Abstract
Introduction
Knowledge of knee kinetics and kinematics contributes to our understanding of the patho-mechanics of knee pathology and rehabilitation and a mobile system for use in the clinic is desirable.
We set out to assess validity and reliability of ambulatory Inertial Motion Unit (IMU) Sensors (Pegasus¯) against an established optoelectronic system (CODA¯).
Pegasus¯ uses inertial sensors placed on subjects' thighs and lower leg segments to directly measure orientation of these segments with respect to gravity. CODA¯) models the position of joint centres based on tracked positions of optical markers placed on a subject, providing 3D kinematics of the subject's hips, knees and ankles in all three planes.
Methods
Intra observer reliability of the Pegasus¯ system was tested on 6 volunteers (4 male; 2 female) with no previous lower limb or knee pathology. IMU's were placed on the long axis of the lateral aspects of both thighs and lower leg segments. A test re-test protocol was used with sagittal data angle collected around a standard circuit.
Inter-observer reliability was tested by placement of IMU's by 5 different testers on a single volunteer.
To test validity, we collected simultaneous sagittal knee angle data from Pegasus¯ and CODA¯ in two subjects. The presence of IMU's did not compromise positioning of optical markers.
Results
Analysis of triplicate measurements showed that intra-observer error is +/− 5°. Inter-observer difference in measurements varied from 3° to 20° absolute values.
Positional error of the Pegasus¯ IMU's was significant in comparison to CODA¯, with absolute offsets in knee angles typically of 10° to 25°. Range of motion differences between the two systems calculated as root mean square (rms) difference of the zero meaned signals were 3.8°-4.8°.
Conclusion
-
The Pegasus¯ system is useful in ambulatory measurement of the range of knee motion in the sagittal plane.
-
In the current configuration there was poor intra and inter-observer reliability possibly related to positional error using the Pegasus¯ system and may be due to fixation method, operator factors, body shape and variability of clothing.
-
Recommendations have been made to the manufacturer.
