Our heart rates fluctuate much more than many people might guess, according to Men's Health. In fact, when your body is stressed (from lack of sleep, stress, or for any other reason), the heartbeat is actually more likely to be regular than when you're at your healthiest. This irregularity is called heart rate variability, or HRV, and some cardiologists think that tracking it could result in more effective workouts. Several professional sports teams have even planned their workout programs around the concept.
As Aaptiv explains, "the heart's ability to vary the duration of time between beats is indicative of its ability to reflect changes in the rest of the body." Men's Health says "you can give yourself an athletic edge by tracking and responding to changes in your HRV," using one of the many wearable fitness trackers available today. The wearables give you an HRV score which indicates whether it's at a high level (you're in good shape) or whether it's been lowered by some stress factor.
When you work out while your HRV is higher, you'll be able to exert more energy and will be more likely to see better results than if you work out on a low-HRV day. On these days, experts advise doing some low-impact activity instead of something more strenuous like you could on a high-HRV day.
Studies have proven this theory. One study conducted on cyclists in Spain found that the athletes who trained based on their HRV experienced 14 percent more improvement than their counterparts who did not take HRV into account for their workouts.