It works by measuring electrical current. Presumably the dumbbell is conductive. So, it isn’t surprising that it thinks it’s lean mass.
highly unlikely as I used a backpack to put the weight in
What you are buying into is the algorithm. It is a math equation about your gender, height, weight and electrical impedance (one more variable than bmi). I would have to assume for cyclists with musculature in the legs the lean mass is possibly overstated. If you have fat in your legs, or around your pelvis, you’ll likely get a higher fat reading (especially if it is not gender aligned fat distribution). The effect of full glycogen stores stacked in excess water might also shift the ratio, weighing before and after a workout would result in possibly wide variation of lean mass depending on how your muscles’ conductivity changes.
This is no different than a scale. A scale over short durations tells you almost nothing. I can gain or lose ~3 lbs in a day easily. It’s the long arc that matters. This is no different in fact than any measurement, by any device.
Consistency is more important to me than accuracy. my bf scale tracks pretty well with what I would expect, I’ve been on a weight loss journey and I can see metrics over the long term that seem to match my expectations and experience. I try to weigh at somewhat consistent times, not only temporally but physically. (mornings before any workout has been done)
I don’t plan on using my bf scale for medical advice. 
but it does give me a sense of the values heading in the right direction or things to look out for. Lately I noticed that my lean mass actually went down (for the first time) which was pretty interesting to me and a sign that maybe I should pay attention to any change in my strength or a real sense of fatigue with sessions I normally feel fine with. I’ll keep an eye on that number to see if it fluctuates back to what I expect. I’m getting into relatively uncommon levels of leanness for me so I want to pay attention.
One thing I will say is that my bmi puts me at overweight, my bf scale says 15% body fat and 171 lbs of lean mass (I don’t look muscle bound, quite the contrary). for me to be in the middle of the healthy bmi zone I would need to weigh (you guessed it) 171 lbs. I haven’t weighed 171 lbs since I was 12.
So in the absence of a real fitness assessment (I want to get one in the spring) I have to go on what I feel and look like, informed by the two options I have available to me, bmi (oof) and bf scale.
Not sure that helps…but it is my anecdotal reference and my personal approach. :cheers:
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My Garmin scale is accurate depending on what “activity class” I set it for. If I set it for 0, meaning I don’t do any exercise, then it is reasonably accurate. If I set it for 10, which is my actual activity class, then it is worthless.
Basically, it isn’t smart at all. So I just use it as an expense scale.
I also have the first gen Garmin Scale. I don’t take any of the measurements as being exact. I only weigh once a week on the same day, same time, and what I’m looking for is a trend one way or the other.
I’m not looking something that will pass inspection at a metrology lab, just trends…
These variations need not be fictitious. E. g. if you drink 1 l of water, your body fat percentage decreases. If I had ramen the day before, I’d gain ~1 kg and body fat percentage decreases.
What matters are longer-term averages, especially if you measure consistently e. g. in the morning after you went to the bathroom.
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When Nate compared is Tanita I mentioned above to DEXA his spreadsheet (posted here a while back) showed that the normal mode was comparable to the DEXA scans, the athlete mode was not. Seems this is an issue with more than just Tanita.
My Tanita, same as his, matched a Bodpod test I had shortly before getting it in normal mode and Athlete mode is always laughable.
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If you can show me the excel formula to plot pictures that would be great.
I have a Withings Body+ (the cheaper one) and I think it is accurate for total weight, but it is all over the place for fat/muscle/water/etc. If I weigh myself right after a shower, the readings will skew towards less fat, but if I weigh an hour after a shower it will skew towards more fat. My theory is better conductivity through the moisturized skin after a shower than when the skin is fully dry.
Even day to day measurements at the same time of the day will give wildly different compositions, as in 5 lb swings in muscle to fat with relatively the same total weight. I’m pretty sure it’s impossible to convert 5lbs of muscle and 0.3lbs of bone to 5.3lbs of fat in 24hours, but my Withings has claimed swings like that multiple times…
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I’ve had good luck with the Garmin. Body fat % seemed high at the start, so I did a DEXA scan and entered that number. Been pretty accurate since.
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This is a cool feature I was just looking into.
I like that my Garmin scale syncs easily with Garmin Connect and I can watch trends over time as I’ve been losing weight. I think I’m going to get a Dexa scan soon as a baseline and adjust my scale if needed.
The non-weight metrics are interesting with my Garmin scale and watch, but I definitely take them with a grain of salt.
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My Renpho was the opposite - %'s in line with the dexa when in “Athlete” mode.
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Good point, hadn’t thought of it that way.
That’s not what I tend to see with the Garmin though. My bf% seems to rise and fall (quickly) with my overall weight, not decrease if I weigh more due to greater water mass and increase if I’m dehydrated.
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Actually, it works by holding the current constant while measuring the impedance, i.e., the AC equivalent of DC resistance.
Given your user name, I would think you would know this bit of trivia…unless you’re a musician? 
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Or a train operator 
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Or a group-discussion session facilitator… 
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FWIW …
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If you care about using Garmin connect to track your body fat %, I’m pretty sure you are stuck using a Garmin scale. I have not seen a way to enter the data manually, and I’m not aware of a way to sync it to Garmin connect.
I’d like to get all my data into Garmin, so if people know of a way I’m interested.
Not important to me. I’ve been typing mass and body fat into a spreadsheet for years, so an app would be nice but not necessary. Still, I understand why you’d want to get all the data into Connect, and I only see the phone app allowing you to enter weight. No idea why… annoying/silly of them.
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