Ten non-athletes (seven males and three females, aged from 19 to 21 years, Group A) and ten athletes (nine males and one female, aged from 14 to 17 years, Group B) were voluntarily enrolled in this study. The inclusion criteria were: 1) age was less than 25 years old; 2) giving informed consent; 3) free of any cardiovascular diseases and free of any other serious diseases. The exclusion criteria were: 1) on any medication; 2) presence of any contraindications to physical exercise. All subjects signed an informed consent form approved by the Ethics Committee of Institute of Physical Education, Chongqing University.

Cardiac Contractility Monitor (CCM, developed by BoJing Medical Informatics Institute, Chongqing, China) was used for this investigation. The hardware of CCM consists of a phonocardiographic sensor, a heart sound signal preprocessing box, a computer, and a printer. CCM uses a sampling rate of 8 kHz with 8 bits per sample (monochannel). The software includes a fundamental heart sound measurement system, the developing environment of which is Visual Basic 6.0 (Microsoft Inc,). The application’s target operating platform is Windows XP.

Since exercise can simultaneously cause a significant change in the inotropic and chronotropic status of the heart, phonocardiogram exercise test (PCGET) was adopted [15,16] in this study. Examination was started after the subject rested for 15 minutes. A PCG sensor was placed on the subjects’ pericardium. Phonocardiograms were recorded in the sitting position at rest and immediately after a step-climbing exercise. The step was 23 cm high. The subjects completed the designed exercise workload and the signals of cardiac cycle and cardiac contractility were simultaneously collected and recorded by CCM. There are many exercise workload protocols for PCGET. A specific multistage incremental protocol was used, which had 4 stages. Exercise test was performed starting at an initial workload of 1750 J. Each subsequent stage had an increment of 1750 J. The peak workload was 7000 J. There were 1-minute intervals between stages. The subjects repeatedly mounted and descended the step. The required number of times of step-climbing was calculated from the target workload, step height, and the subjects’ body weight. The end point of the exercise test was usually limited by subject symptoms, or by achievement of the designed exercise workload. The time taken for completing the workloads 1750 J, 3500 J, 5250 J, and 7000 J was recorded, respectively. During recovery after exercise, heart sound signals were recorded immediately after exercise, and at 1, 5, 10, 15, 20, 25, and 30 minutes after exercise. The ratio of the amplitude of the first heart sound to the amplitude of the second heart sound (S1/S2) on the same phonocardiogram was calculated from the measured data.

Heart sound signal was converted several times during processing, and finally, it was converted to screen coordinates to construct a waveform graph. The basic points concerning heart sound quantitative analysis were: 1) measuring the duration and the amplitude of relevant heart sound components; 2) calculating and analyzing relevant indicators based on the data obtained from the above measurements, including S1/S2 ratio, HR, and power (unit: W). Since the thickness of the chest wall of different subjects is various, the absolute amplitude of S1 cannot be used as an indicator for evaluating cardiac contractility. So, the design of some relative value indicators for evaluating cardiac reserve is very important, such as S1/S2 ratio, D/S ratio, etc, which have dimensionless values. To measure and evaluate heart rate and cardiac contractility change trend, a self-control trial was designed.

In order to show the cardiac reserve mobilization trend during exercise, x-y plots were constructed from the measured data (employing the mean values). S1/S2 ratio, HR, and power (vertical coordinates) were plotted against the work done during exercise (marked on the horizontal axis). The points of S1/S2 ratio, HR, and power corresponding 1750 J were connected to the points of their baseline value, producing 3 lines, S1/S2 line, HR line, and power line (W line), which, then connected to their own various points corresponding to 3500 J, 5250 J, and 7000 J, obtaining 3 polygonal lines, respectively. A higher slope value indicates a steeper incline. In order to show the cardiac function recovery course, a graph was constructed to the right of the graph of cardiac reserve mobilization trend, the vertical axes of which were the same as the left panel, and the horizontal axis changed to a time axis marked with the time points of cardiac function recovery.

Quantitative data are presented as mean ± SD. Statistical comparisons between 2 groups were performed with corresponding t test. A value of P < 0.05 was considered to be statistically significant. Statistical analysis was performed with SPSS software (SPSS Inc).