The Effects of resistance training on the physical capacities of adolescent soccer players

Abstract

This study examined the effects of a progressive resistance training program in addition to soccer training on the physical capacities of male adolescents. Eighteen soccer players (age: 12-15 years) were separated in a soccer (SOC; n = 9) and a strength-soccer (STR; n = 9) training group and 8 subjects of similar age constituted a control group. All players followed a soccer training program 5 times a week for the development of technical and tactical skills. In addition, the STR group followed a strength training program twice a week for 16 weeks. The program included 10 exercises, and at each exercise, 2-3 sets of 8-15 repetitions with a load 55-80% of 1 repetition maximum (1RM). Maximum strength ([1RM] leg press, bench-press), jumping ability (squat jump [SJ], countermovement jump [CMJ], repeated jumps for 30 seconds) running speed (30 m, 10 x 5-m shuttle run), flexibility (seat and reach), and soccer technique were measured at the beginning, after 8 weeks, and at the end of the training period. After 16 weeks of training, 1RM leg press, 10 x 5-m shuttle run speed, and performance in soccer technique were higher (p < 0.05) for the STR and the SOC groups than for the control group. One repetition maximum bench press and leg press, SJ and CMJ height, and 30-m speed were higher (p < 0.05) for the STR group compared with SOC and control groups. The above data show that soccer training alone improves more than normal growth maximum strength of the lower limps and agility. The addition of resistance training, however, improves more maximal strength of the upper and the lower body, vertical jump height, and 30-m speed. Thus, the combination of soccer and resistance training could be used for an overall development of the physical capacities of young boys.

Full text

783
Journal of Strength and Conditioning Research, 2006, 20(4), 783–791
䉷2006 National Strength & Conditioning Association
E
FFECTS OF
R
ESISTANCE
T
RAINING ON THE
P
HYSICAL
C
APACITIES OF
A
DOLESCENT
S
OCCER
P
LAYERS
M
ARIOS
C
HRISTOU
,
1,2
I
LIAS
S
MILIOS
,
1
K
ONSTANTINOS
S
OTIROPOULOS
,
1
K
ONSTANTINOS
V
OLAKLIS
,
1
T
HEOFILOS
P
ILIANIDIS
,
1
AND
S
AVVAS
P. T
OKMAKIDIS
1
1
Department of Physical Education & Sport Science, Democritus University of Thrace, Komotini, Greece;
2
Center
for Leisure, Tourism and Sports Research and Development, Intercollege, Nicosia, Cyprus.
A
BSTRACT
.Christou, M., I. Smilios, K. Sotiropoulos, K. Volaklis,
T. Pilianidis, and S.P. Tokmakidis. Effects of resistance training
on the physical capacities of adolescent soccer players. J.
Strength Cond. Res. 20(4):783–791. 2006.—This study examined
the effects of a progressive resistance training program in ad-
dition to soccer training on the physical capacities of male ado-
lescents. Eighteen soccer players (age: 12–15 years) were sepa-
rated in a soccer (SOC; n⫽9) and a strength-soccer (STR; n⫽
9) training group and 8 subjects of similar age constituted a
control group. All players followed a soccer training program 5
times a week for the development of technical and tactical skills.
In addition, the STR group followed a strength training program
twice a week for 16 weeks. The program included 10 exercises,
and at each exercise, 2–3 sets of 8–15 repetitions with a load
55–80% of 1 repetition maximum (1RM). Maximum strength
([1RM] leg press, bench-press), jumping ability (squat jump [SJ],
countermovement jump [CMJ], repeated jumps for 30 seconds)
running speed (30 m, 10 ⫻5-m shuttle run), flexibility (seat and
reach), and soccer technique were measured at the beginning,
after 8 weeks, and at the end of the training period. After 16
weeks of training, 1RM leg press, 10 ⫻5-m shuttle run speed,
and performance in soccer technique were higher (p⬍0.05) for
the STR and the SOC groups than for the control group. One
repetition maximum bench press and leg press, SJ and CMJ
height, and 30-m speed were higher (p⬍0.05) for the STR group
compared with SOC and control groups. The above data show
that soccer training alone improves more than normal growth
maximum strength of the lower limps and agility. The addition
of resistance training, however, improves more maximal
strength of the upper and the lower body, vertical jump height,
and 30-m speed. Thus, the combination of soccer and resistance
training could be used for an overall development of the physical
capacities of young boys.
K
EY
W
ORDS
. agility, flexibility, growth, running speed, strength,
vertical jump
I
NTRODUCTION
Resistance training is proven to be safe and ef-
fective for adolescents when it is properly de-
signed and supervised. Established scientific
organizations recommend resistance training
for young people to enhance muscular
strength, prevent sport injuries, improve performance in
sports and recreational activities, and affect health and
lifestyle in a positive way (1, 3, 26).
Several studies have shown that resistance training
increases muscular strength more than natural growth in
adolescents (14, 20, 21, 25). However, as far as the effect
of resistance training on motor performance is concerned,
some studies have shown a positive effect on some phys-
ical capacities, whereas other studies have not indicated
a similar effect. For example, after a resistance training
program, vertical jump performance was found to either
increase (21) or not show changes at all (20). When the
effect of resistance training on running speed was ex-
amined, resistance training failed to influence running
performance (20). Similarly, resistance training failed to
affect anaerobic capacity (15, 21). To our knowledge, no
study has examined the effects of resistance training on
flexibility and agility in young boys. Therefore, research
findings concerning the effects of resistance training on
motor performance are either limited or inconclusive, and
further studies of the potential role of resistance training
on physical capacities of adolescents are required to pro-
vide useful information to coaches.
After participating in resistance training programs,
sport performance of youths is expected to improve. An-
ecdotal comments on strength training suggested that
this is enhanced (18). Definitely, sports performance is
the outcome of multiple factors, and it would be difficult
to control and assess the net impact of resistance train-
ing. Nevertheless, Gorostiaga et al. (20) found that hand-
ball throwing velocity in adolescent players increased af-
ter resistance training. Thus, it would be interesting to
examine if strength training would have a positive impact
on specific tasks of soccer involving fast running and ef-
fective handling of the ball at the same time.
On the other hand, it is unknown whether a resistance
training program incorporated with regular soccer train-
ing would enhance the physical capacity of adolescent
players compared with soccer training alone. Soccer is a
sport that requires acceleration, rapid change of direc-
tion, and many powerful movements. Therefore, training
of the sport itself may enhance muscular performance es-
pecially during the developmental period. It would have
been interesting for physical educators and coaches to
recognize if soccer training would have an effect on motor
performance and if resistance training combined with soc-
cer training would have an extra effect on motor perfor-
mance. The purpose of this study was to investigate the
effects of soccer training combined with resistance train-
ing as well as the effects of regular soccer training alone
on physical capacities such as muscular strength, vertical
jump performance, running speed, agility, and flexibility,
and on soccer technique of male adolescent soccer players.
M
ETHODS
Experimental Approach to the Problem
The objectives of this study were to examine (a) the ef-
fects of soccer training on anthropometric characteristics,

784 C
HRISTOU
,S
MILIOS
,S
OTIROPOULOS ET AL
.
T
ABLE
1. Strength training program throughout the 16-week
training period.*
Week Session
Set ⫻
repetitions
Intensity
(%1RM)
112⫻15 55–60
22⫻15 55–60
233⫻15 55–60
42⫻15 55–60
353⫻15 55–60
63⫻15 55–60
472⫻15 55–60
82⫻15 55–60
592⫻12 65–70
10 2 ⫻12 65–70
6113⫻12 65–70
12 2 ⫻12 65–70
7133⫻12 65–70
14 3 ⫻12 65–70
8152⫻12 65–70
16 2 ⫻12 65–70
9172⫻10 70–75
18 2 ⫻10 70–75
10 19 3 ⫻10 70–75
20 2 ⫻10 70–75
11 21 3 ⫻10 70–75
22 3 ⫻10 70–75
12 23 2 ⫻10 70–75
24 2 ⫻10 70–75
13 25 2 ⫻8 75–80
26 2 ⫻8 75–80
14 27 3 ⫻8 75–80
28 2 ⫻8 75–80
15 29 3 ⫻8 75–80
30 3 ⫻8 75–80
16 31 2 ⫻8 75–80
32 2 ⫻8 75–80
*1RM⫽1 repetition maximum.
muscular strength, jumping ability, running speed, agil-
ity, flexibility, and specific technical skills of male ado-
lescent players, and (b) the effects of a combined soccer
and resistance training program for 16 weeks on the
above physical capacities. For these reasons, a team of
male regional soccer players was divided into a strength-
soccer training group (STR) and a soccer training group
(SOC). Soccer training (5 times per week) for the devel-
opment of technical and tactical skills was the same in
both groups. The STR group not only trained in soccer,
but in a strength training program as well, using free
weights and machines twice a week (Table 1). No boy had
regularly participated in any form of resistance training
before this study. To assess the effect of natural growth
on physical capacities, a control group (CON) was used
with boys of a similar age and physical characteristics.
The subjects of this group did not participate in any struc-
tured training program. Anthropometric characteristics,
maximum strength, vertical jump performance, running
speed, agility, flexibility, and soccer-specific technical
skills were measured at the beginning of the training pro-
gram and after 8 and 16 weeks of training.
Subjects
Eighteen soccer players with training experience of 4.3 ⫾
1.9 years and ranging from 12 to 15 years of age volun-
teered to participate in this study. The boys were equally
divided into 2 groups: STR (13.8 ⫾0.4 years; n⫽9) and
SOC (13.5 ⫾0.9 years; n⫽9). In addition, 8 boys were
recruited as a CON group (13.3 ⫾0.7; n⫽8). The mat-
uration status of the boys was determined according to
the development of pubic hair, based on the Tanner 5-
point scale (23), at the beginning, after 8 weeks, and after
16 weeks of strength training by the same investigator.
There were no significant differences between groups for
age or Tanner ratings. The physical characteristics of the
subjects are presented in Table 2.
All boys underwent medical evaluation before the be-
ginning of the study, and the following exclusion criteria
were used: (a) boys with a chronic pediatric disease; (b)
boys with an orthopedic limitation; and (c) boys who were
not classified as Tanner stage 3–5 at the beginning of the
study. Both the boys and their parents were informed
about the scope and the objectives of the study as well as
the risks associated with strength training. A written con-
sent was obtained from the parents. The experimental
protocol was approved by the Institutional Review Board
Committee.
Training Program
Weight-Training Program. The study was performed at
the beginning of the competitive season. The STR group
followed an introductory weight training program of 4
weeks (2 times per week), focusing on proper lifting tech-
niques with low volume and intensity (2 sets of 15 repi-
titions at 艐30–50% of 1 repetition maximum [1RM]). Af-
ter this period, players began the strength training pro-
gram, 2 times per week for 16 weeks with a total of 32
training sessions. Weight-training sessions were separat-
ed by at least 48 hours of rest. Each player performed the
exercises in order: leg press, bench press, leg extension,
peck-deck, leg flexion, overhead press, lat pull-downs, calf
raise, sit-ups, and upper-lower back extension. The spe-
cific configuration of the acute program variables during
the course of the 16-week training period is presented in
detail in Table 1. In summary, the intensity was 55% of
the 1RM at the beginning of the program and progres-
sively increased up to 80% of the 1RM. For each exercise,
2–3 sets of 8–15 repetitions with 2–3 minutes of rest be-
tween sets and 3–5 minutes between exercises were per-
formed. When a boy was able to complete the predeter-
mined number of repetitions with proper form and with-
out help, the load was increased by 5%. Subjects per-
formed up to 2–3 sets of 20–30 repetitions of the body
weight exercises.
Strength training was performed before soccer train-
ing, and the duration of each session lasted approximate-
ly 45 minutes. A warm-up period of 10 minutes, including
low to moderate intensity aerobic exercises and stretch-
ing, was carried out before strength training. In addition,
boys performed stretching exercises for about 1 minute
after each set of exercises and for 5 minutes after the end
of each session. A qualified instructor, who monitored
proper exercise techniques and made adjustments in
training load and repetitions, supervised strength train-
ing sessions. No injuries occurred during the training and
testing sessions.
Soccer Training Program. Both training groups fol-
lowed a soccer training program 5 times a week, with
each session lasting 艐90 minutes. In addition, both
groups participated in a 70-minute official game once a
week (two 35-minute halves). Soccer training focused on
the development of technical and tactical skills first and

R
ESISTANCE
T
RAINING IN
A
DOLESCENTS
785
T
ABLE
2. Anthropometric characteristics for the 3 groups for the 16-week training program (mean ⫾SE).*
Group Pretraining 8 weeks Adjusted means 16 weeks Adjusted means
Body mass (kg)
Strength-soccer (n⫽9) 52.0 ⫾3.3 54.3 ⫾3.4† 56.3 ⫾0.4 55.6 ⫾3.5† 57.5 ⫾0.7
Soccer (n⫽9) 54.1 ⫾2 54.3 ⫾2.0 54.2 ⫾0.4 55.3 ⫾2.0†‡ 55.1 ⫾0.7
Control (n⫽8) 55.8 ⫾4.5 57.4 ⫾4.4 55.6 ⫾0.4 57.5 ⫾4.1 55.8 ⫾0.7
Pretraining (covariate) 54.0
Height (cm)
Strength-soccer (n⫽9) 162.0 ⫾3.8 164 ⫾3.9† 164.7 ⫾0.3 165.2 ⫾3.9†‡ 165.9 ⫾0.4
Soccer (n⫽9) 163.0 ⫾2.5 164 ⫾2.5† 163.7 ⫾0.3 165.3 ⫾2.5†‡ 165.0 ⫾0.4
Control (n⫽8) 163.2 ⫾4.5 164.9 ⫾4.2† 164.5 ⫾0.4 165.6 ⫾4.0† 165.2 ⫾0.5
Pretraining (covariate) 162.8
Maturational status (Tanner 5-pt scale)
Strength-soccer (n⫽9) 4.0 ⫾0.2 4.2 ⫾0.2 4.1 ⫾0.1 4.3 ⫾0.2 4.3 ⫾0.1
Soccer (n⫽9) 3.9 ⫾0.3 4.0 ⫾0.2 4.0 ⫾0.1 4.2 ⫾0.1 4.2 ⫾0.1
Control (n⫽8) 3.8 ⫾0.3 4.0 ⫾0.3 4.1 ⫾0.1 4.1 ⫾0.2 4.2 ⫾0.2
Pretraining (covariate) 3.9
Sum of 4 skinfolds (mm)
Strength-soccer (n⫽9) 30.0 ⫾1.9 28.3 ⫾1.6 38.9 ⫾1.4 28.1 ⫾1.6 37.6 ⫾1.8
Soccer (n⫽9) 40.0 ⫾3.5 38.4 ⫾3.5 40.4 ⫾1.2 37.5 ⫾3.3† 39.2 ⫾1.6
Control (n⫽8) 58.7 ⫾6.6 55.4 ⫾5.7 41.2 ⫾1.6 47.2 ⫾5.5†‡ 34.4 ⫾2.1
Pretraining (covariate) 42.3
Body fat (%)
Strength-soccer (n⫽9) 12.2 ⫾0.9 11.9 ⫾0.7 16.9 ⫾0.7 12.0 ⫾0.7 16.8 ⫾0.8
Soccer (n⫽9) 16.6 ⫾1.5 15.8 ⫾1.6 16.9 ⫾0.6 15.8 ⫾1.5 16.9 ⫾0.6
Control (n⫽8) 24.8 ⫾2.9 23.5 ⫾2.6 17.4 ⫾0.8 20.3 ⫾2.5†‡ 14.6 ⫾0.8
Pretraining (covariate) 17.8
Girth, midthigh (cm)
Strength-soccer (n⫽9) 45.4 ⫾1.1 46.4 ⫾1.4† 48.0 ⫾0.3 48.1 ⫾1.4†‡ 49.4 ⫾0.6
Soccer (n⫽9) 47.7 ⫾0.9 48.3 ⫾0.8 47.6 ⫾0.3 48.9 ⫾0.8† 48.3 ⫾0.6
Control (n⫽8) 47.9 ⫾1.9 49.3 ⫾1.8 48.5 ⫾0.4 48.3 ⫾1.5 47.5 ⫾0.6§
Pretraining (covariate) 47.0
Girth, calf (cm)
Strength-soccer (n⫽9) 32.9 ⫾0.8 33.1 ⫾0.9 34.0 ⫾0.2 33.6 ⫾0.9† 34.4 ⫾0.2
Soccer (n⫽9) 33.9 ⫾0.4 34.1 ⫾0.6 34.0 ⫾0.2 34.4 ⫾0.6† 34.2 ⫾0.2
Control (n⫽8) 34.5 ⫾1.2 34.7 ⫾1.2 34.0 ⫾0.2 34.2 ⫾1.1 33.5 ⫾0.2§
Pretraining (covariate) 33.8
Girth, upper arm (cm)
Strength-soccer (n⫽9) 23.8 ⫾1.0 23.7 ⫾0.9 24.1 ⫾0.2 24.0 ⫾0.9 24.4 ⫾0.2
Soccer (n⫽9) 23.4 ⫾0.4 23.0 ⫾0.4† 23.7 ⫾0.2 23.4 ⫾0.4‡ 24.1 ⫾0.2
Control (n⫽8) 25.2 ⫾1.1 25.4 ⫾1.3 24.3 ⫾0.2 24.8 ⫾1.0 23.8 ⫾0.2
Pretraining (covariate) 24.1
Girth, midbiceps (flexed) (cm)
Strength-soccer (n⫽9) 25.7 ⫾126⫾1 26.7 ⫾0.3 26.4 ⫾1 26.9 ⫾0.3
Soccer (n⫽9) 26.0 ⫾0.5 25.4 ⫾0.5 25.7 ⫾0.3 25.6 ⫾0.5 25.9 ⫾0.3
Control (n⫽8) 27.0 ⫾1.2 26.9 ⫾1.3 26.1 ⫾0.3 26.2 ⫾1 25.5 ⫾0.3
Pretraining (covariate) 26.2
*p⬍0.05.
† From pretraining.
‡ From 8 weeks.
§ Between strength soccer and control.
then on the improvement of physical capacities. Technical
skills, such as dribbling, passing, receiving, shooting, and
heading were performed for 40–50 minutes in at least 3
training sessions a week. In addition, soccer drills and
games were performed in small areas from squads with
a reduced number of players (1 vs. 1, 3 vs. 3, 4 vs. 4) to
work on offensive and defensive strategies and individual
tactics. High-intensity training for the development of
running speed and agility (with and without a ball) was
carried out twice a week, and 1 training session per week
included soccer games or interval training for the devel-
opment of aerobic capacity. A half-field game lasting 20–
30 minutes was played at the end of each training ses-
sion. Stretching exercises for the main muscle groups
were executed in the warm-up and cool-down periods.
Testing Procedures
A standardized 15- to 20-minute warm-up consisting of
submaximal aerobic exercises and stretching exercises
preceded each test.
Anthropometric Measurements. All boys were mea-
sured for height, weight, body fat, and selected girths.

786 C
HRISTOU
,S
MILIOS
,S
OTIROPOULOS ET AL
.
Body fat was measured using triceps and calf skinfolds
as described by Slaughter et al. (33). Girth measures of
the upper arm, midbiceps (flexed), midthigh, and calf
(maximum girth of calf muscle) were obtained with an
anthropometric tape measure as described by Callaway
et al. (11). All anthropometric and body composition mea-
sures were taken from the right side by the same inves-
tigator.
Maximum Strength. All boys were evaluated for 1RM
in the bench press and leg press. The 1RM was typically
determined within 6 trials using the protocol of Ramsey
et al. (29). All testing procedures were closely supervised
(an instructor-to-subject ratio was 1:1), and verbal en-
couragement was given to all boys. All measurements
were performed with a constant position of the body, us-
ing the same resistance equipment by the same test ad-
ministrator. The intraclass correlation coefficient (ICC)
and the coefficient of variation (CV) of the SEM for the
measurement of maximum strength were r⫽0.975 and
2.81%, respectively.
Vertical Jump Performance. Vertical jump performance
was measured with 3 types of jumps: a squat jump (SJ;
initiated from a knee flexion of 90⬚), a countermovement
jump (CMJ), and repeated jumps (RJ) for 30 seconds (9,
10). During the performance of the jumps, the hands of
the boys were placed on the waist. The jumps were exe-
cuted on a platform connected to a digital timer (Ergo-
jump, Psion CM; MAGICA, Rome, Italy) that measured
flight time and calculated jump height. For the SJ and
the CMJ, after 2–3 practice trials, 3 trials were carried
out for each type of jump (1–2 minutes of rest were al-
lowed between jumps), and the best trial was used for
further analysis. For the RJ test, the boys were instructed
to jump continuously using maximal effort. For every
jump, the position of takeoff and landing were the same.
To standardize knee angular displacement during the
contact phase, the boys were instructed to bend the knee
to about 90⬚. Pilot testing revealed an ICC and a CV of
the SEM of r⫽0.971 and 2.47% for the SJ, r⫽0.957
and 3.42% for the CMJ, and r⫽0.932 and 3.44% for the
RJ, respectively.
10- and 30-m Sprint Time. Sprint time was measured
using 3 electronic photo cells connected to a Lafayette
63501 timer (Lafayette Instrument Co. Systems, Lafay-
ette, IN). A photocell was placed at the start, at 10 m, and
at 30 m. The first photocell was positioned at a height of
50 cm from the ground and the photocells of 10 and 30 m
were placed at the height of the head of the boys, in an
attempt to standardize the part of the body breaking the
photocell (5). The boys started on a visual signal from a
standing position and ran the 30-m distance as fast as
possible on an indoor running track. After 1 practice trial,
2 sprints were performed, separated by a 5-minute recov-
ery period, and the fastest was used for subsequent anal-
ysis. All performance times were recorded with an accu-
racy of 0.001 seconds, and the time from 0 to 10 m and
total sprint time from 0 to 30 m was recorded. These
sprint distances were selected because they are the most
common during soccer games (6). The ICC and the CV of
the SEM for the 10-m sprint were r⫽0.958 and 1.46%,
and for the 30-m sprint, r⫽0.979 and 0.83%, respective-
ly.
Agility (10
⫻
5m).Agility was evaluated using a 10
⫻5-m maximal shuttle run on an indoor running track
following the protocol of Eurofit (16). After 1 practice trial
and at least 5 minutes of rest, a maximum test was per-
formed. Performance time was recorded with an accuracy
of 0.01 seconds. The ICC and the CV of the SEM for the
10 ⫻5-m shuttle run test were r⫽0.935 and 1.01%,
respectively.
Flexibility. Flexibility of the lower back and ham-
strings was measured using the sit and reach test as sug-
gested by the American Alliance of Health, Physical Ed-
ucation, Recreation, and Dance (2). Two trials were per-
formed, and the best one was used for further analysis.
The ICC and the CV of the SEM for the sit and reach
test were r⫽0.979 and 2.71%, respectively.
Soccer Technique Test. Technical skills in soccer were
evaluated using a slalom dribble test (35). The boys per-
formed zigzag dribbling with the preferred leg around a
series of 9 cones with a distance of 1.5 m between them
(total distance, 14 m). After 2 practice trials, 2 maximum
efforts of zigzag dribbling were performed separated by a
2-minute rest. The fastest and most successful trial (with-
out losing possession of the ball) was recorded for further
analysis. All performance times were recorded with an
accuracy of 0.001 seconds, using an electronic timer (La-
fayette Instrument Co. Systems). Pilot testing revealed
an ICC and a CV of the SEM of r⫽0.982 and 1.88%,
respectively.
Statistical Analyses
The effects of training on each group were tested using a
repeated-measures analysis of variance and the effect
size ([posttest mean ⫺pretest mean]/SD) for the magni-
tude of treatment effects was determined (12, 30). A two-
way analysis of covariance (ANCOVA), using initial val-
ues as covariate and ␩
2
for effect size, was used to deter-
mine the differences between groups at the 8th and 16th
week of training. Statistical power (P) for the ANCOVA
was determined as suggested by Keppel (22). Significant
differences between means were located with the Tukey
honestly significant difference procedure. The statistical
significance level was set at p⬍0.05.
R
ESULTS
Anthropometric Measurements
Significant main effects (p⬍0.05) were observed for
height and weight, but no differences (p⬎0.05) were
found between groups. No significant changes were ob-
served between groups for percentage of body fat and the
sum of 4 skinfolds. Analysis of covariance revealed that
midthigh and calf circumferences were larger (p⬍0.05)
for the STR group compared with the CON after 16 weeks
of training (Table 2).
Maximal Strength
Leg Press (1RM). Training resulted in significant increas-
es (p⬍0.01) in lower body strength for the STR (58.8%)
and SOC groups (33.8%). A significant increase (17.3%; p
⬍0.05), indicative of growth, was also observed for the
control group (Table 3). For all groups, the greatest in-
crease was observed after the first 8 weeks of training
(37.7, 20.7, and 9% respectively). Further analysis (AN-
COVA) revealed significant differences between groups in
maximum strength (p⬍0.01, ␩
2
⫽0.551, P⫽1). Leg
press 1RM for the STR group was greater than that
achieved by the SOC and CON groups after 8 and 16
weeks of training. Maximum strength of the SOC group

R
ESISTANCE
T
RAINING IN
A
DOLESCENTS
787
T
ABLE
3. Physical performance and soccer technique scores for the three groups for the 16-week training program.*
Group
Pretraining
Means ⫾SE
8 weeks
Means ⫾SE
Effect
size
Adjusted
means
16 weeks
Means ⫾SE
Effect
size
Adjusted
means
Flexibility (cm)
Strength-Soccer (n⫽9) 26.7 ⫾1.8 23.7 ⫾2.2† ⫺0.5 20.1 ⫾1.2 24.6 ⫾1.9 ⫺0.37 21.2 ⫾1.1
Soccer (n⫽9) 22.1 ⫾3.8 23.2 ⫾3.3 0.11 23.5 ⫾1.2 24.7 ⫾2.8 0.26 24.9 ⫾1.1
Control (n⫽8) 18.3 ⫾2.9 18.7 ⫾1.2 0.05 22.1 ⫾1.2 18.4 ⫾2.8 0.01 21.5 ⫾1.2
Pretraining (covariate) 22.5
Sprint time 10 m (s)
Strength-soccer (n⫽9) 2.16 ⫾0.06 2.18 ⫾0.07 0.14 2.14 ⫾0.04 2.09 ⫾0.04 ⫺0.41 2.06 ⫾0.03
Soccer (n⫽9) 2.00 ⫾0.04 2.04 ⫾0.04 0.4 2.13 ⫾0.04 1.98 ⫾0.04 ⫺0.15 2.05 ⫾0.03
Control (n⫽8) 2.18 ⫾0.05 2.20 ⫾0.06 0.12 2.15 ⫾0.04 2.11 ⫾0.04 ⫺0.48 2.07 ⫾0.03
Pretraining (covariate) 2.11
Sprint time 30 m (s)
Strength-Soccer (n⫽9) 5.07 ⫾0.16 5.16 ⫾0.16 0.19 5.13 ⫾0.05 4.94 ⫾0.12†‡ ⫺0.3 4.91 ⫾0.04
Soccer (n⫽9) 4.85 ⫾0.09 4.88 ⫾0.10 0.1 5.04 ⫾0.05 4.85 ⫾0.10 0 5.00 ⫾0.04
Control (n⫽8) 5.20 ⫾0.11 5.26 ⫾0.06 0.22 5.12 ⫾0.05 5.22 ⫾0.10 0.06 5.09 ⫾0.04‡
Pretraining (covariate) 5.04
Agility 10 ⫻5 m (s)
Strength-soccer (n⫽9) 19.92 ⫾0.24 19.25 ⫾0.29† ⫺0.83 19.43 ⫾0.16 18.84 ⫾0.16† ⫺1.74 18.97 ⫾0.19
Soccer (n⫽9) 19.78 ⫾0.21 19.07 ⫾0.22† ⫺1.08 19.34 ⫾0.16 18.99 ⫾0.24† ⫺1.13 19.19 ⫾0.20
Control (n⫽8) 20.86 ⫾0.63 20.89 ⫾0.41 0.02 20.43 ⫾0.18‡§ 21.12 ⫾0.40 0.17 20.78 ⫾0.22‡§
Pretraining (covariate) 20.18
Mean height of repeated jumps for 30 s (cm)
Strength-soccer (n⫽9) 21.6 ⫾1.4 22.4 ⫾1.3 0.2 21.4 ⫾0.5 24.8 ⫾1.4†‡ 0.76 23.7 ⫾0.7
Soccer (n⫽9) 22.2 ⫾1.3 23.0 ⫾1.2 0.21 21.4 ⫾0.5 23.7 ⫾1.2† 0.41 22.0 ⫾0.8
Control (n⫽8) 16.9 ⫾1.4 16.9 ⫾1.1 0 19.5 ⫾0.6 18.3 ⫾1.4 0.36 21.0 ⫾0.9
Pretraining (covariate) 20.21
Soccer technique test (s)
Strength-soccer (n⫽9) 7.91 ⫾0.20 7.51 ⫾0.18 ⫺0.69 7.95 ⫾0.18 7.32 ⫾0.10 ⫺1.22 7.72 ⫾0.19
Soccer (n⫽9) 7.40 ⫾0.24 7.21 ⫾0.24 ⫺0.25 8.08 ⫾0.20 7.10 ⫾0.26 ⫺0.39 7.90 ⫾0.22
Control (n⫽8) 10.27 ⫾0.55 10.32 ⫾0.52 0.03 8.85 ⫾0.26§ 10.30 ⫾0.51 0.02 8.95 ⫾0.28‡§
Pre-training (covariate) 8.52
Squat jump (cm)
Strength-soccer (n⫽9) 24.9 ⫾1.4 28.1 ⫾1.4† 0.76 27.9 ⫾1 32.4 ⫾1.6† 1.65 32.1 ⫾1
Soccer (n⫽9) 23.8 ⫾1.2 25.2 ⫾1.2 0.4 25.7 ⫾1 25.9 ⫾1.1 0.62 26.6 ⫾1¶
Control (n⫽8) 25 ⫾2 26.5 ⫾1.8 0.28 26.2 ⫾1.1 27 ⫾2.1 0.35 26.6 ⫾1¶
Pretraining (covariate) 24.5
Countermovement jump (cm)
Strength-soccer (n⫽9) 29 ⫾1.6 32.9 ⫾1.4† 0.86 33.1 ⫾0.6 35.7 ⫾1.4† 1.49 35.9 ⫾0.8
Soccer (n⫽9) 29.7 ⫾1.7 30.3 ⫾1.5 0.12 30 ⫾0.6¶ 31.1 ⫾1.3 0.3 30.8 ⫾0.8
Control (n⫽8) 29 ⫾2 30.6 ⫾1.4 0.33 30.7 ⫾0.7¶ 31.2 ⫾1.5 0.44 31.3 ⫾0.9
Pretraining (covariate) 29.2
Leg press 1RM (kg)
Strength-soccer (n⫽9) 102.8 ⫾2.5 142.8 ⫾8.5† 1.73 148.7 ⫾4.7 163.9 ⫾7.4† 2.77 170.1 ⫾4.4
Soccer (n⫽9) 106.1 ⫾7 126.7 ⫾6.7† 1 118.9 ⫾4.8¶ 139.4 ⫾6† 1.7 132.5 ⫾4.6¶
Control (n⫽8) 93.8 ⫾5.2 102.5 ⫾6.7† 0.52 108.3 ⫾5§ 110 ⫾6.8† 0.95 115.2 ⫾4.8§㛳
Pretraining (covariate) 100.9
Bench press 1RM (kg)
Strength-soccer (n⫽9) 36 ⫾1.6 45.9 ⫾2.5† 1.57 50.6 ⫾1.1 55 ⫾3.1† 2.54 60.6 ⫾1.5
Soccer (n⫽9) 48.3 ⫾3.2 47.8 ⫾2.6 ⫺0.01 41.4 ⫾1.2¶ 45.8 ⫾3.4 ⫺0.25 38.2 ⫾1.5¶
Control (n⫽8) 39.4 ⫾2.7 41 ⫾2.4 0.22 42.7 ⫾1.1§ 40.5 ⫾2.5 0.15 42.5 ⫾1.5§
Pretraining (covariate) 41.2
*p⬍0.05.
† From pretraining.
‡ From 8 weeks.
§ Between strength-soccer and control.
㛳Between soccer and control.
¶ Between strength-soccer and soccer.

788 C
HRISTOU
,S
MILIOS
,S
OTIROPOULOS ET AL
.
F
IGURE
1. Differences between the strength-soccer (STR),
soccer (SOC), and control (CON) groups in 1RM leg press and
bench press. *p⬍0.01 between STR and SOC, †p⬍0.01 be-
tween STR and CON, ‡p⬍0.05 between SOC and CON.
F
IGURE
2. Differences between the strength-soccer (STR),
soccer (SOC), and control (CON) groups in countermovement
jump (CMJ) and squat jump (SJ). *p⬍0.05 between STR and
SOC, †p⬍0.05 between STR and CON.
was greater compared with the CON group only at the
end of the 16-week training period (Figure 1).
Bench Press (1RM). After 8 and 16 weeks of training,
bench press 1RM was significantly higher (p⬍0.01) for
the STR group than the SOC and CON groups (p⬍0.01,
␩
2
⫽0.79, P⫽1; Figure 1). In particular, upper body
strength increased only in the STR group (52.3%; p⬍
0.01), whereas no significant differences (p⬎0.05) were
observed for the SOC and CON groups (-5.4 and 3.3%
respectively; Table 3).
Vertical Jump Performance
Squat Jump. After 16 weeks of training, SJ height was
higher (p⬍0.05, ␩
2
⫽0.33, P⫽0.83) for the STR group
compared with the SOC and CON groups (Figure 2). SJ
height was significantly improved by 13.5 and 31% for the
STR after 8 and 16 weeks of training, respectively (p⬍
0.05). The increases observed for the SOC (9.8%) and
CON (9.6%) groups were not significant (p⬎0.05; Table
3).
Countermovement Jump. After 8 and 16 weeks of train-
ing, CMJ height was greater (p⬍0.01, ␩
2
⫽0.49, P⫽1)
for the STR group compared to the SOC and CON groups
(Figure 2). CMJ height improved by 14.4 and 24.6% for
the STR after 8 and 16 weeks of training, respectively (p
⬍0.05). No significant changes were observed for the
SOC (6.3%) and CON (9.5%) groups (Table 3).
Repeated Jumps for 30 Seconds. After 16 weeks of
training, the STR and the SOC groups improved signifi-
cantly in average height during repeated jumps for 30
seconds (15.8 and 7.2%, respectively; p⬍0.05), whereas
the CON group showed a nonsignificant improvement
(9.8%; p⬎0.05). No significant differences were observed
between groups after the end of the training period (p⫽
0.07, ␩
2
⫽0.21, P⫽0.57; Table 3).
Sprint and Agility Times
Running Speed. There were no significant improvements
for 10-m acceleration speed in any group during the
study, although the STR group showed a higher speed
(3.1%) than the SOC (0.7%) and CON (2.7%) groups. No
differences were observed between groups (p⫽0.99, ␩
2
⫽0.01, P⫽0.07). Significant improvements in 30-m
sprint time (p⬍0.05) occurred only with the STR group

R
ESISTANCE
T
RAINING IN
A
DOLESCENTS
789
(2.5%) after 16 weeks of strength training, whereas no
significant differences were found for the SOC (0.04%)
and CON (⫺0.5%) groups. Sprint velocity was higher for
the STR group compared with the SOC and CON groups
at the end of the training period (p⫽0.049, ␩
2
⫽0.24, P
⫽0.65; Table 3).
Agility Time. Improvements were observed in the 10
⫻5-m shuttle run for the STR and SOC groups after 8
(3.4 and 3.5%, respectively; p⬍0.05), and 16 weeks (5.4
and 4%, respectively) of training, whereas no changes
were observed in the CON (⫺1.6%) group. The ANCOVA
revealed significant differences between groups in agility
time (p⬍0.01, ␩
2
⫽0.65, P⫽1) with the STR and the
SOC groups being faster than the CON group (Table 3).
Flexibility
Sit and reach scores decreased significantly by 8.2% in
the STR group (p⬍0.05), whereas they increased, but
not in a significant way, in the SOC and CON groups.
The 3 groups did not differ in flexibility scores throughout
the course of the study (p⫽0.07, ␩
2
⫽0.22, P⫽0.62;
Table 3).
Soccer Technique Test
The STR and SOC groups improved their performance
(6.8 and 4.0%, respectively) in the soccer technique test
after training, but this improvement was not significant
(p⬎0.05). No changes were observed in the CON group
(⫺0.4%; p⬎0.05). The scores in the soccer technique test,
at the end of the training period, were better for both the
STR and SOC groups compared with the CON group,
whereas no differences were observed between them (p⬍
0.05, ␩
2
⫽0.34, P⫽0.83; Table 3).
D
ISCUSSION
The soccer training applied in this study improves agility,
flexibility, and strength of the lower extremities. Strength
training twice a week in addition to soccer training (5
times per week) caused greater increases in upper- and
lower-body strength and improved more vertical jump
performance compared with soccer training alone. Fur-
thermore, strength-soccer training improved agility time
and 30-m running speed compared with subjects who did
not get any structured training. Flexibility decreased
(8%), and 10-m acceleration time was not affected by
strength training.
All 3 groups increased their lower-body strength (leg
press). However, upper-body strength (bench press) im-
proved only in the STR group, whereas the SOC and CON
groups showed minor increases. These results, based on
the principle of specificity, confirm the findings of other
studies in children (29) and adolescents (20, 21). Lower-
body muscles are used more in daily physical activities
and are strengthened faster than the upper-body muscles
that may be used less frequently (29). Therefore, strength
training for the upper-body muscles of adolescents seems
to be necessary for an optimal growth of the whole body
and especially for participants in sports such as soccer,
where strength stimuli for the upper body are rare.
The high acceleration of sprint, the stopping move-
ments after a sprint, and the changes in direction are
actions that occur frequently in soccer. These activities
require the development and application of high forces
from the legs, causing a greater leg strength increase in
our soccer training group than natural growth. The spe-
cific effect that soccer has on leg strength is supported by
the fact that these adaptations are limited to the leg mus-
culature, because maximum strength of the arms, which
are not used in soccer, did not increase.
Specific neural adaptations, such as increased motor
unit recruitment and coordination, and improved coordi-
nation of the involved muscle groups have been reported
after resistance training programs in children and ado-
lescents (28, 29). The STR group improved significantly
in 1RM bench press without any increases in their flexed
midbiceps circumferences, indicating that strength in-
creases were mainly caused by neural adaptations. How-
ever, the possibility of muscular hypertrophy in adoles-
cents caused by strength training should not be over-
looked (8, 21, 25). The strength improvements of the low-
er body muscles (1RM leg press) for the STR group were
accompanied by an increase in midthigh and calf circum-
ferences, with minor changes in midthigh and calf skin-
fold sites. Although circumference increases may follow
natural growth and development (24), our data suggest
the possibility of muscular hypertrophy caused by
strength training. The use of advanced techniques, such
as magnetic resonance imaging, would provide better re-
sults on muscular hypertrophy in adolescents after
strength training.
Vertical jump performance (SJ and CMJ) increased
significantly only in the STR group. Our results confirm
previous studies with prepubescent active men (31, 36)
and pubescent athletes (21). It seems that the increase in
the maximal muscle force, as a result of strength training,
also improves muscular power, despite the absence of spe-
cific exercises for the improvement of jump performance.
To our knowledge this is the first study that shows sig-
nificant improvement in jumping ability as a result of
strength training in adolescent soccer players.
Strength training combined with soccer training or
soccer training alone had no effects on anaerobic capacity,
evaluated with the average height of repeated jumps for
30 seconds. Similarly, other studies using the Wingate
test in preadolescents and adolescent subjects did not find
changes in anaerobic capacity, after strength training (15,
21). In the STR group, SJ and CMJ performance in-
creased to a greater rate than in the SOC and CON
groups. Although no significant differences were observed
between groups for repeated jump performance, the STR
group showed higher performance than the SOC and
CON groups. The results may be caused by the different
neuromuscular and metabolic components of anaerobic
performance. Short-term power output (SJ and CMJ) de-
pends more on the degree of neuromuscular activation
than on energy provision from anaerobic metabolism (27).
Besides the need of high neuromuscular activation, the
30-second all-out test (intermediate anaerobic perfor-
mance) activates anaerobic glycolysis and causes lactate
production because of the length of the protocol. Thus,
when increasing their maximum strength, children and
adolescents can benefit to a greater degree on maximum
anaerobic power.
Running speed has been found to correlate with max-
imum strength and VJ performance (4, 21). However, an
improvement in maximum strength does not assure an
improvement in sprinting velocity as well (13, 37). In a
study concerning a similar age group with ours, Hetzler
et al. (21) did not find an improvement in a 40-yd sprint
in boys who followed strength training. In this study,

790 C
HRISTOU
,S
MILIOS
,S
OTIROPOULOS ET AL
.
however, where the STR group followed strength and
sprint training, a significant improvement in 30-m sprint
time was observed, along with an improvement in maxi-
mum strength and vertical jump performance. Soccer
training did not lead to an improvement in sprint despite
the fact that specific sprint training took place 2 days a
week. It seems that a combination of strength and speed
training may be more beneficial for speed development.
It should be noted, however, that based on the data of our
study, the transfer of strength gain to speed was rather
small (58.8% increase in leg strength vs. 2.5% in 30-m
speed), and a low correlation was observed between the
percent increase in leg strength and 30-m speed (r⫽
0.24).
Agility was affected in a similar way in both the STR
and SOC groups, indicating the specificity of soccer train-
ing. Soccer training drills and games involved continuous
changes of direction. Thus, agility performance improved
with this type of training, without any further contribu-
tion from strength training. Furthermore, it is interesting
to note that the soccer training group improved only dur-
ing the agility test, which involved fast running with
changes in direction, and not in 30-m speed that involved
forward fast running. This probably reflects the specific-
ity of soccer training and the need for targeted speed
training for an overall improvement of soccer perfor-
mance. To our knowledge, the only study that examined
the effects of 12 weeks of strength training in agility in-
volving preadolescent boys was that of Falk and Mor (19)
and it is in agreement with our result. Despite these lim-
ited data, it seems that strength training has a minor
effect on agility performance of young people. There is a
minor transfer of the strength gain to agility, which prob-
ably involves a motor control pattern that is not strongly
influenced by the neural adaptations, which occur with
resistance training.
Flexibility increased in the soccer training group. Al-
though it was not significantly different from the control
group, it seems that the participation in soccer training
does not impair flexibility development when proper care
is taken to include stretching exercises in the training
program. Studies performed in adults reported that if
stretching exercises are part of the strength training pro-
gram, flexibility would not be impaired and it may even
increase (34). In this study, stretching exercises were per-
formed before, during, and after the strength training
sessions. However, flexibility of the hamstrings and the
lower back muscles decreased (8%) in the STR group. Pre-
vious studies involving children and prepubescents re-
ported either increases or no changes in flexibility after
strength training when using programs with low to mod-
erate volume and intensity (17–19, 32, 36). The high
volume and intensity of resistance training or the com-
bination of soccer and resistance training in our study
probably interfered with flexibility. More emphasis was
probably required on specific stretching exercises. Fur-
thermore, our subjects were adolescents and the phase of
their physical development might also interact with flex-
ibility levels. It should be noted that, even in the control
group, a slight decrease in flexibility was observed. Stud-
ies reported that inadequate flexibility might limit joint
mobility and predispose the muscles or the connective tis-
sue to injury and reduced performance (7, 34). However,
no injury was observed during the course of this study in
the STR group and other physical performance compo-
nents improved in this group. In any case, it seems that
additional stretching exercises or sessions for the main-
tenance or improvement of flexibility are necessary to
prevent muscles from becoming tight during strength
training. Further research is needed to determine which
factors might impede flexibility or not when adolescent
soccer players lift weights.
Resistance training had no effects on the soccer tech-
nique test used in this study. The nature of the test (drib-
bling with a ball between cones) is such that resistance
training would have a minor effect in comparison with a
training program focusing more on motor performance
development. This does not mean that resistance training
has no effect on any soccer-related action. If we had used
another soccer-specific test (e.g., speed of the ball after a
shot), we might have seen some sort of effect through the
increased leg force after strength training. For example,
Gorostiaga et al. (20) found that handball throwing ve-
locity in adolescent handball players increased after re-
sistance training. Furthermore, resistance training may
improve performance during a soccer game through a
greater speed and jumping ability, which help the player
to get control of the ball. In any case, it should be noted
that in this study we applied a low-frequency resistance
training program of moderate to submaximal intensity,
using basic core exercises and not sport specific strength
exercises, during the competition period. The aim was not
to maximize soccer performance but to develop the neu-
romuscular system of the athletes and help them to tol-
erate the mechanical stress of the various soccer actions
more and protect them from possible injuries. This might
be even more important than an improved performance
in the sport.
P
RACTICAL
A
PPLICATIONS
Soccer is a popular sport worldwide, and many youths
participate in soccer training programs. Soccer training
in adolescents provides a pleasant pastime and seems to
increase the parameters of physical capacity and soccer-
related skills. Soccer training alone improves maximal
strength of the lower-body muscles and agility, and when
stretching is included in the training program flexibility
also increases. The application of soccer training com-
bined with low frequency of moderate to submaximal in-
tensity resistance training offers an overall development
of physical capacities by improving in a greater degree
soccer-related abilities such as maximal strength of the
upper- and lower-body muscles, VJ performance, and 30-
m speed compared with regular soccer training. More em-
phasis should be given, however, to stretching exercises
when the soccer training program is combined with resis-
tance training. This may prevent any possible inconve-
nience caused after resistance training on flexibility dur-
ing the developmental period.
R
EFERENCES
1. A
MERICAN
A
CADEMY OF
P
EDIATRICS
. Committee on Sports
Medicine and Fitness. Strength Training by Children and Ad-
olescents. Pediatrics. 107:147–1472. 2001.
2. A
MERICAN
A
LLIANCE FOR
H
EALTH
,P
HYSICAL
E
DUCATION
,R
EC
-
REATION AND
D
ANCE
.Health Related Physical Fitness Test
Manual. Reston, VA: AAHPERD, 1980.
3. A
MERICAN
C
OLLEGE OF
S
PORTS
M
EDICINE
.ACSM’S Guidelines
for Exercise Testing and Prescription (5th ed.). Baltimore: Lip-
pincott, Williams and Wilkins, 1995.

R
ESISTANCE
T
RAINING IN
A
DOLESCENTS
791
4. B
AKER
, D.,
AND
S. N
ANCE
. The relation between running speed
and measures of strength and power in professional rugby
league players. J. Strength Cond. Res. 13:230–235. 1999.
5. B
ALSOM
P.D., J.Y. S
EGER
,B.S
JODIN
,
AND
B. E
KBLOM
. Physi-
ological responses to maximal intensity intermittent exercise.
Eur. J. Appl. Physiol. 65:144–149. 1992.
6. B
ANGSBO
J., L. N
OOREGARD
,
AND
F. T
HORSOE
. Activity profile
of competition soccer. Can. J. Sport Sci. 16:110–116. 1991.
7. B
EEDLE
, B., C. J
ESSE
,
AND
M.H. S
TONE
. Flexibility character-
istics among athletes who weight train. J. Appl. Sport Sci. Res.
5:150–154. 1991.
8. B
LIMKIE
, C.J.R. Resistance training during preadolescence.
Sports Med. 15:389–407. 1993.
9. B
OSCO
, C., P.V. K
OMI
,J.T
IHANYI
,G.F
EKETE
,
AND
P. A
POR
.
Mechanical power test and fiber composition of human leg ex-
tensor muscles. Eur. J. Appl. Physiol. S1:129–135. 1983.
10. B
OSCO
, C., P. L
UHTANAN
,
AND
P.V. K
OMI
. A simple method for
measurement of mechanical power in jumping. Eur. J. Appl.
Physiol. 50:273–282. 1983.
11. C
ALLAWAY
, C.W., W.P. C
HUMLEA
,C.B
OUCHARD
, J.H. H
IMES
,
T.G. L
OHMAN
, A.D. M
ARTIN
, C.D. M
ITCHELL
, W.H. M
UELLER
,
A.F. R
OCHE
,
AND
V.D. S
EEFELDT
. Circumferences. In: Anthro-
pometric Standardization Reference Manual. T.G. Lohman,
A.F. Roche, and R. Martorell, eds. Champaign, IL: Human Ki-
netics, 1988. pp. 41–53.
12. C
OHEN
,J.Statistical Power Analysis for the Behavioral Scienc-
es. (2nd ed.). Hillside, NJ: L. Erlbaum Associates, 1988. pp. 51–
52.
13. D
ELECLUSE
C., H.
VAN
C
OPPENOLLE
,E.W
ILLEMS
,M.
VAN
L
EEMPUTTE
,R.D
IELS
,
AND
M. G
ORIS
. Influence of high-resis-
tance and high-velocity training on sprint performance. Med .
Sci. Sports Exerc. 27:1203–1209. 1995.
14. D
E
R
ENNE
, C., R.K. H
ETZLER
, B.P. B
UXTON
,
AND
K.W. H
O
. Ef-
fects of training frequency on strength maintenance in pubes-
cent baseball players. J. Strength Cond. Res. 10:8–14. 1996.
15. D
OHERTY
, D., H.A. W
ENGER
,
AND
M.L. C
OLLINS
. The effects of
resistance training on aerobic and anaerobic power of young
boys. Med. Sci. Sports Exerc. 19:389–392. 1987.
16. E
UROFIT
.European Tests of Physical Fitness. Rome: Council of
Europe, Committee for the Development of Sport, 1988.
17. F
AIGENBAUM
, A., L. Z
AICHKOWSKY
,W.W
ESTCOTT
,L.M
ICHELI
,
AND
A. F
EHLANDT
. The effects of a twice per week strength
training program on children. Pediatr. Exerc. Sci. 5:339–346.
1993.
18. F
AIGENBAUM
, A.D., W.L. W
ESTCOTT
, A.R. M
ICHELI
, C.J. O
UT
-
ERBRIDGE
, C.J. L
ONG
,R.L
A
R
OSA
-L
OUD
,
AND
L.D. Z
AICHKOW
-
SKY
. The effects of strength training and detraining on chil-
dren. J. Strength Cond. Res. 10:109–114. 1996.
19. F
ALK
, B.,
AND
G. M
OR
. The effects of resistance and martial
arts training in 6- to 8-year old boys. Pediatr. Exerc. Sci. 8:48–
56. 1996.
20. G
OROSTIAGA
, E.M., M. I
ZQUIERDO
,P.I
TURRALDE
,M.R
UESTA
,
AND
J. I
BANEZ
. Effects of heavy resistance training on maximal
and explosive force production, endurance and serum hor-
mones in adolescent handball players. Eur. J. Appl. Physiol. 80:
485–493. 1999.
21. H
ETZLER
, R., C. D
E
R
ENNE
, B.P. B
UXTON
, K.W. H
O
, D.X. C
HAI
,
AND
G. S
EICHI
. Effects of 12 weeks of strength training on an-
aerobic power in prepubescent male athletes. J. Strength Cond.
Res. 11:174–181. 1997.
22. K
EPPEL
,G.Design and Analysis: A Researchers Handbook. (3rd
ed.). Upper Saddle River, NJ: Prentice-Hall, 1991. pp. 63–91,
221–224.
23. M
ALINA
, R.M.,
AND
C. B
OUCHARD
.Growth, Maturation, and
Physical Activity. Champaign, IL: Human Kinetics, 1991. pp.
229–249.
24. M
ALINA
, R.M.,
AND
C. B
OUCHARD
.Growth, Maturation, and
Physical Activity. Champaign, IL: Human Kinetics, 1991. pp.
371–390.
25. M
ERO
, A., K. HA
¨
KKINEN
,
AND
H. K
AUHANEN
. Hormonal profile
and strength development in young weight lifters. J. Hum. Mov.
Stud. 16:255–266. 1989.
26. N
ATIONAL
S
TRENGTH AND
C
ONDITIONING
A
SSOCIATION
. Youth
resistance training: Position statement paper and literature re-
view. Strength Cond. J. 18:62–75. 1996.
27. N
INDL
, B.C., M.T. M
AHAR
, E.A. H
ARMAN
,
AND
J.F. P
ATTON
.
Lower and upper body anaerobic performance in male and fe-
male adolescent athletes. Med. Sci. Sports Exerc. 27:235–241.
1995.
28. O
ZMUN
J.C., A.E. M
IKESKY
,
AND
P.R. S
URBURG
. Neuromuscu-
lar adaptations following prepubescent strength training. Med.
Sci. Sports Exerc. 26:510–514. 1994.
29. R
AMSEY
, J., C.J. B
LIMKIE
,K.S
MITH
,S.G
ERNRER
, J.D. M
AC
-
D
OUGALL
,
AND
D.G. S
ALE
. Strength training effects in prepu-
bescent boys. Med. Sci. Sports Exerc. 22:605–614. 1990.
30. R
HEA
, M.R. Determining the magnitude of treatment effects in
strength training research through the use of the effect size. J.
Strength Cond. Res. 18:918–920, 2004.
31. R
IANS
, C.B., A. W
ELTMAN
, B.R. C
AHILL
, C.A. J
ANNEY
, S.R. T
IP
-
PETT
,
AND
F.I. K
ATCH
. Strength training for prepubescent
males: Is it safe? Med. Sci. Sports Exerc. 15:483–489. 1987.
32. S
ERVEDIO
, F.J., R.L. B
ARTELS
, R.L. H
AMLIN
,D.T
ESKE
,T.
S
HAFFER
,
AND
A. S
ERVEDIO
. The effects of weight training, us-
ing Olympic style lifts, on various physiological variables in
pre-pubescent boys. Med. Sci. Sports Exerc. 17:288. 1985.
33. S
LAUGHTER
, M.H., T.G. L
OHMAN
, R.A. B
OILEAU
, C.A. H
OR
-
SWELL
, R.J. S
TILLMAN
, M.D. V
AN
L
OAN
,
AND
D.A. B
EMBEN
.
Skinfold equations for estimation of body fatness in children
and youth. Human Biol. 60:709–723. 1988.
34. T
HRASH
, K.,
AND
B. K
ELLY
. Flexibility and strength training.
J. Appl. Sport Sci. Res. 1:74–75. 1987.
35. V
AN
R
OSSUM
J.H.A.,
AND
D. W
IJBENA
. Soccer skills technique
tests for youth players: Construction and implications. In: Sci-
ence and Football II. T. Reilly, J. Clarys, and A. Stibbe, eds.
London: Taylor and Francis, 1991. pp. 313–318.
36. W
ELTMAN
, A., C. J
ANNEY
,C.R
IANS
,K.S
TRAND
,B.B
ERG
,S.
T
IPPIT
,J.W
ISE
,B.C
AHILL
,
AND
F. K
ATCH
. The effects of hy-
draulic resistance strength training in prepubertal males. Med.
Sci. Sports Exerc. 18:629–638. 1986.
37. W
ILSON
, G.J., R.U. N
EWTON
, A.J. M
URPHY
,
AND
B.J. H
UM
-
PHRIES
. The optimal training load for the development of dy-
namic athletic performance. Med. Sci. Sports Exerc. 21:1279–
1286. 1993.
Address correspondence to Savvas P. Tokmakidis,
stokmaki@phyed.duth.gr.