A measuring device (M', M") comprising: a displacement sensor assembly (100, 400, 500, 800, 900) configured for use in measuring a biological parameter at the skin of a user, the assembly (100, 400, 500, 800, 900) comprises: a contact surface (102, 402, 502, 802, 902) facing towards an exterior of the assembly (100, 400, 500, 800, 900) and configured for contacting the skin of a user, and movable along a central axis of the assembly (100, 400, 500, 800, 900) when pressure is applied thereto by the skin of a user; a sensing unit (109, 800, 900) disposed within an interior of the assembly (100, 400, 500, 800, 900) and having an optical axis lying in a plane perpendicular to the central axis of the assembly (100, 400, 500, 800, 900); the sensing unit comprises a light source configured to emit light along the optical axis and a light detector facing towards the light source and configured to detect light emitted by the light source directly towards the detector; and a light blocking member operative to change the amount of light reaching the detector directly from the light source in consequence with displacement of the contact surface (102, 402, 502, 802, 902), whereby the light received by the light detector being indicative of the displacement of the contact surface (102, 402, 502, 802, 902); the measuring device (M', M") being characterized in that it further comprises at least one biological sensor which is fixed to the contact surface (102, 402, 502, 802, 902) and faces in the same direction as the contact surface (102, 402, 502, 802, 902), wherein the biological sensor is configured for optically measuring at least one biological parameter of a user, optionally at least one of a heart rate and heart rate variability, said sensor assembly (100, 400, 500, 800, 900) being configured for increasing accuracy of said biological sensor, the contact surface constituting part of a structure, which holds the at least one biological sensor. a displacement sensor assembly (100, 400, 500, 800, 900) configured for use in measuring a biological parameter at the skin of a user, the assembly (100, 400, 500, 800, 900) comprises: a contact surface (102, 402, 502, 802, 902) facing towards an exterior of the assembly (100, 400, 500, 800, 900) and configured for contacting the skin of a user, and movable along a central axis of the assembly (100, 400, 500, 800, 900) when pressure is applied thereto by the skin of a user; a sensing unit (109, 800, 900) disposed within an interior of the assembly (100, 400, 500, 800, 900) and having an optical axis lying in a plane perpendicular to the central axis of the assembly (100, 400, 500, 800, 900); the sensing unit comprises a light source configured to emit light along the optical axis and a light detector facing towards the light source and configured to detect light emitted by the light source directly towards the detector; and a light blocking member operative to change the amount of light reaching the detector directly from the light source in consequence with displacement of the contact surface (102, 402, 502, 802, 902), whereby the light received by the light detector being indicative of the displacement of the contact surface (102, 402, 502, 802, 902); the measuring device (M', M") being characterized in that it further comprises a contact surface (102, 402, 502, 802, 902) facing towards an exterior of the assembly (100, 400, 500, 800, 900) and configured for contacting the skin of a user, and movable along a central axis of the assembly (100, 400, 500, 800, 900) when pressure is applied thereto by the skin of a user; a sensing unit (109, 800, 900) disposed within an interior of the assembly (100, 400, 500, 800, 900) and having an optical axis lying in a plane perpendicular to the central axis of the assembly (100, 400, 500, 800, 900); the sensing unit comprises a light source configured to emit light along the optical axis and a light detector facing towards the light source and configured to detect light emitted by the light source directly towards the detector; and a light blocking member operative to change the amount of light reaching the detector directly from the light source in consequence with displacement of the contact surface (102, 402, 502, 802, 902), whereby the light received by the light detector being indicative of the displacement of the contact surface (102, 402, 502, 802, 902); the measuring device (M', M") being characterized in that it further comprises at least one biological sensor which is fixed to the contact surface (102, 402, 502, 802, 902) and faces in the same direction as the contact surface (102, 402, 502, 802, 902), wherein the biological sensor is configured for optically measuring at least one biological parameter of a user, optionally at least one of a heart rate and heart rate variability, said sensor assembly (100, 400, 500, 800, 900) being configured for increasing accuracy of said biological sensor, the contact surface constituting part of a structure, which holds the at least one biological sensor.