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Runners are often told to set their treadmill at a base incline of 1 percent. The reason is that, due to the lack of wind resistance, heart rates are slightly lower on the treadmill than they are outdoors. The 1 percent “correction” is intended to make running on the treadmill as challenging as running outside.
There’s only problem: Heart rate does not determine how challenging a run is; conscious perception of effort does. And it just so happens that perception of effort is higher during treadmill running than it is outdoors at any given speed and incline. For this reason, it’s unlikely that you would be able to match your outdoor 5K time (or your time for any other distance) on a treadmill.
Setting your treadmill at a base incline of 1 percent will only exacerbate this discrepancy. So instead leave it at 0 percent unless you’re looking for the specific challenge of running uphill.