The subject invention is directed to a computer-implemented method, a device and a system for detecting and scoring exercises, including maintaining, by a mobile device, a library of motion signatures, in which a motion signature for an exercise is a sequence of characteristic features and a characteristic feature is a movement in the exercise; receiving, by a mobile device, a time series of data from a sensor device, the sensor device attached to the head or torso of a user, the data comprising accelerometer data from an accelerometer included in the sensor device, detecting a single repetition of a designated exercise performed by the user and calculating a motion score for the single repetition of the detected exercise.