Imagine this scenario: You’re 20 seconds into a hard 400-meter lap on the track when your watch flashes a live pace 15 seconds per mile slower than your target. You surge to fix it. A few seconds later, the reading shows you’re now going 10 seconds faster than your goal, so you back off. By the end of the rep, you have spent more energy reacting to your screen than actually settling into the intended effort.

Sound familiar? Ultramarathon runner and USA Triathlon-certified coach Nick Farr , owner of P3 Fitness, calls this the “yo-yo runner” problem.

On most watches, current/live pace is based on recent GPS data, so it can lag behind rapid changes in speed.…