INTRODUCTION

• Traditional motion capture uses markers to measure movement

• Markers are prone to soft tissue artifact and experimental error [1] and may possibly impede natural motion

• Optical motion capture systems cannot be used in a variety of natural settings such as a patient’s home, on the sports field, or in

public.

• Has led to the development of markerless motion capture [2]

• The accuracy of marker-based systems has been analyzed extensively [3-6] whereas markerless motion capture has not

• System accuracy is critical to determine so the best clinical decisions on treatment can be made

• Marker-based and markerless motion capture have been compared [7], but this only provides a measure of relative accuracy

• Markerless systems have been evaluated but they used an array of cameras and were not systems such as the Microsoft Kinect

[8], which relies on just two cameras: a depth map and a color camera.

Goal of study: Is the Kinect as accurate as a marker-based motion capture system?

Can Human Movement be Measured Accurately with a Microsoft Kinect?

1Brian Noehren, 1Anne Schmitz, 2Mao Ye, 3Robert Shapiro, 2Ruigang Yang, 1Division of Physical Therapy

2Department of Computer Science3Department of Kinesiology and Health Promotion

METHODS

• Jig built to simulate leg (Fig. 1)

• Jig placed in 6 different poses: flexed, hyperextended, abducted, adducted, internally rotated, externally rotated

• Accuracy of joint angles assessed and compared between systems (Fig. 2)

UNIVERSITY OF KENTUCKY

RESULTS

REFERENCES

ACKNOWLEDGEMENTS: funded by NSF IIS 1231545

1.Leardini, A., et al. Gait & Posture 21, 212, 2005.

2.Mundermann, L., et al. J Neuroengr and Rehab 3, 2006.

3.Richards, J. G. Human Move Sci 18, 589-602, 1999.

4.Miranda, D. L., et al. J Biomech 46, 567-573, 2013.

5.Holden, J. P., et al. Gait & Posture 5, 217-227, 1997.

6.Kiran, D., et al. Phys Ther Sport 11, 81-5, 2010.

7.Mündermann, L., et al. Proc of ASB, Cleveland, OH, 2005.

8.Microsoft. http://www.microsoft.com/en-us/kinectforwindows/

9. Noehren, B., et al. Clin Biomech 27, 366-371, 2012.

10. Noehren, B., et al. Med Sci Sports Exerc in press, 2013.

11. Butler, R. J., et al. Physical Therapy 91, 1235-1243, 2011.

DISCUSSION

• This is one of the first studyies to quantify the performance of a single markerlerss motion capture system in evaluating joint angles

• Is this accuracy good enough for clinical applications?

• Patients with patellofemoral pain show 3 deg difference in hip adduction [9]

• Patients with a reconstructed anterior cruciate ligament show 4 deg difference in hip flexion [10]

• Patients with knee osteoarthritis show 4-5 deg difference in knee adduction [11]

• Yes. We expect the Kinect to be able to discern clinically relevant differences between patients and healthy controls.

• Next step is to evaluate the accuracy of the Kinect to measure joint angles in healthy subjects

Figure 2: Method used to compare a marker-based motion capture system with a Kinect

KinectFusion Alignment

Articulated Iterative Closest Point Algorithm for Alignment

Surface model with marker positions

Surface model

Kinect global marker positions for pose of interest

Inverse Kinematics using Visual3D

Global marker positions from MotionAnalysis cameras

Kinect Joint Angles

Marker-based Joint Angles

Kinect image and depth map for pose of interest

Kinect image and depth map for a neutral pose

thigh

shank

Ball and socket

Figure 1: Jig built to simulate a leg

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• For flexion-extension and ab-adduction:

• Angles typically altered in rehabilitation

programs

• Accuracy of the Microsoft Kinect is less

than 0.5 deg different than the marker-

based system (Fig. 3)

• For axial rotation, the systems agree

within 2 deg (Fig. 3)

Figure 3: Accuracy results