A NEW BIOMECHANICAL MEASUREMENT AND TESTING METHOD
FOR TURNS IN ALPINE SKIING
O. Rađenović*, B. Nemec**, and V. Medved***
*Zagreb Sport Association, Zagreb, Croatia
**Department of Automation, Biocybernetics and Robotics, Jožef Stefan Institute, Ljubljana,
Slovenia
*** Faculty of Physical Education, University of Zagreb, Zagreb, Croatia
E-mail: [email protected]
Abstract: We have measured electromyographic
(EMG) signals from four left leg muscles: m.rectus
femoris (m.r.f.), m.vastus lateralis (m.v.l.), m.vastus
medialis (m.v.m.) and m.biceps femoris (m.b.f.) and
ground reaction force below ski boots, during
performance of wedge turns. Standard and carving
skis were used and two types of wedge turns were
performed, the Croatian and the Austrian variant.
Video recordings were used to monitor kinematics.
EMG signals and ground reaction forces were
compaired for both types of skis and both types of
turns. The least strain develops while performing the
Austrian variant of the wedge turn using carving
skis. These results may have implications for the use
of this type of skis in teaching basic skiing
techniques.
Introduction
In teaching basic and advanced skiing techniques,
performance of the elements of the ski-school depends
mostly on the skiing knowledge and physical
preparation of the skier 1.
The wedge turn is one of the most essential and basic
elements of the ski-school. Therefore, the way this
element is taught will influence all subsequent steps and
elements of basic and advanced ski technique.
One of the first researches involving EMG in the
biomechanics of skiing was done by Asang et al. 2,
where the authors have monitored EMG from four leg
muscles in conjunction with force measurements using a
dynamometer between the binding and the boot during six
elements ranging from a herringbone step to a parallel
turn. They reported substantional activity from the
anterior tibialis and adductors relative to the quadriceps
femoris, and low levels of activity from the triceps surae.
Exstensive research has been done on 21 Austrian ski
instructors that have performed a progression of turns
based on ski teaching curricula 3. The instructors skied
in a variety of snow conditions and over different terrains.
Measurements included EMG from eight muscles, force
from strain gauges mounted on the skis, knee angls from
goniometers, and qualitative kinematics from film. Turns
were partitioned into initiation and steering phases and
distinguished by unique characteristics in the initiation
phase. Involvement of the anterior tibialis and peroneus
longus in edging, and gluteus maximus and quadriceps
muscles in extension of the knee and the hips were among
the general patterns of muscle activity reported.
Clarys et al. 4 examined the effects of three types of
skis (racing, soft, and compact) on muscular activity
during three types of teaching turns and a straight run.
They monitored six leg muscles on eight ski instructors
and reported mean EMG activity of all muscles
combined. They concluded that since the soft ski elicited
the least muscular activity, it was the best ski for
recreational and competitive use.
Clarys and Cabri 5 have summarized the research
that has been done using EMG in different sports: 13
different studies using EMG have been done in alpine
skiing, during slalom, giantslalom, supergiantslalom and
different competition conditions.
In this report, we present biomechanical analysis of
the wedge turn using video information, EMG and a
system for ground reaction forces determination 6,7.
This approach has permitted an assesment of strains that
arise during performance of this element using either
carving or standard skis, and the comparison of the
Croatian and Austrian variants of this turn.
Materials and methods
The measurements were performed on a member of
the Croatian Ski Demo Team (age 30, 165 cm height, 68
kg mass). Surface EMG signals were measured using
Muscle Tester ME3000 Professional 8. Measurements
were performed on Rogla ski resort in Slovenia. The
skier had the electrodes of the Muscle Tester ME3000P
put directly on the skin, above the muscles tested 9,
and the device was used in a terrain mode. After A/D
conversion, EMG signals were smoothed (full-wave
rectified and low-pass filtered-100 ms time constant).
Portable parts of the both devices: for ground reaction
force measurements and EMG were put on a belt that
was attached to the skier's waist. Kinematic information
was simultaneously acquired by a VHS-video camera.
The measurement begins when a performer pulls a
trigger: the EMG, the unit for ground force reaction
measurement, as well as the flash, are connected to the
trigger. The time is recorded on the VHS-video camera
when the flash goes on, and that moment is considered a
zero point of the measurement.The procedure is exactly
the same and the end of the measurement. The terrain
task consisted in the following: at the ski slope of 50 m
in length and an incline of 15-17, and the slope was
groomed daily to maintain consistent snow conditions.
The skier performed eight wedge turns, four using the
Croatian variant of the turn, and another four using the
Austrian one. The procedure was repeated three times.
This paradigm was performed twice, using standard and
carving skis.
Ground reaction force was determined by a device
described in 6. The force transducers are composed of
two measuring plates, each plate containing two
sensors. The measuring plates were mounted onto the
ski boot sole (Fig.1.). The scheme of the tensiometric
plate is shown in Fig.2.
tpy  l1 
d 1 ( f 11 f 12)l1 ( f 21 f 22)(l1 d 1) F 02(l1l 2  d 2)

Fr
2
tpx 
d2
( f 11  f 21)d 2

2
f 11  f 12  f 21  f 22
, (1)
where F01 and F02 are the forces of the binding, and fij
are the forces from the corresponding sensors according
to Figure 2. The binding forces can be calculated by
measuring the force foij on sensors loaded only with ski
binding force, using the equations:
F 0  f 11  f 12  f
21 
f
22
F 01 
F 0l 2  ( f 011  f 012)d 1
d 1  l1  l 2
F 02 
F 0l1  ( f 021  f 022)d 1
d 1  l1  l 2
.
(2)
If we insert eq. 2 into eq. 1, we get a simplified relation
in the form of:
Fr  f 11  f 011  f 12  f 012  f 21  f 021  f 22  f 022
tpy  l1 
tpx 
Fig.1. Ski boot instrumented with four force sensors.
d 1 ( f 11  f 011  f 12  f 012)l1  ( f 21  f 021  f 22  f 022)(l1  d 1)

2
Fr
( f 11  f
011 
f
Fr
21 
f
021) d 2
.
(3)
The above equation shows that it is not necessary to
calculate the binding forces F01 and F02 as we can
instead subtract the signal of each force sensor loaded
only with ski binding force 6.
During analysis of measurement records, in each
video record three time instants were identified (Figs. 3.
and 4.): 1. the entrance, 2. the steering and 3. the exit.
They represent corresponding phases. The second phase
of the turn is the most important during performance; as
shown in Figs. 3 and 4, and the most marked difference
in technique between the Croatian and Austrian variants
is precisely in this phase. Video recordings were used to
determine precisely the beginning and the end of each
phase.
Results and discussion
Fig.2.
Tensiometric
corresponding acting forces.
plate,
sensors,
and
From the sensor data, fij (see Fig. 2), it is possible to
estimate the total vertical force, Fr, in the z axis and the
torques around the x and y axes, or in a more convenient
form, the point of the application of the vertical forces,
tpx and tpy, using the following equations derived from
the balance of angular momentum:
Fr = f11 + f12 + f21 + f22 - F01 - F02
Differences between Croatian and Austrian variants
of the wedge turn can be seen from kinematic records in
Figs. 3 and 4. When performing the Croatian variant, in
the second phase of the turn the outside (dominant) leg
is mostly loaded, in contrast to the Austrian turn, in
which during the same phase both legs are loaded
almost equally.
Tables 1, 1a, 2 and 2a show the average results of
six independent measurements during performance of
both variants of the wedge turn, on standard and carving
skis, in three phases of the right turn.
As shown in Tables 1, 1a, 2 and 2a, the developed
muscle force estimated via EMG signals is almost the
same for the m.b.f. for both variants of the turn and both
types of skis. The wedge turn, however, is a relatively
static element, with minimal vertical movements of the
body, and the activity of m.b.f. is very low.
As shown in Tables 1, 1a, 2 and 2a the more intense
strain in m.v.m. and m.r.f. develops during performance
of the Croatian variant on standard skis. The m.v.l. is
maximally loaded in the Croatian variant on carving
skis, which is influenced by performing technique.
instant 1
instant 2
instant 3
Slightly higher activities of this muscle are seen in
the Austrian variant of the turn on both types of skis,
because of different technique used, namely the centre
of gravity is closer to the centre of the turn (see Fig. 4.,
instant 2). More important are the activities of m.v.m.,
which drags the knee to the centre of the turn, and of
m.v.l. and m.r.f., which are required to strengthen the
knee during conduction of the wedge turn. As shown in
Figs. 6. and 7., the large scattering implies that the
difference across selected instants is large for standard
skis; also the maximum strain is large when performing
the turn on standard skis. Importantly, on standard skis
the muscles are constantly loaded and the difference
between phases is minimal. The ground reaction force
(Fl.l. - left leg force, and Fr.l. - right leg force), is higher
during performance of the wedge turn on standard skis
(Figs. 6. and 7.). For carving skis (Figs. 6. and 7., the
right side of the graph.), the reaction force is smaller
and equally distributed on both legs which results in less
strain in contrast to standard skis for which just one leg
is loaded.
Conclusions and perspectives
Fig. 3. Croatian variant of the turn with EMG
signals for each phase.
instant 1
instant 2
instant 3
We have measured and studied the performance of
the wedge turn, which is one of the basic elements of
the ski school. The skier who performed the
measurements is a member of the Croatian Ski Demo
Team and uses model technique, which makes our
results a plausible model for the performance of this
element Since carving skis are being widely used even
by begginers, we have chosen to analyze the wedge turn
which is often the first element taught in the ski school
and compaired its performance on standard and carving
skis, using the Croatian and the Austrain variant of the
turn. Our results show that the least strain develops
when performing the Austrian variant of the wedge turn
using carving skis. Similar measurements can be done
for other elements of the ski school and changes in
teaching ski technique can be made accordingly to,
hapefully, reduce time and effort required to learn new
sking skills.
Table 1. Average values of smoothed EMG signals
and ground reaction force measurements in selected
time instants during performance of the Croatian variant
on carving skis.
measure
muscles [µV]
m. r. f.
m. v. m.
m. v. l.
m. b. f.
carving ski
Croation variant
instant 1 instant 2 instant 3
34.00
149.00
165.50
30.50
46.25
480.25
238.75
28.25
35.75
7.50
37.00
9.00
392.33
441.60
501.51
391.43
500.37
442.55
g.r.f. [N]
Fig. 4. Austrian variant of the turn with EMG
signals for each phase.
left leg
right leg
Table 1a. Average values of smoothed EMG signals
and ground reaction forces measurements in selected
time instants during performance of the Croatian variant
on standard skis.
measure
standard ski
Croatian variant
muscles [ µV ] instant 1 instant 2 instant 3
m. r. f.
m. v. m.
m. v. l.
m. b. f.
18.00
85.00
66.25
37.25
51.75
618.00
123.50
50.00
24.50
273.75
207.50
18.75
468.58
327.73
1030.61
747.95
866.23
230.67
g.r.f. [N]
left leg
right leg
Table 2. Average values of smoothed EMG signals
and ground reaction forces measurements in selected
time instants during performance of the Austrian variant
on carving skis.
Fig.6.
Graphical
representation
for
the
measurements of the Austrian variant on carving (the
right side of the column) and standard skis.
measure
carving ski
Austrian variant
muscles [ µV ] instant 1 instant 2 instant 3
m. r. f.
m. v. m.
m. v. l.
m. b. f.
65.75
204.50
144.00
18.00
36.75
221.50
129.75
54.00
31.75
21.00
56.00
9.75
427.08
360.10
403.34
369.32
440.16
327.35
g.r.f. [N]
left leg
right leg
Table 2a. Average values of smoothed EMG signals
and ground reaction forces measurements in selected
time instants during performance of the Austrian variant
on standard skis.
measure
standard ski
Austrian variant
muscles [ µV ] instant 1 instant 2 instant 3
m. r. f.
m. v. m.
m. v. l.
m. b. f.
25.25
75.50
54.00
31.00
52.50
223.25
170.25
75.25
43.75
205.75
112.75
20.75
717.58
304.19
722.88
313.44
714.65
294.52
g.r.f. [N]
left leg
right leg
Acknowledgment
This study was supported by The Ministry of
Science and Technology of the Republic of Croatia
(Project No. 034-004).
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