Your Garmin gives you a pile of running dynamics and almost no explanation of any of them. Vertical oscillation. Stride length. Ground contact time. And that one that says something like 50.4% / 49.6% with a coloured bar next to it.

Is one leg stronger than the other? Is something wrong? Should I be doing anything about it?

Ground contact time balance is the one I actually pay attention to, and after twelve months of Achilles surgery and rehab it’s been the most useful number on my watch. Here’s how to read it.

What it’s measuring

Ground contact time is how long your foot is on the floor each stride, measured in milliseconds. Usually somewhere between 200ms and 300ms, and it gets shorter as you get faster.

The balance figure splits that between your left and right leg and shows it as a percentage. Perfectly even is 50.0% / 50.0%.

Worth knowing before you go looking for it: not every watch reports this. Depending on your model you’ll need a compatible chest strap or a running dynamics pod, and some newer watches do it from the wrist. If you’ve never seen the figure, that’s why.

What counts as normal

Almost nobody is exactly 50/50, and you don’t need to be. Most runners sit within about one per cent either way, and Garmin’s own colour banding treats that range as balanced.

• Inside 1% — normal. Leave it alone
• 1% to 2% — worth watching, not worth worrying about
• Over 2%, consistently — that’s telling you something

The word doing the work there is consistently. One run means nothing.

Why one run tells you nothing

Loads of things shift this figure that have nothing to do with you being lopsided.

• Camber. Run down one side of a road and the crown of it tips you. Run out and back on the same road and it’ll swap
• Bends. Track sessions skew it every time, always the same way
• Pace. Balance usually tightens up as you speed up
• Fatigue. Late in a long run you’ll drift, which is actually the interesting bit
• Where the strap sits. Move it and the number moves

So don’t read a single run. Look at a month.

How to test yourself properly

If you want a number you can trust, control the variables.

Pick a flat, straight, out-and-back stretch. Run it at steady effort for twenty minutes. Do the same route, same pace, same time of day, once a week for a month. Then compare.

If the same leg is consistently doing more of the work across four sessions on a flat road, you’ve found something real. If it wanders about, you’ve found road camber.

What a real imbalance usually is

Almost always one of three things.

A strength difference. Extremely common and most people have no idea. Try eight single leg calf raises on each side and count honestly. Then split squats. The gap is usually embarrassing and it’s the most fixable thing on this list.

Something you’re protecting. If a leg hurts, even slightly, you’ll spend less time on it without deciding to. This is the useful early warning. A balance figure drifting before anything hurts is worth more than the same figure after.

An old injury you think you’ve got over. This is mine. Surgery on one heel in July 2025, and for months afterwards my body was quietly favouring the other side long after I’d decided I was fine. The watch knew before I did.

What to actually do about it

Not much on the run itself. You can’t consciously fix your gait by thinking about it, and trying usually makes you run worse.

You fix it in the gym.

• Single leg everything. Split squats, step-ups, single leg deadlifts. Start with your weaker side and match the stronger one to it, not the other way round
• Heavy calf raises, one leg at a time. Straight leg and bent knee
• Give it months. Strength differences don’t close in a fortnight

And if it’s pain rather than weakness, that’s a physio job, not a watch job.

The honest summary

It’s a proxy, not a diagnosis. It’s noisy, it’s affected by the road you’re on, and chasing 50/50 for its own sake is a waste of your time.

But as an early warning that something’s changed before it becomes something that hurts, it’s the most useful number your watch gives you. Mine’s been telling me the truth about my Achilles all the way to Valencia.

Watch the trend, not the run.

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If a left-right gap is what you’ve found, my strength and conditioning plan is built round single leg work and asks about injury history, so it comes back written for the side that needs it. £30.