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In the nineteenth century, the legendary physicist Lord Kelvin remarked, “to measure is to know” and “when you can measure what you are speaking about, and can express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind”. Even though Lord Kelvin did not have epidemiology in mind when he made these statements, they hold true in the world of epidemiology—where measurement is the key. The importance of physical activity as a determinant for health and diseases—and as an adjuvant in medical treatment and rehabilitation—is increasingly valued. And, it has been highlighted by this journal many times.