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; and a stationary sensor (30, 30') 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 (30,30') 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 (30, 30') and in that the stationary sensor (30,30') 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; and a stationary sensor (30, 30') 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 (30,30') 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 (30, 30') and in that the stationary sensor (30,30') measures a force applied by the measurement member.