Spot Check: The simplest way to estimate your black-hole zone is to do the "talk test." Go for a run or a ride with a heart-rate monitor and a buddy, and slowly pick up the pace. Have a conversation or, if alone, say the Pledge of Allegiance. At the point where it becomes difficult to talk in complete sentences, check your heart rate. That's pretty close to your lactate threshold. Take that rate, add 6 percent, and that's the range to steer clear of. (For example, if your threshold is 150 bpm, your black hole will be 150 to 160 bpm.) Next, set your heart-rate monitor to beep when you get close to that zone, so you know when to back off.
Full Test: To get a more precise sense of your black-hole zone, follow these steps:
1. Do a 30-minute all-out effort wearing a heart-rate monitor.
2. Average your heart rate over the last 20 minutes of the workout. That number is your lactate threshold.
3. Take that heart rate, add 6 percent, and stay out of the range in between.