I recently bought a fitbit Ionic sports smartwatch.
My watch is an Adidas edition which had two arm bands included: A short and long one.
:: To use this extension you need both pairs because of the two thorn closes you need, see the pictures.
It seems the Ionic uses a camera like sensor to watch the tiny skin veins for heart rate.
However when I cycle my hand is quite shaky. I think that might cause inaccurate results.
Other heart rate sensors are attached to the upper arms.
This band extension attaches the watch to my upper arm using one part of the unused watchband.
The files got very big, because I cloned original sports band design the best I could. All the tiny holes are round at one seide and hexagon on the upper side. It fits perfectly into the original thorn closes.
Update: After around 24km of testing the fitbit ionic now works awesome! The Ninjaflex is so soft that I dont feel the extension in a bad way. See the "same route twice.jpg"-picture within the comments for a heart rate comparison. The watch works much better on the upper half of my biceps
Ninjatek Ninjaflex TPU 85 Shore A (very soft, elastic)
Rectlinear, only one angle 90°
I printed the extension using the probably best and softest TPU material I own: Ninjatek Ninjaflex. It is available in a various number of colors.
I used ~25% Infill unidirectional in 90° to the length of the extension band to have it very soft. See the picture which shows the infill. You might not need to do that, but I wanted a very soft watchband to save lifetime of the watchband connectors.
(PS: If you print things that you want to wear you should not just use any material without Safety Data Sheets (SDS). Ninjatek provides that. It is very resistant to many chemicals as well.)