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Ambulatory measurement of knee motion and physical activity: preliminary evaluation of a smart activity monitor

James Huddleston13*, Amer Alaiti2, Dov Goldvasser2, Donna Scarborough2, Andrew Freiberg1, Harry Rubash1, Henrik Malchau1, William Harris1 and David Krebs2

Author Affiliations

1 Harvard Medical School Harris Orthopaedic Biomechanics and Biomaterials Laboratory Massachusetts General Hospital 55 Fruit Street, GRJ 1126 Boston, MA 02114–2696

2 Harvard Medical School Massachusetts General Hospital Biomotion Laboratory MGH Institute of Health Professionals Charlestown Navy Yard 36 First Avenue, #223 Boston, MA 02129–4557

3 Department of Orthopaedic Surgery Stanford University School of Medicine 300 Pasteur Drive, R-105 Stanford, CA 94305–5341

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Journal of NeuroEngineering and Rehabilitation 2006, 3:21  doi:10.1186/1743-0003-3-21

Published: 13 September 2006



There is currently a paucity of devices available for continuous, long-term monitoring of human joint motion. Non-invasive, inexpensive devices capable of recording human activity and joint motion have many applications for medical research. Such a device could be used to quantify range of motion outside the gait laboratory. The purpose of this study was to test the accuracy of the modified Intelligent Device for Energy Expenditure and Activity (IDEEA) in measuring knee flexion angles, to detect different physical activities, and to quantify how often healthy subjects use deep knee flexion in the ambulatory setting.


We compared Biomotion Laboratory (BML) "gold standard" data to simultaneous IDEEA measures of knee motion and gait, step up/down, and stair descent in 5 healthy subjects. In addition, we used a series of choreographed physical activities outside the BML to confirm the IDEEA's ability to accurately measure 7 commonly-performed physical activities. Subjects then continued data collection during ordinary activities outside the gait laboratory.


Pooled correlations between the BML and IDEEA knee flexion angles were .97 +/- .03 for step up/down, .98 +/- .02 for stair descent, and .98 +/- .01 for gait. In the BML protocol, the IDEEA accurately identified gait, but was less accurate in identifying step up/down and stair descent. During sampling outside the BML, the IDEEA accurately detected walking, running, stair ascent, stair descent, standing, lying, and sitting. On average, subjects flexed their knees >120° for 0.17% of their data collection periods outside the BML.


The modified IDEEA system is a useful clinical tool for evaluating knee motion and multiple physical activities in the ambulatory setting. These five healthy subjects rarely flexed their knees >120°.