Twenty participants is a pretty small study. When conducting research, having a large number of people to analyze is always better. With so many variables at play, a bigger group helps eliminate anything that might be influencing the results. For example, those who might have a genetic predisposition for leg pain or chronic back pain might be particularly susceptible to the effects of sitting and standing. When you only have a small number of people in the study, having a disproportionate number of people with those predispositions could really sway the results. The bigger and more randomly selected your group is, the more likely you are to get something close to a representative sample of the whole population. So, generally speaking, we wouldn't recommend taking the findings of any study with just 20 participants as gospel.