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Popular and Useful Technology Used to Train During Runs
This is an excerpt from Triathlon 2.0 by Jim Vance.
As popular as power meters are in the cycling world, GPS (global positioning system) units are even more popular in the running world. Just about every serious heart rate monitor brand on the market offers a GPS or some type of speed-distance device with it. And just about every GPS unit on the market now offers a heart rate monitor with it as well. If it doesn't use GPS satellites, it likely uses an accelerometer to measure speed and distance, usually a foot pod placed in the shoe that measures the accelerations and decelerations of the foot, to determine stride length and stride rate. In running, stride rate is the revolutions per minute (rpm) based on how many times the foot strikes the ground in a single minute. Running rpm are also referred to as cadence.
This chapter covers popular and useful technology used to train during runs, along with some useful terminology that will help you interpret and understand the data the technology provides.
Global Positioning System (GPS)
Most athletes understand GPS units a lot more than power meters and power data because the information they provide is easier to conceptualize, for example, how fast one is running. If you produce more watts on the bike, you're not necessarily going faster (such as when going uphill), but if you push the pace on a run, you are definitely going faster. If you produce less watts on the bike, you're not necessarily going slower either (such as when coasting downhill or with a strong tailwind). If you run easier, you are almost always running slower than if you run hard.
A GPS device uses a network of 24 satellites that have been placed into orbit by the U.S. Department of Defense. GPS works in any weather, anywhere in the world, 24 hours a day without subscription or fee (Garmin 2015). These GPS satellites circle the earth transmitting information back to earth where GPS devices use triangulation to calculate the user's exact location.
The GPS receivers are very accurate due to their multichannel design. Some of the top GPS watches use 12 parallel channel receivers to initially lock onto the satellites and maintain a strong lock in settings such as dense foliage or among tall buildings. In general, the common GPS watches today are accurate to within an average of 15 meters.
Most GPS watches generate data files that can be uploaded to a third-party software suite for postworkout analysis, through external sensors like wireless ANT+ protocol, usually by using a USB cable or Bluetooth technology.
Speed-Distance Devices (Accelerometers)
These devices are similar to GPS watches, but instead of using satellites to determine speed and distance, they use a motion sensor (accelerometer). Most commonly this is a foot pod attached to your shoe.
Because accelerometers do not rely on GPS signals, they are popular for their use indoors, usually on treadmills.
Foot Pods
A foot pod uses an accelerometer, which measures the time the foot is in contact with the ground and counts the number of strides. With this, the foot pod calculates running pace, which gets transmitted to the watch.
The foot pod should be calibrated to each user, which accounts for individual running biomechanics and running shoes. Some athletes have reported as much as a 12 percent or more difference in calibration factors between racing in flats versus trainers.
GPS units or other speed-distance devices help to show the changes in grade (terrain) and the intensity of the effort athletes are putting forth. Both of these variables (grade and intensity) contribute to the physiological demands of running on terrain that is not flat.
In chapter 1, we discussed training with heart rate, showing the pros and cons while on the bike. The same basic principles apply with running, as heart rate is not an output-based metric. Remember, HR has a linear response to an increase in intensity, but it will only increase up to 
O2max, which is the maximal volume of oxygen the body can utilize. This is due to an athlete's 
O2max being limited to a large extent by the cardiovascular system's ability to pump blood. Once we reach that limit, HR can no longer increase to match an increase in intensity.
Long before GPS watches, most runners used the distance covered and the time it took to determine pace, which was the primary method for tracking training load and training intensity. This basic approach works really well on flat terrain, but when examining the intensity and training load generated by varied-pace running on rolling or hilly terrain, there is a lot that can be missed.
Simply using distance and time to measure the quality of intense running works really well on a track, but what if you are not doing all of your intervals on a track? What if the other runs in your training are on hilly courses?
Some of the most important runs in your training are recovery runs. Using distance and time to accurately measure intensity and training load while on open or uneven terrain is simply not as accurate as we would like it to be, which can lead to overdoing a recovery run without knowing it.
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