Training & Testing 647
Comparison of Ground Reaction Forces and Contact
Times Between 2 Lateral Plyometric Exercises in
Professional Soccer Players
Authors
A liations
Key words
strength
football
force
agility
training
D. P. Wong1, A. Chaouachi2, A. Dellal2,3, A. W. Smith1
1
Department of Health and Physical Education, The Hong Kong Institute of Education, Hong Kong
Tunisian Research Laboratory “Sport Performance Optimisation”, National Center of Medicine and Science in Sports, Tunis,
Tunisia
3
Department of Fitness Training and Research, Olympique Lyonnais FC (Soccer), Lyon, France
2
Abstract
There are no studies which have examined the
di erences in kinetics between lateral plyometric exercises and the selection of these exercises
is largely based on the experience and observation of coaches. This study aimed to compare
ground reaction forces (GRF) and contact times
(GCT) between 2 lateral plyometric exercises:
lateral alternative leg hopping (HOP), and speed
lateral footwork (SPEED). 16 professional male
soccer players (age: 24.6 ± 5.5 years; and BMI:
21.7 ± 2.2 kg.m 2) participated in this within-participant repeated measures study. 3-dimensional
GRF data were measured by force platform. Our
Introduction
accepted after revision
January 16, 2012
Bibliography
DOI http://dx.doi.org/
10.1055/s-0032-1304588
Published online:
April 17, 2012
Int J Sports Med 2012; 33:
647–653 © Georg Thieme
Verlag KG Stuttgart · New York
ISSN 0172-4622
Correspondence
Dr. Del P. Wong
Department of Health and
Physical Education
The Hong Kong Institute of
Education
Tai Po
Hong Kong
Tel.: +852/2948 64 21
Fax: +852/2948 78 48
[email protected]
Agility is defined as a rapid whole-body movement with changes in velocity and/or direction in
response to a stimulus [4, 18]. Agility is one of the
important abilities in many athletic performances [18, 19] and may be considered as a separate fitness factor [4] as there are only low to
moderate correlations between agility and
sprinting. Agility is one of the factors that discriminates athletes with respect to competition
levels [9, 14] and positional roles [7], for instance,
elite soccer players have better agility performance than their sub-elite counterparts [16] and
better rugby tacklers tend to have better agility
[8]. To be agile, athletes must demonstrate a
combination of physical ability (movement speed
and strength), cognitive processing (perception
and decision making), and technical skills (footwork and movement technique) [18]. To optimize physical ability and technical skills related
to agility, preplanned training exercises allowing
athletes to practice closed agility skills in a predictable or stable environment have been used
[3, 19]. In this regard, coaches typically use pre-
study revealed significant di erences between
the 2 lateral plyometric exercises in all kinetics
parameters (F = 573.7, P < 0.01). HOP produced
significantly longer GCT (0.45 s vs. 0.23 s, P < 0.01,
large e ect), significantly higher values (P < 0.05,
large e ect) in peak force (3.31 vs. 2.47 Body
Weight [BW]), peak rate of force development
(0.94 vs. 0.29 BW/s), and impulse (0.76 vs. 0.31
BW.s) except for peak force in the medial-lateral
(P < 0.05, medium e ect) and impulse in the
anterio-posterior direction (not significant, small
e ect). Therefore, SPEED is an exercise that aims
to increase step frequency because of its short
GCT (< 0.25 s) while HOP increases leg strength
and power.
planned lateral plyometric exercises targeting 2
types of improvements: explosive leg power (e. g.
lateral alternative leg hopping) and high step frequency (e. g. speed lateral footwork), respectively
[3, 12, 19]. Specifically, explosive leg power exercises employ the stretch-shortening cycle, which
improves movement initiation and acceleration
performance, whereas the high step frequency
exercises involve short ground contact time
(GCT) ( < 0.25 s), which improves changes of
direction and movement frequency [19, 21].
Intervention studies have shown positive training e ects in agility performance. In this regard,
Bloomfield et al. [3] conducted speed, agility, and
quickness (SAQ) training in a variety of sports,
including soccer, tennis, hockey, basketball,
rugby, and netball. The participants received a
range of 10–18 h training over 6 weeks that
resulted in 50–73 % improvement in speed, agility, power, and dynamic balance after training.
Agility performance is not only influenced by
speed and quickness but also by muscular power
as demonstrated in a previous study [5], which
found that agility T-test time was highly corre-
Wong DP et al. Kinetics of lateral plyometric exercises … Int J Sports Med 2012; 33: 647–653
648 Training & Testing
lated with horizontal jumping distance (the 5 jump test,
r = 0.61, p < 0.05).
To date, there are no studies which have examined the di erences in kinetics between lateral plyometric exercises. The
choice of lateral plyometric exercises is largely based on the
experience and observation of coaches, with little empirical evidence supporting their choices. Therefore, the purpose of our
study was to compare the di erences in ground reaction force
(GRF) and GCT patterns between 2 commonly used lateral plyometric exercises: lateral alternative leg hopping (HOP) and speed
lateral footwork (SPEED). The results of our study will be useful
for coaches in the selection of agility training exercises and will
facilitate sport scientists’ understanding of the mechanism of
agility performance in athletes. Taken together, this knowledge
will lead to an improvement in the quality of training programs
and will optimize athletic performance. From our practical
experience, we hypothesized that SPEED aims to increase step
frequency and therefore has short GCT ( < 0.25 s) while HOP aims
to improve leg strength and power and therefore has high peak
force, peak rate of force development, and impulse.
Methods
Design
In this within-subject repeated measures study, all players visited the Human Performance Laboratory and performed 2 lateral plyometric exercises (HOP and SPEED) on a force platform in
a counterbalanced order. To compare the 2 lateral plyometric
exercises, we recorded the 3-dimensional (3D) GRF, i. e., anterioposterior (AP), medio-lateral (ML) and vertical (V) forces. All
players were familiar with these exercises as they perform them
regularly during on-field training. We scheduled the data collection sessions during the 5th week of the 6-week pre-season preparation period and we asked players to refrain from any vigorous
physical training 48 h prior to the test.
record their BW, which was used to normalize GRF data. Players
completed a standardized warm-up that consisted of 5-min of
jogging at 10 km.h 1 on the motorized treadmill, followed by
5-min of static (quadriceps, hamstrings, and calf muscle groups)
and dynamic stretching (walking knee to chest, high knee, butt
kicks, hip adduction and abduction). Following this, players
were given a minimum of 5 practice trials to familiarise themselves with the testing procedure which required them to step
on the force platform ( Fig. 1, 2) at approximately 75, 85, and
100 % of individual’s perceived maximal speed and power while
performing the 2 lateral plyometric exercises. Testing began
5 min after the familiarization session.
Lateral plyometric exercises
We instructed each player to perform HOP exercise such that
they achieved maximal diagonal distance in each step using
alternative legs in a smooth, continuous manner ( Fig. 1). In
addition, we instructed each player to perform the SPEED exercise to achieve maximal movement speed ( Fig. 2). These
instructions were in accordance with the exercise guidelines
[15]. The movement of “1 foot out and 2 feet in” as described in
a previous study was used [21]. The force platform was located
in the middle of the pathway so that the GRF and GCT of 1 step
were recorded during each trial. For each exercise, we allowed
1-min passive recovery between trials and recorded 5 successful
trials on the dominant leg (determined by the ball kicking preference) where the player’s foot landed completely on the force
platform without any alteration in their movement and speed.
The mean values of each exercise were used in the analysis.
Kinetics measurement
We recruited 16 male professional soccer players to participate
in our study (age: 24.6 ± 5.5 years; height: 1.79 ± 0.05 m; weight:
687.8 ± 58.2 N; and BMI: 21.7 ± 2.2 kg.m 2). Prior to the start of
the league season, there were 6 weeks of pre-season training
during which the players had 6–8 soccer training sessions per
week each lasting for approximately 90 min. Each training session generally consisted of a 10-min warm-up, 30-min technical
training, 30-min tactical training, 15-min simulated competition, and 5-min cool-down.
Our study was conducted according to the Declaration of Helsinki and the protocol was fully approved by the Clinical Research
Ethics Committee before the commencement of the assessments. Moreover, our study met the ethical standards of the
International Journal of Sports Medicine [10]. We received written informed consent from all players following an explanation
of the general nature of our study to all players without giving
them its detailed aims so as to eliminate any biases during data
collection. We also explained the benefits and risks involved
with this investigation. We told players that they were free to
withdraw from our study at any time without penalty.
GRF data were collected from a Kistler piezoelectric force platform (Model: 9281C, 400 mm × 600 mm), which measured the
3D forces applied to the platform’s surface. It was mounted on
the floor and oriented such that the X, Y, and Z axes of the
platform corresponded to AP (positive = anterior), V (positive = upward) and ML (negative = lateral) forces applied by the
players, respectively. We sampled signals from the force platform at 1 000 Hz and stored on disk using SMART Capture program
(BTS S.p.A, Milan, Italy). Force data were not filtered. The player’s
GCT was determined by examining the vertical ground reaction
force trace in each trial from landing to take-o ( Fig. 3a). We
wrote a customized SMART Analyzer (BTS S.p.A, Milan, Italy)
protocol to identify GCT (s) from the force data and calculate
peak forces (N) and rate of force development (N/s). The latter
was calculated by determining the peak value of the first derivative of the force with respect to time. The GRF data ( Fig. 3b)
were represented in the XYZ directions (components) and in
overall (resultant). Individual GRF data were then normalized
using the player’s BW, and subsequently we calculated individualized peak forces (BW), peak rate of force development (BW/s)
and impulses (BW.s). Force data in each direction were di erentiated with respect to time using central di erences formula and
the maximum value of the first derivative was taken as the peak
rate of force development. During each step on the force platform there were phases of braking and propulsive shear (horizontal) forces. In our study, the propulsive phases were visually
identified and used in the analysis.
Warm-up
Statistical analysis
Before testing, we recorded each player’s age, height, and body
weight (BW). Players stood motionless on the force platform to
Data are expressed as mean ± SD. The normal distribution of the
data was confirmed using the Shapiro-Wilk test. Measurement
Participants
Wong DP et al. Kinetics of lateral plyometric exercises … Int J Sports Med 2012; 33: 647–653
Training & Testing 649
a
b
F
Start: right leg lead
Fig. 1 Movement illustration a and experimental set up b of lateral alternative leg hopping-HOP.
reliability and variance were indicated by calculating the intraclass correlation coe cient (ICC) and coe cient of variance (CV),
respectively. Repeated-measure MANOVA was used to examine
the di erences between the 2 lateral plyometric exercises by considering all kinetics parameters. We used paired-sample t-test to
compare the di erence in each kinetics parameter between
SPEED and HOP exercises. E ect sizes (Coden’s d) were calculated
to determine the practical di erence between the 2 lateral plyometric exercises. E ect size (ES) values of 0–0.19, 0.20–0.49, 0.50–
0.79 and 0.8 and above were considered to represent trivial, small,
medium and large di erences, respectively [6]. Pearson product
moment correlation coe cient was used to assess the relationship between the kinetics parameters. The magnitude of the correlations was determined using the modified scale by Hopkins
[11]: trivial: r < 0.1; low: 0.1–0.3; moderate: 0.3–0.5; high: 0.5–
0.7; very high: 0.7–0.9; nearly perfect > 0.9; and perfect: 1. Significance level was defined as P < 0.05.
Repeated-measure MANOVA showed significant di erences
between the 2 lateral plyometric exercises in all kinetics parameters (F = 573.7, P < 0.01). Paired sample t-test showed that SPEED
exercise induced significantly shorter GCT as compared to HOP
exercise (P < 0.01, large e ect: 1.48, Table 2). Furthermore, as
compared to SPEED, HOP exercise induced significantly higher
values (P < 0.05, large e ect) in peak force (ES: 1.68–1.71), peak
rate of force development (ES: 1.46–1.87), and impulse (ES:
2.61–3.49) parameters except for the peak force in medial-lateral (P < 0.05, medium e ect: 0.54) and impulse in anterio-posterior direction (not significant, small e ect: 0.43).
When the data of SPEED and HOP were pooled together, signifiTable 3) were observed between
cant correlations (P < 0.01,
GCT and peak force (very high), peak rate of force development
(very high), and impulse (nearly perfect).
Discussion
Results
The reliability and variance of our kinetics measurement are
shown in Table 1. GCT had high ICC ( > 0.90) and acceptable CV
( < 15 %). Overall, impulses had higher ICC and lower CV as compared to peak force and peak rate of force development ( Table 1).
The purpose of our study was to compare the di erences in GRF
and GCT between HOP and SPEED plyometric exercises. This is
the first reported 3D GRF-based study of lateral plyometric exercises. Specifically, we measured the GCT, peak force, peak rate of
force development, and impulse, and each of the kinetics parameters were normalized by BW. Furthermore, these values were
Wong DP et al. Kinetics of lateral plyometric exercises … Int J Sports Med 2012; 33: 647–653
650 Training & Testing
a
b
F
Start: right leg lead
Fig. 2 Movement illustration a and experimental set up b of speed lateral footwork-SPEED.
represented as overall resultant value, and respectively in 3D
components. The results support our hypotheses and therefore,
the SPEED exercise could be considered as an exercise that aims
to increase foot step frequency because of its short GCT ( < 0.25 s),
while the HOP exercise aims to increase leg power because it
induces higher peak force, peak rate of force development, and
impulse [19]. Likewise, SPEED is classified as fast stretch-shortening cycle exercise whereas HOP is slow stretch-shortening
cycle exercise [4]. However, it was found that the reliability (ICC)
of impulse at the anterio-posterior direction during HOP was
moderate and the variance (CV) was quite high. This may be the
result of di erent braking and propulsive techniques being used
in response to the landing and take-o impulse.
As compared to the SPEED exercise in our study, longer GCTs
(0.41 s) were reported in a previous study using a lateral cutting
movement during the custom agility test [2]. This can be attributed to the higher lateral movement speed and the associated
braking movement immediately before the change of direction
in the study of Barnes et al. [2]. Furthermore, GCT appears to be
relevant to agility performance in that Barnes et al. [2] demonstrated that higher division volleyball players have shorter GCT
in drop jump test (0.42 s vs. 0.44 s) and simultaneously better
performance during agility test (5.93 s vs. 6.1 s) than those in the
lower division. Collectively, GCT should be minimized if the
training purpose is to increase step frequency (i. e. the SPEED
exercise in our study) and subsequently improve the agility performance.
The GRF profiles of the SPEED and HOP trials showed some differences ( Fig. 3b). Specifically, the magnitudes of the peak
vertical GRF in the SPEED were greater than the HOP whereas
the anterio-posterior and medio-lateral GRF peaks were greater
in the HOP. Additionally, vertical and medio-lateral GRF of HOP
had plateaus from around 20 % to 80 % of the GCT. This may be
related to the fact that the subjects spent more time in contact
with the force platform in HOP as they were attempting to generate maximal impulse in contrast to the SPEED where they
were trying to maximize movement speed.
Agility performance is characterized by the ability to change
direction and therefore greater improvement has been shown
when training is specific to the movements in horizontal mediallateral and anterio-posterior directions [4, 13]. Our results
provide empirical evidence supporting the fact that lateral plyometric exercises have higher forces in the medial-lateral and
anterio-posterior directions as compared to vertical jump exercise that involve lower horizontal forces [16]. Specifically, the
HOP exercise has ~1.1 times higher peak force in the mediallateral direction, and ~1.9 times higher in the anterio-posterior
direction, as compared to the SPEED.
The peak normalized vertical force of HOP in our study was
comparable to a drop jump from 0.6 m and in agreement with a
Wong DP et al. Kinetics of lateral plyometric exercises … Int J Sports Med 2012; 33: 647–653
Training & Testing 651
b
SPEED
Peak force
1600
Force (N)
1400
1200
Force (N)
SPEED:
1000
800
600
500
400
300
200
100
0
–100
–200
–300
0
20
400
200
2.1
2.4
2.5
HOP
1600
Ground
contact time
1400
1200
600
400
200
0
Impulse = 350.1 Ns
2
2.1
2.2
2.3
Time (s)
2.4
2.5
0
–100
–200
–300
–400
–500
–600
–700
100
0
20
40
60
GCT (%)
80
100
Z
Force (N)
800
80
Y
1000
Force (N)
Force (N)
2.2
2.3
Time (s)
1600
1400
1200
1000
800
600
400
200
0
40
60
GCT (%)
Force (N)
2
Force (N)
0
HOP:
X
Force (N)
a
0
20
40
60
GCT (%)
80
100
500
400
300
200
100
0
–100
–200
–300
1600
1400
1200
1000
800
600
400
200
0
0
–100
–200
–300
–400
–500
–600
–700
X
0
20
40
60
GCT (%)
80
100
Y
0
20
40
60
GCT (%)
80
100
80
100
Z
0
20
40
60
GCT (%)
Fig. 3 a Vertical peak force and ground contact time (GCT); b Anterio-posterior (X), vertical (Y) and medio-lateral (Z) ground reaction force traces of the 2
plyometric exercises.
Table 1 Measurement reliability and variance in 2 lateral plyometric
exercises (N = 16).
ICCa
SPEED
c
contact time (s)
0.91
peak force (BW)
– anterio-posterior (X)
0.71
– vertical (Y)
0.57
– medial-lateral (Z)
0.85
– resultant
0.55
peak rate of force development (BW/s)
– anterio-posterior
0.69
– vertical
0.67
– medial-lateral
0.53
– resultant
0.69
impulse (BW.s)
– anterio-posterior
0.62
– vertical
0.94
– medial-lateral
0.95
– resultant
0.94
a
CVb ( %)
SPEEDc
HOPd
0.93
10.20
14.46
0.83
0.53
0.82
0.60
44.24
15.11
11.64
12.96
22.66
21.37
15.20
19.05
0.91
0.87
0.90
0.89
36.42
33.09
31.00
30.59
24.09
31.08
27.60
26.37
0.44
0.92
0.89
0.92
73.92
10.74
10.84
10.02
67.92
11.59
8.53
10.61
HOP
d
ICC: intraclass correlation coe cient; b CV: coe cient of variance; c SPEED: Speed
lateral footwork; d HOP: Lateral alternative leg hopping
previous study of female basketball players performing similar
side step pivoting movement (ranged 2.34–2.68 BW) [1, 21]. On
the other hand, SPEED has lower normalized peak vertical force
than drop jump from 0.2 m (2.2 vs. 2.6 BW) [1].
In their review of agility literature, Sheppard and Young [18]
suggested that reactive strength is one of the factors in leg muscle quality category that a ect agility performance. Specifically,
reactive strength is the ability to change rapidly from an eccentric to concentric action. It is considered to be high when concentric contractions can generate high power or long distance
jumping (either in vertical or horizontal direction) within the
shortest GCT. During agility movement and other plyometric
exercises, the active muscle is usually pre-stretched and immediately followed by concentric contraction (i. e., the stretchshortening cycle). Our results showed that the HOP exercise
induced ~3.2 times higher resultant peak rate of force development than the SPEED exercise. Therefore, HOP is an appropriate
training exercise to improve the reactive strength with an
emphasis on the power generation.
Impulse is defined as the change in momentum, the product of
mass and velocity, resulting from a force being applied to a body
over a period of time [19]. Thus, changes in impulse result
directly in changes in the player’s velocity since the player’s
mass does not change during the exercises. To increase impulse,
players can apply greater force over the same time; or apply the
same force for a longer period of time; or a combination of
increased force and application time. This information has practical training uses for coaches. For example, in cases where players are working against their own body weight, coaches can
design drills that will increase the GCT to produce higher
impulses. On the other hand, if additional external loads are
used, for example weight belts, pulled sleds and similar, coaches
can either increase the external load, increase the loading time
or do both to increase the training impulses.
Previous studies [2, 21] only measured the peak force during lateral plyometric exercises. However, our study showed larger
e ects in peak rate of force development and impulse when
comparing the 2 lateral plyometric exercises. It is reasonable
because these parameters consider both the peak force and the
GCT. Therefore, it is suggested to use peak rate of force development and impulse in future studies to di erentiate among lateral plyometric exercises. Specifically, using peak rate of force
development and impulse to analyze the plyometric exercises
provide higher sensitivity in the comparison, and these 2 parameters indicate the peak force and GCT at the same time point.
Wong DP et al. Kinetics of lateral plyometric exercises … Int J Sports Med 2012; 33: 647–653
652 Training & Testing
SPEED
contact time (s)
0.23 ± 0.03b
peak force (BW)
– anterio-posterior (X)
0.44 ± 0.16b
– vertical (Y)
2.19 ± 0.23b
– medial-lateral (Z)
1.05 ± 0.15a
– resultant
2.47 ± 0.23b
peak rate of force development (BW/s)
– anterio-posterior
0.15 ± 0.05b
– vertical
0.23 ± 0.08b
– medial-lateral
0.10 ± 0.03b
– resultant
0.29 ± 0.10b
impulse (BW.s)
– anterio-posterior
0.003 ± 0.008
– vertical
0.28 ± 0.05b
– medial-lateral
0.13 ± 0.03b
– resultant
0.31 ± 0.05b
Ratio (HOP/SPEED)c
HOP
E ect size
0.45 ± 0.14
2.02
1.48
0.83 ± 0.20
2.98 ± 0.43
1.15 ± 0.14
3.31 ± 0.42
1.89
1.38
1.10
1.34
1.71
1.68
0.54
1.86
0.36 ± 0.16
0.81 ± 0.39
0.27 ± 0.11
0.94 ± 0.40
2.40
3.52
2.70
3.24
1.46
1.69
1.55
1.87
0.008 ± 0.010
0.70 ± 0.14
0.29 ± 0.03
0.76 ± 0.14
2.67
2.50
2.23
2.44
0.43
2.61
3.49
2.78
a
represent a significant di erence between exercises at P < 0.05; b represent a significant di erence between exercises at P < 0.01;
c
SPEED: Speed lateral footwork; HOP: Lateral alternative leg hopping
pooled
– contact time
– peak force (resultant)
– peak rate of force development (resultant)
SPEED
– contact time
– peak force (resultant)
– peak rate of force development (resultant)
HOP
– contact time
– peak force (resultant)
– peak rate of force development (resultant)
a
Peak force
Peak rate of force
Impulse
(resultant)
development (resultant)
(resultant)
0.75b
0.80b
0.89b
0.95b
0.82b
0.90b
0.46
0.04
0.32
0.81b
0.17
0.01
0.53a
0.82b
0.81b
0.99b
0.56a
0.84b
Table 2 Kinetics comparison
between 2 lateral plyometric
exercises (N = 16).
Table 3 Relationships between
kinetics parameters (pooled
data of the 2 lateral plyometric
exercises).
represent a significant correlation at P < 0.05; b represent a significant correlation at P < 0.01
Furthermore, there are many lateral plyometric exercises being
used in training that have not been quantified [3, 21]. Further
studies in this topic are necessary, including studies that quantify the 3D kinematics and kinetics of the players performing
lateral plyometric exercises to assist coaches in selecting the
most appropriate sport-specific training exercises that eventually maximize specific training e ect and improve the e ciency
of the whole training program.
As mentioned, the lateral plyometric exercises used in our study
involved both physical abilities (movement speed and strength)
and technical skills (footwork and movement technique) but did
not address cognitive processing skills (perception and decision
making) [18]. Recent studies have developed agility exercises
and tests that take the cognitive processing into consideration
[10, 20], and further studies are required in regard to these complex agility exercises [19].
In conclusion, the present kinetics study found that HOP produced longer GCT, higher peak forces, peak rate of force development and impulses, whereas SPEED produced shorter GCT,
lower peak forces, peak rate of force development and impulses.
Therefore, SPEED is considered an exercise that aims to increase
foot step frequency while the HOP exercise aims to increase leg
power, which both improve agility performance. With this
knowledge, quality of training programs and athletic performance could be improved.
Acknowledgements
The authors thank Miss Lo Ka Kai for her assistance in statistical
calculation.
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