Why might the conclusions of scientific studies, later be viewed as invalid?

The conclusions of some scientific studies could later be viewed as invalid because there could be more studies done that disprove the findings of earlier studies. This would make the earlier studies be viewed as invalid because they no longer are correct in what they state.

Explanation:

the more fit you are, the faster the recovery. your heart rate drops most sharply in the first minute after you stop exercising; it should then fall about 20 beats a minute—a drop of less than 12 beats a minute is considered abnormal. this “recovery heart rate” is measured as part of an exercise stress test.