Wireless instrumented klapskates for long-track speed skating

Abstract

In the current project we aim to provide speed skaters with real-time feedback on how to improve their skating performance within an individual stroke. The elite skaters and their coaches wish for a system that determines the mechanical power per stroke. The push-off force of the skater is a crucial variable in this power determination. In this study we present the construction and calibration of a pair of wireless instrumented klapskates that can continuously and synchronously measure this push-off force in both the lateral and normal direction of the skate and the centre of pressure of these forces. The skate consists of a newly designed rigid bridge (0.6 kg), embedding two three-dimensional force sensors (Kistler 9602, Kistler Group, Winterthur, Switzerland), which fits between most individual skate shoes and Maple skate blades. The instrumented klapskates were calibrated on a tensile testing machine, where they proved to be unaffected to temperature conditions and accurate up to a RMS of 42 N (SEM = 1 N) in normal and up to a RMS of 27 N (SEM = 1N) in lateral direction. Furthermore the centre of pressure of these forces on the blade was determined up to a mean error of 10.1 mm (SD = 6.9 mm). On-ice measurements showed the possibility of recording with both skates simultaneously and synchronously, straights as well as curves The option to send data wirelessly and real-time to other devices makes it possible to eventually provide skaters and coaches with visual real-time feedback during practice.

Full text

TECHNICAL NOTE
Wireless instrumented klapskates for long-track speed skating
E. van der Kruk
1
•O. den Braver
1
•A. L. Schwab
1
•F. C. T. van der Helm
1
•
H. E. J. Veeger
1
ÓThe Author(s) 2016. This article is published with open access at Springerlink.com
Abstract In the current project, we aim to provide speed
skaters with real-time feedback on how to improve their
skating performance within an individual stroke. The elite
skaters and their coaches wish for a system that determines
the mechanical power per stroke. The push-off force of the
skater is a crucial variable in this power determination. In
this study, we present the construction and calibration of a
pair of wireless instrumented klapskates that can continu-
ously and synchronously measure this push-off force in
both the lateral direction and normal direction of the skate
and the centre of pressure of these forces. The skate con-
sists of a newly designed rigid bridge (0.6 kg), embedding
two three-dimensional force sensors (Kistler 9602, Kistler
Group, Winterthur, Switzerland), which fits between most
individual skate shoes and Maple skate blades. The
instrumented klapskates were calibrated on a tensile testing
machine, where they proved to be unaffected to tempera-
ture conditions and accurate up to an RMS of 42 N
(SEM =1 N) in normal and up to an RMS of 27 N
(SEM =1 N) in lateral direction. Furthermore, the centre
of pressure of these forces on the blade was determined up
to a mean error of 10.1 mm (SD =6.9 mm). On-ice
measurements showed the possibility of recording with
both skates simultaneously and synchronously, straights as
well as curves. The option to send data wirelessly and real-
time to other devices makes it possible to eventually pro-
vide skaters and coaches with visual real-time feedback
during practice.
Keywords Speed skating Calibration Force
measurements Instrumentation design Real-time
feedback
1 Introduction
Force and power production are crucial factors in any
performance sport. Insight in the force pattern produced by
an athlete and the related relevant velocities can help
understand the technique and performance of that athlete.
Ideally, we would like to provide speed skaters with real-
time feedback on their skating performance within an
individual stroke. The Dutch elite skaters and their coaches
would like to have access to a system that determines the
power per stroke, which is a familiar variable for them
from cycling practices. The push-off force of the skater is a
crucial variable in this power determination [1]. For power
estimations, the velocity of the skater and the orientation of
the skate in the global frame are a necessity. Therefore, to
fulfil the wish of the skaters, accurate measurement sys-
tems that can capture the kinetic and kinematic data of a
skater, and preferably send it real-time to a device for
feedback (e.g., phone, tablet, smart glasses), are essential.
Since the accuracy of all these measurements are of utmost
importance for the quality of the feedback, we will deal
with these in separate studies. In this paper, we present the
construction and validation of a set of wireless instru-
mented klapskates that can measure the forces applied by
the skater. The validation and measurements of the orien-
tation of the skate, which will be performed with an IMU
and self-designed filter, and the velocity of the skater are
presented in a follow-up study.
Publication on instrumented skates started in the early
90s of the last century. At that time, speed skaters skated on
&E. van der Kruk
e.vanderkruk@tudelft.nl
1
Department of Biomechanical Engineering, Faculty of
Mechanical Engineering, Delft University of Technology,
Mekelweg 2 2628 CD, Delft, The Netherlands
Sports Eng
DOI 10.1007/s12283-016-0208-8

the conventional fixed skates, where the blade of the skate
was fixed to the shoe. Two studies were published on
instrumented fixed skates. The pioneers in this field aimed
at measuring normal forces in speed skating on the
straights and in a curve [2,3]. They constructed a right
skate with three temperature compensated strain gauges
that could measure in normal direction and longitudinal
direction in a local coordinate system [4]; (Fig. 1). Their
study on-ice friction [5] measured the longitudinal forces
on the blade and demonstrated that the maximal forces in
longitudinal direction were 10 N, which is less than 1 % of
the force in normal direction. From these studies, we
conclude that measuring this small component of force is a
technical challenge, while it is negligible relative to the
total force.
Yuki et al. [6] measured the normal and lateral forces in
speed skating on the straight. For this study, two sensor-
skates were built, which consisted of strain gauges between
the shoe and the blade. Their study aimed at measuring the
horizontal (F
h
, Fig. 1) and vertical (F
v
) blade reaction
forces for two different skating velocities, slow (9.1 m/s)
and fast (11.5 m/s), respectively. Their study was the first
to also report the centre of pressure of these static resultant
forces on the skate blade (COP). Their results, based
on one-skilled skater (n=1), indicated that the peak
lateral forces were considerable, namely between the
F
L
=0.3–0.6 BW (BW =body weight), which is equal to
25–45 % of the peak normal force. Since the lateral force
proved to be considerable, measuring this component
seems inevitable when analysing speed skating forces.
Furthermore, a relationship was found between the per-
formance of the speed skater and the COP. This makes the
COP, an esteemed variable for feedback.
In the mid-90s, fixed skates were replaced by klapskates,
which incorporate a hinge between the blade and the shoe.
The first constructed instrumented klapskate measured
forces in normal direction, thereby again neglecting the
forces in lateral direction [7]. Not long after, Yuda et al. [8]
constructed a (left) instrumented klapskate that could
measure in normal direction and lateral direction. The skate
was equipped with two quartz voltage sensors which
measured three components between the boot and the
bridge of the skate. The data were logged on a computer
carried on the skater’s back. Their study reports on mea-
surements in the curve with the left instrumented skate.
Unfortunately, no data on the calibration or accuracy of the
force measurements were reported. Drawback of the design
of this instrumented klapskate was the necessity for the
skaters to carry a laptop to log the data (3.3 kg). In addi-
tion, the sensors were placed between the skate shoe and
the blade as a result of which every participant had to wear
(and fit) the same (only left) skate shoe. To perform true
measurements and preserve participation of many skaters, a
skater needs to be able to wear his own skate shoe, and
preferably blade, during measurements.
Three previous studies have described calibration pro-
cedures for instrumented skates in long-track speed skating
[4–6]. In all three studies, which use static calibration with
weights, the accuracy determination focused on the cross-
talk between the force directions rather than the accuracy
of the force directions itself. Since also the relationship
between the forward velocity (performance) and the mea-
sured forces had not completely been grasped yet, literature
did not provide us with an adequate benchmark for the
accuracy requirement for an instrumented skate. One study
has been published on the construction and calibration of
an instrumented ice hockey skate, which measured the
forces in normal direction and lateral direction, with strain
gauges, and reported on the calibration accuracies in the
separate directions [9]. Their skates were calibrated with an
error of 68 N in normal direction and 40 N in lateral
direction.
In this study, we present the construction and calibration
of a pair of instrumented klapskates that can continuously
and synchronously measure both the lateral and normal
forces in a local frame and the COP of these forces. The
skate is designed to fit most individual skate shoes and
Maple skate blades [10]. In addition, the developed system
can log data wirelessly and locally through a logger at the
rear of the skate. In the design, the longitudinal force is
neglected, since this is assumed to be less than 1 % of the
normal force. The skate is built to be part of the
Fig. 1 Normal force (F
N
) and lateral force (F
L
) are the forces
measured in the local skate coordinate system (as in [2,4,7]). The
forces F
v
and F
h
are the vertical and horizontal components (as in [6,
8]) of the push-off forces F
N
and F
L
.ais the lean angle of the skate
and describes the angle between the skate and the ice. bis de angle
between the shank and the skate and describes the eversion of the
ankle. Given is the rear view of the right skate, the force F
L
is for both
the right and the left skate negative in medial direction. The lean
angle is positive, while the blade is on the medial side
E. van der Kruk et al.

instrumentation to provide speed skaters with real-time
feedback during practice on improving their performance
within a single stroke.
2 Method
In this section, first, the design of the instrumented klap-
skates is described. Subsequently, the calibration set-up,
the calibration routine, and calibration analyses are
explained. The section closes with the description of the
on-ice skating measurements.
2.1 Instrumented klapskates
The instrumented klapskate consists of a custom-made
rigid aluminium bridge (mountable on Maple skates), with
two three-dimensional piezoelectric force sensors with
integrated electronics (Kistler 9602, Kistler Group, Win-
terthur, Switzerland) (Fig. 2). Due to the design of the
bridge, the sensors are not aligned with the bridge. The
voltage output of the front (V
fx
,V
fy
,V
fz
) and rear (V
rx
,V
ry
,
V
rz
) sensor are logged on an SD card and sent over Blue-
tooth via a data logger [11]. The logger is further equipped
with an accelerometer, gyroscope, and magnetometer
(IMU), and logs temperature for temperature-compensa-
tion. The IMU will be used in future application for ori-
entation measurements; these are, however, not discussed
in this study. The force sensors are powered by recharge-
able Li-Ion batteries. Synchronisation between the two
skates is done over Bluetooth. To enable synchronisation
with external measurement devices, a digital start–end
pulse can be logged. The weight of the instrumented bridge
and electronics is 600 g. The instrumented bridge replaces
the normal bridge in a klapskate. It can be attached to any
Maple blade via the hinge mechanism, and any skate shoe
can be placed on it. The instrumented bridge does not
increase the height of the skate (Fig. 3).
2.2 Calibration
The calibration of the instrumented skates was performed
using a tensile testing machine (Zwick Z100, Zwick Roell,
Ulm, Germany, principal accuracy 1 N). The aim of the
calibration was to calibrate the forces on the instrumented
skates in normal direction and lateral direction and the
centre of pressure of these resultant forces on the blade
(COP). Since it was uncertain whether the measurements of
the skate were influenced by the low environmental tem-
perature on the ice rink, ranging between -5to0°C on the
ice and 0 to 5 °C just above the ice, first, the temperature
dependency was determined.
The tensile testing machine was placed in a climate-
controlled chamber to regulate the temperature. Each skate
was positioned blade up in four different positions (P1–P4)
on the fixed lower head of the testing machine. The ref-
erence force F
LC
, exerted by the movable upper head of the
machine ([50 mm), was applied directly to the blade via a
constant displacement, up to a maximum force, and
Fig. 2 Overview of the instrumented bridge and an attached blade.
The lower part of the bridge is only attached to the blade via the
hinge. The upper part of the bridge is attached to the lower part via
the Kistler bolts under pre-stress. The skater can place its own shoe on
the bridge via the shoe mounting nuts. The small white box near the
heel is the recording and transmission unit for the measured data
Fig. 3 Calibration Set-up; athe tensile strength machine exerts the
force F
LC
, due to the tilting angle kunder which the skate is
positioned, this results in a force in normal direction and lateral
direction. bThe upper head moves down to exert force on the blade
of the instrumented skate. The skate can be placed in five different
positions (P1–P5) to calibrate the COP and can be placed under an
angle k(see Fig. 2) to calibrate the normal and lateral forces
Wireless instrumented klapskates for long-track speed skating

released by the same constant displacement. A wedge was
designed to place the skate under five tilting angles (k
ta
)
with k
ta
=-7°,7°,0°,-20°,20°, to distribute the applied
force F
LC
into a normal (F
N
) force and a lateral (F
L
) force
on the skate.
2.2.1 Temperature
The temperature dependency test was conducted under four
different temperatures (-5, 0, 5 and 19 °C) at position P2,
by applying a rising force up to 1100 N. From each con-
dition, 12 samples were taken (every 100 N) and the
combined output voltages V
fz
?V
rz
were compared
between temperature conditions. Results showed that the
output voltages were uncorrelated to the temperature con-
ditions (R=-0.015, p=0.89 for left skate and
R=-0.0037, p=0.9712 for the right skate). Therefore,
the remaining calibration could be performed at room
temperature (19 °C).
2.2.2 Normal force and lateral force
The calibration in normal direction and lateral direction
was performed at room temperature (19 °C), in four dif-
ferent positions (P1–P4) under the five tilting conditions
k
-7°
,k
7°
,k
0°
,k
-20°
, and k
-20°
. At each position, under each
condition, 12 samples were taken from the force data,
equally divided from zero up to the maximum measured
force. The maximum forces differed between conditions
due to the applied tilting angle, but ranged from 1600 N to
2500 N in normal direction and from -610 N up to 610 N
in lateral direction.
Calibration was done using a forced entry regression
analysis with a second-order fit in both the normal direction
and lateral direction. This second-order fit was believed to
be necessary due to non-linearity in the material defor-
mations of the bridge, the heel cup and the spring and to
intercept any differences in pre-stress of the sensors. The
characteristics of the heel cup and the spring mainly
influence the force transition in lateral direction and its
influence varies with the position of the applied forces in
lateral direction. Therefore, the ratio RV was introduced as
a measure of this position:
RV ¼Vfz Vrz
Vfz þVrz
ð1Þ
This ratio was introduced as an input into the lateral
regression, to improve the lateral force estimation. Overall,
input for the second-order regression analysis in normal
direction was the voltages measured in the vertical plane
(V
fz
and V
rz
). Input to the second-order model in lateral
direction was the voltages measured in the lateral direction
(V
fx
and V
rx
) and the ratio RV.
2.2.3 Centre of pressure
The COP is the position on the skate blade, where the
resultant force acts on. Input for a forced linear regression
for determination of the COP was the ratio RV:
COP ¼c0þc1RV ð2Þ
where c
0
and c
1
are the variables to be determined by
regression. The assumption was made that the applied force
acts at the centre of the moving upper head, neglecting the
curving of the blade, and that the point of application at the
blade is equal for the normal and lateral applied forces.
2.3 Measurements on the ice rink
To demonstrate the practical use of the measurement sys-
tem, data were collected in 2015 on the indoor ice rink of
Thialf, which is located at Heerenveen in The Netherlands.
Seven Dutch elite speed skaters (5#,2$;22±1.1 years;
77.3 ±6.8 kg; PR at 1000 m: #1.10-1.12 s,$1.16–1.18 s)
were equipped with two instrumented skates, on which
their own skating shoes were positioned. The average
velocity over a straight part or a curve was measured by a
local position measurement system [12]. Skaters familiar-
ized themselves with the equipment before the start of the
test. The test was divided into three parts, each at a dif-
ferent velocity, which each consisted of skating three laps
at a constant velocity. We employed the data at 10.3 m/s
for the current paper. The push-off forces were normalized
to ratio of total body weight and equipment (BW). The
onset of a stroke was defined as the time at which the
normal force attained 100 N [2], the end of the stroke the
time at which the normal force fell back to 100 N. The time
is normalized in units of stroke time.
2.4 Statistics
The statistics applied in this paper are based on three
strokes per participant, per side. Therefore, 21 strokes for
each skate (left and right). To establish any differences in
peak forces or mean COP between left and right, a paired
sample ttest was performed, in which the mean value of
three strokes was used as an input for each participant.
3 Results
3.1 Calibration results
Calibration in normal direction with a second-order
regression equation resulted in correlations of R
2
=0.995
and R
2
=0.997 for the left and the right skate, respec-
tively, with a root-mean-square error (RMS) of 42 N
E. van der Kruk et al.

(SEM =1 N) and 38 N (SEM =1 N) (Fig. 4) (SEM is
the standard error of the mean). Calibration in lateral
direction performed with a second-order regression,
incorporated the factor RV. The correlation for the lateral
direction yielded R
2
=0.991 for both skates with the
corresponding RMS errors of 25 N (SEM =1 N) and
27 N (SEM =1 N) for, respectively, the left skate and the
right skate. The remaining error of the fit proved to be
random. The absolute mean error of the centre of pressure
estimation, based on linear regression (Eq. 2), is 8.5 mm
(SD =6.4 mm) for the left skate and 10.1 mm
(SD =6.9 mm) for the right skate.
3.2 Measurements on the rink
Figure 5shows an example of the force registration for two
straights and a curve. The curve is characterized by a higher
stroke frequency and lower normal forces compared to the
straight parts. Furthermore, the lateral forces of the left
skate change direction when entering the curve, since the
skater changes the push-off for this skate from the medial
side to the lateral side of the blade. Except for normal forces
in the curve, all other forces show a clear peak at the end of
the stroke. Figure 6presents the normalized forces
(mean ±SD of seven participants, each three strokes) with
a velocity of 10.3 (±0.6) m/s for the straight part. The mean
peak values of the normalized normal forces at the straight
part were F
N
=1.35 (±0.09) BW and F
N
=1.38 (±0.08)
BW for the left and right strokes, respectively. The mean
maximum normal forces in the curve (Fig. 7)goupto
F
N
=1.30 (±0.07) BW and F
N
=1.32 (±0.09) BW for the
left and the right strokes, respectively. The lateral forces
showed mean peak values at the straight part of F
L
=0.74
(±0.14) BW and F
L
=0.79 (±0.25) BW for the left and
right skates. There is no significant difference between left
and right for any of these maximum forces (p[0.1);
however, the lateral forces in the curve do have mean peak
values that differ between sides (p\0.00), with F
L
=0.35
Fig. 4 Forces estimated by the right instrumented skate with the
second-order regression functions and the forces measured by the
tensile test machine for each sample. The upper graph shows the
results for the normal direction, the lower graph the results for the
lateral direction. Given are the regression coefficients of the fit
Fig. 5 Example of a force
registration of a female elite
skater (65 kg) on two straights
and a curve part at 10 m/s. The
lateral forces are positive for
both the left skate and the right
skate at the straight parts. In the
curve, the force on the left skate
is applied on the medial side of
the blade
Wireless instrumented klapskates for long-track speed skating

(±0.09) BW for the left stroke and F
L
=0.73 (±0.25) BW
for the right stroke.
The mean COP (Fig. 8) showed no difference between
left and right on the straight part (p=0.58), while in the
curve, there is a clear difference. In the curve, the COP
moves faster toward the front of the blade compared to the
straight, which is most evident in the left skate. The timing
of the peak forces is indicated by the vertical grey bar in
the figure. Except for the left COP in the curve, all peak
forces occur, while the COP moves to the front of the
blade. For the left skate in the curve, the COP already shifts
towards the hinge (at 0 mm) before the occurrence of peak
forces, and levels while the peak forces occur.
4 Discussion
4.1 Construction and calibration
In this paper, we have described the construction and cal-
ibration of two wireless instrumented klapskates that can
measure the lateral and normal forces during high-speed
skating with an accuracy of RMS of 42 N (SEM =1N)in
normal and up to an RMS of 27 N (SEM =1 N) in lateral
direction. As mentioned in the introduction, we cannot
determine yet whether the found accuracy will be sufficient
for the purpose of providing elite speed skaters with ade-
quate feedback on improving their performance, since the
relation between the forward velocity (performance) and
the applied forces is yet to be discovered. This relationship
can, however, be established with for instance a dynamic
model of skating that describes the complete path from
push-off force to forward velocity. The current skate can be
of use in validating such a model [13].
Apart from crosstalk, previous papers on the calibration
of instrumented skates for speed skating did not give an
accuracy indication for separate force directions. The study
on the construction and calibration of an instrumented ice
hockey skate did [9]. The RMS errors of the instrumented
klapskates are similar to those of the ice hockey skates,
with an RMS in normal direction of 42 N for the klapskates
versus 68 N for the ice hockey skates and an RMS in lat-
eral direction of 27 N for the klapskates and 40 N for the
ice hockey skates.
The skates were calibrated in two directions, lateral and
normal, while the sensors are capable of measuring in three
directions. We intentionally chose to neglect the longitu-
dinal direction, due to the relatively small forces acting in
Fig. 6 Mean and standard
deviation for the normal and
lateral forces for seven elite
speed skaters measured at the
straight. The forces are
presented per stroke from each
skater three strokes were
incorporated. The forces are
normalized to body weight and
the stroke is normalized to
stroke time
E. van der Kruk et al.

Fig. 7 Mean and standard
deviation for the normal and
lateral forces for seven elite
speed skaters measured at the
curve. The forces are presented
per stroke from each skater
three strokes were incorporated.
The forces are normalized to
body weight and the stroke is
normalized to stroke time
Fig. 8 Centre of Pressure of the
resultant force (COP) for the
straight part and the curve.
Given are the mean and
standard deviation of the
measured COP of 7 participants,
each three strokes. The grey
area indicates the occurrence of
peak forces. The hinge is
located at 0 mm, the damper at
-205 mm, and the centre of the
heel cup at -220 mm
Wireless instrumented klapskates for long-track speed skating

this direction. Considering the crosstalk of &3 % between
the vertical and transverse plane of the applied sensors, as
reported by the manufacturer, it will be impossible to
observe ice friction forces lower than 1 % of the body
weight with the current design. If in future research, we do
want to determine these forces, a construction needs to be
made, whereby the normal forces are completely decoupled
from the transverse plane, to avoid crosstalk. With the
current design, this will not be feasible.
The aim to build wireless instrumented klapskates was
accomplished in this paper. Although the instrumented
skates function as a klapskate, the skate was only cali-
brated, while it was closed. As soon as the skate opens, the
force measurements are, therefore, no longer reliable.
Previous studies indicate that the opening of the skate
happens at the very last part of the stroke (50 ms before
lifting the skate) and that the forces are small in this time
span [7]. As a consequence, we expect this not to be a
major problem for the power determination. The benefit of
the hinge mechanism in the system is to preserve the
skating movement of speed skaters during testing. The
presented instrumented skate has high resemblance to a
standard klapskate. Still the influence of the added weight
of the instrumented bridge on the skaters technique should
not be disregarded in future analyses. A follow-up research,
in which the skating technique, while skating on the
instrumented skate, is compared to the technique
employed, while skating on common klapskates, can
identify such specific differences.
4.2 Practical results
The recorded normal peak forces at the straight partially
agree with previous studies, where normal peaks between
the 1.2–1.5 BW at 9–11.5 m/s on a conventional skate
were reported [2,6]. In line with the current study, pre-
vious studies report that no clear normal peak forces were
found in the curve. The lateral forces measured in the
curve are higher than the reported lateral forces of Yuki
et al. [6], who found lateral forces on the straight part of
0.4 BW for both sides, and in the curve 0.3 BW for the
left and 0.6 BW for the right skate, for one typical
example. Direct comparison is, however, not feasible,
since the velocity and the skill level of this skater will
influence these values, and these were unfortunately not
mentioned in his study. Nonetheless, both studies showed
that the lateral force is a major component in the push-off
force. The lateral forces are related to the lean angle of
the skate and the eversion of the skate, which causes the
foot and the lower leg to be unaligned. Whether the lat-
eral force is of benefit or counterproductive to the forward
motion of the skater, depends on the orientation of the
skate, determined by both the lean and steer angles. The
lean angle of the skate has a direct effect on the distri-
bution of a push force over the local vertical and trans-
verse components, while the steer angle determines the
direction of push-off in the horizontal plane (forward or
sideways).
4.3 Centre of pressure
The centre of pressure indicates where the resultant mea-
sured forces act on the skate blade. On the straight part, the
centre of pressure of the resultant force levelled at about
20–70 mm ahead of the heel cup, for the first 80 % of the
stroke. Then, the COP shifted towards the hinge of the
skate, in which time the peak forces occurred, followed by
the opening of the skate. Since the hinge is taken as origin,
the skate would open when the COP becomes positive if it
was not for a spring acting on the hinge, which makes it
difficult to interpret the exact opening of the klapskate. The
curve showed different COP patterns for left and right.
Both lack the levelled phase at the start of the stroke, since
the COP continuously shifts towards the front. The left
skate has a faster shift and levels at the end of the stroke,
while the peak forces occur. This pattern arises from the
fact that the skater has to cross his right leg over his left in
the curve. Therefore, the skater is forced to move his left
leg backwards faster, whereby his COP quickly shifts
towards the point of his blade.
4.4 Real-time feedback
Measured outputs were sent wirelessly and real-time over
Bluetooth to a phone carried by the speed skater during the
test. It is possible to link the phone via wifi or a mobile
network to a tablet or smart glasses, to provide both coach
and skater with real-time visual feedback on the force level
and COP during training. Eventually, the forces will be
combined with kinematic measurement systems, to provide
feedback on power per stroke. During this experiment, the
possibility of synchronising the skates with other mea-
surement equipment via a digital end-start pulse proved to
be easy.
5 Conclusion
Two wireless force measuring instrumented klapskates
were constructed and calibrated on a tensile testing
machine, where they proved to be unaffected by tempera-
ture conditions and accurate up to an RMS of 42 N
(SEM =1 N) in normal direction and 27 N (SEM =1N)
in lateral direction. Furthermore, the centre of pressure of
E. van der Kruk et al.

these forces on the blade was determined up to a mean
error of 10.1 mm (SD =6.9 mm). The design of the skate
allows a skater to attach his own shoe and Maple blade to
the bridge. On-ice measurements showed the possibility of
recording with both skates simultaneously and syn-
chronously both straights and curves, and the capability of
the system to send data wirelessly and real-time to other
devices, which makes it possible to eventually provide
skaters and coaches with visual real-time feedback during
practice. With the construction of these instrumented
klapskates, we are one step closer to fulfilling the wish of
the Dutch elite skaters and their coaches for a system
determining the mechanical power per stroke.
Acknowledgments The authors gratefully acknowledge the
mechanical and electronics engineers N. Linskens and G. F. Liqui
Lung for their support, while producing the instrumented skates and
express their gratitude to A. C. Riemslag for the technical support
during the calibration experiment. This study was supported by the
NWO-STW under Grant 12870.
Compliance with ethical standards
Conflict of interest None.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
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