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AT WHAT POINT does a gizmo make you stupid? When you start making cell-phone calls from chairlifts? When you turn to your GPS instead of the local gas station for directions? A gizmo put a “stupid” sign on my back this past November. I was on a run, trying out Polar's brand-spanking-new S510 heart-rate monitor (HRM, in fitness parlance)—$260 worth of chest-belt transmitter and wristwatch—and its myriad functions prompted a frantic button-pushing, run-aborting meltdown. In my own defense, I had reason to get confused: This little beauty, all curvy lines and sexy displays, tells you not only your current heart rate, but your resting heart rate, your estimated maximum heart rate (MHR), and your percentage of MHR, as well as high, low, and average rates. You can program ideal heart rates for various portions of your slog, and the S510 will enforce these zones with pleasant little beeps when you push too hard or, in a fit of slackness, back off too much.

Wait, there's more! You also get cycling-specific functions (average speed, max speed, trip distance, total distance, and an optional cadence feature), as well as the requisite bike attachments to collect the data. There's the first-ever self-administered VO₂ max test (derived from heart rate). There are timers within timers, a bevy of calorie-crunching functions, and a proprietary “SonicLink” that chirps a month's worth of workouts into your personal computer for ever-more-anal record keeping. I'm not sure, but I think the S510 will also file your taxes.

For those who claim such high-tech gewgaws are a sign of civilization in decline, a retreat from the purity of sport, this would surely be Exhibit A. Yet with 5.5 million folks nationwide using HRMs, including everyone from trail runners to circuit trainers, these devices are no longer secret weapons for the fitness elite: They've gone mainstream. Count me among the believers, now that I've achieved oneness with my S510 (no more trailside conniptions). The reason is simple. Conventional gauges of athletic achievement—distance and time—are relatively anemic. So you ran three miles in 25 minutes. Were there hills? Did you start out wiped or fresh? Battle a headwind? You get the idea. What you need is a way to track the intensity of your workout, and your perceived exertion is only part of the equation. HRMs may have a learning curve, but this is the thing—they are nearly perfect measuring devices. “Use it right,” says Ed Burke, author of Precision Heart Rate Training, “and your endurance will skyrocket.”

The philosophy of heart-rate training, in general, is based on the four intensity zones of exercise workload—recovery, aerobic, threshold, and anaerobic. Recovery entails easy work that doesn't significantly increase your breathing or raise your pulse past 60 percent of your maximum heart rate. Aerobic output is exertion with enough oxygen to keep your muscles working efficiently at 70 percent of your MHR, strengthening the lungs and cardiovascular system. Threshold exercise—at “race pace,” roughly 80 percent of your maximum heart rate—takes you to the edge of anaerobic metabolism, when you simply can't take in enough oxygen to meet your body's demand. Here your heart pumps at 85 percent or more of its maximum, and lactic acid, a byproduct of anaerobic metabolism, causes your muscles to burn, cramp, and fail. Train at threshold with enough diligence and control, however, and you can increase your ability to process oxygen at the cellular level—your VO² max. Congrats. You're now stronger at speeds that used to shut you down.

But your HRM is there to tell you what your head and your lungs can't. “If your heart-rate monitor shows you a number that's normally comfortable, but you're now hurting, that means you may have overtrained,” says Jay Blahnik, a nationally known fitness expert based in Laguna Beach, California. “If a heart rate feels better than normal, conversely, it may be time to push yourself.”

Aerobics instructors have been telling people to check their pulses using the finger-on-the-neck method for decades—mostly to keep clients from falling over in a sternum-clutching heap. But the inaccuracy of palpating your carotid artery is one of the dirty secrets of the fitness world—with upward of 2.5 beats to count each second, it can be off by as much as 25 beats per minute—and it's impractical for anyone riding a bike or running a trail. Early HRMs weren't much better—imagine running with a wire clipped to your earlobe or fingertip—and neither were the first wireless models, which had clunky wristwatch units and often picked up signals from other athletes' HRMs. But since their first appearance, in 1977, chest-strap transmitters have become some of the most trusted tools available to sports labs, performing at 99.9 percent of the EKG gold standard. They've gotten cheaper, too. Though you can still pay as much as $700 for a unit with all the bells and whistles, most of the half-dozen brands out there offer midrange models for the price of a pair of running shoes (see chart at left).

First-time buyers should keep it simple—extra functions can be a liability if you don't need them. Save the calorie-crunching for the grocery aisles and the downloaded spreadsheets for your rotisserie football. Look for an accuracy rating of at least 90 percent of the EKG standard, a coded-signal chest strap (so your watch doesn't pick up the arteriosclerotic sack of Spam pumping in the monitor-wearing huffer next to you), big, readable numbers, and programmable training zones.

Eventually, and maybe a little ironically, your aim is to grow so comfortable with your monitor that it is no longer necessary. “The ultimate goal is to be able to do workouts with and without a heart-rate monitor,” says endurance coach Ray Browning. “It should be an educational tool. Something to help you say, 'Oh, this is what a 70-percent effort feels like,' without always being attached to a number. That's using technology to its maximum benefit.” ����

Paul Scott wrote about outdoor winter training in the December issue of ���ϳԹ���.

FS-1 ($200; 800-419-3667; www.fitsense.com): Until now, runners have gotten short shrift when it comes to electronics for accurately gauging pace and distance. Enter FitSense's FS-1, a watch-cum-workout-computer. While the wireless “accelerometer” attaches to your shoelaces, measuring each individual stride, the dashboard-style wristwatch displays total distance, speed, pace, calories burned, and heart rate (with optional chest-strap transmitter). Sure, the watch is about the size of a dashboard, but you gotta start somewhere.

Aquapacer Solo ($100; 011-44-1467-629-360; www.aquapacer.com): Want to swim faster? Don't just swim harder. You need to refine your stroke length and pace to maximize your hydrodynamic efficiency. Mount this doohickey on your goggle strap or tuck it into your swim cap, where the Aquapacer Solo will emit a steady auditory signal to match your target stroke speed (like a metronome, but you're not playing piano, you're doing intervals!). Program the Aquapacer to cue you for complete workouts, including rests, or plug in your optimum stroke frequency (instructions included), turn it on, and go.

UltraCoach NetAthlon Software ($90; 800-400-1390; www.fitcentric.com): It's no mystery why your treadmill, rowing machine, or stationary bike has become your clothes-drying rack: no scenery, no competition, no fun. Fitcentric's NetAthlonaims to change that by linking your basement statuary to your PC, allowing you to ride, run, or row on virtual courses ranging from Arizona bike trails to the course of the Boston marathon. Load the software, patch your machine into the system, and watch your “live” character (and up to six competitors) race along the animated course. UltraCoach displays real-time stats, including pace, distance, and heart rate, and you can store workout performances and race yourself later in the week. Or, if that can't motivate you, use the company's Web link and e-race a friend in Toledo (or Tunisia) on the same virtual course.

BC1 ($300; 800-321-1218; www.stay healthy.com): Forget skin-fold calipers and those oh-so-impractical water displacement tests; simply clasp the silver receptors on both sides of Stayhealthy Inc.'s BC1 (for Body Composition), and it sends an electrical charge through your body to quickly and accurately measure body-fat percentage, muscle mass, and hydration level based on the amount of resistance the charge encounters. Don't worry, it's painless, which is more than we can say for the results. Use the unit's pager-size sister gizmo—the calorie-tracking CT1—to rein yourself in.

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Seriously: The Vivonic Fitness Planner ($230; wwwvivonic.com) is not a toy. Well, OK, it looks like a toy—part Game Boy, part Tamagotchi—but this fitness-specific digital assistant packs your personal trainer, workout buddy, and dietician into one hand-size, 2.2-ounce package. Log the foods you eat, check their calories, protein, carbs, and fat, then calculate the myriad ways you can burn them off. (Erasing that Big Mac at lunch, by the way, is going to require 65 minutes of swimming.) Vivonic's Web site offers downloadable fitness plans, such as a beginning triathlon program, and Fitness Planner software for a Palm handheld ($50). Finally, a reason for gadget geeks and fitness freaks to unite. —James Glave

1) Central to the Fitness Planner is its touch-sensitive screen; use the stylus to input meals, snacks, and workouts. There are no character-recognition snafus á la Palm's units; just tap your way through a series of on-screen lists and prompts. The screen then shows you, with a simple calories-in/calories-out bar graph, whether you are gaining blubber, or dropping it. After a week, you may find yourself more consciously stocking the fridge and sticking to your program. If not, put the Fitness Planner to better use by wedging it under the leg of a wobbly table.

2) Vivonic's focus-group work is most palpable in the Fitness Planner's design. Grasp the ergonomic rubber-wrapped sports-grip housing and admire its decidedly unisex plum color—clearly the R&D team was aiming between jazzy iMac styling and the serious utility of, say, a geiger counter. A single button serves as both power switch and contrast control for the LCD screen, and a rakish waistband holster (included) frees your hands as you pump it up in the weight room, recording sets and reps as you go.

3) This cable (included) connects the Fitness Planner to a desktop mother ship and allows it to “sync,” or swap updated information, with a Windows program. (Vivonic, owned by Windows-oriented giant Intel, claims a Mac version is in the works.) Aside from producing charts and graphs of your fitness progress and dietary intake, the desktop software contains the same food and exercise database as the handheld unit, with full calorie/protein/fat/carb specs on everything from Pop-Tarts to sushi, and burn rates on every activity from leg extensions to lawn darts (an hour of the latter will allegedly burn 217 calories).

4) The plastic stylus feels like a cheap toothpick in big athletic hands, but those who opt for the Vivonic Fitness Planner hardware—and not the software for Palm handhelds—get an onboard bonus: an electronic pedometer. Switch it on, hit the hill, and presto, your walking distance and intensity are immortalized in the log. All that's missing is an altimeter to remind you why your lungs feel like they're collapsing.