What Is a Pedometer?
A pedometer is a device that counts the number of steps a person takes. Pedometers motivate walkers to get fit by helping them calculate their daily steps, usually to reach a defined goal, such as the 10,000 steps recommended by Dr. Catrine Tudor-Locke, assistant professor of health promotion at Arizona State University. More sophisticated (and pricier) pedometers use step counts to estimate distance covered and calorie expenditure based on your stride length, age and weight.
Mechanical Pedometers
Basic pedometers are based on simple pendulum technology that dates back to the nineteenth century. A pedometer strapped to your waist detects hip movement with an internal horizontal arm (pendulum) suspended on a spring or wire. When your foot hits the ground, the impact swings the pendulum, which hits a sensor on a stationary post, completing an electrical circuit. The pedometer registers a step each time the circuit is completed.
Accelerometer Pedometers
High-tech pedometers such as Nike+ use the same accelerometer technology as Nintendo Wii controllers, iPhones and automatic airbag deployment in cars--technology that has been available since the mid-2000s. Rather than a pendulum, these devices use piezoelectric crystals, which are microscopic materials that produce an electrical impulse when they are bent or compressed. When the exerciser takes a step, the piezoelectric materials bend, creating a small electrical current, which is then picked up by sensors and counted as a step.
Piezoelectric pedometers also measure the amount of time that an exerciser's foot is on the ground and use this information to calculate speed and distance covered. The faster a person walks or runs, the less time her feet spend on the ground per step, and the longer her stride length. The pedometer uses foot strike frequency and length data to calculate speed and distance with 95 percent accuracy.
Margin of Error
Even the best pedometers have a 5 percent margin of error, and cheap mechanical versions have up to a 40 percent variance. That means that when your cheap pedometer registers 10,000 steps (about 5 miles), you may have really only taken 6,000 steps (3 miles), or as many as 14,00 steps (7 miles).
Pedometers lose their accuracy at extremes, underestimating steps at high intensity and losing accuracy at a slow walk. Shaking a pedometer is enough to close the circuit and register steps, so bending over, sitting and even having a little extra "jiggle" in your stride can register as steps. Also, because mechanical pedometers function on only one plane of movement, they can't count steps accurately if they are tilted. This is especially a problem for walkers with larger waistlines whose tummies interfere with the pedometer's angle.
To assure the accuracy of your pedometer, fasten it to a belt directly above the knee on your dominant leg, and make sure it sits completely vertically. Accelerometer pedometers avoid registering extra "steps" from the impact of activities like driving or coughing by turning on only after registering a minimum number of steps. If accuracy is your concern (and price is not), consider purchasing a GPS system that measures speed and distance with significantly better accuracy.
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