Pedometer manufacturers choose from several different types of step-counting mechanisms. Less expensive pedometers base their counts on mechanical triggers with the least reliability. You shouldn't expect exceptional accuracy from step-counting pedometers, since measurements of distance depend on an exact stride length and other changing factors. Even GPS pedometers might estimate missing travel data, but offer accuracy unmatched by other pedometer types.
How They Work
The simplest type of pedometer counts a pendulum's movement inside the pedometer's case. When a foot impacts the ground, the pendulum bounces and the pedometer counts a step. Pedometers that depend on physically triggered mechanisms often miss steps unless they are positioned correctly on the belt and fixed at the proper angle. Some mechanical pedometers measure distance traveled by plus or minus 3 percent in tests, while others might err by more than a third of the total distance. Mechanical and piezoelectric pedometers base calculations on stride length, a value entered by the owner. If real stride differs from the programmed value, accuracy suffers. If you travel hilly courses, stairs, or change pace and stride as you exercise, you won't see accurate distance readings.
Springs
Avoid inexpensive hair-spring pedometers if you want the most accurate readings. Hair-spring pendulums depend on one tiny metal strand for the step count. As the strand gradually fatigues, accuracy drops. Coil-spring pendulums give more accurate counts and last longer, but both types depend on precise positioning for proper functioning. If you bump your mechanical pedometer to an odd angle during a brisk walk, it may not record your steps.
Accelerometers
Accelerometer pedometers place the sensor that counts the steps either in the pedometer case or in a separate device attached to the shoe. The strain gauge sensor includes a small piece of piezoelectric material that generates an electrical current if deformed. Footstep impacts cause an electrical pulse no matter where you carry the pedometer or how you position it. Advanced models with shoe sensors measure the time the foot spends on the ground, calculating whether the user walks or runs and estimating the speed of travel.
GPS Pedometers
GPS pedometers don't count steps unless combined with an accelerometer sensor. GPS devices compare the length of time satellite transmissions take to reach the pedometer and calculate position with that information. Accuracy drops when fewer than four satellite transmissions reach the unit. GPS units estimate routes as straight lines if transmissions fade completely.
Most Accurate
Where terrain allows continuous access to the GPS network, GPS pedometers provide the most accurate information. The Garmin Forerunner might include an error of plus or minus 20 feet after a mile of travel, compared to the plus or minus 158 foot per mile error of the best mechanical or accelerometer pedometers such as the Yamax Digi-Walker or Kenz Lifecorder. Garmin's Forerunner allows indoor tracking if used with the Garmin Footpod, an accelerometer sensor that attaches to the user's shoe. If calibrated outdoors using the GPS features as a standard, the Footpod measures distance with 99 percent accuracy.
References
- "Health Hints": Pedometers -- Motivating Fitness; Janet M. Pollard; September 2009
- "Medicine and Science in Sports and Exercise": Accuracy and Reliability of 10 Pedometers for Measuring Steps Over a 400-m Walk; P.L. Schneider, et al.; October 2003
- Runner+: How Does The Nike + iPod Sport Kit Accelerometer Work?
- Smithsonian National Air and Space Museum: How Does GPS Work?
- Garmin: Forerunner 405 Owner's Manual
- Garmin: Forerunner 305 Footpod Quick Reference Guide
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