The Tanita Body Fat Monitor does not measure fat directly. The only two values measured directly are weight and the electrical resistance (impedance) of the body. Body fat percentage is calculated using an equation based on these and other values such as height, gender and body type.
Tanita's equations for standard Adult mode have been calibrated against clinical readings taken from a representative sample of the population, most of whom have sedentary or only moderately active lifestyles.
When developing its body composition equations, Tanita found that a different equation was needed to maintain accuracy in the elite athletic population. People who exercise at high levels over long periods of time exhibit differences in their muscle structure and hydration. Their muscle tissue composition may differ, there will be a difference in their water to muscle ratio and overall their bodies will be denser than the average person, as muscle tissue is denser than fat. If these people were to use the standard Adult mode on a Body Fat Monitor, their readings could be overestimated.
Tanita therefore developed a specific Athlete mode, calibrated against a representative sample of "athletes".
As a general guideline, we recommend Athlete mode for anyone who carries out more than 10 hours of intensive aerobic exercise a week AND who has a resting heartbeat of under 60bpm. (Some people naturally have a low heart rate - it does not apply unless they exercise at that level.) They should also have been exercising at this level for at least 6 months.
It could also apply to people who have a lifetime history of fitness - who used to do more than 10 hours a week but do less now.
There is no exact moment when a person becomes an "athlete" and individuals vary. Some people will fall into a "grey" area between athlete and non-athlete; getting a slight underestimation on athlete mode and a slight overestimation on standard adult mode.
The important factor in Tanita's Body Fat Monitors is that they give consistent repeatable readings over a period of time - allowing you to monitor your health in the long-term. If you are 15% now and 25% in a few years time you will know something is wrong!
Athlete mode is particularly suitable for people in endurance sports like running and cycling and those in cross training. (Professional athletes and body builders may still get an overestimation of their body fat percentage even in Athlete mode.)
Athlete mode can be found on a number of Body Fat Monitors in the Tanita range.