Man of Steel wrote:
Worked in a body comp lab, and the short answer is it depends.
Longer answer; these tools use bioelectrical impedance to estimate total body water, which can be used to estimate fat free mass and percent body fat. So, if you are over or under hydrated this can effect the measurement. Also, they tend to be less accurate at the extremes (very lean, or very fat), although most methods of measurement suffer at the extremes.
From personal experience comparing the scales to other measurement tools (calipers, bodpod, DXA). The scale tends to be more accurate for men and overestimate women (my theory is because women carry more fat in their lower bodies this skews the measurement since it is taken through the feet). The hand held ones are just very hit or miss(again, I think they are affected by fat distribution). But even when they are wrong, they tend to be consistent (as long as hydration is kept relatively constant)so they are fine for tracking changes.
...sorry for the long answer...I like science
Don't be sorry. Like the OP, I've always wondered about these things. Even 2% out is like 4lb in the average 200lb man so that's a hell of a lot of uncertainty. I would guess they would become more unreliable for the SSBBWs and SSBHMs we would like to see on them ...
