Method for determining an instant velocity of a user using a sensing device destined to be worn on an arm of the user and comprising a motion sensor for measuring and delivering a motion signal representative of the user's movements when the sensing device is worn; the method comprising: measuring the motion signal; calculating an activity level of the user by calculating an energy value from the measured motion signal; comparing the calculated energy value with a threshold energy; in the case the calculated energy value is above the threshold energy, detecting segments of the measured motion signal having rhythmical characteristics and, based thereon, estimating a fundamental movement frequency of the user from the measured motion signal; identifying when the user is walking or running by combining the calculated energy value of the user using the measured motion signal and the fundamental movement frequency, identifying when the user is walking or running being based on two features including the amplitude of the motion and the motion frequency and on another feature being the orientation of the sensing device in the gravity field; when the user is walking or running, providing a relationship between the measured motion signal and corresponding motion features that relate to a propulsive phase of a user's step alternating with an impact of the user's foot on the ground by using a bio-mechanical model of human locomotion; wherein said bio-mechanical model comprises an estimation of the counter-moment produced by the arms in relation to the moment produced by the impact and the propulsion phases of the legs; and wherein the method further comprises the step of estimating the angle of the arm's elbow by using said another feature being the orientation of the sensing device, comprising the motion sensor, in the gravity field; estimating a frequency of the user's step and estimating a user-relative step length of the user's step using the motion features; determining a user-relative instant velocity by combining the estimated user's step frequency and the estimated user-relative step length; and determining the instant velocity by multiplying the user-relative instant velocity by a calibration factor. measuring the motion signal; calculating an activity level of the user by calculating an energy value from the measured motion signal; comparing the calculated energy value with a threshold energy; in the case the calculated energy value is above the threshold energy, detecting segments of the measured motion signal having rhythmical characteristics and, based thereon, estimating a fundamental movement frequency of the user from the measured motion signal; identifying when the user is walking or running by combining the calculated energy value of the user using the measured motion signal and the fundamental movement frequency, identifying when the user is walking or running being based on two features including the amplitude of the motion and the motion frequency and on another feature being the orientation of the sensing device in the gravity field; when the user is walking or running, providing a relationship between the measured motion signal and corresponding motion features that relate to a propulsive phase of a user's step alternating with an impact of the user's foot on the ground by using a bio-mechanical model of human locomotion; wherein said bio-mechanical model comprises an estimation of the counter-moment produced by the arms in relation to the moment produced by the impact and the propulsion phases of the legs; and wherein the method further comprises the step of estimating the angle of the arm's elbow by using said another feature being the orientation of the sensing device, comprising the motion sensor, in the gravity field; estimating a frequency of the user's step and estimating a user-relative step length of the user's step using the motion features; determining a user-relative instant velocity by combining the estimated user's step frequency and the estimated user-relative step length; and determining the instant velocity by multiplying the user-relative instant velocity by a calibration factor.