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We’re excited to announce that the AURA Devices R&D team has completed extensive testing of the AURA Strap 2, the advanced Apple Watch replacement band that measures your body composition and hydration level using hand-to-hand bioelectrical impedance analysis (BIA).
It’s common knowledge that the body is made mostly of water, and that water containing impurities (i.e., almost all water) conducts electricity. But there’s an important difference between water that’s inside the cells (intracellular water, or ICW) and water that’s outside the cells (extracellular water, or ECW). Cell membranes are very poor conductors, so the water inside and outside, separated by the membrane insulator, together create a natural capacitor. Therefore, a low-frequency alternating current (AC) can flow through ECW but can’t reach ICW, whereas a high-frequency current can pass through the membranes and into ICW as well. This natural feature of body tissue can be described just like a simple parallel electrical circuit with resistors on both branches (the ECW and ICW), and a capacitor on the ICW branch: The manufacturers claim the AURA Strap 2 to have 95% compared to a DEXA-scan, the gold standard of BIA measurement Image|Aura, Links to more info on research at auradevices.io BIA measures the impedance of this circuit, or how much the tissue resists the electrical current that passes through it. Using the height of the person, the total body water (TBW) can be calculated, and from there, fat-free mass (FFM). Subtracting FFM from total body mass gives fat mass (FM). However, height isn’t enough to get an accurate measurement, since the human body isn’t a perfect cylinder. Laboratory BIA equipment uses a five-cylinder model (arms, legs, and torso), but that means you need to attach electrodes all over your body. AURA uses a novel two truncated cones (TTC) model for the arms as a nice balance to take convenient measurements with the hands without sacrificing too much accuracy. How accurate is BIA?
What is it?
The Aura Biogram is a cool tool to graphically show your progress toward your body composition goal. To truly maximize the benefits of AURA Strap 2, the new AURA Plus subscription service provides members more resources and features, including monthly fitness and nutrition reports, deep analytics of body composition, exclusive fitness content and – coming soon – a live chat function with a personal trainer. You have full control over what information the Aura app can read from your phone or write to it. Using the normal sync to Apple Health, you might want to enable everything except, perhaps, the ability of Aura to write your weight to the phone. A: Yes it will fit pretty much all wrist sizes. The 41mm Apple Watch strap set includes XS, S, M, and L loop sizes and for the 45mm Apple Watch there are M, L, XL, and XXL loop sizes. A more streamlined measurement process now requires only one-touch to initiate measurement, rather than three actions required with the original AURA Strap
For some comparison tests, I used an old Tanita RD953 scale and a newish Garmin Index S2 smart scale. Even using those two comparator devices poses problems right from the start. Aura sends its electrical signal down one arm across the chest and down the other arm, whereas Garmin does a similar thing but its loop is from one foot to the other via your core. Finally, Tanita does a combination of readings with a sensor on each of the feet andhands. The strap and its material have changed from a single adjustable boucle, velcro-like strap to a silicone loop. Aura Strap 2 has every size in the box. The reading takes just over 20 seconds and a nice graphic on the watch shows your progress and gives tips or messages if your hands or elbows are in the wrong place. I achieved an 80% success rate in taking readings, although I was using a beta version of the Apple Watch software. Bearing in mind that I’m definitely significantly both musclier and ‘fatter’ on my legs than my upper body and that the readings might reflect that, let’s see what I found.
The Apple Watch app
Generally, the Aura Strap 2 isn’t ugly or badly designed, but it’s definitely designed for form over function. An adjustable strap would have been nice — or at least an option for one. Aura Strap 2 setup
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