def stress_score(norm_power, threshold_power, duration):
    """Stress Score

    Parameters
    ----------
    norm_power : number
        NP or xPower
    threshold_power : number
        FTP or CP
    duration : int
        Duration in seconds

    Returns
    -------
    ss:
        TSS or BikeScore
    """

    ss = (duration/3600) * (norm_power/threshold_power)**2 * 100

    return ss