with the measurement output element , and in that the measurement gear train amplifies the angular displacement of the measurement output element and in that the sensor is a sensor measuring an angular displacement of the measurement output element ; or in that b.) the measurement member is a rigid member arranged to transmit torque from the measurement output element , and in that the measurement member includes a pivotally-mounted bell crank having a first arm engaged by the measurement output element and a second arm arranged to apply force to the stationary sensor and in that the stationary sensor measures a force applied by the measurement member . <EOS>