TBKFiT BoKi4: Every Strike, Every Limb

Four sensors, one on each wrist and ankle, and an app that turns what they measure into coaching. We took TBKFiT's BoKi4 from an idea to a product in mass production, and we're still building its app.

The Brief

TBKFiT wanted combat athletes to get the feedback a private coach gives: how fast, how hard and how accurately each hand and foot is landing, alongside heart rate, live on their phone.

Most fitness wearables track one wrist. Boxing, kickboxing and MMA need all four limbs measured at once and kept in sync, under wraps and gloves, through rounds of sweat and impact.

The Hardware

Each BoKi4 module carries a 9-axis IMU (accelerometer, gyroscope and magnetometer) sampling at 100Hz, plus a PPG heart-rate sensor with combined red and green illumination sampling at 50Hz. All four modules stream to the phone over Bluetooth 5.3 LE with under 50ms of latency, so a strike shows up in the app almost as it lands.

The module measures 34 × 28 × 9mm and weighs 22g, in an aerospace-grade aluminium housing rated IP68 against sweat, rain and rinse-down. It sits on a breathable elastic strap that fits wrists and ankles from 140 to 220mm. Reading a pulse optically through a strap that's moving at punching speed takes careful work on the sensor window, its placement against the skin and the filtering behind it.

Four sensors means four things to charge, so we designed a case that does it all at once. The sensors drop onto magnetic contacts, an LED shows charge and connection status, and the case's own battery holds five or more full recharges for the whole set. It charges over USB-C.

The App

We built the TBKFiT app for iOS and Android. It pairs all four sensors and breaks every session down limb by limb, showing contact rate, power, speed and hit count for each hand and foot.

Around the data sits the training: programmes from TBKFiT's coaches, an AI trainer that turns your numbers into drills and corrections, a full workout history, and weekly leagues with promotion and demotion zones that give people a reason to train again tomorrow.

Into Mass Production

Plenty of hardware works as a prototype and stalls on the way to the factory. We took BoKi4 through design for manufacture with our production partner in Shenzhen, and we now manage manufacturing on TBKFiT's behalf, including time on the factory floor alongside their team.

Still Building

Launch isn't the end of our involvement. We keep developing the app, adding features and refining the tracking as athletes put BoKi4 through real training, so the product gets better after people have bought it.

BoKi4 is launching soon. Find out more and join the early access list at www.tbkfit.com/boki4.