What is it?
Ground Contact Time (GCT), or Ground Contact, is the amount of time per stride that a runner's foot is touching the ground, in milliseconds. The starting trigger of a runner's ground contact time will vary depending on the runner's gait (heel-strike versus midfoot-strike versus forefoot-strike) but will always end at toe-off (full body removed from the ground, now in the swing phase of the gait cycle).
Why does it matter?
Ground Contact Time has an inverse relationship with metabolic power, meaning a lower Ground Contact Time is typically an indicator of a higher metabolic power (within an individual rather than across different runners). It is best used as a metric to monitor trends, rather than to aim for specific values (i.e. striving to attain a GCT of 165ms). A runner may experience upward trending GCT values as they fatigue near the end of a long run. They may also see Ground Contact Time decrease over the course of a training cycle as the runner becomes stronger. Other factors that will affect a runner's GCT are shoes (cushion level and heel-to-toe offset), pace, and cadence.