The gait metric your phone already records
You did not have to do anything to collect it, which is exactly what makes it useful.
Somewhere in Apple Health, under Mobility, there is a metric called walking asymmetry. It is the percentage of your steps where one leg’s timing differs measurably from the other’s. Your phone has been recording it every time you walk with it in a pocket, for years, and most people have never opened it.
It deserves better, for a reason most consumer metrics cannot claim: gait asymmetry has an actual clinical literature behind it. Menz and colleagues characterised gait variability and its relationship to stability in 2003; Brach and colleagues connected gait variability to functional decline in older adults in 2008. This is not a proprietary score invented to fill a dashboard.
What the numbers mean
Below about 1.5% is symmetrical gait — normal, with day-to-day noise that means nothing. Between 1.5% and 3% is mild asymmetry, the kind that turns up with fatigue, a minor muscle imbalance, or new shoes. Above 3% suggests active compensation: you are protecting something.
Age shifts the whole distribution. In the 20–40 range, 1–3% is typical. From 61–74, 3–7% is normal and not a cause for concern on its own. Past 75, 4–9%. An asymmetry of 5% means something quite different at 30 than at 78, and reading the same threshold across ages is the most common way to misuse the number.
Why passive collection is the whole point
Every metric that requires a test suffers from the same problem: you take the test when you feel like taking it. That selects for good days, and it selects against exactly the periods you most want to see.
Walking asymmetry has none of that. It is collected while you walk to the shop. Its baseline is built from hundreds of ordinary days, which means a deviation from that baseline is a real deviation and not a scheduling artefact.
This is what makes it a good early indicator. Compensation shows up in gait before it shows up in pain you would report. A runner developing a problem on one side will often show a rising asymmetry trend two or three weeks before they would describe anything as an injury — because the compensation is the early stage, and it is unconscious.
How to read it without frightening yourself
Three rules.
Trend, never a day. A single walk on an uneven pavement, carrying a bag on one shoulder, in unfamiliar shoes, will produce a spike that means nothing.
Compare against your own baseline, not the table. The reference ranges tell you what is typical; your own eight-week median tells you what is normal for you. A move from 1.2% to 2.8% is a large personal change while remaining inside the “mild” band.
Read it with its neighbours. Double support time — the fraction of the gait cycle with both feet down — rises when you are being cautious. Walking speed falls. Asymmetry rising while both of those also worsen is a different picture from asymmetry rising alone.
And the boundary that matters: this is a movement-quality signal, not a diagnosis. It can tell you something changed. It cannot tell you what, and a persistent change is a reason to see someone who can examine you, not a reason to read more graphs.
Walk Analytics and the mobility metrics library lay out the reference ranges with the papers they come from.