A phone in your pocket. A watch on your wrist. AirPods playing a song. That's three motion sensors at three points on your body — a distributed gait lab you're already wearing, right now, while you walk to the kitchen. You don't need a new device. You need the software that reads all three at once and turns them into one measured gait signal.
Branch Ergonomics, a work-wellness app on the App Store today, reads the motion sensor inside your AirPods to catch a slouch in real time and buzz you to sit up. It uses CMHeadphoneMotionManager — the exact head-tracking IMU Apple built for spatial audio. The lesson is the whole thesis: the head is now an instrumented body segment on a device hundreds of millions of people already wear. If that sensor is good enough to detect posture, it is good enough to measure how your head moves while you walk. The sensor shipped. The gait application is open.
Single-point sensing gives you a number. Three points at different segments of the body give you coordination — the thing a clinician actually reads when they watch someone walk.
The head sits atop the kinetic chain and houses the vestibular system — the body's own balance sensor. How smoothly the head travels through a stride is a direct read on gait stability.
In a pocket, the phone rides near the body's center of mass — the workhorse location of the entire phone-gait literature. This is where speed and rhythm live.
The wrist captures arm swing — and reduced or asymmetric arm swing is one of the earliest, most sensitive markers of a gait or neurological change.
One IMU tells you how fast. Three IMUs at head, pelvis, and wrist tell you how well — because gait quality is a coordination problem, and coordination only shows up across body segments.
Head-vs-pelvis decoupling — a stable walker's head stays quiet while the pelvis sways; when the head starts tracking every wobble, stability is failing. Arm-swing vs trunk symmetry — a limp or a one-sided change shows as a mismatch. Three independent cadences that agree — confidence you can't get from one clock. This is the difference between a pedometer and a gait assessment.
Apple hands you a pre-cooked walkingSpeed scalar. We ran the method on real Apple Health data and watched that scalar arrive mislabeled — imperial values tagged as metric. Raw tri-IMU means you compute speed, symmetry, and smoothness yourself, validate them against the published method, and never inherit a black box's unit bug. End-to-end signal you control.
Both streams surface through the phone's Core Motion at once — a synchronized two-point capture (pelvis + head) is buildable now, no watch required. The phone is the master clock; AirPods samples arrive phone-timestamped.
The wrist adds arm swing via a small watchOS companion that streams to the phone and aligns to under ~100 ms — ample when footfalls land roughly once a second. A third point, not a blocker to shipping the first two.
AirPods head-motion runs while the AirPods are in and the app is active (~25 Hz; it pauses for calls). So this is a 60-second guided walk — "pocket the phone, keep your AirPods in, walk the hall and back" — not silent background sensing. A weekly measured walk is the clinical cadence anyway.
Three phones-and-earbuds IMUs are not an instrumented gait lab, and this is decision support, not a diagnosis — the same honest posture as the method. It is tuned for trend and coordination, where consumer sensing is genuinely strong.
New sensors don't change the method — they enrich what it reads. The 3-point capture is the differentiated middle tier between the passive phone read and clinical hardware.
Passive, zero effort, pre-cooked Apple Health mobility metrics. The on-ramp — running now at healthgait.com/check.
A guided walk, raw multi-IMU from AirPods + iPhone + Watch, computed from first principles. The layer you already wear — and the one nobody else is building.
Plantar-pressure insoles, powered assist — from partners, when the clinical bar rises. Same method, deeper signal.
Earbuds, a smart watch, a shoe with a sensor — each is a stream of raw motion that, alone, doesn't screen, doesn't bill, and doesn't carry a defensible claim. This capture architecture fuses worn devices into one gait signal and runs it through a published method — a number, a risk band, a decline flag, a reimbursable event. Bring your device, or ride the three your customer already owns.
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