Developing a Motion Analysis System for Aerial Sports by Inertial
Sensors
Heike Katrein
Brock
PhD Candidate
Graduate
School of Media and Governance
Intention
One of the most interesting objectives for the provision of
technological support in sports is to obtain additional performance feedback
for the enhancement of motor skill acquisition and training. With ubiquitous tools
for performance monitoring, more and more training and activity surveillance
applications that provide data insights or recommendations on future activities
appeared over the last years in leisure, health and recreation. They alter and
revolutionize human activity habits in everyday life. In contrast to the
increasing technological supply of the leisure market, performance monitoring
tools are however still less common and often also less developed in
competitive and professional sports so far. This is due to the fact that the
motion data is more complex on the one hand, while on the other hand generally
also more accurate and complex motion information for processing is required.
For an automated, technology-supported training environment it can therefore be
useful to create meaningful machine knowledge on the base of relevant motion
information.
During the last year, I investigated how to evolve and adapt common
machine learning methods, so that motion can be automatically processed and
analyzed. Here, I particularly considered aerial sports for an establishment of
new flight monitoring system for training and competition. Aerial sports still remain
relatively unexplored and are generally subject to direct supervision by
coaches. Motion feedback is given to the athletes by visual or video data analysis
and generally no quantitative data is used to affirm the visual impression. The
development of a flight performance measurement system is consequently not only
important to understand the motion, but also to improve safety issues in aerial
sports.
Research details
For my research, the development of techniques that provide motion
information on an easy-to-use common basis like wearable sensor devices was a
crucial landmark. Especially in sports with extremely large capture volumes as
aerial sports and difficult natural weather and daylight conditions, cheap,
small and light devices such as inertial measurement units (IMUs) are very
beneficial. They capture human motion in a direct way and do not depend on
outer capture conditions so that they can also be used over a wide area. However,
they only measure angular velocities, acceleration and gravity of designated
body parts and can generally not display all information necessary for a
complete motion analysis. Therefore, the main question is to find specific motion
knowledge from the sensor data, so that useful information can be extracted and
provided to users like coaches and athletes. Gradually enhancing sensor
hardware as well as processing methods that determine characteristics and
kinematic performance parameters like angular changes and body positions, the
first step for tuning sensor-based training and performance monitoring tools
into more sophisticated devices for future applications has been made in the
previous years. Now it is time to use the methods and algorithms for meaningful
applications.
The crucial part for this research was to conduct experiments under
real conditions to obtain sufficient aerial motion data for the development of
the intended innovative motion analysis algorithms. I chose ski jumping for the
creation of a main motion data set. In two experiments in summer with several
training sessions and training days, I could collect sufficient fundamental
data for an analysis (more than 180 jumps in total). Figure 1 gives an
impression of the experiments.
Figure 1: Experiment in summer at Myoko Kogen
ski jump stadium. Sensor are attached to the athletefs body and skis and the
athletefs body movements during jump captured.
The winner of a ski jump competition is decided on a scoring system
based on distance, style points awarded by five judges and inrun length and
wind conditions to compensate for the variable outdoor conditions. To obtain a
measure for different jump qualities and to evaluate the following feature
selection strategies, I additionally took note of the jump lengths of every
jump as well as their point scores awarded by an experienced ski jumping judge.
The judge scores were collected on paper in real-time during the data capture
sessions and under real judging circumstances from the judge's tower. They
consequently conform to score results as they are obtained in real
competitions. After data acquisition, all score sheets were digitized to be
used as ground truth in the machine learning and testing step. Figure 2 shows a
sample judging score sheet with point deductions acquired in my experiments.
Figure 2: Sample judge score sheet for the
evaluation of flight performances acquired during the data experiments.
Results
Data accuracy and usability:
The screening of the captured data showed that data could be captured
well in general, whereas not all acquired data could be used in the next step. One
problem was that the wireless data transmission between computer (software) and
sensors did not work flawlessly in every capture process and the transmission
of the start signal sent from the computer could not always reach all sensors. Another
problem were concussions and random noise of the gyro rate sensor modules
within the measurement units attached to the skis as a result of high landing
impact. This noise could be observed as peculiarity and linear deviation from
the zero sensor reading occurring immediately data after a hard landing. Those
concussions of the gyro rate sensor modules led to bias in the sensor data for
generally 30-40 minutes or the rest of a capture session until battery
depletion or the manual turning off the sensor status.
Incomplete jumps with missing or distorted sensor data, have been not
considered within the use and building of the analysis methods, so that from
the 180 captured jumps, approximately 100 jumps have been then selected to form
a ski jump data set. All of the selected ski jumps have then been further
investigated with the processing methods developed in an earlier stage of my
research, whereas 60% of the jumps were used as a training data and the
remaining 40% were chosen to be used to test the results of the newly
implemented algorithms.
As a first step, I computed all positions and orientations for the
jumps in the training data to analyze, compare and evaluate the flight
performances in various motion information retrieval tasks. Looking at the
general orientation data, I could observe huge variations in the heading values
among all jumps in the test data set. Initial heading differences were
especially present in dependence on the sensors used during every specific
capture session, the body segment they were attached to and the date of the
data acquisition. Such differences and errors appeared randomly within the
complete data set and the different capture sessions and seemed to be
influenced by the following properties: different sensitivities in the sensor's
magnetometers (every sensor measures a slightly different magnetic field at the
same position of the ski jump hill), closeness to the inrun slope and jump hill
(sensors attached to the arms are less variant than sensors attached to the
thigh and for example close to the magnetic start gate) as well as differences
in the magnetic field with changing outer weather conditions.
I concluded that variances in the magnetic field are responsible for
the registered heading variations - especially as ski jumping hills are built
from a lot of ferromagnetic materials like steel. As research showed that differences
already occur in less electric and magnetic environments, magnetic bias is
likely to be even larger in artificial, man-made environments with jumping
ramps, slopes and technical equipment and the magnetic measurements cannot be
trusted blindly. Consequently, any processing system should be made invariant
to magnetic differences and variations first. For this, I introduced a new
magnetic bias compensation method, which can drastically reduce the variation
within the magnetic heading to stable and reasonable values.
Figure 3: Visualization of the differences in
heading before (top) and after (bottom) magnetic bias compensation at various sensor
locations. Heading angles are of positive values 0 toΞon the right side of the circle and
of negative values 0 toΞon the left side of the circle, whereas 0 is at the bottom and }pi at the
top of the circle plot. Red pies represent the angular areas in which initial
heading angles occur.
Application in motion analysis and evaluation:
After verifying the robustness and accuracy of the estimates, the data could
then be used for the development of motion analysis methods. For this, it was
necessary to transform the kinematic motion data into data representations that
describe well the motion performance. Ski jumping is subject to two kinds of
motion descriptors: technical determinants of a motion and the aesthetic
overall impression of flight, landing and outrun. A combination of both has
been used in this research as motion descriptors for feature extraction.
The flight characteristics were described by certain style criteria in
the jump phases flight, landing and outrun (as they are also shown in Figure 2
at the filled judging score). Particularly important specifications for the
flight phase are the active use of air pressure and aerodynamic conditions by a
bold and aggressive move at the take-off with a rapid and smooth transition to
the optimal flight position, characterized by steady ski positioning, good
balance and symmetric body and ski positioning. For maximum landing points (or
no point deduction), the landing should take place in a standardized landing in
a squat-like position with one leg slightly shifted in front of the other, the
so-called Telemark, and no other body parts touching the ground. Remaining in
the stable Telemark position for approximately 10 to 15 meters and a stable
outrun until the fall line are the main indicators for little or no point
deduction in the outrun. Faults and deviations are punished in dependence on
their severity and time of occurrence during flight.
For this research, I derived five performance determinants A1-A5 for
the aerial phase and five performance determinants L1-L5 for the landing phase.
After a segmentation into the two jump phases on the base of the raw sensor
signals, I calculated the body kinematics for every jump with respect to the
ten style criteria and transformed the motion descriptions into feature
matrices that contain the information on every jump criteriafs specialties. A
feature matrix for the flight phase then for example contained several single
features like the v-opening angle of the skis, the forward lean angle of the
upper body and the ski-attack angle (ski elevation angle), the feature matrix
for landing the bending angle of the knees, the positional distance between the
two ski boots and the orientation of the arms, as shown in Figure 4.
Figure 4: Visualization of ski jump style
characteristics. The purple angles and distances display indicators of flight
style quality and can be used as feature extractors.
As a next step, I set up an algorithm to evaluate the jumps in the test
database. Reference feature matrices have been built for all style criteria
from all jump files in the training data set for the two cases faulty execution
and non-faulty execution. With the averaged reference motion features, I then
investigated how the similarity between a test jump to the trained reference
features of either error or no error could be measured. In concrete, a
framewise distance for single feature vectors within the reference matrix and
the incoming matrix of the test jump was computed. The distances were then
summed up under the principles of Dynamic Time Warping (DTW), yielding a final
accumulated cost as the similarity measure per every style criteria. I then
labeled an error as existent, if the similarity measure between M and the
faulty reference feature was smaller than the similarity measure between M and the
non-faulty reference matrix and vice versa. Such labeling process was repeated for
every style criteria and the annotations then compared with the annotations
made by the human judges. Every correct and incorrect labeling was counted and
the final count used to determine the precision and recall over all faulty and
non-faulty jumps within the test data set. The distribution of precision and
recall values for all C is shown in Figure 5. As one can see, precision and
recall are of relatively good value, with recall ranging between 70 and 85%
probability of a successful evaluation.
Figure 5: Precision and recall values for the different
style criteria and the probability border of 50% (red line).
Future work
Although precision and recall show an overall satisfying accuracy, it
is not possible to further distinguish within different errors so far. To
really evaluate a performance in a meaningful way, it is now necessary to train
further knowledge that can award point deductions within the different style
categories with respect to the gravity of every occurring error. In future
work, respective learning algorithms shall be implemented that retrieve the
amount of deviation from a non-erroneous motion pattern and therewith create
powerful performance knowledge. Then, it can become possible to numerically analyze
a jump performance - and the framework be used in competition to increase measurability
and objectivity as well in training to gain more reliable insights into motion
executions, to improve performance or to recruit talent.
Next, the algorithms have to be tested with other aerial sports than
ski jumping that might be easier to access and verify, but that might also be
more difficult to evaluate (depending on the certain characteristics of related
sports).
Conference
Presentations
As outcome of my research, I have given an oral presentations at the
annual conference of the International Association of Computer Science in
Sports last autumn in Loughborough, Great Britain.
Heike Brock, Yuji Ohgi. Towards
better measurability - IMU-based feature extractors for motion performance
evaluation. 10th International Symposium on Computer Science in Sport,
Loughborough, UK, 2015.
Two journal papers have been submitted for publication in scientific
journals in 2015 and have already been revised several times. Currently, I am now
awaiting their final acceptance. Furthermore, I am in the process of finishing
a third journal paper by the end of this month, and have planned to write a
fourth paper in March and April this year to sum up my thesis work.