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In the former Soviet Union, global sports domination was so vital to the propaganda machine that the regime employed an army of 5,500 elite sports doctors to analyze every aspect of their athletes, from heart rates to sleep cycles to cortisone levels. “It was very complicated,” says Val Nasedkin, a Russian track-and-field athlete during the 1980s. Nasedkin, who went on to study sports science and became a coach with the , recalls dreading the testing process. “You had to go to the lab, where you’d exercise almost to exhaustion while they took muscle-tissue samples, blood samples, hormone panels, and so on.”

Of course, the system was rudimentary at best. It took weeks to get results, and the data was hard to interpret. Wouldn’t it be great, Nasedkin and his colleagues thought, if the information could be collected instantly, in real time?

Fast-forward 25 years and the dream has become reality. A new generation of fitness-tracking platforms let even amateur athletes effortlessly collect reams of personalized training info. The platforms combine data-collection devices like wristbands and chest-strap monitors with sophisticated apps that track everything from sleep cycles to cardio patterns to blood-sugar levels. Proponents insist the number crunching can make you healthier and happier (and faster and stronger) by allowing you to calibrate your workout regimen to the inner workings of your body. Still, all that data poses a singular question: Which metrics are worth paying attention to? As researchers like Nasedkin will agree, the first step is determining what your goals are.

If what you’re looking for is an overall health boost, the current wave of wristband trackers—, , and —will give you a baseline measure of how much physical activity you’re getting each day. Unlike the cheap pedometers of yore, these devices are powered by robust accelerometers that detect motion in three dimensions. But their biggest advance is in usability: they’re small enough to wear 24 hours a day, and they sync effortlessly with smartphone apps. More important, they provide a simple tally—Nike calls it a Fuel-Score—so users need to compare only a single data point from day to day. “People like to see how they’re progressing,” says Trent Stellingwerff, a physiologist at the Canadian Sports Institute Pacific. That desire alone is enough to get you active.

If you’re more motivated by competition, look for something that quantifies your effort rather than just your distance. ’s new chest-strap-mounted Armour39 tracker combines heart-rate data with motion sensors to calculate a real-time “willpower” score. “Our vision was a single number that tells you how hard you’re working, no matter what the sport is,” says Christy Hedgpeth, Under Armour’s head of digital sports. Nike’s FuelBand and offer similar cross-sport scoring systems, letting you track your fitness output across activities. They also allow you to compare scores and compete with friends and worldwide leaderboards, basically making a game of working out.

But it’s a third category, which aims to help you maximize your training—telling you when to push hard and when to slow down—that represents the boldest leap yet. “This is the holy grail, but it’s also a black hole,” says Shona Halson, who heads the performance-recovery division at the . Over the years, scientists have struggled to pin down the physiological indicators of overtraining, like heart-rate variability (the fluctuations in the time between heartbeats) and stress-related hormones like cortisol.

For coaches, two of the more trusted indicators of overtraining are mood and sleep cycle—and naturally, there are apps for those. With Moodscope, which keeps daily tabs on your emotions, you use a virtual deck of cards to rate feelings like alertness and nervousness. A sustained downward trend is a sign that you should probably back off. For sleep, there are a handful of top-end trackers that detect various stages, like REM and deep sleep, but Halson uses a simple wristband accelerometer to measure sleep time in her athletes. She’ll watch for patterns of disruption and suggest tweaks in bedtime habits, caffeine consumption, and training.

Perhaps the most ambitious personal performance-data technology is Omegawave, a “physiological readiness” tracker that Nasedkin and a group of ex-Soviet sports scientists spent 25 years developing. Strapped around the chest like a heart-rate monitor, Omegawave takes two minutes to measure cardiac variability (which tells you if your heart is still recovering from exercise stress), run a simplified EKG to assess the aerobic and anaerobic systems, and perform a “DC potential” assessment of the nervous system (to gauge electrical activity in the brain and determine exercise-stress levels). It then gives advice about the type and intensity of training you’re prepared to handle.

Of course, focusing too much on data can be counterproductive. For example, many people get stressed about sleep. “If you wake up and go, ‘Oh man, I slept bad,’ you already know you slept bad.” says Halson. “You don’t need something telling you that.”

Halson’s advice? Focus on the trends rather than the daily values, whether you’re measuring step count, sleep time, or even willpower. Does the data indicate that you’re improving compared with last week, last month, and last year? “It’s a fine balance,” Halson says. “You want to pay attention to it but not obsess about it.”     

Stay on track with a few of these techy fitness helpers

BEST FOR: Overall Health Boost
: Wear the wristband to monitor activity and sleep. Use the software—or sync with apps like RunKeeper—to parse the data and spur a healthier lifestyle. ($130)

: The wristband logs distance, calories burned, and hours slept, among others. Upload the data to your PC or smartphone to chart activity over time. ($100)

BEST FOR: Fueling Your Competitive Fire
: A chest strap measures heart rate, calories burned, and movement to calculate a “willpower” score. Compare your efforts over time and post them online. ($150)

: Attach the device to your shoe to collect pace, distance, and acceleration data, then compare with friends and online leaderboards. ($70)

BEST FOR: Maximizing Your Training
: The device tracks seven physiological markers, and the app uses a proprietary formula to assess readiness for various intensities of exercise. ($100)

: It sounds low-tech—a virtual card game measures your moods—but this online app is an effective way of figuring out when to push and when to rest.