The experimental design must have only test one variable and keep all others constant. Experimental design should provide for data collection and data analysis. Many times even careful scientists have published results that cannot be repeated by other labs. Even simple experimental techniques do not always work all the time. There are many causes for this, one is the care needed in executing an experiment the same way every time, making sure the controls are working properly and making sure you are only testing one variable at a time.
Sample size is also important.
Small sample size may provide you with unexpected results. Not everything in this lab will work, but that's part of science as well. Time permitting you are welcome to repeat experiments within the lab as any good scientist would. Cardiovascular fitness can be measured by a variety of variables. The easiest is pulse, those that are more fit generally have a lower pulse rate at the end of the exercise and return to normal more quickly.