Accuracy of ankle- and knee-angle measurements with thin flex sensors
K. Schindelwig, N. Marterer, D. Morawetz, S. Kapferer, R. Csapo, W. Nachbauer
Dept. of Sport Science, Univ. of Innsbruck, Austria,
Introduction
The accurate determination of joint angles is a prerequisite for biomechanical research. Electronical goniometers are cheap, transportable and easy to use. However, in some situations their application is complicated. For instance, in alpine skiing the thickness of goniometers precludes mounting within ski boots, limiting the possibilities to obtain reliable measures of ankle joint angle. Externally fixed goniometers are also prone to damage, e.g. through impacts with external objects such as flex poles. Flex sensors feature a thickness of 0.4 mm only and may, therefore, represent an alternative when the use of goniometers is not feasible. The aim of this study was to determine the accuracy of measurements of knee and ankle angle (flexion and extension) as performed with thin flex sensors during deep squats.
Methods
Three subjects (age 21.3 ± 1.6 yrs, height 1.72 ± .09 m, mass 68.1 ± 3.1 kg) performed six series of 15 deep squats. Motion speed was gradually increased after completion of two series from 15 to 45 repetitions per minute. The motion speed was controlled through the use of an electronic metronome. The flex sensors (Spectra Symbol, Flex Sensor 11.43 cm (SEN-08606), Salt Lake City, UT) were attached on both legs, centrally over the back of the foot and laterally on both sides of the popliteal fossa. The attachment sites were chosen to allow for longitudinal motion of the sensors. Data were sampled at 1500 Hz and transmitted wirelessly to a recording system (Telemyo 2400T G2, Noraxon, USA). Reference measures were acquired with an active three camera motion capturing system sampling at 100 Hz (AS 200, LUKOtronic, Innsbruck, Austria). Kinematic data of the right and left leg were separately recorded and manually synchronized with the flex sensor recordings. The maximum and standard deviation (SD) of differences between the data obtained with both systems were calculated as measure of accuracy.
Results
Across the six deep squat series, the mean ranges of movement in ankle and knee joint were 39° and 94°, respectively. For the ankle and knee joint, the mean of the maximum differences between the measures obtained by flex sensors and motion analysis were 9.1° (23.6%) and 13.1° (14.0%), respectively. The mean of the SDs of differences were 2.9° (7.5%) and 4.0° (4.3%). Measurement accuracy was independent of movement speed.
Discussion
The flex sensors yielded a satisfactory accuracy for measurements of ankle- and knee angle. Therefore, they may be used as low-cost system for angle measurements that may be applied where usage of electronic goniometers is not feasible due to space constraints or risk of damage. However, it should be noted that goniometers feature superior accuracy and should be used when high measurement precision is required.