Pedometer accuracy under scrutiny.

many of today's pedometers do a great deal more than record the number of steps you take. If you enter some personal information, such as stride length and weight, they also calculate how far you traveled and how much energy you used to do it. Although the information pedometers give you about your physical activity is abundant, is it accurate?

A study at the University of Tennessee tested the accuracy of ten different makes of pedometer at five different walking speeds: 54, 67, 80, 94 and 107 meters per minute (m/min). For each combination of speed and pedometer, participants walked on a treadmill for five minutes while pedometers recorded the number of steps taken, distance traveled, and kilocalories (kcal) expended.

Simultaneously, investigators counted steps, calculated distance traveled by multiplying the counted steps by the participant's average stride length, and used indirect calorimetry to measure energy expenditure. Pedometer performance was then compared against these highly accurate forms of measurement.

In general, pedometers were most accurate recording steps taken and less accurate calculating distance. At slower walking speeds, pedometers underestimated steps taken and overestimated distance traveled. Among the pedometers tested, 20 minutes per mile (80 m/min) walking pace was the most accurate for distance traveled. As the pace increased, they became more accurate at measuring steps taken (most were within + or - 1%), but began to underestimate distance. For kcal expended, pedometers did not have high levels of accuracy, regardless of speed.

What does this mean for runners? First, keep in mind that the measurements were taken for five-minute walks, so these findings may not extend to running or longer periods of activity. If the pattern is any indication, however, because most pedometers underestimated distance traveled at quicker paces, you may be running farther than you think. To test yours out, run where you know the distance, such as on a track, and compare it to your pedometer's reading. Also, stride length increases with speed, which may be the reason for underestimation at faster walking speeds. To improve your pedometer's ability to accurately calculate the distance you've run, enter your running stride length rather than your walking stride length. Once you know your stride length, you can calculate distance fairly accurately.

If you are mostly interested in how much energy you're expending, look for a pedometer that operates with an accelerometer. This type considers form of activity (e.g., walking versus running) in the calculations, so it will be more accurate for runners than a pedometer that does not use this technology. Bear in mind, however, that pedometers are not particularly reliable sources of caloric expenditure.

(Med. Sci. Sports & Exerc., 2003, Vol. 35, No. 8, pp. 1455-1460)