Except they control for overall health in these types of studies.
For something measuring physical activity they won't have participants that are mobility limited. They will only use people who are able to be active. Practically all medical studies put participants through an exam before accepting them to ensure they don't have conditions that would skew the results.
For something measuring physical activity they won't have participants that are mobility limited. They will only use people who are able to be active. Practically all medical studies put participants through an exam before accepting them to ensure they don't have conditions that would skew the results.