1. Portable pulse rate detecting device ( 1 ) intended to be worn on and in contact with the human body tissue ( 5 ), comprising a light-emitting source ( 10 ) for emitting radiant energy at the surface of or through the human body tissue ( 5 ); at least first and second light detectors ( 21, 22, 23, 24 ) for detecting the intensity of said radiant energy after propagation through the human body tissue ( 5 ) and for providing first and second input signals (y 1 (t), y 2 ( t )) representative of said propagation; a device ( 40 ) for measuring motion of the detecting device ( 1 ) on and with respect to the human body tissue ( 5 ) and for providing a motion reference signal (a x (t), a y (t), a z (t)) representative of said motion; and processing means ( 50 ) for removing, from said first and second input signals (y 1 (t), y 2 (t)), motion-related contributions due to movement of the detecting device ( 1 ) on and with respect to the human body tissue and producing first and second enhanced signals, said processing means being arranged to elaborate a model of said motion-related contributions based on said motion reference signal (a x (t), a y (t), a z (t)) and to subtract this model from each of said first and second input signals (y 1 (t), y 2 (t)).