Every endurance athlete fears the dreaded bonk. There you are, running without a care in the world, when all of a sudden your body crashes. Abruptly, you metamorphose from a capable person into an immobile sea anemone, your useless tentacles waving wildly.
Marathoners call it “hitting the wall”, but it can happen in any sport for any number of reasons. Maybe your blood sugar crashed or you forgot to sip water. Maybe the wind was much harder than it was in training, or your new protein bars don’t have as many calories. Figuring out the precise amounts of fuel you need, and when, can take years of experimenting.
Fitness wearable company Polar thinks it has figured it out. Its latest sports watch is geared toward serious endurance athletes, with a new feature called FuelWise that allows you to program smart or manual reminders to eat or drink. It has a few big flaws, but I still like the Grit X a lot.
If all fitness wearables were high school stereotypes, Garmin would be the burly jock, Fitbit the prom queen, and Polar the lovable nerd busily programming their graphing calculator.
The Grit X has built-in GPS and Glonass, a compass, altimeter, and optical heart rate monitor. It records extremely granular data in its accompanying web and app service, called Polar Flow, which syncs with popular health platforms like Apple Health, Strava, and Nike+ Run Club.
Polar Flow isn’t the most attractive app or the easiest one to use, but on the watch, it’s one of my favorite training platforms. The training program, called FitSpark, is adaptive, which means it adjusts according to the exercise you've done or want to do.
First, you take the onboard fitness test, which estimates your aerobic fitness by measuring your heart rate and heart-rate variability while you're lying down, and takes into account other factors like your age, gender, height, and body weight. It's a (very rough) estimate of your VO2 max, a measurement that our editor in chief, Nick Thompson, calls "the size of your car's engine."
Once the Grit X has measured your fitness level, it will determine which workouts can maintain or improve it. It remembers my previous exercises and suggests that today might be a great day for strength conditioning versus cardio or stretching. If I click Yes to improve my strength today, it suggests bodyweight training. If I don't have my own routine, it will suggest specific exercises, like squats or sit-ups.
Unlike most fitness platforms that are very prescriptive—you either pay attention to the recommendations or you don't—the Grit X will accommodate any decisions I choose to make, whether I want another day of cardio or different strength-building exercises. It also took into account my sleep patterns (or “nightly recharge” in Polar-speak) to recommend what kind of exercise and intensity of workout to do.