Step activity monitor: Increased accuracy in quantifying ambulatory activity

This study compares the accuracy of a two-dimensional accelerometer worn on the ankle (a step activity monitor) with that of an electronic, digital pe domrter worn on the belt line. Twenty-nine human subjects were evaluated wh ile they briskly walked 400 M, slowly walked 10 M, and ascended and descend ed a flight of stairs. The step activity monitor had less error in all acti vities: its mean absolute error was 0.54%, whereas that of the pedometer wa s 2.82%. The difference was more pronounced in obese subjects (body mass in dex greater than 30), with an overall mean absolute error of 0.48% for the step activity monitor and 6.12% for the pedometer (nearly 13 times that of the step activity monitor). For subjects with a body mass index less than 3 0, the step activity monitor had an overall error of 0.56% and the pedomete r had an overall error of 1.56% (less than 3 times that of the step activit y monitor). The absolute error of the pedometer was positively correlated w ith body mass index (r = 0.792, p < 0.0001) and weight (r = 0.753, p < 0.00 01), whereas the error of the step activity monitor was not significantly c orrelated with either. Neither device was significantly biased by age, gend er, or the presence of a lower-extremity joint prosthesis. The accuracy and additional capabilities, including a real-time memory record of activity. of the step activity monitor make it well suited for objectively quantifyin g ambulatory activity, especially for obese subjects.