Ground Contact Time and Left/Right Asymmetry in Running

Of all the metrics a modern running watch throws at you, ground contact time is one of the few worth looking at, and the one runners most consistently misread. It is the number of milliseconds each foot spends on the ground, typically somewhere around 200 to 300 for a recreational runner at easy pace and considerably lower for elites at race pace. It is easy to measure, it responds to training, and it connects directly to how efficiently you run.

It is also routinely turned into a target, which is where it goes wrong. The useful signal in stance time is not “make this number smaller.” It is the difference between your left and your right, and what that difference costs you.

What ground contact time actually is

Stance time, or ground contact time, is the interval from initial contact to toe-off for one foot. Its counterpart is flight time, the airborne phase. Together they make up your stride, and cadence is just the rate at which the whole cycle repeats.

Contact time falls as you speed up, which is the first thing to understand before comparing your number to anyone else’s. A 300 ms contact on a recovery jog and a 300 ms contact at 5k pace mean completely different things. Any comparison, including a comparison with your own previous runs, has to hold pace roughly constant or it is measuring pace, not mechanics.

Is shorter ground contact time better?

Broadly yes, but not as a target. Faster runners do have shorter contact times, and shorter contact tends to accompany better running economy. A study of female Kenyan runners found a significant relationship between ground contact time and running economy, and contact time was among the few stride characteristics that tracked with both economy and maximal speed.

The complication is that the relationship is not a straight line. Work looking directly at metabolic cost found a U-shaped relationship between contact time, leg stiffness and energy cost, consistent with runners self-optimising toward their own best combination. Push contact time below where your current leg stiffness wants it and cost goes up, not down.

Which gives you the practical rule. Shorter contact time is an outcome of getting stronger, stiffer and faster. It is a bad thing to chase directly, in the same way that trying to consciously hold a lower contact time on a run mostly produces a tense, expensive stride you cannot maintain.

The number that actually matters: the gap between your legs

Here is where stance time earns its place. Contact time is a timing measurement, which makes it far more robust than joint angles. It does not need calibration, it does not care much about camera angle, and both of your legs generate one every stride under identical conditions. That makes left-versus-right contact time one of the cleanest comparisons available to a runner without a lab.

And the gap is not free. In a controlled study, runners were driven to specific step time asymmetries with a metronome while their metabolic rate was measured. Every 10% increase in ground contact time asymmetry raised net metabolic power by 7.8%. Every 10% of step time asymmetry cost 3.5%.

Sit with the size of that. A 7.8% increase in energy cost is far larger than the effect of the best racing shoes ever made. Asymmetry is expensive.

Your asymmetry is a tax you pay on every single stride, and you pay it in energy long before you pay it in tissue.

Does asymmetry cause injury?

Probably not directly, and the honest answer here matters because plenty of apps sell the opposite.

The best available test comes from a secondary analysis of a randomised trial: 836 recreational runners were measured on an instrumented treadmill and followed for six months. Gait asymmetry in spatiotemporal and kinetic variables did not increase running-related injury risk. Runners who went on to get injured did not show meaningfully different baseline asymmetry.

That is a real result and we are not going to talk around it. Asymmetry is not a disease, and finding some in your data is not a prediction that you are about to break.

What asymmetry is, and this is still worth a great deal:

1. A measurable performance cost, as above.
2. A pointer to a capacity gap. Legs do not spend different amounts of time on the ground for no reason. The usual causes are a genuine strength or stiffness difference, a previous injury the body has quietly worked around, or a hip that is not controlling the pelvis as well on one side.
3. The most trustworthy thing a phone camera can tell you, because it is a within-clip comparison rather than an absolute measurement.

So treat it as a lead, not a verdict. It tells you where to point your strength work, not that you are injured.

How much asymmetry is normal?

Some. Perfect symmetry is not a thing that exists in runners, and chasing it is a mistake.

When researchers quantified bilateral asymmetry across competitive, recreational and novice runners, symmetry indices varied enormously by variable, from under 1% for stride time up to more than 20% for loading rate. Timing variables are the most symmetric things about human running; force variables are much noisier.

That gives you a rough reading frame. For contact time specifically, small differences of a percent or two are ordinary and should be ignored. A consistent, repeatable gap that shows up across multiple runs, especially one that widens as you tire, is worth investigating. And a step change from your own baseline matters more than any absolute threshold, which is the argument for measuring it more than once.

Where a stance-time gap comes from

Four common sources, roughly in order of how often they turn out to be the answer:

• Unilateral strength difference. Usually calf and glute. The weaker side spends longer on the ground because it cannot generate force as quickly. This is the most common and the most fixable.
• A previous injury you think you have finished with. Ankle sprains and calf strains in particular leave behind strength deficits that persist long after pain resolves, and the body compensates silently.
• Poor lateral hip control on one side. If the pelvis drops during stance, the leg effectively lengthens the time it needs. See contralateral pelvic drop.
• Leg length difference or a structural asymmetry. Real, but far less common than the internet suggests, and usually the explanation of last resort after the trainable ones are ruled out.

Notice that three of the four are capacity problems, which is exactly why the response to an asymmetry finding is strength work rather than a form cue.

How to measure it

With a watch or pod. Garmin’s running dynamics, Stryd, Coros and others report ground contact time and a left-right balance. It is convenient and continuous, which is its real advantage: you get it on every run rather than in a one-off test. Take a single run’s balance figure with scepticism and look at the trend over ten runs.

With a phone. Film yourself side-on at 60fps or higher, at the end of a run, and count the frames each foot is down. At 60fps each frame is about 17 ms, which is coarse but adequate for spotting a real difference. Do several strides per side and average, because stride-to-stride variation is large.

Test tired. This is the part people skip. Asymmetry tends to be smallest when you are fresh, which means a first-mile test is the one most likely to tell you everything is fine. Film at the end of a normal run, not the start of one.

What to do about it

Train the difference out rather than trying to run more evenly.

1. Go unilateral. Single-leg Romanian deadlifts, split squats, step-downs, single-leg calf raises. Bilateral lifts let the strong side hide the weak one, which is precisely the pattern you are trying to break.
2. Do the weak side first, and give it more. A common approach is to work to a number of quality reps on the weaker side, match it on the stronger, then add an extra set on the weaker only.
3. Include calf work specifically. Heavy bent-knee calf raises for the soleus. It is the tissue most directly responsible for how long your foot stays down and the one runners neglect most.
4. Add strides or short hills. Contact time responds to fast running, and hills load the propulsive chain hard without the impact cost of track work.
5. Retest in six to eight weeks, under the same conditions and at the same pace. Same run, same fatigue level, same camera position.

The full programming detail is in strength training for runners.

Keep it honest

Ground contact time is a genuinely useful metric that gets ruined by being treated as a score. Shorter is not automatically better, symmetry is not a diagnosis, and no watch number tells you why one leg behaves differently from the other.

What a left-right gap does do is point. It says look at this side, and it says the reason is more likely to be capacity than technique. That is a much better place to spend your effort than another lap of the shoe shop, and it is the same logic behind everything in running gait analysis.

RunGait reads exactly this kind of comparison from a short phone video: it tracks both legs in the same clip, reports the difference between them rather than a falsely precise absolute number, flags each signal with the confidence it actually deserves, and turns the strong ones into a strength block for the side that needs it. Then you film it again in six weeks and find out whether the gap closed.