Knee alignment measurements are one of the most extended indicators of knee-complex injuries such as anterior cruciate ligament injury and patellofemoral pain syndrome. The Frontal Plane Projection Angle (FPPA) is widely used as a 2-D estimation of knee alignment. How- ever, traditional procedures to measure this angle suffer from practical limitations, which leads to huge time investments when evaluating mul- tiple subjects. This work presents a novel video analysis system aimed at supporting experts in the dynamic measurement of the FPPA in a cost-effective and easy way. The system employs Kinect V2 depth sensor to track reflective markers attached to the patient leg joints to provide an automatic estimation of the angle formed by the hip, knee and ankle joints. Information registered by the sensor is processed and managed by a computer application that simplifies expert’s work and expedites the analysis of the test results.

1. First Approach to Automatic Measurement
of Frontal Plane Projection Angle During
Single Leg Landing Based on Depth Video
UCAmI 2016 (Las Palmas de Gran Canaria, Spain)
Carlos Bailon1, Miguel Damas1, Hector Pomares1 and Oresti
Banos2
1Department of Computer Architecture and Computer Technology, CITIC-UGR
Research Center, University of Granada, Spain
2Telemedicine Cluster of the Biomedical Signal and Systems Group,
University of Twente, Netherlands

3. Knee alignment
• Grade of alignment of the hip, knee and ankle joints.
• Commonly used as a risk indicator of many biomechanical injuries
related to knee joint when measured during the performance
dynamic tasks.
Anterior Cruciate
Ligament (ACL) injuries
Patellofemoral Pain
Syndrome (PFPS)
Potential misalignments
during dynamic exercises are
the most common injury
mechanisms.

4. Quantification of knee alignment
Projection of the angle
formed by the hip, knee and
ankle joints over the frontal
plane of the body.
Frontal Plane
Projection Angle
(FPPA)
Wilson et al. “Core strength and lower extremity
alignment during single leg squats” Medicine &
Science in Sports & Exercise (2006)

5. Key limitations of existing techniques for
FPPA measuring
Inertial sensor-based systems 3D motion tracking video
systems
2D offline video analysis
 Accurate 3D rotations
Possible motion
restriction
Non-deliberated
sensor displacement
 Tridimensional motion tracking
 High sampling rate
Need of high number of
cameras
Costly and space demanding
 One camera needed
 Portable and easy-to-
use equipment
Elevated time for
analysis
Prone to human
errors
2D analysis

6. Objectives of the project
• Automatic estimation of FPPA during the
performance of dynamic tasks (ideally any 2D
biomechanics angle)
• Single-camera solution.
• No external light sources.
• Inexpensive and easy-to-use system.
• Real-time visualization of the FPPA.
• Automatic analysis of the data.

7. Overview of the proposed system

8. Why do we use markers?
Although Kinect is well-known for being a markerless system, we
introduce the tracking of three retro-reflective markers.
This method increases the accuracy of the pose estimation algorithm
of Kinect and allows for tracking points that are not necessarily joints.
The blue line shows the
data registered during a
single leg landing using
markers.
The red line shows the data
registered using the Kinect
pose estimation algorithm.
RMSE = 8.498º

9. Reflective markers tracking
• Kinect’s depth sensor captures
the infrared intensity value for
each pixel of the image (512 x
424 resolution).
• An empirical intensity threshold
(high-pass filter) is used to select
candidate marker’s pixels.
• Kinect’s pose estimation algorithm
is used to classify each marker
position.
• Retro-reflective elements not
belonging to a marker are
ignored.
• Markers coordinates are

10. Application

11. Application

12. Experimental results
 High concordance among the measurements
 Proposed approach saves up to 10 minutes per
assessed subject
Comparison between Kinovea (2D offline analysis tool,
expert oriented) and the proposed system.
FPPA evaluated for 10 healthy subjects from a
professional football team

13. Conclusions
• Proposed a novel system to perform an automatic
estimation of dynamic FPPA, by a single-camera, cost-
effective and portable solution.
• The system uses a depth sensor to track the position of
three retro-reflective markers attached to the subject’s
hip, knee and ankle joints.
• Designed a user interface which simplifies the expert’s
routine and expedites the analysis of the results.
• Experimental results show the interrater reliability of the
proposed system, as well as the limitations of the 2D
analysis (limited joint rotation measurement).

14. THANKS!

15. Application description

16. Application implementation

17. Data storage
Local database
engine
Why?
• On-disk database file.
• Not very large dataset.
• No concurrent writers.
• Data easily exported to CSV files for external analysis.
Data is stored in two tables, differentiating patient personal
information and data collected. Both tables are related by a
personal ID.