80/20 Running: Run Stronger and Race Faster by Training Slower

by Matt Fitzgerald

Cardiac lag has important implications for heart rate monitoring during workouts that include changes in pace. For example, suppose you are doing a run that features six intervals of thirty seconds in Zone 5 with two-minute recoveries in Zone 1 between intervals. When you start the first interval, you will accelerate abruptly, and your heart rate will begin to climb. But chances are your heart rate will not actually reach Zone 5 until the very end of the thirty-second interval, if even then. That doesn’t mean you failed to do the interval at the right intensity. As long as you were running fast enough that your heart rate would have reached Zone 5 if you had continued at that pace, then you did it correctly. Likewise, when you slow down for recovery at the end of your first thirty-second Zone 5 interval, your heart rate will begin to decrease. But because of cardiac lag, no matter how much you slow down, your heart rate might not get all the way back down to Zone 1 before your two-minute recovery ends and it’s time to start the next interval. Again, this doesn’t mean you screwed up. As long as you jogged slowly enough that your heart rate would have dropped down to Zone 1 eventually, you did it correctly.