European Journal of Cardiovascular Medicine

Cardiorespiratory fitness requires a combination of the circulatory, respiratory, and muscular systems to supply oxygen to the working tissues during physical activity ^[1]. VO_2max is the main indicator used for assessing cardiopulmonary capacity and determining respiratory circulation functions such as exercise intensity. ^[2] VO_2max can be assessed with direct or indirect procedures ^[3,4]. Direct measures provide the most precise assessment of cardiopulmonary fitness and are obtained by ventilatory gas analysis at maximal exertion during a graded exercise test ^[3,5]. Indirect methods estimate VO_2max from maximal exercise duration, the peak load or heart rate response achieved during sub-maximal exercise, or the time required to walk, jog, or run a specified distance ^[4,5].

Cardiopulmonary exercise testing with direct VO₂ (CPET) measurement is widely considered a golden standard for maximum oxygen uptake (VO_2max) and fitness level assessment. This method is expensive, requires specialized staff, and highly demanding preconditions need to be met.  Under such circumstances, especially in large population studies, indirect VO_2max prediction needs to be implemented.^[6]

Cardiorespiratory fitness (CRF) levels, mainly determined

by recent moderate-to-vigorous physical activity, are important for aerobic performance and engagement in daily exercise activities. It is also a strong predictor of several non-communicable diseases and all-cause mortality. ^[7,8,9].

Hence this study is undertaken to analyse the cardiopulmonary fitness of individuals by using different methodologies and also to find out the economically feasible methods amongst them in the Indian healthcare setup.

The present cross-sectional study was conducted in the Department of Physiology at Kodagu Institute of Medical Sciences in Madikeri. After obtaining approval from the institutional ethical committee, 20 female subjects were randomly selected from the general population using a stratified random sampling method. The participants were aged between 20 and 40 years and provided written informed consent prior to participating in the study. Individuals with diagnosed congenital conditions affecting the cardiovascular, respiratory, or musculoskeletal systems, as well as those who are specially-abled, pregnant, lactating, or on long-term medication for any illness, were excluded from the study.

Study protocol

Participants were instructed not to consume beverages, eat a heavy meal, or engage in any vigorous activity for 24 hours prior to the test. Proper instructions regarding the experiment were provided. A thorough history was taken, and a physical examination was conducted to rule out any contraindications for exercise testing. The tests were performed between 10 AM and 4 PM on two separate days. Anthropometric measurements were recorded before testing. A resting ECG was obtained using a wireless ECG machine, and blood pressure was recorded.

YMCA 3- minute step test

Participants were instructed to step up and down on a 30 cm platform 72 times within a 3-minute.A metronome set at 96 beats per minute indicates stepping frequency, where four clicks correspond to a step cycle. The stepping rate is 24 steps per minute. The subject must stop immediately upon completing the test and then sit down and remain still. After 5 seconds, a wireless ECG machine monitors the subject's heart rate for one minute. VO_2max is calculated using an equation formulated by the Korean Institute of Sport Science as follows: (table:1)

Males: VO_2max=70.597-(0.246 x age) +(0.077 x height in cm) -(0.222 x weight in kg) -(0.147 x heart rate).

Female: VO_2max=70.597-( 0.185 x age) +( 0.097 x height in cm) -(0.246 x weight in kg) – (0.122 x heart rate).

Table 1: YMCA 3 min step grading

Treadmill bruce protocol

Each participant warmed up for 1 minute on the treadmill by walking at their own pace with a 0% gradient. According to the Bruce protocol, the speed and incline were increased every 3 minutes until the subject reached exhaustion. (Table 2)ECG was monitored continuously.

Table 2: Bruce treadmill protocol

The endpoint of the test was complete exhaustion of the subjects or attainment of 90% of the predicted maximum heart rate (220-age in years). Total time of exercise test (T) was noted in a fraction of a minute.( table:3)

-Female: VO_2max=4.38 x T - 3.9

Table 3: VO_2max grading according to indirect treadmill bruce protocol

Statistical analysis

Statistical Package for Social Sciences [SPSS] for Windows Version 22.0 Released 2013.Armonk, NY: IBM Corp., will be used to perform statistical analyses. Descriptive analysis of all the explanatory and outcome parameters will be done using frequency and proportions for categorical variables, whereas Mean & SD for continuous variables. Independent Student t-test was used to compare the mean VO_2max values by YMCA 3min step test and Treadmill Bruce protocol based on the BMI status of subjects. Pearson's correlation test was used to determine the relationship between VO_2max values by the YMCA 3-minute step test and treadmill Bruce protocol.  Simple Linear Regression Analysis was used to predict the VO_2max values by Treadmill Bruce protocol using the YMCA 3 min step test among study subjects. The level of significance was set at P<0.05.

20 female subjects were taken for this study. Their age group ranges between 20 to 36 years. The mean age group of subjects is 25.05±4.39. 12 of them were of the age group less than 25 which is around 60%. 8 of them were of the age group more than 25 years which is around 40%. (table no 4), Fig no 1.

Table no 4. Distribution of age among study subjects

70% of the subjects were under normal BMI. BMI of 30% of subjects were overweight (pre-obese) (table no 5)., fig no 2. Out of 20 subjects, the mean height is 162.35±4.69cm, mean weight is 59.10±5.41 kg. mean BMI is 22.35±1.63 kg/m² (table no 6)        

Table no: 5- Distribution of BMI Status among Study Subjects

Table no: 6 Descriptive analyses for BMI Parameters among study subjects

Table no 7 shows the VO_2max values obtained from the YMCA 3-minute step test and Treadmill Bruce protocol. VO_2max   obtained from the YMCA 3min step test was 38.99±2.31 ml/kg/min and VO_2max   obtained from the Treadmill Bruce protocol was 41.80±2.32 ml/kg/min. The VO_2max    value obtained from these two tests shows similar results.

Table no: 7 Descriptive analyses for VO_2max Parameters among study subjects

Table no 8 shows the comparison of mean VO_2max values by the YMCA 3min step test & Treadmill Bruce protocol based on the BMI status of subjects using an Independent Student t-test.  For subjects having normal BMI, VO_2max obtained from the YMCA 3min step test is 38.618±2.568 ml/kg/min, and VO_2max   obtained from the Treadmill Bruce protocol is 41.551±2.505 ml/kg/min. The p-value is 0.29. For subjects having overweight BMI, VO_2max obtained from the YMCA 3min step test is 39.852±1.395 ml/kg/min, and VO_2max   obtained from the Treadmill Bruce protocol is 42.383±1.876 ml/kg/min. The p-value is 0.48.

Table no: 8 Comparison of mean VO_2max values by YMCA 3min step test & Treadmill Bruce protocol based on the BMI status of subjects using Independent Student t-Test

In the Pearson correlation test, the relationship between VO_2max values obtained by the YMCA 3 min step test and treadmill Bruce protocol a significant very strong positive correlation (r=0.86) with VO_2max  values by Treadmill Bruce protocol, and the relationship was statistically significant at p<0.001. (table no 9)

Table no: 9 Pearson correlation test to determine the relationship b/w VO2max values by YMCA 3min step test & Treadmill Bruce protocol

           * - Statistically Significant

Table no 10 shows the simple linear regression analysis to predict VO_2max values by treadmill Bruce protocol using YMCA 3 min step test. The test results demonstrate that for every 1 value increase in VO_2max values by the YMCA 3min step test, there is a corresponding significant increase of 0.858 VO_2max values by Treadmill Bruce protocol. The finding was statistically significant at p<0.001. The variability in VO_2max values Treadmill Bruce protocol test will be explained by the VO_2max values of the YMCA 3-minute step test by 72%.

Table no: 10 Simple Linear Regression analysis to predict VO_2max values by Treadmill Bruce protocol using YMCA 3min step test

           * - Statistically Significant

The current study shows that the VO_2max value obtained from the YMCA 3-minute step test is similar to that of the value obtained from the Treadmill Bruce Protocol. Out of 20 female subjects 12 of them were of the age group less than 25 and 8 of them were of the age group more than 25 years. The mean age group of subjects is 25.05±4.39. 70% of the subjects were under normal BMI. BMI of 30% of subjects were overweight (pre-obese).

Cardio-respiratory fitness, also called cardiovascular fitness or maximal aerobic power, is theoverall capacity of the cardiovascular and respiratory systems and the ability to carry outprolonged strenuous exercise ^[10]. Insufficient physical activity is a risk factor for non-communicable diseases such as cardiovascular disease, diabetes mellitus, stroke, and cancers and health outcomes such as mental health injuries and obesity ^[11]. VO_2max is the main indicator for assessing cardiopulmonary capacity and determining respiratory circulation functions such as exercise intensity ^[12]. The magnitude of VO_2max depends on the oxygen transport by the cardiorespiratory system from the atmosphere to exercising muscles, followed by the utilization of oxygen by metabolically active tissue ^[13]. VO_2max can be determined by using direct and indirect tests.in this study, VO_2max is determined by two indirect tests using the YMCA step test and the treadmill Bruce protocol.

VO_2max   obtained from the YMCA 3min step test was 38.99±2.31 ml/kg/min and VO_2max   obtained from the Treadmill Bruce protocol was 41.80±2.32 ml/kg/min. The p-value is 0.29. A study conducted by Beutner et.al reported a strong correlation between VO_2max values predicted from a linear regression model including age, sex, and 1-minute heartbeat count (HBC) from the YMCA 3MST, and actual measurements of VO_2max during the treadmill test (r = 0.86).^[14]  

In the Pearson correlation test, the relationship between VO_2max values obtained by the YMCA 3 min step test and treadmill Bruce protocol a significant very strong positive correlation (r=0.86) with VO_2max values by Treadmill Bruce protocol, and the relationship was statistically significant at p<0.001. The test results demonstrate that for every 1 value increase in VO_2max values by the YMCA 3min step test, there is a corresponding significant increase of 0.858 VO_2max values by Treadmill Bruce protocol. The finding was statistically significant at p<0.001. The variability in VO_2max values Treadmill Bruce protocol test will be explained by the VO_2max values of the YMCA 3-minute step test by 72%.

This study shows that the VO_2max value obtained from the YMCA 3-minute step test is almost the same as that obtained from the Treadmill Bruce protocol. The equation for the YMCA 3-minute step test is a valid submaximal test for predicting maximum aerobic capacity. This test is cost-effective and can be used anywhere without requiring significant investment. It can be used as a tool for assessing the physical fitness level in a clinical setting and can serve as a screening tool for non-communicable diseases.

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