Short ground contact time isn't always faster — and neither is low vertical bounce
The wearable industry trained a generation of runners to chase a single number: get your ground contact time below 210 milliseconds, like the pros. The science is messier than that.
A 2019 study in the Journal of Experimental Biology split sub-elite runners into two groups by duty factor — basically the share of each stride spent on the ground. Low-duty-factor runners had short ground contact and bounced higher. High-duty-factor runners had longer contact and stayed flatter. Same pace. Same energy cost. Two completely different strategies, both economical.
The short-contact group recycled elastic energy through more vertical motion and stiffer legs — they ran like springs. The long-contact group kept oscillation low and pushed forward instead of up. Neither is wrong. They're different solutions to the same problem.
Where the data does matter is in asymmetry. Studies of GCT imbalance — one foot consistently on the ground longer than the other — find a strong link to impaired economy. That's a useful signal because it points to a real cause: a weak glute, a tight hip, an old injury you're guarding around. Chasing absolute GCT numbers won't fix it. Chasing balance will.
Vertical oscillation is the same story. A few studies that asked runners to consciously reduce bounce found running economy got worse, not better, because they were fighting their natural elastic mechanics.
Practical takeaway: stop trying to copy elite biomechanics. Watch for asymmetry between left and right, watch for your numbers drifting late in a run, and address those with strength work and form drills — not with a GCT target.
Source: Patoz, A., Lussiana, T., Gindre, C., & Mourot, L. (2019). Journal of Experimental Biology, 222(6).
RunNerd reads ground contact time and vertical oscillation from your watch on every run, but it doesn't push you toward an "elite" GCT number. Instead it tracks left/right GCT balance and how those numbers drift inside a single workout — GCT imbalance and intra-run drift are the signals that actually correlate with breakdown. If your easy-pace GCT starts creeping up 20+ ms over the second half of a long run, the coach flags it as form fatigue and prescribes shorter intervals or hip-stability work before recommending more mileage.