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INTRODUCTION
Jump rope is a sport in which players jump over a rope while
swinging the rope with their upper body and hands to pass it under
their feet and over their heads (Miyaguchi, Demura & Omoya, 2015),
and is one of the most effective aerobic exercises for cardiorespiratory
health (Jones, Squires & Rodahl, 1962). Neuromuscular coordination
is essential for accurate timing and maintenance of rhythm, which are
critical for effective jump rope performance, and good neuromuscular
coordination improves dynamic stability while performing jump rope
(Ozer, Duzgun, Baltaci, Garacan & Colakoglu, 2011). Furthermore, jump
rope effectively enhances muscle strength, endurance, balance, explo-
sive power, speed, cardiopulmonary endurance, and flexibility, and is
therefore used as a warm-up exercise and training program in various
sports (Hawkins & Kennedy, 1980; Orhan, 2013; Solis & Thompson, 1988;
Trampas & Kitisios, 2006). Jump rope is an essential component of pro-
fessional training programs designed to enhance athletic performance
by elite junior athletes, and several studies have reported beneficial
effects on physical fitness, growth, and development in these athletes
(Baker, Côté & Abernethy, 2003; Miyaguchi, Sugiura & Demura, 2014).
Elite athletes often use plyometric training programs with box jumps
or hurdle jumps to improve explosive power and jump performance
(Komi, 1984). However, these training programs inflict a heavy load on
elite junior athletes who are still undergoing physical development, and
increase their risk of injury (Komori, Zushi, Konishi & Komori, 2012).
For this reason, it is important to develop alternative training programs
that offer effective training with a relatively low risk of injury. Jump rope
is one such alternative. Jump rope has the properties of plyometric
exercise, as the sport involves repeated contractions and extensions
of the quadriceps femoris, gastrocnemius, and soleus muscles, thereby
improving explosive power and jumping ability (Komi, 1984; Norman
& Komi, 1979; Miyaguchi et al., 2014, 2015). Accordingly, jump rope is
utilized in training programs for a variety of other sports, such as
volleyball, basketball, soccer, gymnastics, rhythmic gymnastics, boxing,
wrestling, tennis, and martial arts (Ozer et al., 2011; Trecroci, Cavaggioni,
Caccia & Alberti, 2015).
Korean Journal of Sport Biomechanics 2017; 27(3): 171-179
http://dx.doi.org/10.5103/KJSB.2017.27.3.171
http://e-kjsb.org eISSN 2093-9752
ORIGINAL
Analysis of Kinematics and Kinetics According to Skill Level and
Sex in Double-under Jump Rope Technique
Dae Young Kim1, Kyeong Hui Jang1, Myeoung Gon Lee1, Min Ji Son2, You Kyung Kim3, Jin Hee Kim1, Chang Hong Youm4
1
Department of Health Science, Graduate School of Dong-A University, Busan, South Korea
2
Department of Medicine, Graduate School of Dong-A University, Busan, South Korea
3
Department of Taekwondo, Graduate School of Dong-A University, Busan, South Korea
4
Department of Health Care and Science,
Donga University, Busan, South Korea
Received : 30 July 2017
Revised : 25 August 2017
Accepted : 03 September 2017
Corresponding Author
Chang Hong Youm
Department of Health Care &
Science, 37 Nakdong-Daero 550
Beon-gil, Saha-gu, Busan, 49315,
South Korea
Tel : +82-10-4572-2521
Fax : +82-51-200-7505
Email : chyoum@dau.ac.kr
Objective:
The purpose of this study was to perform a kinematic and kinetic analysis of double
-
under
jump rope technique according to skill level
and sex.
Method: Participants comprised a skilled group of 16 (9 males, 7 females), and an unskilled group of 16 with
6 months or less of experience (9 males, 7 females). Five consecutive double-under successes were regarded
as 1 trial, and all participants were asked to c
omplete 3 successful trials. The data for these 3 trials were
averaged and analyzed after collecting the stable third jump in each trial. The variables used in the analysis
included phase duration, total duration, flight time, vertical toe height, stance width, vertical center of mass
displacement, and right lower limb ankle, knee, and hip joint angles in the sagittal plane during all events.
Results: The skilled group had a shorter phase and total duration and a shorter flight time than the unskilled
grou
p. The vertical center of mass displacement and ankle dorsiflexion angle were significantly smaller in
the skilled group. The male group had a shorter phase duration than the female group. The vertical toe
height was greater, the stance width was smaller,
and the ankle and hip flexion angles were smaller in the
male group.
Conclusion:
Variables that can be used to distinguish between skill levels are phase and total duration,
flight time, vertical center of mass displacement, and ankle dorsiflexion angle.
Differences between sexes
in double-under jump rope technique may be related to lower limb flexion angle control.
Keywords: Double-under, Jump rope, Biomechanics, Stance width, Motion capture, Plyometric training
Copyright
○
C 2017 Korean Journal of Sport Biomechanics
This is an Open Access article distributed under the terms of
the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which
permits
unrestricted noncommercial use, distribution, and reproduction in any medium, provi ded the original work is properly cited.
172 Dae Young Kim, et al. KJSB
Korean Journal of Sport Biomechanics
In addition to its use in training programs, jump rope is becoming a
competitive sport. The first World Jump Rope Championship was held
in in 1997, and about 1,200 athletes from 21 countries competed in
the 2014 World Championship (International Rope Skipping Federation,
2016), while about 1,500 athletes competed in the 2015 Korean National
Jump Rope Competition (Korea Rope Skipping Association, 2016). Among
various jump rope skills, double under is a technique in which the
jumper passes the rope twice per jump. It is one of the major events in
jump rope competitions, in which athletes compete based on the num-
ber of double unders performed in 30 seconds (Korea Rope Skipping
Association, 2016; Miyaguchi et al., 2014). Double-under technique is
performed in competition in kindergarten, elementary, middle, and high
school, and adult levels (Korea Rope Skipping Association, 2016).
Despite rapid advances in the sport, studies on jump rope are largely
limited to physiological aspects (Baker, 1968; Baker, Côté & Abernethy,
2003; Jette, Mongeon & Routhier, 1979; Miyaguchi et al., 2014; Quirk &
Sinning, 1981; Town, Sol & Sinning, 1980) and training methods and
effects (Buyze et al., 1986; Hatfield et al., 1985; Hawkins & Kennedy,
1980; Myles, Dick & Jantti, 1981; Orhan, 2013; Solis & Thompson, 1988;
Trampas & Kitisios, 2006). A few researchers have analyzed the proper-
ties of jump rope movement. Gowitzke and Brown (1989) analyzed the
kinetic variables of alternating foot exercise in skilled and unskilled
groups and reported that novice jumpers have a larger wrist joint
rotation radius, greater knee and hip flexion, and greater center of mass
variability while performing alternating foot exercise, compared to skilled
jumpers. Pittenger, McCaw and Thomas (2002) found that the peak
vertical ground reaction force (GRF) during the 1-foot jump is greater
than that during the 2-feet jump. Kim and Kim (2015) investigated the
kinetic differences between a 2-feet jump and a 2-feet double jump
(double under) among male jump rope coaches and reported that the
2-feet double jump is associated with greater center of mass displace-
ment, lower limb joint range of motion, and vertical GRF compared to
the 2-feet jump.
Studies that analyzed the properties of basic jump rope movements
focused on kinetic and kinematic aspects, and few studies examined
skills and movements related to jump rope performance. Furthermore,
double-under performance rapidly improves as middle school athletes
move on to high school; this is a period in which the gap between
male and female athletic skill levels and features increases (Kim & Oh,
2010, Korea Rope Skipping Association, 2016). Nevertheless, research
data are lacking on differences in jump rope performance in relation
to skills and sex in high school athletes. In this context, a study in-
vestigating differences in double-under performance in relation to sex
and skill level in high school students would be relevant.
Thus, this study investigated kinetic and kinematic differences in
double-under performance in relation to skill level and sex in high
school students. The data collected can be used to improve double-
under performance and coaching strategies. The authors hypothesized
that the kinetic and kinematic features of double-under technique would
vary according to jump rope skill level and sex.
METHODS
1. Participants
Male and female high school students with no orthopedic condition
within the prior 6 months were enrolled in this study. The participants
were divided into a skilled group (n=16, 9 males 7 females), comprising
skilled jumpers who have won at least one national jump rope com-
petition, and an unskilled group (n=16, 9 males 7 females), comprising
students with a jump rope career of fewer than 6 months. Double under
is a difficult skill to master, and previous studies have noted that at least
6 months of jump rope practice is needed to perform the technique.
Therefore, the unskilled group comprised students with less than 6
months of jump rope experience who were nonetheless capable of
performing 5 consecutive double-under jumps. This study was approved
by the university Institutional Review Board, and informed consent
was obtained from all participants and guardians prior to the experi-
ment. During the study period, the participants were instructed to refrain
from exercise programs and vigorous physical activities other than
activities of daily living. The physical characteristics of the participants
are shown in Table 1.
2. Procedure
An infrared camera (MX-T10, Vicon, UK) and ground force plate
(AMTI OR6-7, Watertown, MA, US) were used for the experiment.
Identical jump ropes and training shoes were used to ensure consistency
of analysis. The experiment was conducted over 2 days. On the first day,
consent forms were collected, physical characteristics were measured,
environmental adaptation training, and double-under practice. On the
second day, the participants performed adequate warm-up stretching,
Table 1. Physical characteristics
Age
(y)
Height
(cm)
Weight
(kg)
BMI
(kg/m2)
Career
(y)
Skilled
(n=16)
Male (n=9) 17.6±0.9 170.6±6.9 65.6±6.8 22.5±1.6 3.7±1.0
Female (n=7) 17.5±0.6 160.1±4.7 54.7±10.9 21.3±4.0 4.3±0.6
Unskilled
(n=16)
Male (n=9) 17.6±0.7 170.8±5.6 61.6±7.9 21.1±4.0 0.5±0.1*
Female (n=7) 17.4±0.5 158.5±5.3 50.8±3.3 20.2±1.5 0.6±0.1*
BMI: Body mass index, independent sample
t
-test comparing skilled and unskilled groups, *
p
<.05
KJSB Analysis of Kinematics and Kinetics According to Skill Level and Sex in Double-under Jump Rope Technique 173
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2-feet jumps, and double-under technique to prevent injuries and
enable completion of the main double-under event. Nine cameras for
motion analysis and 1 force platform were installed in an area with
adequate field of view. With reference to left posterior direction from
the participant, the left and right directions were set as the X axis, the
anterior and posterior directions as the Y axis, and the vertical direction
as the Z axis (Figure 1).
All participants wore T-shirts and shorts made of spandex material
and wore designated training shoes. Heights and weights were meas-
ured with a body composition analyzer (GL-150KT, G-TECH, Korea),
and the measurements were used to calculate BMI. Shoulder width,
elbow width, wrist width, hand thickness, leg length, knee width, and
ankle width were measured with a tape measure and caliper to develop
body models. Body models were created using the Vicon Plug-in Gait
full-body model, with 39 round spherical reflective markers placed on
the participant and 1 reflective tape at the mid-point of the jump rope
(length: 2.7 m, mass: 120 g) (Figure 2). All reflective markers were placed
firmly using double-sided tape and athletic tape to prevent displace-
ment during jumping.
3. Data processing
Image and GRF data for double-under motion were collected with
Nexus software (Vicon, UK), at a sampling frequency of 200 Hz for
image data and 1,000 Hz for GRF data. The collected data were filtered
using a Butterworth low-pass filter with a cut-off frequency of 10 Hz
(Decker et al., 2003; Pappas, Sheikhzadeh, Hagins & Nordin, 2007). Five
consecutive double-under jumps were regarded as 1 successful trial,
and the participants were asked to perform 3 successful trials. The
data from the stable third jump in each trial were collected, and the
average of the 3 trials was computed for analysis.
Double-under motion was analyzed by dividing the motion into 5
events (E1, E2, E3, E4, E5) and 4 phases (P1, P2, P3, P4). E1 was defined
as the point at which the feet contact the ground (when vertical GRF
exceeds 10 N), and E2 was defined as the point at which the vertical
GRF reaches peak. E3 was defined as the point at which the feet left
the force plate (when vertical GRF is less than 10 N), and E4 was defined
as the point at which the center of body mass reached a vertical peak.
E5 was defined as equal to E1. Phases were defined as the intervals
between corresponding events (Figure 3).
Figure 1. Experimental setup
Figure 2. Marker set. Top: Plug-in Gait full-body model; Bottom: jump
rope with attached reflective tape
Figure 3. Events and phases
174 Dae Young Kim, et al. KJSB
Korean Journal of Sport Biomechanics
The kinematic variables for analysis were phase duration, total duration,
flight time, vertical toe height, stance width, vertical displacement of
the center of mass, and right hip, knee, and ankle angles in the sagittal
plane for each event. The kinetic variables for analysis were peak vertical
ground reaction force (VGRF) and load at P1 (peak VGRF/time from
ground contact to peak VGRF).
4. Statistical analysis
The means and standard variations for each variable were calculated
using SPSS software (version 21.0; SPSS. Inc., Chicago, IL, USA), and
normality of each variable was tested with the Shapiro-Wilk test. Main
effects and the interaction effect of skill and sex were assessed using
repeated-measures 2-way analysis of variance. An independent sample
t
-test was performed as post hoc analysis for skill and sex. Statistical
significance (α) was set at .05.
RESULTS
1. Phase duration, total duration
There was a significant main effect of sex on P2 duration (
p
<.000).
Post hoc testing showed that the male group had a shorter P2 than
the female group in both the skilled (
p
=.001) and unskilled (
p
=.017)
groups. There was a main effect of skill on P3 (
p
=.001). Post hoc testing
showed that the skilled group had a shorter P3 than the unskilled
group in both male (
p
=.042) and female (
p
=.010) groups. There was a
significant main effect of skill on P4 duration (
p
=.009). Post hoc testing
showed that the skilled group had a shorter P4 than the unskilled group
in the female group (
p
=.043).
There was a significant main effect of skill on total duration (
p
<.000).
Post hoc testing showed that the skilled group had a shorter total
duration than the unskilled group in both male (
p
=.013) and female
(
p
=.015) groups (Table 2).
2. Flight time
There was a significant main effect of skill on flight time (
p
<.000).
Post hoc testing showed that the skilled group had a shorter flight time
than the unskilled group in both male (
p
=.015) and female (
p
=.003)
groups (Table 3).
Table 2. Phase and total duration (Unit: s)
Male Female
t F
P1 Skilled 0.10±0.01 0.11±0.01 1.809 6.539* (G)
Unskilled 0.11±0.01 0.12±0.01 1.841 5.466* (S)
t
1.349 2.104
0.153 (G×S)
P2 Skilled 0.10±0.00 0.11±0.01 4.059* 21.002* (G)
Unskilled 0.11±0.01 0.12±0.01 2.696* 4.093 (S)
t
1.828 1.100 0.063 (G×S)
P3 Skilled 0.13±0.02 0.13±0.02 0.885 0.690 (G)
Unskilled 0.15±0.01 0.15±0.01 0.233 13.211* (S)
t
2.207* 3.053* 0.285 (G×S)
P4 Skilled 0.16±0.02 0.15±0.02 0.664 1.098 (G)
Unskilled 0.18±0.02 0.17±0.01 0.813 7.946* (S)
t
1.978 2.266* 0.023 (G×S)
Total duration Skilled 0.50±0.04 0.51±0.04 0.328 0.540/ (G)
Unskilled 0.55±0.03 0.56±0.02 0.828 15.783* (S)
t
2.812* 2.841* 0.036 (G×S)
All data are shown as means and standard deviations.
t
: independent
t-
test
comparing skilled and unskilled and male and female groups; P1:
phase 1, P2: phase 2, P3: phase 3, P4: phase 4, G: Main effect of sex, S: Main effect of group, G×S: Interaction effect, *
p
<.05
Table 3. Flight time (Unit: s)
Male Female
t F
Skilled 0.10±0.01 0.11±0.01 1.809
6.539* (G)
Unskilled 0.11±0.01 0.12±0.01 1.841
5.466* (S)
t
1.349 2.104
0.153 (G×S)
All data are shown as means and standard deviations.
t
: independent
t-
test comparing skilled and unskilled and male and female groups; G:
Main effect of sex, S: Main effect of group, G×
S: Interaction effect,
*
p
<.05
KJSB Analysis of Kinematics and Kinetics According to Skill Level and Sex in Double-under Jump Rope Technique 175
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3. Vertical toe height
Main effects of sex (
p
=.006) and skill (
p
=.018) on vertical left toe
height were observed. Post hoc testing showed that male students
had significantly greater vertical left toe height compared with female
students in the skilled (
p
=.041) group. Significant main effects of sex
(
p
=.002) and skill (
p
=.020) on vertical right toe height were observed.
Post hoc testing showed that male students had significantly greater
vertical right toe height compared with female students in both the
skilled (
p
=.039) and unskilled (
p
=.019) groups (Table 4).
4. Stance width
There was a significant main effect of sex on stance width at E4
(
p
=.039). Post hoc testing showed that male students had a significantly
narrower stance width than female students in the skilled group (
p
=.027)
(Table 5).
5. Vertical center of mass displacement
There was a significant main effect of skill on vertical center of mass
displacement (
p
<.000). Post hoc testing showed that the skilled group
had significantly smaller vertical center of mass displacement compared
with the unskilled group in both male (
p
=.012) and female (
p
=.018)
students (Table 6).
6. Lower limb joint angles in the sagittal plane
There was a significant main effect of sex on the hip angle at E1
(
p
=.011). Post hoc testing showed that male students had a significantly
smaller hip flexion angle at E1 compared with female students in the
skilled group (
p
=.022). There was a significant main effect of sex on
the hip angle at E2 (
p
<.000). Post hoc testing showed that male
students had a significantly smaller hip angle at E2 compared with
female students in both the skilled (
p
<.000) and unskilled (
p
=.015)
Table 4. Vertical toe height (Unit: cm)
Male Female
t F
Left toe Skilled 21.04±4.05 16.38±4.19 2.253* 8.803* (G)
Unskilled 25.07±5.37 20.33±3.80 1.975 6.339* (S)
t
1.797 1.848
0.001 (G×S)
Right toe Skilled 21.84±4.43 16.88±4.20 2.275* 12.227* (G)
Unskilled 26.30±4.76 20.21±4.21 2.663* 6.085* (S)
t
2.055 1.486 0.124 (G×S)
All data are shown as means and standard deviations.
t
: Independent
t-
test comparing skilled and unskilled and male and female groups; G: Main
effect of sex, S: Main effect of group, G×S: Interaction effect, *
p
<.05
Table 5. Stance width (Unit: cm)
Male Female
t F
Skilled 5.19±2.30 7.79±4.02 1.635 0.063 (G)
E2 Unskilled 6.79±3.08 4.71±2.06 1.529 0.500 (S)
t
1.246 1.802
5.002* (G×S)
Skilled 3.98±1.75 5.89±1.20 2.472* 4.693* (G)
E4 Unskilled 3.99±1.14 5.01±3.09 0.916 0.409 (S)
t
0.025 0.704 0.442 (G×S)
All data are shown as means and standard deviations.
t
: independent
t-
test comparing skilled and unskilled and male and female groups; G: Main
effect of sex, S: Main effect of group, G×S: Interaction effect, *
p
<.05
Table 6. Vertical center of mass displacement (Unit: cm)
Male Female
t F
Skilled 23.36±3.80
23.02±4.52
0.164 0.000 (G)
Unskilled
28.20±3.45
28.50±2.73
0.194 15.520* (S)
t
2.825* 2.748*
0.062 (G×S)
All data are shown as means and standard deviations.
t
: Independent
t-
test comparing skilled and unskilled and male and female groups;
G: Main effect of sex, S: Main effect of group, G×S: Interaction effect,
*
p
<.05
176 Dae Young Kim, et al. KJSB
Korean Journal of Sport Biomechanics
groups (Table 7).
There were no significant main effects or interaction effects on knee
angles in the sagittal plane.
There was a significant main effect of sex (
p
=.002) and skill (
p
=.007)
on ankle angle at E2. Post hoc testing showed that male students
had significantly less dorsiflexion compared with female students in the
skilled group (
p
=.011), and that the skilled group showed significantly
lower dorsiflexion compared with the unskilled male group (
p
=.012).
There was a significant sex and skill interaction effect on ankle angle
at E3 (
p
=.039). Post hoc testing showed that male students showed
significantly smaller dorsiflexion angles at E3 compared with female
students in the unskilled group (
p
=.032) (Table 8).
Table 8. Right ankle angle in the sagittal plane (Unit: °)
Male Female
t F
E1 Skilled 3.89±8.61 1.55±11.75 1.070 0.017 (G)
Unskilled 1.31±8.30 5.01±10.50 1.349 0.039 (S)
t
1.304 1.102
2.895 (G×S)
E2 Skilled 16.85±6.60 25.86±5.38 2.929* 11.719* (G)
Unskilled 24.80±5.19 29.47±4.90 1.831 8.344* (S)
t
2.838* 1.33 1.174 (G×S)
E3 Skilled 15.22±7.29 12.29±11.25 0.633 1.684 (G)
Unskilled 13.27±8.58 24.95±11.02 2.388* 2.521 (S)
t
0.521 2.126 4.703* (G×S)
E4 Skilled 11.02±10.04 10.45±9.81 0.113 3.438 (G)
Unskilled 14.48±9.04 0.60±17.74 2.221* 0.810 (S)
t
0.768 1.443 2.958 (G×S)
All data are shown as means and standard deviations. Positive value is flexion, Negative value is extension,
t
: Independent
t-
test comparing skilled
and unskilled and male and female groups; E1: event 1, E2: event 2, E3: event 3, E4: event 4, G: Main effect of sex, S: Main effect of group, G×
S:
Interaction effect, *
p
<.05
Table 7. Right hip angle in sagittal plane (Unit: °)
Male Female
t F
E1 Skilled 15.45±5.58 23.22±6.50 2.572* 7.331* (G)
Unskilled 15.95±6.42 20.94±8.08 1.380 0.141 (S)
t
0.178 0.580
0.348 (G×S)
E2 Skilled 15.01±6.04 28.49±4.09 5.054* 23.781* (G)
Unskilled 16.92±9.16 30.30±10.08 2.777* 0.455 (S)
t
0.521 0.440 0.000 (G×S)
E3 Skilled 13.50±4.35 16.81±6.21 1.255 1.373 (G)
Unskilled 11.67±6.11 13.52±8.00 0.525 1.353 (S)
t
0.731 0.858 0.110 (G×S)
E4 Skilled 17.98±4.59 21.99±10.96 0.999 1.752 (G)
Unskilled 16.4±9.86 21.78±12.311 0.911 0.044 (S)
t
0.340 0.033 0.023 (G×S)
All data are shown as means and standard deviations. Positive value is flexion, Negative value is extension,
t
: Independent
t-
test comparing skilled
and unskilled and male and female groups; E1: event 1, E2: event 2, E3: event 3, E4: event 4, G: Main effect of sex, S: Main effect of group, G×
S:
Interaction effect, *
p
<.05
KJSB Analysis of Kinematics and Kinetics According to Skill Level and Sex in Double-under Jump Rope Technique 177
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7. Peak vertical ground reaction force and loading rate
There were no significant main effects or interaction effects on peak
VGRF and loading rate (Table 9).
DISCUSSION
This study analyzed the kinematic and kinetic characteristics of the
double-under technique of jump rope according to skill level and sex.
With regard to phase duration, total duration, and flight time according
to skill level, male and female students in the skilled groups showed
significantly shorter durations of P3, defined as the time from takeoff
(E3) to the time at which the center of body mass reaches peak vertical
height (E4), than the male and female students in the unskilled group.
The female students in the skilled group showed significantly shorter
P4, defined as the period from the point at which the center of body
mass reaches peak vertical height (E4) to the point at which the feet
contact the force plate (E5), than the female students in the unskilled
group. In addition, the male and female students in the skilled group
showed significantly shorter total duration and flight time than the
male and female students in the unskilled group.
Because athletes compete in terms of the number of double-under
jumps within a 30-second period, the recommended strategy for double
under is to increase the number of jumps by shortening total duration
and flight time as much as possible (Korea Rope Skipping Association,
2016; Miyaguchi et al., 2014). For this reason, the skilled group in this
study is likely to have attempted to increase the number of jumps by
shortening phase duration, total duration, and flight time. In other words,
the skilled and unskilled groups differ in their double-under strategies,
in that the former perform their motions as quickly as possible by main-
taining low jumps, while the latter take relatively higher jumps than
the former.
With regard to the differences in phase duration, total duration, and
flight time according to sex, both skilled and unskilled male students
showed significantly shorter P2, defined as the period from the peak
VGRF (E2) to the point at which the feet take off from the force plate
(E3), than skilled and unskilled female students. As shown here, male
students in this study showed significantly shorter P2 (take off) in double
under compared with female students, regardless of skill level, indicating
that they use strategies for minimizing flight time as much as possible.
Conversely, female students performed double unders with relatively
longer flight times than males.
Skilled male students showed significantly higher vertical left toe
height compared with skilled female students. Both skilled and unskilled
male students showed significantly greater vertical right toe height
compared with skilled and unskilled female students. The difference
in vertical toe height according to sex is attributable to the difference
in restriction of ankle flexion and extension at takeoff, which ultimately
induces stiffer landing and affects overall jump rope performance
(Pittenger et al., 2002). The male students in this study used a strategy
that minimized anterior-posterior ankle movement by keeping the soles
of the feet parallel to the ground as much as possible. This strategy
increases the vertical toe height, which in turn reduces the duration of
takeoff (P2).
With regard to skill level-specific vertical center of mass displacement,
the skilled male and female students showed significantly smaller
displacement compared with the unskilled male and female students.
Gowitzke and Brown (1989) reported that novice jumpers show large
vertical displacements when performing alternating foot exercise. Main-
taining a consistent vertical center of mass contributes to preserving
dynamic energy, ultimately increasing the efficiency of jump rope per-
formance (Brancazio, 1984). The skilled group in this study maintained
efficiency of jump rope performance by minimizing vertical center of
mass displacement. Therefore, vertical center of mass displacement
during a double under is believed to be a meaningful indicator of jump
rope skill level, and strategies that maintain low jumps by minimizing
vertical center of mass displacement are recommended for better
performance.
Previous studies reported that the mean distance between the medial
malleoli in an upright posture is about 9 cm (Murray, Seireg & Sepic,
1975; Perry & Burnfield, 2010), and the mean stance distance during
walking in men and women is 7 cm and 8 cm, respectively (Murray,
Drought & Kory, 1964; Murray, Kory & Sepic, 1970; Perry & Burnfield,
2010). These findings suggest that individuals increase the base of
support in an upright posture to maintain static stability, while decreasing
the base of support when walking to ensure dynamic stability and
Table 9. Kinetic variables
Male Female
t F
PVGRF Skilled 50.30±5.20 48.97±2.85 0.606 0.495 (G)
Unskilled 50.19±8.33 48.63±4.35 0.450 0.012 (S)
t
0.174 0.033
0.003 (G×S)
Loading rate Skilled 485.80±68.41 442.18±42.86 1.471 3.445 (G)
Unskilled 463.27±111.71 438.82±90.29 1.251 1.143 (S)
t
0.516 1.550 0.052 (G×S)
All data are shown as means and standard deviations. Positive value is dorsiflexion, Negative value is plantarflexion,
t
: Independent
t-
test comparing
skilled and unskilled and male and female groups; VGRF: Vertical ground reaction force, G: Main effect of sex, S: Main effect of group, G×
S:
Interaction effect
178 Dae Young Kim, et al. KJSB
Korean Journal of Sport Biomechanics
momentum (Shin, Youm & Son, 2013). In the present study, stance
width at E2 (peak VGRF) of double under was 5.2 cm in skilled male
students and 7.3 cm in skilled female students. Compared to the stance
width in an upright posture found in a prior study (9 cm), these are
about 42% and 13% narrower, respectively. Moreover, stance widthat
E4 (point at which vertical center of mass reaches maximum) was 4.0
cm in skilled male students and 5.9 cm in skilled female students.
Compared to the stance width in an upright posture, these are about
56% and 35% narrower, respectively. Motions such as landing after
jumping require dynamic stability, and a lack of dynamic stability may
lead to an injury (Wright, Arnold & Ross, 2016). In particular, jump rope
is a sport that requires minimization of energy consumption while per-
forming repeated motions. In this context, narrowing stance width
during a double under would be beneficial for maintaining consistency
of movement and increasing dynamic stability.
With regard to limb joint angles in the sagittal plane according to
skill level, skilled male students showed significantly less ankle dorsi-
flexion at E2 (peak VGRF) compared with unskilled male students.
Jump rope features a stiff landing, in which the dorsiflexion range
of motion of the ankle in the contact phase is controlled to limit ankle,
knee, and hip flexion (Pittenger et al., 2002). Increasing the stiffness
within an ideal range increases the stability of lower limb joints in the
early contact phase, contributes to generating maximum energy at
takeoff, and reduces the risk of injury (Butler, Crowell III & Davis, 2003).
The skilled group in this study maintained ankle dorsiflexion not only
at E2, when the VGRF reaches peak after landing, but also at E3, when
the feet take off from the force plate; this increases the ankle joint
stiffness, thereby improving the efficiency of double-under technique.
Although both feet are used for takeoff and landing in a double under,
maintaining ankle dorsiflexion is thought to improve double-under per-
formance, and ankle dorsiflexion angle is deemed a meaningful indicator
of jump rope skill level.
With regard to lower limb joint angles in the sagittal plane according
to sex, the skilled male students showed significantly smaller hip angles
at E1 (ground contact) compared with the skilled female students. At E2,
when the GRF reaches peak, the skilled male students showed signifi-
cantly less ankle dorsiflexion compared with the skilled female students.
Furthermore, both the skilled and unskilled male students showed
significantly less hip flexion compared with the skilled and unskilled
female students. At E3, when the feet take off from the force plate, the
unskilled male students showed significantly less ankle flexion com-
pared with the unskilled female students. At E4, when the center of
mass reaches peak vertical height, the unskilled male students showed
significantly smaller ankle dorsiflexion compared with the unskilled
female students. As shown here, compared with female students, male
students restricted ankle and hip flexion more while performing a double
under. These results suggest that male students use a strategy in which
they increase lower limb stiffness by maintaining less lower limb joint
flexion during a double under (Butler et al., 2003). Therefore, the sex-
specific differences in the lower limb joint flexion angles during a double
under are a result of the difference in strategies used to improve
performance.
Load rate for landing is increased by high VGRF and reduced lower
limb joint range of motion (Quatman, Ford Myer, & Hewett, 2006).
Reduced ankle range of motion increases loading rate for landing,
thereby heightening the risk of injury (De Ridder et al., 2015). In this
study, there were no significant differences in peak VGRF and loading
rate for P1 (landing phase); however, the loading rate tended to be
higher in the skilled group compared with the unskilled group and in
male students compared with female students. The higher loading rate
in the skilled and male groups is thought to be the result of a strategy
that maintains ankle dorsiflexion to improve double-under performance,
but the effect of such high load rate on the risk of injury is unclear.
In summary, the skilled group employed a strategy in which they
increased jump rope speed by reducing flight time and jump height
as much as possible with dorsiflexed ankles during a double under.
Based on these findings, double-under training programs should aim
at training athletes to speed up rope rotations while maintaining low
and fast jumps for better performance. Furthermore, we found that
male students used strategies to minimize ankle and hip flexion by
maintaining short flight time, while female students showed higher
ankle and hip movement and greater flight time. Such differences in
strategies have an impact on the stiffness of lower limb joints, and
the appropriate flight time and lower limb joint movement for each
sex should be identified to help athletes achieve optimal performance.
Future studies should recruit a larger sample and investigate the asso-
ciations between double-under performance and potential injury using
variables such as joint stiffness, joint work, and joint contribution.
CONCLUSION
This study analyzed the kinematic and kinetic characteristics of
double-under technique according to skill level and sex. The skilled
group displayed significantly shorter phase duration, total duration, and
flight time, and less vertical center of mass displacement compared
with the unskilled group. Further, the skilled group maintained signifi-
cantly less ankle dorsiflexion compared with the unskilled group. Skilled
male students displayed significantly shorter takeoff, but a significantly
greater left and right toe height at takeoff compared with the skilled
female students. In addition, male students exhibited significantly smaller
ankle, knee, and hip flexion angles during a double under compared
with female students. In conclusion, phase duration, total duration, flight
time, vertical center of mass displacement, and ankle dorsiflexion angle
were found to be meaningful indicators of double-under skill level.
Moreover, the control of lower limb joint flexion was found to be an
indicator of sex-specific differences in double-under strategy. These
findings may contribute to improving double-under performance and
coaching strategies.
ACKNOWLEDGEMENTS
This study was supported by the Dong-A University research fund.
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