cardio - pulmonary fitness is related to the ability to perform large muscle , dynamic , moderate - to - high exercises for a prolonged period . the performance of such exercises depends on the functional state of the cardiovascular , respiratory , and skeletal muscle systems . a variety of tests have been used to check cardio - pulmonary fitness and some of these tests are quite popular , such as , the cooper 12-minute test . the procedure of this test involves the participants running with maximal effort for 12 minutes exactly . the advantages of cooper test are that the whole team can participate together , and compete against each other . the test is relatively easy to carry out , and does not involve too much time or equipment . it can also be used easily for training ; and , any improvements can be easily appreciated . the disadvantages include that it is an indirect test , the results of which are not very accurate . also , it is suitable only for well trained individuals due to the fact it requires maximal effort for the entire length of the test . another disadvantage is that the test does not give actual maximum oxygen consumption values ; it only gives a classification of poor to excellent . hence , it has been suggested that this test is best for comparing athlete 's individual scores against each other to monitor improvements , rather than comparing the scores of the whole team . in the laboratory , maximum oxygen consumption is the gold standard in the assessment of cardio - pulmonary fitness . normally , heart rate is controlled by autonomic nervous system activity . heart rate variability ( hrv ) hrv demonstrates a relationship between autonomic nervous system function ( sympathetic or vagal activity ) and cardiovascular system . on the other hand , hrv has been recognized as a predictor of cardiovascular events , both in symptomatic and asymptomatic population.[36 ] also , it was found that reduced hrv predicts hypertension and hyperglycemia in the future . the simplest variable is the standard deviation of normal - to - normal qrs intervals ( sdnn ) in a continuous electrogram ( ecg ) recording . traditionally , hrv is recorded over a longer period ( 24 hour ) ; however , some investigators have reported that ultra - short hrv ( 2 - 15 minutes ) is strongly correlated with the 24-hour hrv . the aim of this study was to assess the cardio - pulmonary fitness by ultra - short hrv . study population was divided into 3 groups : group-1 ( n = 40 ) consisted of military sports man who volunteered to participate in this study . group-2 ( n = 40 ) were healthy age - matched sedentary male subjects who had a normal bmi [ bmi = 19 - 25 kg / m ) . group-3 ( n = 40 ) consisted of healthy age - matched obese male subjects [ bmi > 29 the subjects of the research were all free of cardiovascular and pulmonary disease , alcohol use , and diabetes mellitus ; and , were not taking any kind of medication , and did not present with any abnormal electrocardiographic patterns . , all the subjects rested in a supine position for a minimum of 15 minutes in a room with a constant temperature of 25 degree c. for the holter recording , standard digital devices were used . after holter recording , bruce protocol treadmill test was used . at timed stages during the run , the slope of the treadmill is increased as detailed in table 1 . maximum oxygen consumption ( vo2max ) , which has been shown to be an objective criterion of cardiopulmonary fitness , was calculated by foster equation : bruce protocol treadmill test vo2max ( ml / kg / min ) = 14.8  [ 1.379  t ] + [ 0.451  t ]  [ 0.012  t ] t is the total time of the test expressed in minutes and fractions of a minute . comparisons between groups were made using student 's t - test and mann - whitney u test as appropriate . study population was divided into 3 groups : group-1 ( n = 40 ) consisted of military sports man who volunteered to participate in this study . group-2 ( n = 40 ) were healthy age - matched sedentary male subjects who had a normal bmi [ bmi = 19 - 25 kg / m ) . group-3 ( n = 40 ) consisted of healthy age - matched obese male subjects [ bmi > 29 the subjects of the research were all free of cardiovascular and pulmonary disease , alcohol use , and diabetes mellitus ; and , were not taking any kind of medication , and did not present with any abnormal electrocardiographic patterns . before the test , all the subjects rested in a supine position for a minimum of 15 minutes in a room with a constant temperature of 25 degree c. for the holter recording , standard digital devices were used . after holter recording , bruce protocol treadmill test was used . at timed stages during the run , the slope of the treadmill is increased as detailed in table 1 . maximum oxygen consumption ( vo2max ) , which has been shown to be an objective criterion of cardiopulmonary fitness , was calculated by foster equation : bruce protocol treadmill test vo2max ( ml / kg / min ) = 14.8  [ 1.379  t ] + [ 0.451  t ]  [ 0.012  t ] t is the total time of the test expressed in minutes and fractions of a minute . comparisons between groups were made using student 's t - test and mann - whitney u test as appropriate . the mean age was identical between 3 groups as might be expected from the matching . baseline characteristics of the 3 groups involved in the study relationship between sdnn and estimated vo2 max are demonstrated in table 3 . relation between standard deviation of normal - to - normal qrs intervals ( sdnn ) and maximum oxygen consumption ( vo2max ) of the three groups involved in the study when the study population was divided into quartiles of sdnn ( first quartile : < 60 msec ; second quartile : > 60 and < 100 msec ; third quartile : > 100 and < 140 msec ; and fourth quartile : > 140 msec ) , progressive increase was found in vo2max ; and , sdnn was significantly linked with estimated vo2max values [ figure 1 ] . maximum oxygen consumption ( vo2max ) of study population compared by quartiles of standard deviation of normal - to - normal qrs intervals ( sdnn ) . when the study population was divided into quartiles of sdnn ( first quartile : < 60 msec ; second quartile : > 60 and < 100 msec ; third quartile : > 100 and < 140 msec ; and fourth quartile : > 140 msec ) , progressive increase was found in vo2max . the mean age was identical between 3 groups as might be expected from the matching . relation between standard deviation of normal - to - normal qrs intervals ( sdnn ) and maximum oxygen consumption ( vo2max ) of the three groups involved in the study when the study population was divided into quartiles of sdnn ( first quartile : < 60 msec ; second quartile : > 60 and < 100 msec ; third quartile : > 100 and < 140 msec ; and fourth quartile : > 140 msec ) , progressive increase was found in vo2max ; and , sdnn was significantly linked with estimated vo2max values [ figure 1 ] . maximum oxygen consumption ( vo2max ) of study population compared by quartiles of standard deviation of normal - to - normal qrs intervals ( sdnn ) . when the study population was divided into quartiles of sdnn ( first quartile : < 60 msec ; second quartile : > 60 and < 100 msec ; third quartile : > 100 and < 140 msec ; and fourth quartile : > 140 msec ) , progressive increase was found in vo2max . previously , 24 hour was determined as an optimal cut point for hrv recording and analysis . bigger et al . found that measures of rr variability calculated from short ( 2 to 15 minutes ) ecg recordings are remarkably similar to those calculated over 24 hours . hrv has been recognized as a predictor of cardiovascular events both in symptomatic and asymptomatic population.[36 ] it was also used in risk stratification for sudden cardiac death and diabetic autonomic neuropathy . they used hrv as a tool for noninvasive testing of autonomic nervous system activity with exercise training . they showed that regular exercises result in a significant improvement of all hrv indices . in the present study , we have analyzed cardio - pulmonary fitness ( vo2max ) and ultra - short hrv ( sdnn ) . we showed that sdnn is a simple cardio - pulmonary fitness test which just requires 15 minutes , and involves no exercise such as in the treadmill or cycle test . previously , 24 hour was determined as an optimal cut point for hrv recording and analysis . bigger et al . found that measures of rr variability calculated from short ( 2 to 15 minutes ) ecg recordings are remarkably similar to those calculated over 24 hours . hrv has been recognized as a predictor of cardiovascular events both in symptomatic and asymptomatic population.[36 ] it was also used in risk stratification for sudden cardiac death and diabetic autonomic neuropathy . they used hrv as a tool for noninvasive testing of autonomic nervous system activity with exercise training . they showed that regular exercises result in a significant improvement of all hrv indices . in the present study , we have analyzed cardio - pulmonary fitness ( vo2max ) and ultra - short hrv ( sdnn ) . we showed that sdnn is a simple cardio - pulmonary fitness test which just requires 15 minutes , and involves no exercise such as in the treadmill or cycle test . in conclusion , the results of this study demonstrate that exercise training improves cardio - respiratory autonomic function ( and increase heart rate variability ) . improvement in cardio - respiratory autonomic function seems to translate into a lower rate of long term mortality . ultra - short hrv is a simple cardio - pulmonary fitness test which just requires 15 minutes , and involves no exercise such as in the treadmill or cycle test .