an aspect of the present invention is related to a method for determining a right intensity for fitness level or exercise stress that includes a first step of inspection a first change of amplitudes of a first heart sound with regard to a second change of intensities of exercise stress , a second step of inspection of a third change of ratios of a first length of a cardiac dilation period relative to a second length of one cardiac cycle with regard to the second change , a third step of identification of a folding point of the first change , and a fourth step of determination of an intensity of the intensities at the folding point as the right intensity of exercise stress if a ratio of the ratios at the intensity is higher than a predetermined value .