Vol. 87, Issue 3, 1141-1147, September 1999
Development of muscle strength in relation to training level
and testosterone in young male soccer players
L.
Hansen1,
J.
Bangsbo1,
J.
Twisk2, and
K.
Klausen1
1 Department of Human
Physiology, Institute of Exercise and Sport Sciences, University of
Copenhagen, DK-2100 Copenhagen, Denmark; and
2 Institute for Research in
Extramural Medicine, Vrije Universiteit, 1081 Amsterdam, The
Netherlands
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ABSTRACT |
Isometric and
functional strength of ninety-eight 11-yr-old male soccer players at an
elite (E) and nonelite (NE) level were determined (3-4 times)
through a 2-yr period, and the changes were related to growth and
maturation. The initial isometric strength for extension with dominant
leg [1,502 ± 35 (E) vs. 1,309 ± 39 (NE) N],
extension with nondominant leg (1,438 ± 37 vs. 1,267 ± 45 N),
extension with both legs (2,113 ± 76 vs. 1,915 ± 72 N), back muscles (487 ± 11 vs. 414 ± 10 N), abdominal muscles
(320 ± 9 vs. 294 ± 8 N), and handgrip (304 ± 10 vs. 259 ± 8 N) increased by 15-40% during the period. Broad jump
increased (P < 0.05) by 15 (E) and
10% (NE). The E players had higher (P < 0.05) initial isometric strength and broad jump performance
compared with NE players, and differences were maintained throughout
the period (multiple ANOVA for repeated measures) also when adjustment
was made for age, dimensions, testosterone, and insulin-like growth factor I (generalized estimating equations analyses). The development of strength for both E and NE players together was significantly (P < 0.001) related to changes in
serum testosterone concentrations. The present data indicate that
testosterone is important for development of strength in young boys and
that, independent of serum testosterone concentration, E players have
developed greater muscle strength compared with NE players.
elite and nonelite players; age; dimensions
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INTRODUCTION |
IN ADULTS, substantial knowledge is present about
factors that determine muscle strength and its change with training
(e.g. Refs. 13, 15), but less information is available about
development of muscle strength in children. Rochcongar et al. (22)
found that young French elite soccer players had greater isokinetic leg
strength compared with high school students, indicating that soccer
training has an effect on the development of muscle strength. In
contrast, Maffulli et al. (18) found that athletic boys (including soccer players) until the age of 15 yr had similar isometric quadriceps strength as did nonathletic boys, and after this age the strength of
the athletic group was significantly higher compared with nonathletic boys. The latter finding may indicate that the training responses are
affected by maturation.
Muscular strength increases more or less linearly with age from early
childhood in boys. Strength is known to be related to the physiological
cross-sectional area of the muscle and hence, according to a
dimensional analysis, related to the second power of body height.
During growth the cross-sectional area would then be expected to
increase, with the square of the increase in the linear dimension. Some
studies have shown that strength development in boys improves more than
can be explained by increase in height squared (1, 8), indicating that
factors other than quantitative changes play a role in the development
of strength. Thus the marked acceleration of muscle strength during
puberty observed in boys is possibly related to the elevated levels of
circulating androgen hormones in adolescents. In a cross-sectional
study of 11- to 13-yr-old athletic boys, Mero et al. (20) found that
muscle fiber area correlated well with serum testosterone. Maturation of the metabolic response to exercise might be related to the hormonal
changes [increases in testosterone, estradiol, growth hormone,
and insulin-like growth factor I (IGF-I)] occurring during puberty (9, 19). On the basis of a difference in increase in strength
between boys and girls, Parker et al. (21) suggested that testosterone
may stimulate muscle growth. The growth-promoting effect of growth
hormone is mediated by somatomedins, particularly IGF-I (16). However,
no longitudinal study has measured changes in blood hormone
concentrations and related them to changes in muscle size and strength
of children and adolescents (23).
As indicated by Rochcongar et al. (22), soccer training at an elite
level might increase leg strength, but it is also possible that the
boys selected for the elite level are stronger because of higher levels
of circulating hormones.
The aim of this investigation was to study the development in strength
of boys playing soccer at an elite and a nonelite level and to examine
the association between the development in strength and testosterone
concentration. Both questions were addressed with correction for age,
body size dimensions, and IGF-I.
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METHODS |
Subjects.
One hundred and ten young male soccer players from seven successful
clubs in the area of Copenhagen, at the highest level in their age
category, were recruited as subjects. All participants and their
parents gave their informed consent, and the study was approved by the
Ethics Committee of Copenhagen, Denmark (KF
01-132/95). Clubs that had at least four teams in the
same age categories were selected (to ensure differences between elite
and nonelite players). The boys were included in the study at the age
of 10-12 yr according to the selection age at the competition
levels. One-half of the boys were recruited from the best team to which
they were selected by the coach (elite players), and the other one-half were recruited from the lowest ranked team (nonelite players) from the
same club. Measurements were taken three times at 0.5-yr intervals for
all the boys, and, in addition, 28 of the subjects (16 elite, 12 nonelite) were also studied a fourth time. Eight boys were excluded
from the study because they were not members of the same team during
the whole study, two because they did not want to participate after the
first test and two because they moved to another part of the country.
Thus 98 subjects were included with 48 boys in the elite group and 50 boys in the nonelite group. During the study some boys stopped playing
soccer and were then excluded from subsequent tests. Thus 87 boys were
tested three times, and 28 of these were also tested a fourth time.
The players included in the study had participated in organized soccer
for 6.3 (elite) and 4.4 (nonelite) yr with a significant difference
(1.9 yr; P < 0.05) between the
groups. As shown in Table 1, the elite
players were playing soccer for more hours per week and were in general
more physically active compared with the nonelite players. The leisure
time activity registered consisted mostly of soccer with friends but
also roller-skating and high-activity play as well as participation in
other organized sports.
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Table 1.
Amount of time with organized soccer training (including competitions)
and with physical activity in leisure time, including participation
in other sports
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The age of the subjects was assessed to the nearest 0.01 yr. Standing
and sitting height were measured by a stadiometer to the nearest 0.1 cm, and body weight was measured to the nearest 0.1 kg by using a
spring balance. The body mass index (BMI) was calculated
as body weight (kg) divided by height (m) squared. Bicipital,
tricipital, subscapular, and suprailiacal skinfolds were measured with
a Harpenden skinfold caliper, and the sum of these four skinfolds was calculated.
The pubertal developmental stages were recorded by one experienced
pediatric endocrinologist on the basis of assessment of secondary sex
characteristics by using the criteria of Tanner (25) and from
testicular volume estimated from measurements of the size of the testes
by using a Prader orchidometer (29). Blood samples were drawn from an
antecubital vein between 1600 and 1730 and were centrifuged. Serum was
stored at 
20°C and later analyzed for levels of testosterone
and IGF-I. The sensitivity of the assay for IGF-I was 0.041 µg/l
(12), and the sensitivity for testosterone was 0.23 nmol/l. Values less
than assay sensitivity were assigned the value of assay sensitivity.
Strength measurements.
The subjects were all familarized with the testing procedures as well
as with the investigators before the test. All subjects had a
standardized warm-up period, including 5 min of cycling on a Monarck
bike, before the strength measurements. All subjects started with the
broad jump followed by measurements of isometric strength. Broad jump
was performed as a two-foot takeoff and landing. The takeoff was from
behind a line on the floor, and the landing was on a 2-cm-thick mat on
which the subjects were instructed to land on their feet. The distance
from the takeoff to the point where the nearest heel touched the mat
was measured, and the best of three recorded trials was used as the
performance score (cm). The maximal voluntary isometric strength
[maximal voluntary contraction (MVC)] of the leg extensors
was measured by using a strain-gauge dynamometer in a standardized
seated position with support of the back (4). To measure
MVC of the trunk muscles in a standing position, a strain-gauge
dynamometer was connected to a frame placed around the trunk 20 cm
below the shoulders by the use of two straps (4). Grip strength was
measured with a hand dynamometer in subjects while seated and for the
dominant arm only. The boys were all encouraged to the highest effort
by the investigators, and the best of three attempts was accepted as maximal.
The training regimen and competition intensity were evaluated for a
subgroup of the original subjects (n = 30, 10 teams). The competition intensity was evaluated from heart rate
measurements by a heart rate monitor (Polar) both during competition
and while the subjects ran on a treadmill with simultaneous
measurements of oxygen uptake. The elite players had higher relative
oxygen uptake in competition compared with the nonelite group (79.4 ± 5.3 vs. 67.3 ± 10%; P < 0.05). For the elite and nonelite players, 27 vs. 7% of the training
consisted of fitness training (sprint run, etc.), 61 vs. 39% was
technical training, and 12 vs. 54% was play. No supplemental weight
training was used.
Statistics.
To assess the longitudinal relationship between strength and soccer
ability (elite or nonelite), two analyses were carried out.
1) In the first analysis, the
differences in longitudinal development of strength measurements
between elite and nonelite soccer players were analyzed with multiple
ANOVA (MANOVA) for repeated measures (SPSS; Ref. 24).
2) In the second analysis, the
longitudinal relationship between strength and soccer ability (elite or
nonelite) was analyzed with generalized estimating equations (GEEs)
(30), a longitudinal linear regression technique that is extensively
described elsewhere (26, 27). The advantages of using this method are
that all available longitudinal data are used to estimate the
regression coefficients and that the method is suitable for designs
with unequally spaced time intervals. Furthermore, it allows a
correction for both time-dependent and time-independent covariates and
the method takes into account that the repeated observations on each
individual are not independent. GEE analysis was also carried out to
analyze the longitudinal relationship between strength parameters and
testosterone concentration. For all GEE analysis, a univariate analysis
was carried out first. After that, separate analyses were carried out
correcting for age, body size dimensions, and IGF-I concentration. All
GEE analyses were carried out with the Statistical Package for
Interactive Data Analysis (10). Significance was accepted at
P 
0.05.
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RESULTS |
The characteristics of the subjects are presented in Table
2. The elite players were slightly older
than the nonelite players (0.4 yr; P < 0.05). When adjustment was made for age, the elite players were
significantly taller (P < 0.05) and
had lower values for skinfold measurements
(P < 0.05). The elite players had
greater testicular volume than did the nonelite players and higher
serum testosterone concentration (Fig. 1;
P < 0.05). No significant differences between the groups in BMI or IGF-I (Fig.
2) were present.
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Fig. 1.
Serum testosterone concentrations. Measurements were taken at 0.5-yr
intervals starting when the subjects had a mean age of 11.9 ± 0.5 (SD) (elite) and 11.6 ± 0.7 yr (nonelite). Values are means ± SE; n, no. of subjects. Dotted line
indicates that the last test round included a reduced number of players
(n = 28). Difference (adjusted for
age) between groups is significant for 4 test rounds
(P = 0.015) with a tendency for
significance present for 3 test rounds
(P = 0.076).
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Fig. 2.
Insulin-like growth factor I levels in elite and nonelite young male
soccer players. Values are means ± SE;
n, no. of subjects. No significant
difference between elite and nonelite players was
present.
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Figures
3-6
show the development in strength parameters for elite and nonelite
players. Differences in development between the two groups were
analyzed by MANOVA for repeated measures. The results of these analyses
(Table 3) showed that elite players had
higher values (P < 0.05) compared
with nonelite players for all strength parameters throughout the
measurement period. The increase in strength over time was, however,
similar in both groups, i.e., no significant elite/nonelite-time
interaction for any of the strength parameters. No significant
differences in development between the groups according to Tanner
stages were present (MANOVA for repeated measures).
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Fig. 3.
Isometric strength for abdominal and back muscles measured in standing
elite and nonelite young male soccer players. Values are means ± SE. Measurements were taken at 0.5-yr intervals starting when the
subjects had a mean age of 11.9 ± 0.5 (SD) (elite) and 11.6 ± 0.7 yr (nonelite). Values are means ± SE;
n, no. subjects for elite
players/nonelite players in each test round. Difference between the
groups is significant: abdominal, P = 0.011; and back, P = 0.001.
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Fig. 4.
Isometric handgrip strength (dominant hand) in elite and nonelite young
male soccer players. Values are means ± SE;
n, no. of subjects. Differences
between the groups are significant, P = 0.002.
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Fig. 5.
Isometric strength for leg extensors measured in seated elite (E) and
nonelite (N-E) young male soccer players. Values are means ± SE; n, no. of subjects for elite players/nonelite
players in each test round. Differences between the groups are
significant: both legs, P = 0.004;
dominant leg, P = 0.002; and
nondominant leg, P = 0.01.
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Fig. 6.
Broad jump performance in elite and nonelite young male soccer players.
Values are means ± SE; n, no. of
subjects. Difference between the groups is significant,
P = 0.002.
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Table 3.
Results of MANOVA for 3 test rounds, carried out at 0.5-yr intervals on
isometric strength and broad jump, testing the effects of team
(elite/nonelite), time by team, and time of measurement
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The results from the GEE analysis regarding the longitudinal
relationship between being an elite player/nonelite player and strength
development are presented in Table 4. In
univariate analysis a significant positive relationship was found
between all strength parameters and being an elite player. In general, with an adjustment for age, the regression coefficients for
elite/nonelite players decreased slightly; a more marked decrease was
found for the relationship with leg extension by using both legs.
Adjustment for body dimensions also led to a decrease in regression
coefficients. When the relationships between elite/nonelite player and
strength were adjusted for height, weight, and sum of skinfolds, as
well as for numbers of years of training in organized soccer, only the
relationships with back muscles and handgrip remained significant (P < 0.05). When adjustment was made
for serum testosterone and IGF-I, a small decrease in regression
coefficients was observed; however, the positive relationships between
all strength parameters and being an elite player remained highly
significant.
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Table 4.
Longitudinal relationships between being an elite or a nonelite player
and development of isometric strength/broad jump, adjusted for
confounding factors
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The results of the GEE analysis regarding the longitudinal relationship
between strength and serum testosterone are presented in Table
5. Univariate analysis showed a significant
positive relationship between development in all strength parameters
and serum testosterone concentration. Adjustment for age, body
dimensions, and IGF-I showed more or less the same picture as for the
longitudinal relationships between being an elite/nonelite player and
strength, i.e., in general, a decrease in regression coefficients. The
adjustment for body weight, height, and sum of skinfolds led to a
dramatic decrease in regression coefficients; i.e., none of the
relationships between serum testosterone and strength parameters was
significant.
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Table 5.
Longitudinal relationships, for both elite and nonelite players as one
group, between serum testosterone levels and development of isometric
strength/broad jump, adjusted for confounding factors
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DISCUSSION |
The present study showed that both elite and nonelite players through a
2-yr period had an increase in strength that was related to the levels
of serum testosterone, indicating that testosterone is important for
development of muscle strength in young boys. In addition, a strong
positive relationship between being an elite player and the level of
strength was observed. This relationship was independent of
testosterone and IGF-I, indicating that the greater strength was not
solely due the level of serum testosterone.
Increases in muscle strength with age in young boys cannot simply be
explained by growth, because it has been shown in both prepubertal and
pubertal boys that strength increases more rapidly than does height
(6). It is more likely to be due to an interrelationship between
several factors such as age, stature, weight, muscle size, and
maturation (endocrine and neurological). From experimental data and
from the recognition that testosterone has a prominent anabolic effect,
it has been suggested that testosterone is responsible for the increase
in strength in male individuals at puberty (5). In the present study
this is confirmed by a significant positive relationship between
development in all strength parameters and serum testosterone
concentration. This relationship is dependent on anatomic dimensions
and skinfold thickness, indicating that these factors also play a role
in the development of strength. Asmussen and Heebøll-Nielsen (2)
suggested that, besides dimensions, age per se has a positive influence
on muscular strength, especially in tests that require a high degree of
neuromuscular coordination. We found that the relationship between
strength development and changes in serum testosterone was independent
of age, except for leg extension with both legs. It has been shown for
both children and adults (14, 28) that MVC for leg extension with both
legs are less than the sum of MVC for each leg, indicating a limit in
neural output. Furthermore, in the present study the percentile difference between the sum of the strength of each leg and both legs
decreased with age [from 39 to 15% (elite) and from 34 to 22%
(nonelite)], which is in agreement with Asmussen and
Heebøll-Nielsen (3), who showed a gradual decrease of this difference
in male subjects aged 15-35 yr. Thus dependency of age when
examining extension with two legs (with some degree of neuromuscular
coordination) could be explained by a requirement for a neuromuscular
maturation possibly related to age.
The nonelite players, while not as strong as the elite players, had
almost the same strength values as did Danish schoolboys, aged 11 yr,
who were examined in 1981 (11). It seems likely that the development of
leg muscle strength in particular would give an advantage to the elite
soccer player. Leatt et al. (17) showed a greater isokinetic and
explosive strength in Canadian national soccer players who were under
18 yr old compared with the national players who were under 16 yr old.
The elite players in the present study were also stronger compared with
the nonelite players when a correction was made for the small
difference in age between the two groups. The elite players were taller
and more mature compared with the nonelite players, so the increase could be due to growth, maturation, or competition level. To examine this relationship, the GEE analysis was carried out. The difference in
strength between elite players and nonelite players appears not to be
due to the difference in height between the two groups because the
difference was independent of dimensions, except for leg extension with
both legs and broad jump, which only revealed a tendency for
independence (P = 0.073 and
P = 0.058, respectively). The
relationship between strength development and elite/nonelite was
independent of serum testosterone and IGF-I, indicating that the
development in strength was related to factors associated with being an
elite player independent of testosterone concentration. The reason for
this increase in strength may be due to a greater relative increase in
muscle mass of the elite players and thus a larger cross-sectional area
of the muscles. Alternatively, it may have been caused by qualitative
changes in the muscles such as a lower ratio of connective tissue to
muscle tissue so that the same mass of musculature may be brought to
produce more tension in the elite players. Leatt et al. found that
elite players had more lean body mass compared with normal subjects. In
the present study no difference in BMI was found between the two
groups, but the elite players had less subcutaneous fat evaluated from
skinfold measurements, which indicate a larger lean body mass in the
elite players, possibly caused by muscle hypertrophy as a response to training. When the sum of skinfolds was included with dimensions in the
GEE analysis of relationships between elite/nonelite and strength
development, a dependency was found for leg extensions, abdominal
muscles, and broad jump but not for back muscles and handgrip strength.
This indicates that the development in strength is related to some
extent to a hypertrophy of the muscles. It is also plausible that part
of the difference in strength may result from a better mastery of the
neuromuscular system in the elite players caused by the training
regimen that the elite players were exposed to from an early age. The
fact that the elite players were initially stronger compared with the
nonelite players could partly explain why the increase in strength for
the elite players during the test period did not lead to an increase in
the difference between the two groups because strength is known to
increase more from lower initial levels (e.g., Ref. 7). It cannot,
however, be excluded that the differences between the elite and
nonelite players are due to an early selection of boys with higher
strength for the elite group.
In summary, development of isometric strength and performance in broad
jump was related to changes in serum testosterone concentrations but
also influenced by body size, indicating that testosterone is important
for development of strength in young boys. Furthermore, elite players
were stronger than nonelite players independent of testosterone
concentration also with correction for body size, indicating that being
an elite player per se affected the development of strength.
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ACKNOWLEDGEMENTS |
The authors gratefully acknowledge Dr. Med. Jørn Müller
for doing the measurements of testicular volume and the Tanner stage evaluations and further for the valuable discussions regarding the
interpretations of the maturational data.
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FOOTNOTES |
This study was supported by grants from the Danish Association of
Soccer, the Danish Association of Sports, and Team Danmark.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: L. Hansen,
Institute of Exercise and Sport Sciences, Dept. of Human Physiology,
Universitetsparken 13, DK-2100, Copenhagen, Denmark (E-mail:
L1Hansen{at}aki.ku.dk).
Received 9 April 1998; accepted in final form 26 April 1999.
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