What does it mean to be healthy? Exercise and eating right. Most would rattle these off as if they’re the answer to a third-grade addition problem. But wearable fitness trackers (and people with PhDs) measure health numerally. Asking a small device to pull the weight of a medical-grade machine is a tall order, and all should taken with a pinch of skepticism. But a select set have proven themselves a good first line of defense against complacency.

Just like they’re different in style, wearables vary in which metrics they measure: heart rate, calories burned and blood pressure are just a few metrics vital to determining fitness. Prospective buyers need to ask themselves two questions: will you actually wear it? And will you understand it? More data means nothing if you don’t know what it means or how it’s measured. Take a look at our breakdown of the different metrics and which wearables utilize each.

Deciphering-Data-Gear-Patrol-HR

Heart rate

What is it?
The number of times your heart beats per minute.
Why is it important?
A healthy heart rate will improve sleep quality, stress levels and caloric burn.
How is it measured?
Typically measured in beats per minute (bpm), heart rate will vary between individuals according to their physicality. Most wearable devices “check the pulse” through the wrist or finger. There are limitations to those methods; the most accurate way to measure heart rate is with EKGs, which monitor the electrical activity right near the heart, but the majority of wearable devices use optical sensors to track blood flow. They’re less accurate because the areas from which they measure pulse (the capillaries in the wrist or fingers) are farther away from the heart. Also, most wearable devices aren’t able to read heart rate while “on the run”.

Optical Flow: Basis Carbon Steel, Samsung Gear Fit, LG G Watch R, Apple Watch, Withings Pulse O2, Fitbit Charge HR, Fitbit Surge, Suunto Ambit3, Motorola 360
EKGs: Suunto Dual Comfort Belt
Bioimpedance: Jawbone UP3 (upcoming)

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Sleep

What is it?
When you’re James Bond riding a T-rex. No really, sleep is technically defined as the state where the body’s brain activity and voluntary muscles slow down and people enter an alternate consciousness.
Why is it important?
Sleep is a rejuvenation period for the immune, skeletal, muscular and nervous systems. You’ll die without it.
How is it measured?
When people sleep, their circulatory systems don’t work as hard because the body’s metabolic needs are less. Wearable fitness devices sense the change in heart rate and blood pressure, and are then able to determine length and quality of sleep. The devices are broken down into two categories: devices that automatically monitor sleep and devices that require manual initiation and termination.

Automatically: Basis Carbon Steel, Fitbit Charge, Fitbit Charge HR, Fitbit Surge
Manually: Samsung Gear Fit, Samsung Galaxy Gear S, LG G Watch R, Fitbit Flex, Jawbone UP24, Misfit Shine, Withings Pulse O2

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Movement

What is it?
Traveling from one point to another. Soon to be accomplished solely by Segway and/or jetpack.
Why is it important?
Moving uses energy, plain and simple. Even if it’s just walking around, it’s healthy for a grocery list of reasons: it burns calories, it boosts energy and morale, it improves sleep quality and it prevents disease.
How is it measured?
In general, all these wearable devices are partly pedometers. But in terms of general movement tracking, the market is split in two: devices that measure all general movement and afterward give the option to plug in the exact activity, and devices that can automatically distinguish between walking, running, biking and swimming (if the device is waterproof).

Distinguishes Movement: Basis Carbon Steel, Samsung Gear Fit, Samsung Galaxy Gear S, LG G-Watch R, Apple Watch, Apple Watch, Motorola 360, all Fitbit devices
General Movement: Suunto Ambit3, Jawbone UP24, Misfit Shine, Withings Pulse O2

Deciphering-Data-Gear-Patrol-Calories

Calories Burned

What is it?
A calorie is the amount of energy required to raise the temperature of 1 gram of water by a single degree Celsius (1.8 degrees Fahrenheit).
Why is it important?
In order to do anything — move, think, eat, breath — every muscle in our body needs to burn calories. Calories are the currency of the health world.
How is it measured?
In 2012, WIRED reported that using an indirect calorimeter, a device costing over $30,000, to analyze your oxygen consumption was the only way to accurately measure calories burned. So today’s wearables won’t be spot on, but their estimates are usually accurate to around 10 percent. Most devices use proprietary algorithms that factor in the wearer’s age, height, weight and gender to give people a closest estimate.

Proprietary Algorithms: Basis Carbon Steel, Samsung Gear Fit, Samsung Galaxy Gear S, LG G-Watch R, Apple Watch, Apple Watch, Motorola 360, Suunto Ambit3, Jawbone UP24, Misfit Shine, Withings Pulse O2, all Fitbit devices

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GPS Location

What is it?
A satellite’s approximation of a person’s current location. Wave to the NSA!
Why is it important?
Very few wearable fitness trackers have GPS capabilities. GPS navigation can more accurately track distances, elevations, and energy consumed.
How is it measured?
A fitness tracker with a GPS is able to gauge speed, distance and location. These metrics help determine to amount of calories burned and the overall success of a workout. Still an estimate, the GPS in a wearable device is capable of tracking the wearer’s path throughout the day. The idea is to compare and improve on each day’s results.

Built-in GPS: Fitbit Surge, Samsung Galaxy Gear S, Suunto Ambit3

Deciphering-Data-Gear-Patrol-Sweat

Perspiration

What is it?
Sweat. The body excretes a mixture of water and salt to naturally lower its body temperature.
Why is it important?
We sweat for a variety of reasons, the most common being to cool off (second most common: the IRS). Despite being sort of a defense mechanism, sweating is healthy. People who exercise regularly have higher oxygen levels (more fluid) in their blood, making it easier for them to perspire. More sweat means the body is able to cool down faster and exercise for longer periods of time.
How is it measured?
Sweat sensors, or electrodermal response sensors, measure the flow of electricity on a person’s skin.

Perspiration sensors: Basis Carbon Steel, BodyMedia FIT LINK Armband

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Skin Temperature

What is it?
The temperature of our epidermis, also known as skin.
Why is it important?
The sweat glands are our skin’s primary weapon against overheating. The glands release moisture and cool down our bodies’ core temperatures. Exercise causes skin temperatures to rise. Working in tandem with sweating, the more fit a person is, the quicker their temperature will return to normal and the more they’re able to work out. Monitoring skin temperature is important because
How is it measured?
The ability to measure skin temperature is rare in the world of wearable devices. The ones that can, like the Basis Carbon Steel, have thermometer sensors to measure temperature.

Skin sensors: Basis Carbon Steel, BodyMedia FIT LINK Armband

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Blood Pressure

What is it?
The force with which blood pushes against the arteries’ walls.
Why is it important?
Every heartbeat pumps blood throughout the body; blood pressure jumps when a beat occurs and lessens in the time between beats, as the blood flow slows. When exercising, the heart beats faster and harder so that more oxygen is carried to muscles; blood pressure increases as a result. People who exercise more often have stronger hearts because it is used to pumping harder and more often; as such, healthy people have lower blood pressure because the heart doesn’t have to work as hard. Measuring blood pressure is important if you want to monitor how hard your heart is working.
How is it measured?
Blood pressure is measured as a fraction, a normal score being 120/80 mm Hg (millimeters of mercury displaced). The top number is a person’s blood pressure when the heart beats; the bottom is between beats. Not many trackers have explored the metric. A sphygmomanometer and stethoscope are traditionally used to restrict blood flow and measure pressure around a person’s upper arm. Withings makes a Wireless blood pressure monitor that works with a smartphone app (which replaces the stethoscope). But the device is still worn around the upper arm, not the wrist.

Blood Pressure monitor: Withings Blood Pressure Monitor

Deciphering-Data-Gear-Patrol-Glucose

Blood Glucose

What is it?
The amount of sugar in our bloodstream.
Why is it important?
The body breaks down fats into sugar, which the bloodstream carries out to the body’s cells. During activities, the body’s glucose levels will rise; because the body requires more energy, it breaks down more fat. Shortly after exercising, the body’s blood-glucose levels will be decrease because the energy is used up. Understanding and managing the body’s blood-glucose level will help people know their limits. Overly high and low blood-glucose levels are dangerous.
How is it measured?
A glucose sensor uses non-evasive scattered light, or lasers, to keep track of the blood’s sugar and dehydration levels.

Blood Pressure monitor: No small wearable device