Just about every smartphone, not to mention the coveted Apple Watch Sport and a plethora of other trendy fitness-specific watches and electronic wristbands, can help runners log their miles and fine-turn their foot-pounding workouts. But when it comes to trail running, Vail Vitality Center’s Ellen Miller believes that less is more, as far as gadgetry goes.

“To me, the heart-rate monitor is kind of the backbone gadget of coaching and training,” says Miller, who is the only American woman to have climbed Mount Everest from both Nepal and Tibet—and who, after hip replacement surgery in 2008, notched Lhotse and Nuptse to become the first American woman to complete the “Everest trilogy,” summiting three of the world’s highest peaks. “Also, physiological testing and a lot of metabolic testing are based on a person’s heart rate. Those are the numbers by which effort is valued. It’s reliable; it’s tested; it’s not gimmicky. I wouldn’t even call a heart-rate monitor a gadget. I would call it a tool.”

"The heart-rate monitor is kind of the backbone gadget of coaching and training. it’s reliable; it’s tested; it’s not gimmicky. I wouldn’t even call a heart-rate monitor a gadget. I would call it a tool.”

On twice-weekly summertime interval-training trail runs up Vail Mountain (7 a.m. Tue. & Thu., vailvitalitycenter.com) the Vail Vitality Center endurance coach encourages novice to elite trail runners, trekkers, and hikers to purchase and wear the Polar FT1, which the club sells at its store. With this simple, inexpensive monitor (a chest harness that wirelessly transmits heart-rate info to a wristwatch receiver), Miller first determines each client’s resting heart rate and theoretical maximum (roughly, age subtracted from 220 beats per minute). Then she uses those baselines on the trail to stay abreast of whether each athlete is working hard enough or too hard during minutes-long work intervals and is recovering quickly enough during rest periods. If someone logs abnormal numbers, she knows immediately that he or she may be dehydrated, sleep-deprived, or ill.

“If they can’t drop down below 110 after two minutes of rest, that’s something I need to know as a coach,” Miller says. “The more they learn about what their own body is doing, the better. I want to empower my athletes, teach them the science.”

(josiahmiddaugh.com) uses a software program called Training Peaks to track and analyze workout statistics. To document his own stats, he relies on the Suunto Ambit3 Sport HR watch.

Like Miller, Middaugh doesn’t put much stock in numbers such as mileage or calories burned; instead, he tracks his heart rate and the pace he keeps during various stages of his workouts. Lately, though, he’s been synching his watch with a brand-new trail-running gadget called Stryd. The device, designed in a lab in Boulder and instantly a Kickstarter rage this spring, attaches to a waistband and tracks power output using a complex algorithm measuring the force of footfalls over different types of terrain.

“A lot of those devices, the Fitbit, the Jawbone … are more for the fitness person trying to stay active and lose weight: it quantifies a number for how much they’ve done,” Middaugh says. “But for a competitive athlete, that’s not precise enough to tell you how hard you’re working.”

Indeed, most fitness apps don’t account for factors specific to trail running, such as how one’s pace can slow significantly when running uphill. Once it hits the fitness market after its Kickstarter testing phase, Middaugh predicts, the Stryd will become a trail-running game-changer for pro athletes like him, as well as for ordinary enthusiasts you’ll find huffing up and down every footpath in this valley.

“It’s so new now, but in the future I think I’ll be prescribing intensity levels based on that device,” Middaugh says. “My theory is that it’s going to be an easier way to quantify how hard people are working while they’re running. It’s the best we can do because pace doesn’t mean much for running unless we’re on a track.”

So until then, to track how hard you’re running, both Middaugh and Miller recommend using a simple heart-rate monitor. But, they add, don’t forget to tune into your own body first and foremost.

“People become so dependent on technology,” observes Miller. “It’s important for my athletes to have endurance days. I make them take all the technology off and say, ‘We’re going to connect with nature, what’s beneath our feet, what’s around us. No headphones, no monitors.’ I believe that trail running is a soulful activity.”

Either augmented with or unshackled from technology, it’s also clearly good for the heart.