1. ACCURACY OF ANKLE- AND KNEE-ANGLE MEASUREMENTS WITH THIN FLEX SENSORS
Schindelwig K1, N Marterer1, D Morawetz1, S Kapferer1, R Csapo2,
W Nachbauer1,2
1) Department of Sport Science, University of Innsbruck, Austria
2) Technology Centre of Ski- and Alpine Sports

2. Introduction
A multitude of biomechanical standard methods for the assessment of body segment positions and angles are available
Lab measurements: active and passive markers, very precise
Field measurements: IMUs, goniometers, flex sensors
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

3. Introduction Classical goniometers Advantage: accurate, cheap,
Disadvantage: Externally fixed goniometers prone to damage at some types of sport, e.g. Alpine skiing
Flex sensors
Thickness of 0.4 mm  alternative if classical goniometers are not feasible
Thickness of classical goniometers e.g. precludes mounting within ski boots
The following four slides show recent work about wearable goniometers and flex sensors for measuring the ankle and knee angles. Additionally, the accuracy of the sensors is given.
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

4. Introduction: Literature
Flexible polymer component - knee angle RMS = 1.3°
Development and evaluation of a novel low-cost
sensor-based knee flexion angle measurement system
Saggio et al., 2014, The Knee
Fiber Optic - knee angle max. error = 8°
Knee angle Strain Gage max. error = 10°
Comparison of Strain-Gage and Fiber-Optic Goniometry
for Measuring Knee Kinematics During Activities of Daily
Living and Exercise
Abeer et al., 2012, Journal of Biomechanical Engineering
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

5. Introduction: Literature
Spectra flex sensor - knee angle max. error = 11°
Evaluation of Bend Sensors for Limb Motion Monitoring
Masdar, et al The 2013 Biomedical Engineering
International Conference
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

6. Goal Determine the accuracy of thin flex sensors - ankle angle
- knee angle
during deep squats for the application in skiing
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

7. Method 3 Subjects Motion 6 series of 15 deep squats
Speed Increased from 15 to 45 repetitions per minute
Speed control with metronome
4 Flex sensors Spectra Symbol, Flex Sensor cm
(SEN-08606), Salt Lake City, UT
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

8. Method Flex sensor position - on both legs
- central anterior between shank and foot
- central at the hollow of the knee
Patch with padding to allow longitudinal motion of the sensors
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

9. Method Data recording - 1500 Hz sample rate
- transmitted wirelessly to a recording system
Telemyo 2400 G2, Noraxon, USA
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

10. Method Reference measure - active three camera motion capturing system
AS 200, LUKOtronic, Innsbruck, Austria
- 100 Hz sample rate
- kinematic data of right and left leg
- manual synchronization
Data analysis
- maximum differences and standard deviations
between both systems
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

11. Results

12. Results

13. Results Ankle angle Knee angle Mean range flex sensors 39° 94°
Mean of the maximum differences (flex sensor – reference)
9.1°
13.1°
Mean of the standard deviations of differences (flex sensor – reference)
2.9°
4.0°
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016,Vienna - Austria

14. Discussion
Flex sensors yielded a satisfactory accuracy for measurements of ankle- and knee angle
 Could be used as low-cost system for angle measurements
Especially, if usage of classical goniometers is not feasible due to space constraints or risk of damage
However, flex sensors do not achieve the high accuracy of electronic goniometers
21th annual Congress of the EUROPEAN COLLEGE OF SPORT SCIENCE, 6th –9th July 2016, Vienna - Austria

15. Practical application: Skiing
Ankle angle (°) Knee angle (°)