An electrocardiographic (ECG) device for sensing cardiac activity in a user comprises a set of electric potential (EP) sensors 101, 102 provided within an insulating mat 105 and a set of indicia on the surface of the mat indicating an optimal location and orientation of a user's hands. The hands of the user are placed so that the sensors 101, 102 lie under the ball of the thumb of each hand. The indicia may be an outline of a pair of hands or indentations in the surface of the mat. The mat may include further EP sensors 103, 104 and indicia indicating the position of a user s feet. A control unit 112 processes the data from the sensors and may determine whether the user is optionally engaged with the device and indicate how to improve the engagement. The invention may involve controlling the interaction of a user with an electromyographic device. A similar mat may be used for an infant to determine whether the infant is sleeping in a safe position. The control unit 112 may determine the most likely position in which the user is engaged with the sensors and analyze the data accordingly.
