The question that haunts runners after a race—good or bad, personal best or worst—is, “Could I have gone faster?” Even after Health - Injuries, fans wondered if she could have chased down Brigid Kosgei of Kenya for the silver. After all, Kosgei finished just 10 seconds ahead in 2:27:36. The answer might be found on Seidel’s Strava, where she posted an unofficial summary of her race in Sapporo. According to the heart-rate data captured by Seidel’s Coros Apex watch, it doesn’t look like she could have pushed much harder for those last few miles. Her heart rate soared past 180 bpm, holding steady at her near maximum right until she crossed the line. In theory, Seidel was firing on all cylinders.
More recently, at the Millrose Games, some athletes wore Whoop monitors while competing, which broadcast their heart rates for TV viewers. As a result, race announcers could speculate as to whether an athlete had enough gas in the tank to catch a competitor on the bell lap, or if they were totally spent by the final stretch. But heart rate is just one measure of effort that we can use to assess how hard we’re working. Another option is a feature new to Garmin’s Fenix 7 line of GPS watches. It’s called Real-Time Stamina, a data screen that shows you how long you can hold your current speed. It basically estimates your capacity. Push too hard, you’ll blow up sooner. Go too easy and you’ll end up with a slower finishing time. Of course, that raises questions: Will it make us all robotic, following a prescribed metric? And does already knowing our finish time take all the fun out of racing?
How Does Real-Time Stamina Work?
Health & Injuries: VO2 max, maximum heart rate, and your recorded log of activities. To get the most precise estimates, you’ll want to ensure those inputs are as accurate as possible. Anytime you go for a run using GPS, the watch captures your pace and heart rate to ballpark your VO2 max—that’s the minimum information required to start estimating your stamina. Manually input your max heart rate for the best results. And finally, wear the watch as much as possible. That means recording all your activities, whether it’s a run or a yoga session, and keeping it on your wrist when you snooze. Why? The device tracks your daily strain and training load, as well as your sleep quality, to assess how well you’ve recovered, and accounts for that fatigue the next day. It’s unlikely you’ll start every run with full bars.
The stamina data screen gives you a few metrics to view in real time. The first is “potential stamina,” which is how much capacity you have at a moderate effort level. It’s expressed as a percentage and shown as a black bar. Think of this as going for an easy jog. Then there’s “stamina,” which is more detailed, based on how hard you’re running—go faster and this percentage drops faster, too. (This green bar overlays the black bar as seen below.) Percentages, however, aren’t all that helpful without context, so the watch can also show you distance or time remaining at your current effort level.
Benefits for Racing
If you’re running a marathon, you might see that easing off the pace could help you avoid hitting the wall. Runner-in-Chief Jeff Dengate has been testing the stamina function on the short-but-steep hills of Easton, Pennsylvania, while training for mountain races this summer. For Jeff, pace becomes meaningless when the road slopes upward, so in the past, he would use heart rate to lock into an effort. Now, with the Fenix 7, he tries to keep the green bar from dipping too far away from the black bar as he climbs.
Where some runners will find the real value in this feature is during longer races that demand a high-intensity effort. The The Best Running Watches for Mapping and Pacing, for example, is an all uphill climb to the top of the tallest peak in the Northeast, with 4,650 vertical feet of climbing. It’s 7.6 miles long and hits portions of 22 percent grade. When Jeff ran it in the past, he tried to keep his heart rate around 160 bpm—any higher and he’d soon slow to a hike. But, finding that sweet spot came as a result of a lot of practice. With the stamina feature, Jeff can zoom in on a portion of the climb and judge whether he can maintain his steady rhythm on the stretches that get steeper, or if doing so will bottom out his potential stamina bar and tack minutes onto his finish time.
Smarter Fueling
Garmin’s real-time stamina measurement can take some guesswork out of race efforts and training paces. It may even be able to predict your finish time. But what if it’s something else, like your fueling, that you want to keep tabs on? Selene Yeager (a.k.a. Bicycling’s “Fit Chick”) has been an endurance athlete for two decades. She’s competed in everything from Ironmans to gravel bike races longer than 200 miles, and her biggest barrier to racing success was proper fueling. For her, a Continuous Glucose Monitor (CGM) was a game-changer.
CGMs have been around for quite some time, mostly used by people with diabetes to manage their blood sugar levels throughout the day. But athletes are leveraging the devices to determine what to eat, when. In a sense, a CGM can act almost like a fuel gauge in your car to show how much gas you’ve got in your tank.
The CGM itself is worn on the back of the upper arm and registers the concentration of glucose circulating in the fluid surrounding your body’s cells. The quarter-sized device uses a hair-thin needle that feels more like a mosquito bite than a shot when applied. What’s new isn’t the wearable monitor itself, but the smartphone apps that interpret the data. Yeager spent months testing Supersapiens, a CGM app awaiting FDA approval, with the capability to show glucose levels in real time on your smartphone screen.
Why Use a CGM?
Understanding big spikes and drops can help you eliminate GI issues or the feelings of a bonk, like fatigue and dizziness. Most of the time, you’ll want to keep your levels stable—but not always. For example, you’ll want to top off your muscles’ glycogen stores before a race, which usually requires a rise in your blood sugar. But you need to time your prerace meal correctly so you don’t toe the line with a full stomach or as you’re crashing from a sugar high.
You can also see the impacts of different fueling methods, like “dripping” versus “dumping.” This means taking in small bites of food or gels very regularly instead of eating larger amounts all at once. Ultrarunners usually figure out which strategy works best for them through trial and error—and more than a few pitstops in the bushes. (Courtney Dauwalter finds the steady-drip strategy works best for her; the opposite proves effective for Coree Woltering.) A CGM can help you more easily figure out the technique that works best for you personally, since two athletes can eat exactly the same foods and experience different blood sugar responses.
“If you’re cruising along several hours into a long event and you start to feel off, checking your glucose levels can help you determine if you need food or if it’s something else,” Yeager said. “That way you don’t automatically shovel in a packet of energy blocks and find yourself with a case of gut rot.” Having less food in the stomach means there’s less to digest, which lets blood flow go elsewhere in the body.
Of course, more than just blood sugar affects running performance. For example, glucose levels are neither indicative of glycogen storage (except when glucose is critically low), nor a strict measure of energy availability. Even when your glucose levels are in ideal ranges, you can still feel lousy during a workout. “Sometimes I’d feel draggy toward the end of a run and see my glucose falling in real time. Other times, the sensations in my legs—heavy or light—didn’t match the glucose readings,” Yeager said.
For that reason, Yeager thinks the real value of a CGM is its ability to help you design a fueling strategy around your training. (Using it in real time can be useful, but it’s not always convenient to check your glucose midrun.) Pin down your best prerace meal, determine how many gels you actually need during a marathon, nail your favorite recovery snack, and then go back to training and racing CGM-free. That said, if you get hooked on the tech, there’s no reason you can’t keep wearing one on a regular basis.
Best Running Shoes 2025 Runner's World, guiding the brand's shoes and gear coverage. A true shoe dog, he's spent more than a decade testing and reviewing shoes. In 2017, he ran in 285 different pairs of shoes, including a streak of 257 days wearing a different model.
