Cardiorespiratory fitness denotes the capacity of an individual to perform large muscle, dynamic, moderate to vigorous physical activity for a prolonged period of time. It depends upon the integrated functional status of the cardiovascular, respiratory, metabolic and musculoskeletal systems. The assessment of cardiorespiratory fitness is of pivotal importance as it gives us a general idea of the fitness of the individual and is a vital part of any preventive or rehabilitation program. The criterion used to assess the cardiorespiratory fitness is maximal oxygen uptake (VO₂max) expressed in liters/minute (L/min) as an absolute value or in milliliters/kilogram/minute (ml/kg/min) as relative VO₂max. It is the maximal rate at which one can consume oxygen during physical activity and the true physiological limit reached where no further increase in the oxygen uptake will occur on continuing the exercise. The gold standard for measuring VO₂max is graded maximal exercise testing, performed in a lab using gas analysis. Maximal exercise testing protocols mandate the requirement of an expensive laboratory setup of equipments, monitoring systems and emergency equipments, require trained personnel, are time consuming and labor intensive and risk the precipitation of unwarranted symptoms which may lead to adverse events. When compared to maximal exercise testing, submaximal exercise tests have a greater applicability to physiotherapists in assessment of cardiorespiratory fitness and exercise prescription due to their low-risk, low-cost, low-supervision nature and objective evaluation of VO₂max.

The six minute walk test is a commonly used self paced, level ground walking submaximal exercise test performed across a 100 feet hallway for six minutes. The 6MWT results and the anthropometric variables of the individual are used to estimate the VO₂max using the equation given by Burr et al. The Chester Step Test is a submaximal incremental metronome based multistage step test. The total test time is of 10 minutes comprising five stages lasting for 2 minutes each.  The step height is adaptable depending upon the age and the physical abilities of the individual. The VO₂max is estimated by plotting a graph of the heart rate points achieved at the end of each stage on the specified Chester step test data record sheet. Thus the current study was undertaken to assess the agreement between the VO₂max estimated by these two submaximal exercise tests namely the six minute walk test and Chester step test.

80 normal healthy adults (40 males and 40 females) between the age group of 18-40 years and able to complete Stage 3 of Chester step test were recruited for this study. Individuals engaged in regular sports or vigorous physical activity and those with BMI greater than 23 kg/m² were excluded. The participants were explained about the study protocol, the purpose of the study and written consent was obtained from them. The six minute walk test was performed on the first day followed by the Chester Step Test on the next day for all odd numbered participants and vice versa for even numbered participants. A practice test and then the final test were performed on the same day with a gap of minimum 1 hour between the two tests. 

The mean VO₂max-6MWT and VO₂max-CST were 43.53 ± 4.65 ml/kg/min and 38.34 ± 4.94 ml/kg/min respectively. The Bland and Altman analysis revealed that the mean difference between VO₂max-6MWT and VO₂max-CST was 5.19 ± 5.50 ml/kg/min which exceeded the maximum acceptable difference of 3 ml/kg/min decided a priori. The standard error of mean was 0.61 ml/kg/min and the standard error of limits of agreement was 1.06 ml/kg/min. It was therefore concluded that an agreement did not exist between VO₂max-6MWT and VO₂max-CST in the given study population and that the tests cannot be used interchangeably.