I 3D printed a Fitbit Air bicep mod to wear it under my sleeve

Recently, the tech community has found a novel way to customize and adapt these minimalist devices, driven by an unlikely source: open-source design blueprints and 3D printing. Following Google’s decision to make the design guidelines and 2D computer-aided design files for the Google Fitbit Air publicly available, users have begun fabricating custom accessories, housings, and adapters that push the boundaries of how and where fitness wearables can be worn.

The movement has gained significant traction among hobbyists utilizing desktop 3D printers, such as the Bambu Lab P2S, to craft personalized hardware solutions. While the market offers a wide array of official and third-party accessories, the availability of official blueprints has allowed users to bypass commercial limitations and tailor their fitness trackers to specific athletic routines and personal style preferences.

3D Printing to Keep It Classy

The versatility of the Google Fitbit Air has unlocked a wave of creativity within maker communities, leading to the development of custom straps, protective cases, and mounting hardware. Traditional fitness bands often rely on flexible thermoplastic polyurethane, or TPU, to secure the tracker to the body. However, for intense athletic activities like heavy weight training, standard TPU materials can lack adequate breathability, trapping sweat and causing discomfort during prolonged exertion.

Faced with these practical limitations, tech enthusiasts have explored alternative housing methods. By searching through community-driven platforms and repositories like the Bambu Studio app, users have discovered innovative cross-compatibility solutions, including adapters designed to bridge the gap between different wearable ecosystems. One such adaptation involves modifying the Fitbit Air to fit within the structure of a Whoop 4.0 band.

The appeal of the Whoop strap lies in its longer, highly expandable fabric design, which lends itself well to alternative placements, such as higher up on the arm. Because official retail channels do not currently offer dedicated bicep straps specifically for the Google Fitbit Air, users have turned to additive manufacturing. By printing custom adapters out of rigid materials like polylactic acid, or PLA plastic, enthusiasts can securely house the Fitbit Air inside a Whoop band, creating a seamless hybrid wearable that can be worn comfortably beneath a shirt sleeve.

This configuration appeals particularly to individuals who prefer the classic aesthetic of a traditional analog wristwatch on their wrist while still capturing comprehensive biometric data. By moving the screenless tracker to the bicep or upper arm, users can maintain their preferred fashion choices without sacrificing continuous health monitoring.

Whoop, Whoop

Transitioning a fitness tracker from the wrist to alternative anatomical locations inevitably introduces questions regarding data accuracy and metric consistency. Before relocating the Fitbit Air to the bicep, comparative analyses against other established health-monitoring hardware, such as the Oura Ring 4, revealed minor discrepancies in daily metrics. For instance, the Fitbit Air initially recorded slightly higher sleep durations and a higher total step count than the Oura Ring 4 under identical conditions.

Furthermore, relocating a fitness tracker to the upper arm or bicep fundamentally alters how physical movement is interpreted by the device’s internal sensors. Step tracking, which relies heavily on rhythmic wrist motion and acceleration swings, inherently loses a degree of precision when the tracker is secured higher on the arm. Comparative studies evaluating step-tracking accuracy across various wearables—including the Fitbit Air, the Google Pixel Watch, and Whoop models—have consistently demonstrated that traditional wrist placements yield the most reliable step counts during walking and running activities.

Despite these trade-offs, some users report alternative benefits depending on their specific athletic discipline. Online enthusiast communities have long debated whether chest or bicep placements improve heart rate monitoring fidelity during strenuous physical exertion. While tests with the Fitbit Air on the bicep revealed only modest improvements in heart rate tracking accuracy compared to standard wrist placement, the hidden profile of the device offers distinct practical advantages.

Athletes participating in contact sports, such as soccer, often find that wearing a screenless tracker on the upper arm protects the hardware from direct impacts, pushes, and shoves during competitive play. Similarly, weightlifters utilizing bicep configurations can avoid snagging the device during barbell exercises, provided they use a secure, dedicated bicep strap rather than a standard band that might loosen under tension.

These experimental setups, however, are often born out of circumstance. Many users who previously subscribed to alternative wearable ecosystems found themselves with unused hardware lying dormant after ending their subscription services, prompting repurposing experiments that breathe new life into older straps and accessories.

It’s a 3D World

Google’s decision to release the Fitbit Air’s design guidelines and 2D CAD files has catalyzed a broader ecosystem of user-generated hardware modifications that extend well beyond straps and bands. Because the charging puck included with the tracker is exceptionally compact, it is prone to misplacement in desk drawers and gym bags. To combat this, makers have designed and shared files for custom charging stands, allowing users to dock their devices securely on a nightstand or workspace.

Other more experimental designs include analog watch strap adapters, which sandwich the Fitbit Air tracker between a traditional leather or metal watch strap and the wearer’s wrist. While some users find this dual-device configuration slightly bulky or uncomfortable, the existence of such designs highlights the diverse demands of the consumer base. Community contributions have even spanned into more unconventional territory, including specialized holders designed to secure the tracker to undergarments, pointing toward an expanding frontier of hidden wearable placements.

Yet, these hardware modifications highlight a notable limitation in the current software ecosystem. Companion mobile applications typically assume a standard wrist placement and do not currently provide native settings within the app to adjust for bicep, torso, or alternative configurations. This software limitation means that algorithms calculating metrics like caloric expenditure, step counts, and activity recognition continue to interpret sensor data through the assumption of wrist-based movement.

As a growing segment of fitness enthusiasts continues to experiment with unorthodox hardware placements and 3D-printed modifications, there is an underlying hope within the community that future software updates might introduce broader customization options. Until then, makers and tech hobbyists will continue to bridge the gap between manufacturer specifications and personalized utility using the power of open-source design.