This was a prospective cross-sectional pilot study, approved by the Institutional Ethical Committee of RUHS College of Medical Sciences, Jaipur. (Approval letter no RUHS-CMS/Ethics Comm/2018/148 dated-21/12/18) and conducted from January 2019 to August 2019.

Inclusion criteria: A total of 100 apparently healthy young medical students (Pilot-study) of both gender, aged between 18-25 years were recruited randomly for the study after obtaining their consent form.

Exlusion criteria: Individuals suffering from Hypertension, Diabetes, Cardiac, Respiratory and musculoskeletal disorders, Smokers, alcoholic and history of hospitalisation in the last 3 months were excluded.

Everything was explained to students regarding aim of the study and how the procedure will be performed. All the students were tested under similar laboratory conditions (27-29°C temperature and relative humidity ranging between 75%-80%). Subjects were also instructed not to have heavy meals/tea/coffee at least 2 hours and not have performed any exercise before the test. Detailed history was taken and clinical examination was done. The Anthropometric data which included age, height, weight, skin-fold thickness was measured as accordance to National Health and Nutrition Examination Survey (NHANES) [10] and Body Mass Index (BMI) and body fat percentage [11] were calculated.

VO₂ max using direct method Analog-Digital (AD) instrument gas analyser (model-ML206): Test protocol was explained to the subject and demonstrated the same. The subject was made to wear the mask which is in turn was connected to the equipment to measure the total amount of gases inhaled and exhaled during the test. Now we started the test procedure i.e.,. the treadmill graded exercise test protocol, in which subject was asked to walk for 3 minutes at level grade followed by jogging at self selected speed (b/w 4.3-7.5 mph) at level grade for 3 minutes and then with a constant speed, treadmill grade was increased by 2.5% every minute until the subject achieves fatigue and is unable to continue the exercise. Here, during the procedure, the equipment was connected to a monitor screen, which showed us various values e.g., Volume of Oxygen (VO₂), Volume of Carbon-dioxide (VCO₂), Resting Energy Expenditure (REE), Respiratory Exchange Ratio (RER), Metabolic Equivalent of Tasks (METs) etc., of every 10 seconds [12].

VO₂ max using indirect method (QCT) [13]: Test protocol was explained to the subject and demonstrated the same. The subject was then asked to rest in a supine posture for 30 min. The resting heart rate and the blood pressure were measured. The height of the step is to be kept at 16.25 inches and the stepping cadence is to be set at:

Male: 24 cycles/min Female: 22 cycles/min

Since there are 4 steps in each cycle, the metronome (electrical device that produce a regular repeated sound) is to be set at 88 ticks/min for females or 96 ticks/min for males. The stepping protocol was followed as: both feet on the ground, first foot on the step, step-up with the second foot on the step, both foot on the step, first foot back on the ground, and finally the second foot on the ground. The subject was asked to perform the test for 3 min. At the end of the test, the subject has to remain standing and the number of heart beats between the 5^th and 20^th second after the cessation of the test, is to be noted down. The pulse rate per minutes is calculated by multiplying it by a factor of 4. This is used to predict the VO₂ max by using the following formula [13].

Male: VO₂ max(mL/kg/min)=111.33-(0.42×pulse rate in beats/min)

Female: VO₂ max(mL/kg/min)=65.81-(0.1847×pulse rate in beats/min)

Collected data were entered in Microsoft-Excel (version-16) and Unpaired and Paired t-test was used to compare VO₂ max results measured by using direct and indirect methods.

The study was conducted on 100 apparently healthy students (65 males and 35 females). Paired t-test was used to compare VO₂ max results measured by using direct and indirect methods. [Table/Fig-1] showed that Mean value of VO₂ max for males was 47.56±8.37 mL/kg/min and for females it was 37.62±7.40 mL/kg/min, which was found significantly higher in males than in females (p<0.0001).

[Table/Fig-2] showed that the mean value of VO₂ max (mL/kg/min) using direct method was 44.66±9.25 and that of using indirect method was 48.00±8.02. There was statistically very significant difference between two methods of VO₂ max estimation (p=0.0032).

VO₂ max measurement using gas analyser has been used as the gold standard method for assessing cardio-respiratory fitness. Queen college step test was originally formulated for Caucasian population but is being widely used for measuring VO₂ max even in Indian population, which differs from the ethnicity of Caucasian population. The present study measured Cardio-respiratory fitness by estimating VO₂ max and compared the result of VO₂ max obtained using direct (Gas Analyser) and indirect method (QCT), were found statistically very significant (p=0.0032). The actual VO₂ max is lower than the predicted VO₂ max from the QCT which suggest that the prediction equation is not found to be applicable to the studied population. In earlier studies, Chattarji S et al., reported that original predictive equation of QCT cannot be applied on Indian population due to difference of VO₂ max value using direct and indirect method [8,9].

In this present study, the mean value of VO₂ max for males was 47.56±8.37 mL/kg/min and for females was 37.62±7.40 mL/kg/min, which was significantly high in males (p<0.0001). Findings of this study are in agreement with other studies [14,15]. Modala S et al., in their study found that the mean value of VO₂ max was 51.76±3.8 in males and 44.07±4.2 in females [15]. Bandyopadhyay A and Bandyopadhyay P reported that males have superiority than their female counterpart [16]. The males are higher to females in cardio-respiratory fitness because, lower fat percentage and others factors mainly linked to the cardiac size and oxygen-carrying capacity (i.e., left ventricular inner diastolic diameter, maximal heart rate (HR max) and maximal stroke volume) [17], The reason for reduced VO₂ max in the female subjects of the present study, than the male subjects could be because of the significantly decreased physical activity level and significant higher level of body fat percentage.

On comparison of the VO2 max value in the present study with the standard VO₂ max classification, this study subjects fitted in the category of good on cardio-respiratory fitness (VO₂ max) scale [18]. Mean body fat percentage of males was 15.63±4.75 and for females it was 26.73±4.19, which was found statistically significantly higher in females than in males (p<0.001). These results are consistent with the findings of several studies in the literature [19,20]. Sharma M et al., reported a high negative correlation between VO₂ max and body fat percent in healthy males [19]. Mondal H and Mishra SP, also reported that increased body fat percentage is associated with decreased level of VO₂ max in adults [20]. The assessment of cardio-respiratory fitness is valuable while educating individuals about their overall fitness status, developing exercise programs and quantifying cardio-vascular risk.

This study is not applicable to all age groups. So, further studies can be done in larger sample size of all age groups and new regression equation should be developed for studied population.

The present study concludes that Direct method of VO₂ max using gas analyser provides more valid and accurate result than Indirect method (QCT). This study suggests that QCT in its original form cannot be applied in this study population. QCT is quickest, easiest and safest as well as feasible way to measure until a new modified equation is developed to measure VO₂ max for this studied population.