The impact of 2years of additional athletic training on the jump performance of young athletes

Abstract

Objectives

An early-established strength training regimen brings many benefit young athletes, especially those engaged in competitive sports. Strength training also leads to improve athletic performance, strength training is used for rehabilitation and injury prevention. The aim of this study was to evaluate how 2 years of additional strength training and plyometric training in elite 9- to 12-year-old soccer players (n = 70) affects their performance in the squat jump (SJ), countermovement jump (CMJ) and drop jump (DJ).
Equipment and methods

Therefore, subjects were divided into two groups. The difference between the first group (CG = soccer) and the second group (STG) was the additional strength training (1 to 2 times per week) and athletic units for the second group (STG = soccer and strength and plyometric training). For the analysis, performance gains within a group and for pair-wise comparisons between 2 groups, analysis of variance was performed with repeated measures with group and time factors.
Results

The results show in most variables a significant (P < 0.05) positive effect on performance in the SJ, CMJ and DJ tests for both groups over the time. The results also show in most variables significant (P < 0.05) better performances in the STG compared to the CG.
Conclusion

Therefore, long-term resistance training is recommended as early as childhood and adolescence.

Full text

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
Science
&
Sports
(2013)
xxx,
xxx—xxx
Disponible
en
ligne
sur
www.sciencedirect.com
ORIGINAL
ARTICLE
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes
L’impact
de
deux
années
supplémentaires
d’entraînement
sportif
sur
la
performance
en
saut
de
jeunes
athlètes
M.
Keiner∗,
A.
Sander,
K.
Wirth,
D.
Schmidtbleicher
Institute
of
Sport
Science,
Johann
Wolfgang
Goethe-University
Frankfurt,
Ginnheimer
Landstr.
39,
60487
Frankfurt
am
Main,
Germany
Received
12
April
2013;
accepted
13
July
2013
KEYWORDS
Strength
training;
Plyometric
training;
Soccer;
Children;
Young
athletes
Summary
Objectives.
—
An
early-established
strength
training
regimen
brings
many
benefit
young
ath-
letes,
especially
those
engaged
in
competitive
sports.
Strength
training
also
leads
to
improve
athletic
performance,
strength
training
is
used
for
rehabilitation
and
injury
prevention.
The
aim
of
this
study
was
to
evaluate
how
2
years
of
additional
strength
training
and
plyometric
training
in
elite
9-
to
12-year-old
soccer
players
(n
=
70)
affects
their
performance
in
the
squat
jump
(SJ),
countermovement
jump
(CMJ)
and
drop
jump
(DJ).
Equipment
and
methods.
—
Therefore,
subjects
were
divided
into
two
groups.
The
difference
between
the
first
group
(CG
=
soccer)
and
the
second
group
(STG)
was
the
additional
strength
training
(1
to
2
times
per
week)
and
athletic
units
for
the
second
group
(STG
=
soccer
and
strength
and
plyometric
training).
For
the
analysis,
performance
gains
within
a
group
and
for
pair-wise
comparisons
between
2
groups,
analysis
of
variance
was
performed
with
repeated
measures
with
group
and
time
factors.
Results.
—
The
results
show
in
most
variables
a
significant
(P
<
0.05)
positive
effect
on
perfor-
mance
in
the
SJ,
CMJ
and
DJ
tests
for
both
groups
over
the
time.
The
results
also
show
in
most
variables
significant
(P
<
0.05)
better
performances
in
the
STG
compared
to
the
CG.
Conclusion.
—
Therefore,
long-term
resistance
training
is
recommended
as
early
as
childhood
and
adolescence.
©
2013
Elsevier
Masson
SAS.
All
rights
reserved.
∗Corresponding
author.
Swimming
Federation
of
the
State
Niedersachsen,
Ferdinand-Wilhelm-Fricke-Weg
10,
30169
Hannover,
Germany.
E-mail
address:
michaelkeiner@gmx.de
(M.
Keiner).
0765-1597/$
–
see
front
matter
©
2013
Elsevier
Masson
SAS.
All
rights
reserved.
http://dx.doi.org/10.1016/j.scispo.2013.07.010

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
2
M.
Keiner
et
al.
MOTS
CLÉS
Musculation
;
Entraînement
pliométrique
;
Football
;
Enfants
;
Jeunes
athlètes
Résumé
Objectifs.
—
Un
programme
d’entraînement
de
résistance
précoce
apporte
de
nombreux
avan-
tages
aux
jeunes
athlètes,
en
particulier
à
ceux
engagés
dans
les
sports
de
compétition.
La
musculation
conduit
également
à
l’amélioration
de
la
performance
athlétique,
la
muscula-
tion
est
utilisée
pour
la
réhabilitation
et
la
prévention
des
blessures.
Le
but
de
cette
étude
était
d’évaluer
la
fac¸on
dont
deux
années
supplémentaires
de
musculation
et
d’entraînement
pliométrique
sur
des
joueurs
de
football
d’élite
de
neuf
à
12
ans
(n
=
70)
ont
une
influence
sur
leur
performance
dans
le
squat
jump
(SJ),
countre-mouvement
(CMJ)
et
le
drop
jump
(DJ).
Matériels
et
méthodes.
—Par
conséquent,
les
sujets
ont
été
divisés
en
deux
groupes.
La
dif-
férence
entre
le
premier
groupe
(CG
=
football)
et
le
second
groupe
(STG)
a
été
l’entraînement
supplémentaire
en
musculation
(1
à
2
fois
par
semaine)
et
les
unités
de
sport
pour
le
second
groupe
(STG
=
football
et
musculation
et
entraînement
pliométrique).
Pour
l’analyse
des
gains
de
performance
au
sein
d’un
groupe
et
pour
des
comparaisons
par
paires
entre
les
2
groupes,
l’analyse
des
écarts
a
été
réalisée
avec
des
mesures
répétées
avec
des
facteurs
de
groupe
et
de
temps.
Résultats.
—
Les
résultats
montrent
pour
la
plupart
des
variables
une
augmentation
significative
(p
<
0,05)
de
la
performance
dans
les
tests
SJ,
CMJ
et
DJ
pour
les
deux
groupes
au
cours
du
temps.
Les
résultats
montrent
également
pour
la
plupart
des
variables
une
amélioration
significative
(p
<
0,05)
des
performances
chez
STG
par
rapport
à
CG.
Conclusion.
—
Par
conséquent,
la
musculation
à
long
terme
est
recommandée
dès
le
plus
jeune
âge
et
l’adolescence.
©
2013
Elsevier
Masson
SAS.
Tous
droits
réservés.
1.
Introduction
Strength
training
is
established
as
a
complementary
train-
ing
method
in
childhood
and
adolescence
for
only
a
few
sports.
Strength
training
often
seems
to
be
simply
a
means
to
increase
muscle
mass
and
definition
or
to
improve
appearance.
In
addition
to
these
esthetic
benefits,
an
early-
established
strength
training
regimen
offers
many
benefits
for
young
athletes,
especially
those
engaged
in
competitive
sports.
In
addition
to
the
obvious
goal
of
getting
stronger,
strength
training
also
leads
to
improved
athletic
perfor-
mance
[1—3].
Furthermore,
strength
training
is
used
for
rehabilitation
and
injury
prevention.
Many
parents,
athletes
and
coaches,
however,
are
unsure
of
the
age
at
which
chil-
dren
can
start
strength
training.
An
investigation
by
Sewall
and
Micheli
(in
1986)
[4]
showed
that
a
9-week
progres-
sive
strength
training
program
lead
to
high
strength
gains
without
loss
of
mobility
for
girls
and
boys
aged
10—11
years.
Faigenbaum
et
al.
(in
1999)
[5]
found
that
5-
to
12-year-old
girls
and
boys
exhibited
increased
strength
after
an
8-week
strength
training
program.
Sadres
(in
2001)
[6]
found
that
children
with
an
average
age
of
9
years
old
showed
signifi-
cant
power
increases
after
21
months
of
free
weight
training
(weight
lifting
exercises).
Even
our
own
studies
suggest
that
trainability
is
very
good
in
childhood
and
adolescence
[1,7].
In
childhood,
strength
training
leads
mainly
to
strength
gains,
which
can
be
explained
by
neural
adaptations.
However,
morphological
adaptations,
such
as
hypertrophy
effects,
cannot
be
excluded.
Faigenbaum
et
al.
(in
2009)
[8]
argue
that,
based
on
the
current
literature,
an
increase
in
muscle
thickness
as
an
adaptation
to
strength
training
cannot
be
ruled
out
for
pre-pubertal
children.
The
reasons
that
the
authors
give
are
that
the
study
period
is
usually
too
short,
that
the
exercise
intensities
used
in
the
training
pro-
gram
are
too
low
and
that
there
is
insufficient
sensitivity
in
the
analysis.
This
view
is
supported
by
studies
in
which
the
effects
of
hypertrophy
could
be
observed
even
in
children
aged
6
to
11
years
[9,10].
In
addition,
adjustments
to
the
passive
musculoskeletal
system,
such
as
positive
adjust-
ments
to
the
bones
and
tendons,
can
be
expected
[11—13].
Other
possible
adaptations
to
resistance
training
include
the
prevention
of
injuries
[14].
Studies
show
that
young
players
who
have
strength
training
experience
tend
to
sustain
fewer
injuries
[14,15].
Incidence
of
injury
in
strength-trained
youngsters
is
approximately
one-third
that
of
young
athletes
without
any
strength
training
experience
[16].
In
addition
to
reducing
overall
incidence
of
injury,
strength
training
can
also
help
to
reduce
the
severity
of
injuries.
However,
those
injuries
that
are
caused
by
overuse
or
pour
physical
conditioning
may
be
significantly
reduced
through
strength
training
[15].
After
an
injury,
strength-trained
young
play-
ers
also
respond
better
to
rehabilitation,
resulting
in
a
more
rapid
return
to
training
and
competition
[14,17].
Fur-
ther
positive
psychological
changes
can
be
expected
after
a
strength
training
intervention
[18,19].
Therefore,
according
to
the
recommendations
of
various
organizations,
includ-
ing
the
National
Strength
and
Conditioning
Association,
the
Committee
on
Sports
Medicine
and
Fitness
and
the
Cana-
dian
Society
for
Exercise
Physiology,
strength
training
should
be
started
as
early
as
possible
in
children
(ages
6
and
up)
[8,20,21].
These
recommendations
are
based
on
the
low
risk
of
injury
due
to
strength
training
compared
to
the
risk
associated
with
other
sports
[22—25].
A
four-year
ret-
rospective
study
of
1109
weightlifters
(age
12—20
years)
who
participated
in
national
and
international
competitions
showed
no
injuries
(i.e.,
to
the
epiphyseal
plates)
that
required
surgical
treatment
or
hospitalization
[26].
There
are
several
published
studies
of
adolescents
and
adults
that
show
the
positive
effect
on
athletic
performance
[3,7].
In
the
literature,
however,
there
are
only
short-term
studies

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
Athletic
training
effects
on
jump
performance
3
of
the
impact
of
additional
weight
training
on
athletic
per-
formance
in
children,
and
the
results
of
the
literature
review
were
somewhat
ambiguous.
Thus,
Faigenbaum
et
al.
(1996)
[27]
demonstrated
that
it
is
possible
to
detect
significant
increases
in
leg
strength
in
7-
to
12-year-old
girls
and
boys
after
8
weeks
of
strength
training,
but
that
no
improvements
in
vertical
jump
were
observed
[28].
A
study
by
Weltman
et
al.
(in
1986)
[2]
with
a
strength
training
intervention
using
hydraulic
strength
training
machines
for
14
weeks
showed
significantly
improved
vertical
jump
performance
compared
to
the
control
group.
Long-term
studies
on
the
development
of
athletic
performance
in
children
are
lacking.
In
addition
to
strength
training,
plyometric
training
seems
to
be
beneficial
to
the
performance
in
squat
jumps
(SJ),
countermovement
jumps
(CMJ)
and
drop
jumps
(DJ)
in
childhood
[15,29].
Meylan
and
Malatesta
(in
2009)
[30]
and
Ingle
et
al.
(in
2006)
[31]
reported
faster
times
in
a
linear
sprint
in
adolescent
soccer
players
after
plyomet-
ric
training,
but
Thomas
et
al.
(in
2009)
[32]
did
not.
The
adaptions
to
plyometric
training
can
be
explained
primar-
ily
by
neural
adaptations.
Nevertheless,
further
adjustments
to
the
musculoskeletal
system
and
positive
adjustments
to
the
skeletal
system
can
be
expected
[11—13,33].
Neverthe-
less,
studies
of
children
for
this
purpose
are
lacking.
Similar
to
strength
training,
plyometric
training
under
competent
supervision
has
a
low
risk
of
injury
[34].
A
plyometric
train-
ing
may
also
alter
landing
strategies,
lead
to
decreased
varus
and
valgus
torques
at
the
knee
[29,35,36].
These
changes
are
considered
conducive
to
a
reduced
risk
of
knee
injury.
Some
studies
show
that
a
combination
of
jump
and
strength
training
seems
to
be
the
most
effective
[3,37].
Wirth
et
al.
(in
2011)
[3]
showed
that
the
combination
of
jump
training
and
strength
training
yielded
the
greatest
increases
in
CMJ
in
sports
students,
outperforming
strength
training
or
jump
training
in
isolation.
Likewise,
Kotzamanidis
et
al.
(in
2005)
[37]
showed
that
combining
strength
train-
ing
and
sprint
training
for
young
soccer
players
resulted
in
significantly
better
results
than
did
isolated
strength
train-
ing.
The
aim
of
this
study
was
to
evaluate
how
2
years
of
addi-
tional
strength
training
and
plyometric
training
in
elite
9-
to
12-year-old
soccer
players
affects
their
performance
in
the
SJ,
CMJ
and
DJ.
2.
Methods
To
estimate
the
effects
of
a
2-year
strength
training
program
on
athletic
performance,
70
male
young
soccer
players
were
followed
for
2
years.
Measurements
were
taken
at
the
begin-
ning
of
the
study
(T1),
after
1
year
(T2),
and
after
2
years
(T3).
The
T1
measurements
were
taken
in
July
2009,
before
the
initial
training
period,
and
the
T2
and
T3
measurements
were
taken
in
May
2010
and
May
2011,
2
to
3
weeks
after
the
last
game.
The
independent
variable
was
the
additional
strength
and
plyometric
training
(STG,
CG),
and
the
depend-
ent
variables
were
the
performances
in
the
jump
tests
(SJ,
CMJ,
DJ).
Each
subject
and
their
parents
were
informed
of
the
experimental
risks
involved
with
the
research.
All
subjects
provided
written
informed
consent
to
participate.
Informed
consent
was
also
obtained
from
the
subjects’
parents
where
subjects
were
less
than
18
years
of
age.
The
research
design
was
approved
by
institutional
review
board.
The
study
was
carried
out
with
respect
for
the
use
of
human
subjects.
2.1.
Subjects
The
soccer
players
were
recruited
from
two
youth
train-
ing
centres
affiliated
with
professional
teams
in
the
second
and
third
divisions
in
Germany.
The
2
groups
were
children
and
young
people
who
had
been
active
in
soccer
and
par-
ticipated
in
a
minimum
of
5
hours
of
training
per
week.
To
determine
the
training
volume,
information
on
training
fre-
quency
and
duration
obtained
from
the
head
coaches
were
used.
The
difference
between
the
first
group
(CG
=
soccer)
and
the
second
group
was
the
additional
strength
training
(1
to
2
times
per
week)
and
athletic
units
for
the
second
group
(STG
=
soccer
and
strength
and
plyometric
training).
The
implementation
of
the
strength
and
athletic
training
caused
by
the
STG
depended
on
age,
with
an
average
of
1
hour
more
training
per
week
relative
to
the
training
vol-
ume
of
the
first
group.
In
the
data
analysis,
subgroups
were
classified.
The
athletes
who
were
11
and
12
years
old
at
T1
were
classified
as
subgroup
D.
The
athletes
who
were
9
and
10
years
old
at
T1
were
classified
as
subgroup
E.
The
ath-
letes
were
combined
into
1
subgroup
because
the
training
volume
and
contents
of
these
groups
coincided
well.
This
classification
according
to
age
is
normal
and
common
in
Ger-
many.
The
anthropometric
data
are
presented
in
Table
1
(Table
1).
At
beginning
of
the
study,
the
ages
of
the
chil-
dren
in
the
STG
were
9.5
±
0.5
years
old
for
subgroup
E
and
11.5
±
0.5
years
old
for
subgroup
D.
The
average
ages
of
the
CG
were
9.6
±
0.5
and
11.5
±
0.5
years
old
for
subgroups
E
and
D,
respectively.
2.2.
Testing
Protocol
The
subjects
completed
a
10-minute
standardized
warm-up.
All
participants
completed
the
following
series
of
tests,
in
the
order
presented:
•
SJ:
the
SJ
is
a
vertical
jump
from
the
crouched
posi-
tion
without
momentum
(test-retest
correlation,
r
=
0.87,
P
<
0.01).
The
knees
are
bent
to
90◦,
the
body
is
upright,
and
the
hands
remain
at
the
hips;
•
CMJ:
the
CMJ
is
a
vertical
jump
with
momentum
(test-
retest
correlation,
r
=
0.94,
P
<
0.01).
The
jump
is
initiated
from
an
upright
position,
and
the
center
of
the
body
is
lowered
until
the
knees
are
bent
at
a
90◦angle.
The
CMJ
is
a
rapid
movement
with
no
pause
between
eccentric
and
concentric
phases;
•
DJ:
(test-retest
correlation,
r
=
0.85—0.88,
P
<
0.01):
the
DJ
was
performed
from
different
heights
(16
cm
[DJ16],
24
cm
[DJ24]).
The
participants
step
off
of
a
box,
and
when
both
feet
contact
the
ground,
the
participant
jumps
as
high
as
possible.
The
reactive
power
of
the
participant
is
improved
with
a
shorter
duration
of
ground
contact
(ms)
and
a
higher
jump
(cm).
From
these
data,
a
per-
formance
index
(LI)
was
calculated
(LI
=
jump
height
in
millimeters/contact
time
in
milliseconds
×
100).

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
4
M.
Keiner
et
al.
Table
1
Anthropometric
data.
Group
Weight
(kg) Height
(cm) BMI
T1
T2
T3
T1
T2
T3
T1
T2
T3
STG
D
(n
=
19)
38.7
±
6.3
41.7
±
8.3
46.4
±
9.2
146.3
±
7.5
150.9
±
8.9
156.7
±
8.8
17.9
±
1.5
18.2
±
1.7
18.7
±
1.8
E
(n
=
19)
35.7
±
7.0
37.9
±
7.1
41.6
±
8.3
140.0
±
9.1
144.3
±
8.4
148.9
±
9.1
18.1
±
2.0
18.1
±
2.0
18.6
±
2.2
CG
D
(n
=
16)
37.8
±
3.7
42.5
±
5.4
47.8
±
7.1
146.8
±
4.8
152.5
±
5.3
160.6
±
6.7
17.5
±
1.4
17.2
±
1.4
18.5
±
1.6
E
(n
=
16)
31.6
±
4.8
34.3
±
4.9
37.1
±
5.2
137.7
±
7.1
142.2
±
7.3
147.1
±
8.1
16.6
±
1.3
16.9
±
1.3
17.1
±
1.3
STG
=
training
group;
CG
=
control
group;
T1
=
pretest;
T2
=
test
after
1
year;
T3
=
test
after
2
years;
D
=
D
subgroup;
E
=
E
subgroup.
Every
participant
was
allowed
up
to
3-trial
jumps
for
each
jump
type.
Then,
5
test
jumps
were
completed.
The
best
performance
was
used
for
statistical
analysis.
A
contact
mat
was
used
to
evaluate
the
variables
(Refitronic,
Schmit-
ten,
Germany).
Every
participant
was
familiarized
with
the
tests
by
performing
a
pretest
1
week
before
testing.
Subjects
did
not
participate
in
a
fatiguing
training
session
during
a
minimum
of
2
days
before
testing.
None
of
the
participants
reported
any
injury
at
the
time
of
testing.
Anthropomet-
ric
and
performance
measurements
were
collected
by
the
same
researchers
at
the
same
time
on
each
testing
day,
and
all
participants
were
asked
to
wear
the
same
clothing
and
footwear.
All
participants
were
asked
to
eat
and
drink
a
suf-
ficient
amount
until
1
hour
before
testing.
One
week
before
the
testing,
all
subjects
underwent
a
familiarization
test.
2.3.
Training
protocol
The
athletic
training
of
subgroup
E
consisted
of
2
training
sessions
per
week
of
20
minutes
each.
The
training
program
consisted
of
jumping,
sprinting
and
throwing
exercises,
as
well
as
lunges
and
strength
training
exercises
for
the
trunk.
Each
week,
there
was
1
training
session
using
the
strength-
ening
exercises
and
1
plyometric
training
session.
During
the
plyometric
training
session,
only
1
to
2
exercises
were
per-
formed,
but
a
different
plyometric
exercise
was
used
at
every
training
session
(see
Table
2).
The
young
athletes
in
both
subgroups
engaged
in
plyo-
metric
training
sessions
with
rotating
exercises
1
time
per
week
for
20
minutes
(see
Table
2).
During
another
train-
ing
session
each
week,
the
young
athletes
participated
in
strength
training,
which
consisted
of
weightlifting
exer-
cises
(see
Table
2)
for
approximately
1
hour.
The
aim
of
the
strength-training
program
was
to
develop
perfect
tech-
nique.
Thus,
the
loads
used
by
the
young
athletes
were
low
but
were
slightly
increased
during
the
2
years
of
train-
ing.
However,
the
correct
technique
of
the
exercises
was
always
the
center
of
focus.
Only
3
to
4
exercises
were
used
during
each
training
session,
but
the
exercises
changed
weekly.
2.4.
Statistical
analysis
The
data
were
analyzed
using
the
Kolmogorov-Smirnov
test
for
normal
distributions.
Additionally,
the
data
were
tested
for
homogeneity
of
variance.
Differences
in
performance
between
the
2
groups
at
baseline
were
controlled
by
analysis
of
variance.
For
the
analysis
of
performance
gains
within
a
group
and
for
comparisons
between
2
groups,
analysis
of
variance
with
repeated
measures
was
performed
with
group
and
time
factors.
The
data
are
presented
as
the
means
±
standard
deviations.
If
significant
F-values
were
calculated
the
Scheffé
test
was
performed
post
hoc.
3.
Results
All
parameters
yielded
significant
(P
<
0.05)
results
in
the
Kolmogorov-Smirnov
test,
indicating
that
the
data
were
normally
distributed.
The
lack
of
significance
(P
<
0.05)
in
Levene’s
test
demonstrated
that
the
variances
were
equal.
At
the
time
of
the
pretest,
there
was
no
significant
(P
<
0.05)
difference
between
the
STG
and
CG
(both
subgroups)
with
respect
to
the
performance
in
the
SJ,
CMJ
and
DJ
tests.
3.1.
E
subgroup
In
the
SJ
test,
the
changes
between
T1
and
T2
did
not
differ
significantly
between
the
2
groups;
however,
the
dif-
ferences
between
groups
with
respect
to
the
performance
changes
from
T2
to
T3
and
from
T1
to
T3
were
significant
(P
<
0.05).
For
the
CMJ
test,
the
performance
gains
did
not
differ
between
the
2
groups.
For
the
DJ
16
test,
the
perfor-
mance
increases
of
the
2
groups
differed
significantly
only
from
T2
to
T3
(Figs.
1
and
2).
Significant
(P
<
0.05)
increases
in
performance
were
found
for
both
groups
with
time
in
all
parameters,
except
performance
of
CG
in
DJ
from
T2
to
T3
was
significant
(P
<
0.05)
getting
worse.
3.2.
D-
Subgroup
In
SJ,
CMJ
and
DJ16
LI,
the
performance
gains
between
T1
and
T2
did
not
differ
significantly
between
the
2
groups;
however,
the
performance
gains
from
T2
to
T3
and
from
T1
to
T3
did
differ
significantly
(P
<
0.05).
In
the
DJ24
test,
the
performance
increases
from
T1
to
T2
were
already
signifi-
cantly
different.
The
differences
in
the
performance
gains
in
this
test
from
T2
to
T3
and
from
T1
to
T3
were
also
sig-
nificant
(P
<
0.05)
(Figs.
3—5).
Significant
(P
<
0.05)
increases
in
performance
were
found
for
both
groups
with
time
in
all
parameters.

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
Athletic
training
effects
on
jump
performance
5
Table
2
Training
exercises
of
the
subgroups
E
and
D
of
the
athletic
training
group.
E
D
Exercise
Strength
training
Lunges
with
additional
weight
Sit-ups
Overhead
squat
Back
squat
Front
squat
Dead
lift
Standing
row
Bench
press
3—5
sets
of
10—15
reps,
2
mins
rest
Jump
training
Vertical
jumps
Horizontal
jumps
Lateral
jumps
Vertical
jumps
Horizontal
jumps
Lateral
jumps
5
sets
of
6—8
jumps,
2
mins
rest
Sprint
training
Linear
sprints
up
to
15
m
Change-of-direction
sprints
up
to
15
m
Linear
sprints
up
to
15
m
Change-of-direction
sprints
up
to
15
m
5
sets
of
6—8
sprints,
2
mins
rest
Medicine
ball
training
Medicine
ball
throws
Medicine
ball
jump-throws
Medicine
ball
throws
Medicine
ball
jump-throws
5
sets
of
6—8
jumps/throws,
2
mins
rest
E
=
E
subgroup;
D
=
D
subgroup;
reps
=
repetitions;
mins
=
minute.
4.
Discussion
This
study
shows
that
a
2-year
supplemental
strength
and
plyometric
training
program
has
a
positive
effect
on
perfor-
mance
in
the
SJ,
CMJ
and
DJ
tests.
Even
at
a
low
training
volume
in
the
E
subgroup,
significant
differences
in
the
increases
in
the
performance
in
the
SJ
and
DJ
tests
were
Figure
1
Mean
and
standard
deviation
of
the
SJ
und
CMJ
results
for
subgroup
E
from
the
beginning
of
the
study
(T1)
and
after
2
years
(T3).
*
=
group
differences
(P
<
0.05)
in
the
absolute
changes
from
T1
to
T3;
STG
=
group
with
additional
strength
and
plyometric
training;
CG
=
group
without
additional
strength
and
plyometric
training;
SJ
=
squat
jump;
CMJ
=
countermovement
jump;
cm
=
jump
high
in
centimeter.
evident.
In
addition,
a
significant
difference
was
found
for
subgroup
D
of
the
STG.
This
difference
was
even
greater
than
that
of
subgroup
E
and
was
observed
for
all
measured
variables
(SJ,
CMJ,
DJ).
To
the
best
of
our
knowledge,
no
published
study
has
focused
on
children
who
participate
in
junior
competitive
sports
and
who
completed
a
strength-
training
program
over
a
study
period
as
long
as
that
used
for
this
study.
Therefore,
the
results
can
only
compared
with
Figure
2
Mean
and
standard
deviation
of
the
DJ16
results
for
subgroup
E
from
the
beginning
of
the
study
(T1)
and
after
2
years
(T3).
*
=
group
differences
(P
<
0.05)
in
the
absolute
changes
from
T2
to
T3;
STG
=
group
with
additional
strength
and
plyometric
training;
CG
=
group
without
additional
strength
and
plyometric
training;
DJ16
=
drop
jump
16
centimeters;
LI
=
performance
index
of
drop
jump.

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
6
M.
Keiner
et
al.
Figure
3
Mean
and
standard
deviation
of
the
SJ
und
CMJ
results
for
subgroup
D
from
the
beginning
of
the
study
(T1)
and
after
2
years
(T3).
*
=
group
differences
(P
<
0.05)
in
the
absolute
changes
from
T1
to
T3;
STG
=
group
with
additional
strength
and
plyometric
training;
CG
=
group
without
additional
strength
and
plyometric
training;
SJ
=
squat
jump;
CMJ
=
countermovement
jump;
cm
=
jump
high
in
centimeter.
data
from
shorter
longitudinal
studies
or
longitudinal
studies
with
young
populations.
The
limitations
of
this
study
include
the
absence
of
exer-
cises
mimicking
the
strength
tests
in
the
training
program
(i.e.,
squats),
which
was
due
to
organizational
reasons.
This
limitation
complicates
the
discussion
of
whether
the
Figure
4
Mean
and
standard
deviation
of
the
DJ16
results
for
subgroup
D
from
the
beginning
of
the
study
(T1)
and
after
2
years
(T3).
*
=
group
differences
(P
<
0.05)
in
the
absolute
changes
from
T1
to
T3;
STG
=
group
with
additional
strength
and
plyometric
training;
CG
=
group
without
additional
strength
and
plyometric
training;
DJ16
=
drop
jump
16
centimeters;
LI
=
performance
index
of
drop
jump.
Figure
5
Mean
and
standard
deviation
of
the
DJ24
results
for
subgroup
D
from
the
beginning
of
the
study
(T1)
and
after
2
years
(T3).
*
=
group
differences
(P
<
0.05)
in
the
absolute
changes
from
T1
to
T3;
STG
=
group
with
additional
strength
and
plyometric
training;
CG
=
group
without
additional
strength
and
plyometric
training;
DJ24
=
drop
jump
24
centimeters;
LI
=
performance
index
of
drop
jump.
changes
in
the
performance
parameters
of
the
training
group
result
from
athleticism
or
strength
training.
In
this
study,
therefore,
only
the
influence
of
the
combination
of
the
2
types
of
training
sessions
was
estimated.
Furthermore,
it
must
be
noted
that
an
additional
control
group
of
untrained
young
people
was
not
included
due
to
the
excessive
orga-
nizational
effort
required
to
include
such
a
control
group.
Therefore,
we
could
not
filter
out
the
effects
on
the
perfor-
mance
tests
of
playing
soccer.
Faigenbaum
et
al.
(1996)
[27]
have
shown
that
in
7-
to
12-
year-old
girls
and
boys,
there
are
significant
increases
in
leg
strength
after
8
weeks
of
strength
training,
but
no
improve-
ments
in
vertical
jump
were
observed
[5].
It
is
possible
that
the
intervention
period
was
too
short.
A
study
by
Weltmann
et
al.
(in
1986)
[2]
showed
that
the
group
who
completed
a
strength
training
intervention
for
14
weeks
had
significantly
better
results
in
the
vertical
jump
than
the
control
group.
Studies
of
adolescents
have
also
shown
positive
performance
changes
in
response
to
long-term
strength
training
inter-
ventions
[7].
After
approximately
1
year
of
supplemental
strength
training
in
young
soccer
players,
the
performance
in
the
SJ,
CMJ,
and
DJ
tests
significantly
differed
from
the
performance
of
the
control
group.
These
results
are
primarily
due
to
the
lower
training
volume
and
intensity
compared
to
the
training
programs
used
for
adults,
espe-
cially
the
shorter
duration
of
the
supplementary
strength
training
intervention.
In
addition
to
the
strength
training,
the
plyometric
train-
ing
could
also
have
influenced
the
tested
parameters.
A
study
of
10-year-old
by
Michailidis
et
al.
(in
2012)
[38]
found
that
after
a
14-week
training
intervention
consisting
of
jumps,
there
were
significant
improvements
relative
to

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
Athletic
training
effects
on
jump
performance
7
the
control
group
in
the
SJ,
CMJ
and
linear
sprint
tests.
A
sec-
ond
study
by
McKay
and
Henschke
(in
2012)
[34]
found
that
plyometric
training
improved
the
jump
height
and
running
speed
of
8-
to
14-year-old
children.
The
data
obtained
this
study
is
in
line
with
the
current
data.
However,
in
the
first
year,
no
significant
differences
in
the
SJ,
CMJ
and
DJ
16
results
were
observed
between
the
STG
and
the
CG.
Only
in
the
D
subgroup
was
a
significant
group
difference
found
for
the
DJ
24
test
after
1
year.
One
possible
explanation
are
the
low
intensities
of
the
strength
training
and
athletic
units
at
the
beginning
of
the
study;
low
intensities
were
used
so
that
the
children
could
perfect
the
techniques.
After
perfecting
the
techniques,
the
intensity
of
the
strength
training
and
the
plyometric
training
could
be
increased.
The
continuous
increase
in
the
intensity
explains
why
significantly
better
performances
were
measured
only
in
the
second
year
of
intervention
for
almost
all
parameters
when
comparing
the
STG
and
the
CG.
When
comparing
sub-
group
D,
the
STG
was
clearly
superior
with
respect
to
the
results
of
the
SJ
and
CMJ
tests
after
a
2-year
strength
train-
ing
intervention,
but
in
subgroup
E,
significant
differences
were
observed
only
for
the
SJ
test.
Increased
neuromuscu-
lar
efficiency
as
a
result
of
the
strength
and
athletic
training
program
used
by
the
STG
was
most
likely
sufficient
to
gen-
erate
significant
differences
in
the
SJ
compared
to
CG
for
subgroup
E,
but
most
likely
this
increased
neuromuscular
efficiency
not
sufficient
to
result
in
significant
differences
between
the
groups
in
the
more
complex
task,
CMJ.
The
small
increases
in
neuromuscular
performance
may
have
resulted
from
the
lack
of
lifts.
It
is
conceivable
that
children
will
benefit
more
from
strength
training
with
complex
exer-
cises
such
as
squats,
deadlifts
or
clean
and
jerks
[5,8,19].
It
is
possible
that
the
long-term
cautious
increase
in
the
loads
used
in
the
weightlifting
exercises
in
subgroup
E
may
have
led
to
better
performance
gains
in
the
SJ
and
CMJ
tests.
The
significant
differences
between
groups
in
the
DJ
can
be
explained
primarily
by
the
improved
stabilization
of
the
knee
and
hip
joints
and
by
the
increased
upper
body
strength.
This
improved
stabilization
of
the
knee
and
hip
joints
may
have
further
improved
the
performance
in
the
concentric
phase
of
the
DJ.
In
addition,
changes
to
the
connective
tissue
structures
of
the
tendomuscular
sys-
tem
are
likely
to
have
occurred,
and
these
changes
may
have
improved
the
performance
in
the
DJ
test
[39,40].
In
addition
to
the
changes
to
the
tendomuscular
system,
an
optimized
monosynaptic
stretch
reflex
may
have
led
to
the
performance
improvements
in
the
DJ
test.
Another
possible
explanation
is
the
increased
muscular
activity
in
the
eccen-
tric
phase
and
the
speed
of
the
movement
in
the
reverse
phase
[41,42].
In
summary,
it
can
be
stated
that
a
long-term
periodized
strength
and
athletic
training
intervention,
even
with
low
volumes
and
intensities,
in
the
age
range
of
9
to
14
years
is
not
only
the
basis
for
future
athletic
performance
but
also
results
in
an
improved
performance
in
the
SJ,
CMJ
and
DJ
tests.
5.
Conclusion
A
holistic
education
in
youth
sports
should
include
not
only
technical
and
tactical
training,
but
also
early
training
for
all
training
exercises.
The
training
curricula
for
junior
per-
formance
athletes
often
lack
the
long-term
periodization
of
training
exercises.
Strength
and
flexibility
are
often
neglected
in
training
schedules
or
are
only
given
limited
attention.
Because
of
the
advantages,
supplementary
strength
and
plyometric
training
should
be
incorporated
into
training
for
competitive
sports
as
early
as
possible
(from
the
age
of
approximately
6—7
years)
[8].
If
this
recom-
mendation
were
implemented,
young
people
17—18
years
old
would
already
have
over
10
years
of
training
experi-
ence.
Individuals
in
these
age
groups
can
be
trained
more
effectively
than
young
people
who
first
start
weight
training
at
the
age
of
16
years
because
the
correct
techniques
will
have
been
mastered
and
the
training
can
start
at
a
higher
level.
There
is
no
need
to
rapidly
increase
the
training
volume
and
intensity
before
the
change
to
the
senior
level.
Disclosure
of
interest
The
authors
declare
that
they
have
no
conflicts
of
interest
concerning
this
article.
References
[1]
Keiner
M,
Sander
A,
Wirth
K,
Caruso
O,
Immesberger
P,
Zawieja
M.
Trainability
of
adolescents
and
children
in
the
back
and
front
squat.
J
Strength
Cond
Res
2013;27:357—62.
[2]
Weltman
A,
Janney
C,
Rians
CB,
Strand
K,
Berg
B,
Tippitt
S,
et
al.
The
effects
of
hydraulic
resistance
strength
training
in
pre-pubertal
males.
Med
Sci
Sports
Exerc
1986;18:629—38.
[3]
Wirth
K,
Bob
A,
Müller
S,
Schmidtbleicher
D.
Vergleich
unterschiedlicher
Belastungsintensitäten
zur
Steigerung
der
Schnellkraft.
Leistungssport
2011;41:36—42.
[4]
Sewall
L,
Micheli
LJ.
Strength
training
for
children.
J
Pediatr
Orthop
1986;6:143—6.
[5]
Faigenbaum
AD,
Westcott
WL,
Loud
RL,
Long
C.
The
effects
of
different
resistance
training
protocols
on
muscular
strength
and
endurance
development
in
children.
Pediatrics
1999;104:e2.
[6]
Sadres
E.
The
effect
of
long-term
resistance
training
on
anthropometric
measures,
muscle
strength
and
self
concept
in
pre-pubertal
boys.
Pediatr
Exerc
Sci
2001;13:357—72.
[7]
Keiner
M,
Sander
A,
Wirth
K,
Schmidtbleicher
D.
Ergänzendes
Krafttraining
im
Nachwuchsleistungssport.
Schweizerische
Zeitschrift
für
Sportmedizin
und
Sporttraumatologie
2012;60:8—13.
[8]
Faigenbaum
AD,
Kraemer
WJ,
Blimkie
CJ,
Jeffreys
I,
Micheli
LJ,
Nitka
M,
et
al.
Youth
resistance
training:
up
dated
position
statement
paper
from
the
national
strength
and
conditioning
association.
J
Strength
Cond
Res
2009;22:60—79.
[9]
Fukunaga
T,
Funato
K,
Ikegawa
S.
The
effects
of
resistance
training
on
muscle
area
and
strength
in
prepubescent
age.
Ann
Physiol
Anthropol
1992;11:357—64.
[10]
Mersch
F,
Stoboy
H.
Strength
training
and
muscle
hypertrophy
in
children.
In:
Oseid
S,
Carlsen
K,
editors.
Children
and
exercise
XIII.
Champaign,
IL:
Human
Kinetics
Books;
1989.
[11]
Fleck
SJ,
Falkel
JE.
Value
of
resistance
training
for
the
reduc-
tion
of
sports
injuries.
Sports
Med
1986;3:61—8.
[12]
Heinonen
A,
Sievanen
H,
Kannus
P,
Oja
P,
Pasanen
M,
Vuori
I.
High-impact
exercise
and
bones
of
growing
girls:
a
9-month
controlled
trial.
Osteoporos
Int
2000;11:1010—7.
[13]
Morris
FL,
Naughton
GA,
Gibbs
JL,
Carlson
JS,
Wark
JD.
Prospec-
tive
ten-month
exercise
intervention
in
premenarcheal
girls:

Please
cite
this
article
in
press
as:
Keiner
M,
et
al.
The
impact
of
2
years
of
additional
athletic
training
on
the
jump
performance
of
young
athletes.
Sci
sports
(2013),
http://dx.doi.org/10.1016/j.scispo.2013.07.010
ARTICLE IN PRESS
+Model
SCISPO-2798;
No.
of
Pages
8
8
M.
Keiner
et
al.
positive
effects
on
bone
and
lean
mass.
J
Bone
Miner
Res
1997;12:1453—62.
[14]
Faigenbaum
AD,
Schramm
J.
Can
resistance
training
reduce
injuries
in
youth
sports?
National
Strength
and
Conditioning
Association
2004;26:16—21.
[15]
Lehnhard
RA,
Lehnhard
HR,
Young
R,
Butterfield
SA.
Monitor-
ing
injuries
on
a
college
soccer
team:
the
effect
of
strength
training.
J
Strength
Cond
Res
1996;10:115—9.
[16]
Bompa
T.
Total
training
for
young
champions.
Champaign,
IL:
Human
Kinetics;
2000.
p.
1—20.
[17]
Kraemer
WJ,
Fleck
SJ.
Strength
training
and
your
child.
In:
Strength
training
for
young
athletes.
2nd
ed.
Champaign,
IL:
Human
Kinetics;
2005.
p.
1—17.
[18]
Faigenbaum
AD.
Psychosocial
benefits
of
prepubescent
strength
training.
Strength
Cond
J
1995;17:28—32.
[19]
Faigenbaum
AD,
Zaichkowsky
LD,
Micheli
LJ,
Outerbridge
AR.
Psychological
effects
on
strength
training
in
children.
J
Sport
Behav
1997;20:164—75.
[20]
Behm
DG,
Faigenbaum
AD,
Falk
B,
Klentrou
P.
Canadian
Society
for
exercise
physiology
position
paper:
resistance
train-
ing
in
children
and
adolescents.
Appl
Physiol
Nutr
Metab
2008;33:547—61.
[21]
Washington
RL,
Bernhardt
CD,
Gomez
J,
Johnson
MD,
Martin
TJ,
Rowland
TW,
et
al.
Strength
training
by
children
and
ado-
lescents.
Pediatrics
2001;107:1470—2.
[22]
Hamill
BP.
Relative
safety
of
weightlifting
and
weight
training.
J
Strength
Cond
Res
1994;8:53—7.
[23]
Myer
GD,
Quatman
CE,
Khoury
J,
Wall
EJ,
Hewett
TE.
Youth
versus
adult
‘‘weightlifting’’
injuries
presenting
to
United
States
emergency
rooms:
accidental
versus
nonaccidental
injury
mechanisms.
J
Strength
Cond
Res
2009;23:2054—60.
[24]
Nicholl
JP,
Coleman
P,
Williams
BT.
The
epidemiology
of
sports
and
exercise
related
injury
in
the
United
Kingdom.
Br
J
Sports
Med
1995;29:232—8.
[25]
Raske
A,
Norlin
R.
Injury
incidence
and
prevalence
among
elite
weight
and
powerlifters.
Am
J
Sports
Med
2002;30:248—56.
[26]
Lavallee
ME,
Balam
T.
An
overview
of
strength
training
injuries:
acute
and
chronic.
Curr
Sports
Med
Rep
2010;9:307—13.
[27]
Faigenbaum
AD,
Westcott
WL,
Micheli
LJ,
Outerbridge
AR,
Long
CJ,
LaRosa-Loud
R,
et
al.
The
effects
of
strength
training
and
detraining
on
children.
J
Strength
Cond
Res
1996;10:109—14.
[28]
Faigenbaum
AD,
Milliken
LA,
Loud
RL,
Burak
BT,
Doherty
CL,
Westcott
WL.
Comparison
of
1
and
2
days
per
week
of
strength
training
in
children.
Res
Q
Exerc
Sport
2002;73:416—24.
[29]
Hewett
TE,
Riccobene
JV,
Lindenfeld
TN,
Noyes
FR.
The
effect
of
neuromuscular
training
on
the
incidence
of
knee
injury
in
female
athletes:
a
prospective
study.
Am
J
Sports
Med
1999;27:699—706.
[30]
Meylan
C,
Malatesta
D.
Effects
of
in-season
plyometric
training
within
soccer
practice
on
explosive
actions
of
young
players.
J
Strength
Cond
Res
2009;23:2605—13.
[31]
Ingle
L,
Sleap
M,
Tolfrey
K.
The
effect
of
a
complex
train-
ing
and
detraining
programme
on
selected
strength
and
power
variables
in
early
pubertal
boys.
J
Sports
Sci
2006;24:
987—97.
[32]
Thomas
K,
French
D,
Hayes
PR.
The
effect
of
two
plyomet-
ric
training
techniques
on
muscular
power
and
agility
in
youth
soccer
players.
J
Strength
Cond
Res
2009;23:332—5.
[33]
Johnson
BA,
Salzberg
CL,
Stevenson
DA.
A
systematic
review:
plyometric
training
programs
for
young
children.
J
Strength
Cond
Res
2011;25:2623—33.
[34]
McKay
D,
Henschke
N.
Plyometric
training
programs
improve
motor
performance
in
prepubertal
children.
Br
J
Sports
Med
2012;46:727—8.
[35]
Irmischer
BS,
Harris
C,
Pfeiffer
RP,
De-Beliso
MA,
Adams
KJ,
Shea
KG.
Effects
of
a
knee
ligament
injury
prevention
(KLIP)
exercise
program
on
impact
forces
in
women.
J
Strength
Cond
Res
2004;18:703—7.
[36]
Myer
GD,
Ford
KR,
Palumbo
JP,
Hewett
TE.
Neuromus-
cular
training
improves
performance
and
lower
extremity
biomechanics
in
female
athletes.
J
Strength
Cond
Res
2005;19:51—60.
[37]
Kotzamanidis
C,
Chatzopoulos
D,
Michailidis
C,
Papaiakovou
G,
Patikas
D.
The
effect
of
a
combined
highintensity
strength
and
speed
training
program
on
the
running
and
jumping
ability
of
soccer
players.
J
Strength
Cond
Res
2005;19:
369—75.
[38]
Michailidis
Y,
Fatouros
IG,
Primpa
E,
Michailidis
C,
Avloniti
A,
Chatzinikolaou
A,
et
al.
Plyometrics’
trainability
in
pre-adolescent
soccer
athletes.
J
Strength
Cond
Res
2012
[Pub-
lished
ahead
of
print.
PMID:
22450257].
[39]
Kubo
K,
Kanehisa
H,
Fukunaga
T.
Effects
of
resistance
and
stretching
training
programmes
on
the
viscoelastic
proper-
ties
of
human
tendon
structures
in
vivo.
J
Physiol
2002;538:
219—26.
[40]
Kubo
K,
Ikebukuro
T,
Yata
H,
Tsunoda
N,
Kanehisa
H.
Time
course
of
changes
in
muscle
and
tendon
properties
dur-
ing
strength
training
and
detraining.
J
Strength
Cond
Res
2010;24:322—31.
[41]
Komi
PV.
Strength
and
power
in
sport,
volume
III
of
the
ency-
clopaedia
of
sports
medicine.
Oxford,
UK:
John
Wiley
&
Sons;
2003.
[42]
McBride
JM,
McCaulley
GO,
Cormie
P.
Influence
of
preactiv-
ity
and
eccentric
muscle
activity
on
concentric
performance
during
vertical
jumping.
J
Strength
Cond
Res
2008;22:
750—7.