A method and device are provided for determining a physical activity state of a human. The method comprises receiving a signal representing a heartbeat of the human as a function of time. Based on that signal, multiple heartbeat rates at multiple points in time are determined. Based on the determined heartbeat rates, a first variability rate of the heartbeat rates over time is determined. If the first variability of the heartbeat rates decreases below a pre-determined level, it is determined that the body of the human moves from a first activity state to a second activity state. This method provides a more accurate determination of crossing the aerobic threshold than using the theories available and employing the formulas they provide. The analysis provides an output that depends on the state of the specific monitored body at that very moment.
