Development and evaluation of a novel taekwondo chest protector to improve mobility when performing axe kicks

Abstract

The axe kick, in Olympic style taekwondo, has been identified as the most popular scoring technique aimed to the head during full contact competition. The first purpose of this study was to identify and investigate design issues with the current World Taekwondo Federation approved chest protector. A secondary purpose was to develop a novel chest protector addressing the identified design issues and to conduct a biomechanical analysis. Fifteen male elite Taekwondo players were selected to perform three different styles of the axe kick, i.e., front, in-out, and out-in axe kick five times each for a total of 45 kicks. Two-way repeated measures ANOVA showed significant differences between the novel and existing chest protector conditions for vertical height of the toe, downward kicking foot speed, hip flexion angle and ipsilateral shoulder flexion extension range of motion (ROM) (p < 0.05). There were no significant differences between the control condition (no chest protector) and the novel chest protector condition for these variables (p > 0.05). These results indicate that the novel chest protector interferes less with both the lower and upper limbs during the performance of the axe kick and provides a more natural, free-moving alternative to the current equipment used.

Full text

Biology of Sport, Vol. 30 No1, 2013 51
Novel taekwondo chest protector
Reprint request to:
Her Jin Gang
Graduate School of Medical &
Therapy Science, Hallym University
Hallym daehak-gil
Chuncheon,Gangwon-do
200-702 Korea
E-mail:jghur7@empas.com
Accepted
for publication
28.09.2012
INTRODUCTION
According to the World Taekwondo Federation (WTF) there are more
than 80 million [8] practitioners of Taekwondo (TKD) in 200 coun-
tries [14]. The WTF has amended earlier rules [15] to now allow
four points for successful spinning kicks to the head. As a direct
result it has been recently demonstrated, in a study of the techniques
used in the WTF nals from 2001 to 2011, that there has been
an increase in the number of kicks executed by athletes to
the head

[5]. Koh and Cassidy [6], in a study of the incidence of
concussions in TKD competitions, concluded that the most common
head strikes observed in competitions in Korea were the axe kick,
roundhouse kick, back kick and spinning hook kick.
The axe kick has been shown to be a highly effective offensive
and defensive scoring technique [6,11] and its purpose is to attack
an opponent’s head with a powerful downward force [13]. In Koh
and Watkinson’s study [7], from a total sample of 1652 males and
676 females, 51.4% of head blows were by the axe kick, followed
by the roundhouse kick, (25.7%). The sequential movements during
the axe kick include the knee raising in an upward in-to-out or out-
to-in arc motion. Another variation to this kick involves the simple
DEVELOPMENT AND EVALUATION OF
A NOVEL TAEKWONDO CHEST PROTECTOR
TO IMPROVE MOBILITY WHEN PERFORMING
AXE KICKS
AUTHORS: Woo J.H
1
, Ko J.Y.
2
, Choi E.Y.
1
, Her J.G.
3
, O’Sullivan D.M.
4
1
Department of Rehabilitation Therapy, Hallym University, Korea
2
Department of Rehabilitation Medicine, CHA University, CHA Bundang Medical, Rep. of Korea
3
Graduate School of Medical & Therapy Science, Hallym University, Korea
4
College of Sports Science, Chung-Ang University, Korea
ABSTRACT: The axe kick, in Olympic style taekwondo, has been identied as the most popular scoring technique
aimed to the head during full contact competition. The rst purpose of this study was to identify and investigate
design issues with the current World Taekwondo Federation approved chest protector. A secondary purpose was
to develop a novel chest protector addressing the identied design issues and to conduct a biomechanical
analysis. Fifteen male elite Taekwondo players were selected to perform three different styles of the axe kick,
i.e., front, in-out, and out-in axe kick ve times each for a total of 45 kicks. Two-way repeated measures ANOVA
showed signicant differences between the novel and existing chest protector conditions for vertical height of
the toe, downward kicking foot speed, hip exion angle and ipsilateral shoulder exion extension range of
motion (ROM) (p<0.05). There were no signicant differences between the control condition (no chest protector)
and the novel chest protector condition for these variables (p>0.05). These results indicate that the novel chest
protector interferes less with both the lower and upper limbs during the performance of the axe kick and provides
a more natural, free-moving alternative to the current equipment used.
KEY WORDS: axe kick, novel chest protector, taekwondo
raising of the leg straight up and down in front of the body, where
the leg is extended above the target and pulled down onto the tar-
get

[1]. Because the axe kick motion requires the kicking foot to be
raised higher than the intended target, the angle created at the hip
joint is found to require a substantially large range of motion[11].
Various studies have been carried out about different aspects of
the chest protector in taekwondo, focusing on protective quali-
ties[3,4,9] and the introduction of the electronic scoring protector
system[2]. The rst published study [9] investigated the protective
qualities of the chest protector in taekwondo for various swing-like
kicks and push-like kicks. The authors demonstrated that without
a chest protector, a kick to the chest can induce thoracic deections
up to 5 cm and peak viscous tolerance values of up to 1.4 m · s
-1
.
This means that if a player is to receive a kick unprotected, there is
a high probability of serious injury. Similarly, there have been two
further studies investigating the protective function of the chest pro-
tector [3,4]. Chi et al. [2] report on the use of wireless force sensing
chest protectors and their role in improving judging in taekwondo
competition. However, there have been no published studies that
Original Paper
Biol. Sport 2013;30:51-55
DOI: 10.5604/20831862.1029822

52
Woo J.H. et al.
examine the ergonomic difculties of having to wear a chest protec-
tor during competition.
At the 2011 Gyungju World Taekwondo Competition the authors
surveyed the 56 Korean professional players (age: 21.8 ± 2.21 years)
using the provided satisfaction survey of the chest protector (Figure

1).
Over 96% of the players agreed with the statement that the current
chest protector interferes with kicking to the head. When the players
were asked about which kick in particular the chest protector interfered
with, over 91% indicated the axe kick. To investigate whether the
chest protector was really interfering with the performance of the axe
kick the following variables were recorded: maximum vertical height
of the toe, maximum downward kicking foot speed, hip exion (rela-
tive angle between the trunk and the thigh), ipsilateral shoulder
exion-extension range of motion and ipsilateral shoulder extension
pulling speed.
The purpose of this study was to discover functional performance
limitations associated with using the current WTF approved chest
protector and to modify the chest protector to t more ergonomically
to enhance axe kick functional kicking performance. The nal objec-
tive was to evaluate the effect of this modied chest protector on
biomechanical kicking parameters, in comparison to two other kick-
ing conditions: wearing the WTF approved chest protector, and
a control condition (no chest protector).
MATERIALS AND METHODS
Development of novel chest protector. Prior to the change in design
of the existing chest protector the authors carried out a survey about
the satisfaction level of the chest protector among the national team
player squad in Korea. Fifty-six of these professional players com-
pleted the questionnaire which included closed and open-ended
questions (Figure 1). When asked if the chest protector interferes
with kicking to the head, over 96% of the players indicated that it
was a limiting factor, and 91% of these respondents indicated that
the axe kick was the most restricted technique. These responses
provided the basis for the follow through study involving the develop-
ment and assessment of the ergonomic ecology of the design.
FIG.
1.
SURVEY OF EXISTING CHEST PROTECTOR
TABLE 1.
DIFFERENCES IN HOGU DIMENSIONS
FIG.
3.
STANDARD SIZE 3 EXISTING CHEST PROTECTORFIG.
2.
DIMENSIONS OF NEW HOGU
Ofcial
Specications
New Hogu
Ofcial Size 3 3
Recommended Weight Range (kg)
51~63 51~63
A Chest circumference (cm) 98 94
B
Length from naval to sternum (cm)
47.5 35.9
C Closure length (cm) 29.5 25
D Shoulder width (cm) 15 14
E Centre of side length (cm) 29.5 20.3

Biology of Sport, Vol. 30 No1, 2013
53
Novel taekwondo chest protector
For sizing of the chest protector, the height, weight, chest circum-
ference, waist front length, front interscye eld length, back interscye
eld length, closure length (iliac crest to scapular inferior angle),
shoulder width, neck-clavicle length (rst rib to inferior part of the
clavicle) and clavicle length of 20 Taekwondo players were measured
using a vernier calipers (NA500-300s, 600s, Bluebird, Korea). The
male (54-62 kg) and female (51-62 kg) players surveyed all used
the ofcially approved size 3. These measurements were performed
both in a standing position and in a standard ghting position. The
measurement changes were based on the measurements shown in
The following changes were applied to the dimensions of the chest
protector as a result of the anthropometric measurements and com-
ments from the survey. The 95th percentile was used for the tting
as the 50th percentile was suggested to be too small [16]. With the
original chest protector 2 cm above the clavicle, the athletes tended
to pull or move the chest protector down approximately 8 cm. This
8 cm was measured between before and after the players adjusted
the chest protector for comfort. As a result of the players moving the
chest protector down, it began to interfere with their kicking and as
they kicked the chest protector would slide back up, hitting their neck.
Therefore to prevent the players from repositioning their chest protec-
tor to a lower position the straight neck part was changed to a V-neck.
During competition, dynamic trunk positions (e.g., trunk exion) force
the clavicle to navel distance to be shortened by 6.0 cm to 35.9 cm.
This altering of the clavicle-navel distance indicates that the chest
protector is too long, thus interfering with the hip range of motion
during various kicking techniques. Side dimensions (C, D and E) were
shortened to allow for more movement of the scapula and more
natural arm movement. The sides were curved to follow more move-
ment for both the legs and the arms accordingly. The lower parts were
shorted to follow the arc above the iliac crest and the higher parts
were shortened to follow the natural curve of the scapula (Figure 4).
In addition, the standard chest protector is made out of 1.0 cm
of polyester sponge and 1.5 cm of ethylene vinyl acetate copolymer
for shock absorption. To increase the pliability and comfort, the pad-
ding was changed to two sheets of memory foam (width = 0.5 cm).
Two sheets of ethylene vinyl acetate copolymer (width = 0.5 cm)
were covered with one sheet of toilon (width = 0.5 cm). A rubber
band was also used to x the chest protector as close as possible to
the body for extra comfort.
Participants
Fifteen elite collegiate level taekwondo players (mass 59.4 ± 4.3kg,
height 1.71 ± 0.04 m, leg length 0.84 ± 0.03 m) were recruited.
Each of these elite participants reported over ten years of experience
in taekwondo. Participants suffering from any musculoskeletal injury
within the last two years were excluded.
Experimental procedure
Participants completed an informed consent form approved by the
Hallym University Institutional Review Board held in accordance with
the Declaration of Helsinki. After completion of the informed consent
form, subjects were instructed to warm up by performing a combina-
tion of ten jumping jacks and light static stretching of the following
muscles: quadriceps, biceps femoris, soleus, gastrocnemius, external
abdominal obliques, erector spinae. In total the warm-up took ap-
proximately 10 minutes. After this warm-up, reective markers were
xed to the relevant landmarks via double sided tape. Each of the
participants was instructed to perform each kick ve times for each
condition. Each player executed a total of 45 kicks, 15 kicks for each
condition, 5 in-out axe kicks, 5 out-in axe kicks and 5 straight axe
kicks for each of the chest protector conditions. Conditions consisted
of wearing the standard WTF approved chest protector, our novel
chest protector, and the control condition (no chest protector). All
kicks and chest protector conditions were randomized prior to testing
and subjects were unaware of the order until execution of each kick
and introduction of each condition. For each of the conditions max-
imum vertical height of the toe, maximum downward kicking foot
speed, hip exion (relative angle between the trunk and the thigh),
ipsilateral shoulder exion-extension range of motion and ipsilateral
shoulder extension pulling speed were measured.
Data collection and statistical analysis
Kinematic data were recorded by eight Qualisys cameras (OQUS
100, Sweden) at a frequency of 150 Hz. Cameras were synchronized
and reective markers were manually labelled using QTM (Qualisys
Track Manager, Sweden). All data were exported as a C3D le and
processed and ltered by a second order Butterworth lter at 15 Hz
using Visual3D version 4.01 (C-motion, Germantown, MD, USA).
Repeated measures ANOVA was used to detect differences between
the three conditions and independent variables. The level of signi-
cance was set at 0.05.
Terminology
1) Chest protector is the standard protective chest guard which must
be worn by all athletes that are participating in Olympic style TKD
competitions sanctioned by the WTF.
FIG.
4.
COMPARISON OF TWO CHEST PROTECTORS (NEW CHEST
PROTECTOR ABOVE)

54
Woo J.H. et al.
2) Leg length is measured from the marker on the anterior superior
inferior spine (ASIS) to the marker on the medial malleolus.
3) Vertical height of the toe is measured by the maximum height in
the z-axis from the positional data of the reexive marker on the
kicking foot’s head of the fth metatarsal.
4) Downward kicking foot speed is measured from the speed of the
marker on the fth metatarsal in the downward direction, i.e. after
the foot has reached its maximum height to impact.
5) Hip exion is the relative angle measured between the thigh seg-
ment and the trunk segment.
6) Ipsilateral shoulder exion-extension range of motion is measured
during the axe kick from the point of maximum exion of the upper
arm to the maximum extension of the upper arm.
7) Ipsilateral shoulder extension pulling speed is the measured speed
at the distal end speed of the upper arm segment.
RESULTS
Table 2 displays the descriptive statistics and results of the two-way
3 (chest protector condition) x 3 (kick type) repeated measures
ANOVA performed on the variables. There were no signicant inter-
actions between the kick type and the chest protector condition.
There were signicant main effects (p<0.05; refer to Table 2) re-
corded between the chest protector conditions for all variables except
the ipsilateral shoulder extension pulling speed for the straight axe
kick. Post hoc analyses using the Bonferroni post hoc criterion were
used to show signicant differences within the groups. Results of the
post-hoc analyses are also included in Table 2.
DISCUSSION
The purpose of this study was to compare the effects of a modied
chest protector and those of a standard chest protector on axe kick
performance. Results of the Korean Taekwondo National Team
players’ responses to the survey indicated that existing chest pro-
tectors interfere with performance of the axe kick. Based on these
results of the satisfaction survey, the chest protector was redesigned
to be more exible and not to interfere with kicks aimed to the
head. The main changes to the chest protector included the change
of padding material and layering to improve pliability and to narrow
the side protection as it restricted thigh and upper arm movements.
The effects of these modications on vertical height of the kicking
toe, downward kicking foot speed, hip exion, ipsilateral shoulder
exion-extension ROM, and ipsilateral shoulder extension pulling
CP types Height (m) Foot speed (
m · s
-1
) Hip exion (°) Shoulder ROM (°)
Shoulder speed (m · s
-1
)
Front axe kick
CP
no
1.76 ± 0.05
2
7.91 ± 1.10
2
146.98 ± 34.50
2
71.83 ± 16.18
2
2.70 ± 0.78
CP
e
1.72 ± 0.05
1,3
7.22 ± 0.83
1,3
140.01 ± 32.52
1,3
60.45 ± 21.16
1,3
2.35 ± 0.40
CP
n
1.75 ± 0.05
2
7.84 ± 0.98
2
145.11 ± 32.91
2
69.67 ± 22.54
2
2.63 ± 0.51
p <0.001 <0.001 0.003 0.010 0.080
F 10.417 17.818 7.413 5.512 3.082
effect size 0.427 0.733 0.346 0.282 -
In-out axe kick
CP
no
1.74 ± 0.05
2
7.72 ± 0.57
2
150.95 ± 34.50
2
71.45 ± 16.22
2
3.00 ± 0.83
CP
e
1.69 ± 0.06
1,3
7.24 ± 0.69
1,3
140.12 ± 33.11
1,3
63.24 ± 15.81
1,3
2.57 ± 0.57
3
CP
n
1.73 ± 0.05
2
7.70 ± 0.84
2
146.81 ± 32.28
2
69.36 ± 19.69
2
2.94 ± 0.65
2
p <0.001 <0.001 <0.001 0.014 0.010
F 36.220 17.546 15.298 5.022 6.715
effect size 0.721 0.556 0.522 0.264 0.508
Out-in axe kick
CP
no
1.72 ± 0.05
2
7.41 ± 0.73
2
145.55 ± 32.40
2
68.93 ± 23.95
2
2.93 ± 0.56
2
CP
e
1.69 ± 0.06
1,3
6.87 ± 0.58
1,3
135.75 ± 33.67
1,3
57.75 ± 25.30
1,3
2.44 ± 0.40
1,3
CP
n
1.71 ± 0.05
2
7.34 ± 0.74
2
143.94 ± 32.02
2
68.39 ± 24.10
2
2.87 ± 0.51
2
p <0.001 <0.001 0.003 <0.001 0.006
F 12.270 13.460 9.765 12.189 7.891
effect size 0.467 0.490 0.600 0.465 0.548
Note: CP = chest protector; CP
no
= no chest protector; CP
e
= existing chest protector; CP
n
= novel chest protector;
Height = maximum vertical height of the toe; Foot speed = maximum downward kicking foot speed; Shoulder ROM = ipsilateral shoulder exion-
extension range of motion; Shoulder speed = ipsilateral shoulder extension pulling speed.
1 signicantly different compared with CP
no
2 signicantly different compared with CP
e
3 signicantly different compared with CP
n
TABLE 2.
COMPARISON OF INDEPENDENT VARIABLES BY THE TYPES OF CHEST PROTECTOR (N=15)

Biology of Sport, Vol. 30 No1, 2013
55
Novel taekwondo chest protector
speed, compared to the standard WTF approved chest protector
and the control condition, were analyzed. The downward kicking
foot speed of the straight axe kick before impact for the control
group, the WTF approved protector group, and the novel chest
protector group was 7.91 ± 1.10
m · s
-1
, 7.22 ± 0.83
m · s
-1
, and
7.84 ± 0.98
m · s
-1
, respectively. The maximum value recorded by
this study was 11.43
m · s
-1
, which was nearly identical to the back
leg axe kick foot velocity (11.3
m · s
-1
) recorded by Sung and col-
leagues [10].
With the importance of the use of the rectus abdominal muscles
and hip exors to raise the kicking leg, the chest protector must be
exible and not restrict hip exion. The kicking foot height was high-
est for the no chest protector condition (1.76 ± 0.05 m) followed
by the novel chest protector (1.75 ± 0.05 m) then the existing chest
protector (1.72 ± 0.05 m), which implied that the existing chest
protector interfered with the hip exion during the kick. There was
a signicant difference in the foot height between the no chest pro-
tector group and the existing chest protector group. However, there
was no signicant difference between the no chest protector group
and the novel chest protector group.
The same order was shown for both the in-to-out axe kick and
the out-to-in axe kick. With the extra height the player had the op-
portunity to develop more downward foot speed. The results showed
signicant differences between the no chest gear condition and the
existing chest protector condition. The results also showed no sig-
nicant differences between the no chest protector condition and the
novel chest protector. Tsai and his colleagues examined how the
upper arm motion affects the lower limb motion while performing
the spinning heel kick [12]. Their data illustrated that the larger
range of motion in the upper limbs, mainly the upper arm, then
the larger the range of motion in the lower limbs. In this study, ipsi-
lateral shoulder range of motion was calculated to observe whether
the chest protector had any effect on the movement of the upper
arm. Similarly, ipsilateral shoulder extension pulling speed was cal-
culated for the same purpose. Our data suggest that even though
the range of motion is larger for the upper arm movement, there were
no signicant differences between the groups in pulling arm speed.
One of the limitations of this study is that the safety performance of
the chest protector was not tested. The authors intend to test the
shock absorption of the chest protector in a future study. It is also
intended to develop more sizes and survey the appropriateness of t
and comfort.
CONCLUSIONS
The ndings of this study, shown by the biomechanical variables,
such as kick height, foot speed, hip exion, shoulder ROM and
shoulder speed, illustrate that using the novel chest protector
enables a player to have more natural axe kicking motion.
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