The present invention provides a sensor arrangement. The signal from the sensor is used to control an auxiliary motor of a vehicle propelled by muscular strength. The auxiliary motor is there integrated into a wheel hub unit of the vehicle. The wheel hub unit comprises a stationary axle and a hub rotatably mounted to the axle. According to the invention, the assembly comprises the sensor and a freewheel of the drive train, which is propelled by muscular strength, mounted rotatable relative to the axle, where the sensor is arranged such that it detects a rotational motion of the freewheel relative to the axle.