Effects of Resistance Training in Youth Athletes on Muscular Fitness and Athletic Performance: A Conceptual Model for Long-Term Athlete Development

Abstract

During the stages of long-term athlete development (LTAD), resistance training (RT) is an important means for (i) stimulating athletic development, (ii) tolerating the demands of long-term training and competition, and (iii) inducing long-term health promoting effects that are robust over time and track into adulthood. However, there is a gap in the literature with regards to optimal RT methods during LTAD and how RT is linked to biological age. Thus, the aims of this scoping review were (i) to describe and discuss the effects of RT on muscular fitness and athletic performance in youth athletes, (ii) to introduce a conceptual model on how to appropriately implement different types of RT within LTAD stages, and (iii) to identify research gaps from the existing literature by deducing implications for future research. In general, RT produced small-to-moderate effects on muscular fitness and athletic performance in youth athletes with muscular strength showing the largest improvement. Free weight, complex, and plyometric training appear to be well-suited to improve muscular fitness and athletic performance. In addition, balance training appears to be an important preparatory (facilitating) training program during all stages of LTAD but particularly during the early stages. As youth athletes become more mature, specificity, and intensity of RT methods increase. This scoping review identified research gaps that are summarized in the following and that should be addressed in future studies: (i) to elucidate the influence of gender and biological age on the adaptive potential following RT in youth athletes (especially in females), (ii) to describe RT protocols in more detail (i.e., always report stress and strain-based parameters), and (iii) to examine neuromuscular and tendomuscular adaptations following RT in youth athletes.

Full text

REVIEW
published: 09 May 2016
doi: 10.3389/fphys.2016.00164
Frontiers in Physiology | www.frontiersin.org 1May 2016 | Volume 7 | Article 164
Edited by:
Johnny Padulo,
University eCampus, Italy
Reviewed by:
Pantelis Theodoros Nikolaidis,
Hellenic Army Academy, Greece
Sébastien Ratel,
Blaise Pascal University, France
*Correspondence:
Urs Granacher
urs.granacher@uni-potsdam.de
Specialty section:
This article was submitted to
Exercise Physiology,
a section of the journal
Frontiers in Physiology
Received: 26 February 2016
Accepted: 18 April 2016
Published: 09 May 2016
Citation:
Granacher U, Lesinski M, Büsch D,
Muehlbauer T, Prieske O, Puta C,
Gollhofer A and Behm DG (2016)
Effects of Resistance Training in Youth
Athletes on Muscular Fitness and
Athletic Performance: A Conceptual
Model for Long-Term Athlete
Development. Front. Physiol. 7:164.
doi: 10.3389/fphys.2016.00164
Effects of Resistance Training in
Youth Athletes on Muscular Fitness
and Athletic Performance: A
Conceptual Model for Long-Term
Athlete Development
Urs Granacher 1*, Melanie Lesinski 1, Dirk Büsch 2, Thomas Muehlbauer 1, Olaf Prieske 1,
Christian Puta 3, Albert Gollhofer 4and David G. Behm 5
1Division of Training and Movement Sciences, Research Focus Cognition Sciences, University of Potsdam, Potsdam,
Germany, 2Department of Game and Combat Sports, Institute for Applied Training Science, Leipzig, Germany, 3Department
of Sports Medicine and Health Promotion, Friedrich-Schiller-University Jena, Jena, Germany, 4Department of Sport and
Sport Science, Albert-Ludwigs-University Freiburg, Freiburg, Germany, 5School of Human Kinetics and Recreation, Memorial
University of Newfoundland, St. John’s, NL, Canada
During the stages of long-term athlete development (LTAD), resistance training (RT) is an
important means for (i) stimulating athletic development, (ii) tolerating the demands of
long-term training and competition, and (iii) inducing long-term health promoting effects
that are robust over time and track into adulthood. However, there is a gap in the literature
with regards to optimal RT methods during LTAD and how RT is linked to biological age.
Thus, the aims of this scoping review were (i) to describe and discuss the effects of RT on
muscular fitness and athletic performance in youth athletes, (ii) to introduce a conceptual
model on how to appropriately implement different types of RT within LTAD stages,
and (iii) to identify research gaps from the existing literature by deducing implications for
future research. In general, RT produced small-to-moderate effects on muscular fitness
and athletic performance in youth athletes with muscular strength showing the largest
improvement. Free weight, complex, and plyometric training appear to be well-suited to
improve muscular fitness and athletic performance. In addition, balance training appears
to be an important preparatory (facilitating) training program during all stages of LTAD but
particularly during the early stages. As youth athletes become more mature, specificity,
and intensity of RT methods increase. This scoping review identified research gaps that
are summarized in the following and that should be addressed in future studies: (i) to
elucidate the influence of gender and biological age on the adaptive potential following
RT in youth athletes (especially in females), (ii) to describe RT protocols in more detail (i.e.,
always report stress and strain-based parameters), and (iii) to examine neuromuscular
and tendomuscular adaptations following RT in youth athletes.
Keywords: weight lifting, children, adolescents, physical fitness, muscle strength, muscle power, muscular
endurance

Granacher et al. Resistance Training in Youth Athletes
INTRODUCTION
The pool of youth with athletic potential to be introduced to
long-term athlete development (LTAD) has become smaller in
western industrialized countries due to demographic change and
secular declines in motor performance (Figure 1). In 1950 for
instance, 30% of Germany’s population was under 20 years of
age. An almost linear decline occurred over the following decades
so that in 2013, only 18% of the German population was 20
years and younger (German Federal Statistical Office, 2015).
In addition to demographic change, secular declines in youth
motor performance were reported. These findings are not limited
to Germany. Between 1981 and 2000, a meta-analysis revealed
rapid performance declines in aerobic endurance (i.e., 0.43%
per year) of children and adolescents aged 6–19 years living
in developed countries (Tomkinson et al., 2003). Performance
deteriorations were most marked in the older age groups but
similar for boys and girls (Tomkinson et al., 2003). Secular
declines were not only reported for aerobic endurance but also
for muscular fitness (Runhaar et al., 2010). In this context,
“muscular fitness” is used as an umbrella term for “muscular
strength,” “local muscular endurance,” and “muscular power”
(Smith et al., 2014). In Dutch children aged 9–12 years, Runhaar
et al. (2010) observed a secular trend in muscular endurance
(i.e., bent-arm hang) over a 26-years period ranging from
−16 to −49%. This finding was supported by Cohen et al.
(2011) who reported 10-years secular changes in measures
of muscular fitness (i.e., hand grip strength [−6%], sit-ups
[−27%], bent-arm hang [−26%]) in English children aged 10–
11 years. From a health-related perspective, these declines are
concerning because findings from a meta-analysis indicate an
inverse association between muscular fitness and total and central
adiposity, cardiovascular disease, and metabolic risk factors in
youth (Smith et al., 2014). Further, positive associations were
observed between muscular fitness and bone health and self-
esteem (Smith et al., 2014).
Besides its function as a marker of health, muscular fitness
is an essential component of athletic performance, which is
why it plays an important role during the stages of LTAD.
LTAD is a structured pathway to optimize the development
from talented children into elite athletes that consists of seven
sequential stages (1. Active Start, 2. FUNdamentals, 3. Learn to
Train, 4. Train to Train, 5. Train to Compete, 6 Train to Win,
7. Active for Life) and considers individual maturational level
rather than chronological age (Balyi et al., 2013). According
to Ford et al. (2011),Lloyd et al. (2015), and Faigenbaum
et al. (2016), muscular fitness should specifically be promoted
at all stages of LTAD to support motor skill acquisition, to
enhance motor performance, to improve markers of health
and well-being, and to reduce the risk of sustaining sports-
related injuries. Thus, youth athletes may benefit in three ways
from implementing muscular fitness enhancing exercises in
their regular training routine. First, by stimulating their athletic
development/career, second, by tolerating the demands of long-
term training and competition, and third, by inducing long-term
health promoting effects that are robust over time and track into
adulthood (stage 7 of LTAD) (Lloyd et al., 2015; Faigenbaum
FIGURE 1 | Impact of demographic change and secular declines in
motor performance on the pool of young talents with athletic potential
to be introduced to long-term athlete development.
et al., 2016). The positive effects of resistance training (RT) on
proxies of muscular fitness, health, sport-related, and everyday
activities have been examined and described in healthy (non-
athletic) children and adolescents in the form of randomized
controlled trials (Granacher et al., 2014), systematic reviews
(Benson et al., 2008), meta-analyses (Behringer et al., 2010), and
position stands (Behm et al., 2008). However, findings from these
studies can only partially be translated to the athletic context
because physiology and proficiency in motor performance differ
markedly between non-athletic and athletic youth (Armstrong
and McManus, 2011). In other words, specific characteristics of
youth athletes’ physiology and level of expertise have an impact
on their trainability (Lloyd et al., 2015). In addition, there is a
gap in the literature with regards to optimal RT methods during
LTAD and how RT is linked to biological age.
Thus, the aims of this scoping review were to describe and
discuss the effects of RT on measures of muscular fitness and
athletic performance in youth athletes (Table 1). In addition, a
conceptual model will be introduced on how to appropriately
implement different types of RT during the stages of LTAD
(Table 2). Finally and in accordance with the principles of
scoping reviews, we aimed at identifying research gaps in the
literature and provided recommendations for future research
(Table 3). Whereas prior reviews have described LTAD stages,
others have summarized pediatric training-induced physiological
adaptations; the present review attempts to integrate and apply
training responses, mechanisms, and prescription through the
stages of LTAD. A specific strength of this scoping review is
Frontiers in Physiology | www.frontiersin.org 2May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
TABLE 1 | Summary of studies that were included in this scoping review on the effects of resistance training on muscular fitness and athletic performance in youth athletes.
Author Characteristics of participants Resistance training protocol Outcomes
N Biological
age
Chronological age
(years)
LTAD stage Sex Sport Type of
resistance
training
Duration
(weeks)
Frequency
(sessions/
week)
Comparator Between-subject ES
MUSCULAR STRENGTH
Chelly et al.,
2009
EG: 11; ACG: 11 n/a EG: 17 ±0.3; ACG:
17 ±0.5
Training to train M Soccer FW 8 2 EG vs. ACG Half back squat (1 RM): ES
=1.74
Klusemann
et al., 2012
EG I: 13; EG II: 11;
ACG: 12
n/a M: 14 ±1; F: 15 ±1 Training to train M and F Basketball EG I: FT
(supervised)
6 2 EG I vs. ACG Push-up test: ES =0.55
Pull-up test: ES =0.32
EG II: FT
(video-based)
6 2 EG II vs. ACG Push-up test: ES =0.54
Pull-up test: ES =0.32
Sander et al.,
2013
EG I: 13; EG II: 30;
EG III: 18 ACG I:
15; ACG II: 25;
ACG III: 33
n/a EG and ACG I: 17;
EG and ACG II: 15;
EG and ACG III: 13
Learning to
train and
training to train
n/a Soccer EG I: FW 80 2 EG I vs. ACG Back squat (1 RM): ES =
2.96
Front squat (1 RM): ES =
4.00
EG II: FW 80 2 EG II vs. ACG Back squat (1 RM): ES =
3.50
Front squat (1RM): ES =
3.82
EG III: FW 80 2 EG III vs. ACG Back squat (1 RM): ES =
3.58
Front squat (1RM): ES =
4.45
MUSCULAR POWER
Hammami
et al., 2016
EG I: 12; EG II: 12 Years from
predicted
PHV (EG I:
–0.7 ±0.3;
EG II: –0.9
±0.4)
EG I: 12.7 ±0.3; EG
II: 12.5 ±0.3
Learning to
train
M Soccer EG I: BT +PT 8 2 EG I vs. EG II;
(“+” in favor of
EG I; “-” in favor
of EG II)
Reactive strength index: ES
=2.04
EG II: PT +
BT
8 2 Absolute leg stiffness: ES =
1.56
Relative leg stiffness: ES =
1.98
Triple hop test: ES =2.07
Y balance test: ES =1.38
(Continued)
Frontiers in Physiology | www.frontiersin.org 3May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
TABLE 1 | Continued
Author Characteristics of participants Resistance training protocol Outcomes
N Biological
age
Chronological age
(years)
LTAD stage Sex Sport Type of
resistance
training
Duration
(weeks)
Frequency
(sessions/
week)
Comparator Between-subject ES
Meylan and
Malatesta,
2009
EG: 14; ACG: 11 n/a EG: 13.3 ±0.6;
ACG: 13.1 ±0.6
Learning to
train
M Soccer PT 8 2 EG vs. ACG CMJ: ES =2.04
SJ: ES =1.29
Reactive strength index: ES
=0.03
Multiple 5 bound test: ES =
1.13
Ramírez-
Campillo
et al., 2014a
EG I: 13; EG II: 13;
EG III: 11; ACG: 14
Prepubertal
assessed
through
Tanner
stage
10.4 ±2.3 Learning to
train
M Soccer EG I: PT (30 s
inter-set rest)
7 2 EG I vs. ACG CMJ: ES =0.40
Reactive strength index (20
cm): ES =0.87
Reactive strength index (40
cm): ES =0.73
EG II: PT (60 s
inter-set rest)
7 2 EG II vs. ACG CMJ: ES =0.48
Reactive strength index (20
cm): ES =0.78
Reactive strength index (40
cm): ES =0.69
EG III: PT
(90 s inter-set
rest)
7 2 EG III vs. ACG CMJ: ES =0.31
Reactive strength index (20
cm): ES =0.66
Reactive strength index (40
cm): ES =0.86
Santos and
Janeira, 2011
EG: 14; ACG: 10 (Post-)
pubertal
assessed
through
Tanner
stage
EG: 15.0 ±0.5;
ACG: 14.5 ±0.4
Training to train M Basketball PT 10 2 EG vs. ACG CMJ: ES =1.28
SJ: ES =2.02
Abalakov test: ES =1.36
Depth jump: ES =1.50
Mechanical power:
ES =0.45
MUSCULAR ENDURANCE
Christou
et al., 2006
EG: 9; ACG: 9 (Post-)
pubertal
assessed
through
Tanner
stage
EG: 13.8 ±0.4;
ACG: 13.5 ±0.9
Training to train M Soccer MB and FW 16 2 EG vs. ACG 30-s repeated jump test: ES
=0.27
DeRenne
et al., 1996
EG I: 7; EG II: 8;
ACG: 6
n/a 13.3 ±1.3 Learning to
train
M Baseball EG I: MB and
FW
12 1 EG I vs. ACG Pull-up test: ES =1.06
EG II: MB and
FW
12 2 EG II vs. ACG Pull-up test: ES =1.33
(Continued)
Frontiers in Physiology | www.frontiersin.org 4May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
TABLE 1 | Continued
Author Characteristics of participants Resistance training protocol Outcomes
N Biological
age
Chronological age
(years)
LTAD stage Sex Sport Type of
resistance
training
Duration
(weeks)
Frequency
(sessions/
week)
Comparator Between-subject ES
Granacher
et al., 2014
EG I: 13; EG II:14 n/a EG I: 13.7 ±0.6; EG
II: 13.8 ±0.
Training to train M and F n/a EG I: FT on
stable
surfaces
6 2 EG I vs. EG II;
(“+” in favor of
EG I; “−” in
favor of EG II)
Ventral TMS test: ES =
−0.24
EG II: FT on
unstable
surfaces
6 2
Dorsal TMS test: ES =1.49
Lateral right side TMS test:
ES =0.04
Lateral left side TMS test:
ES =0.00
Weston et al.,
2015
EG: 10; ACG: 10 n/a EG: 15.7 ±1.2;
ACG: 16.7 ±0.9
Training to train M and F Swimming FT 12 3 EG vs. ACG Prone bridge test: ES =
0.11
ATHLETIC PERFORMANCE
Behringer
et al., 2013
EG I: 12; EG II: 12;
ACG: 12
(Post-)
pubertal
assessed
through
Tanner
stage
15.0 ±1.6 Training to train M Tennis MB 8 2 EG I vs. ACG Tennis service velocity: ES
=0.04
Tennis service precision
test: ES =0.69
PT 8 2 EG II vs. ACG Tennis service velocity: ES
=1.39
Tennis service precision
test: ES =0.51
Prieske et al.,
2016a
EG I: 19; EG II: 18 n/a EG I: 16.6 ±1.1; EG
II: 16.6 ±1.0
Learning to
train
M Soccer EG I: FT on
stable
surfaces
9 2–3 EG I vs. EG II;
(“+” in favor of
EG I; “–” in
favor of EG II)
Kicking velocity: ES =
−0.65
EG II: FT on
unstable
surfaces
9 2–3
Ramírez-
Campillo
et al., 2014b
EG: 38; ACG: 38 (Post-)
pubertal
assessed
through
Tanner
stage
13.2 ±1.8 Training to train M Soccer PT 7 n/a EG vs. ACG Kicking test: ES =0.88
Saeterbakken
et al., 2011
EG: 14; ACG: 10 n/a 16.6 ±0.3 Training to train F Handball FT 6 2 EG vs. ACG Throwing velocity: ES =
1.43
Legend: ACG, active control group; BT, balance training; CMJ, countermovement jump; EG, experimental group; ES, effect size; F, female; FT, functional training (training with own body mass, thera-bands etc.), FW, free weights, LTAD,
long term athlete development; M, male, MB, machine based; n/a, not applicable; N, number of subjects; PHV, peak height velocity; PT, plyometric training; RM, repetition maximum; SJ, squat jump; TMS, trunk muscle strength.
Frontiers in Physiology | www.frontiersin.org 5May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
TABLE 2 | Conceptual model for the implementation of resistance training (RT) programs during the stages of long-term athlete development (LTDA) to
enhance muscular fitness and athletic performance.
Early childhood Late childhood Adolescents Adulthood
CHRONOLOGICAL AGE
Female: 6–8 years Female: 9–11 years Female: 12–18 years Female: >18 years
Male: 6–9 years Male: 10–13 years Male: 14–18 years Male: >18 years
BIOLOGICAL AGE
Tanner stage I Tanner stage I–II Tanner stage III–IV Tanner stage V
MATURITY
Pre-pubertal (pre PHV) Pre-pubertal (pre PHV) Pubertal (mid PHV) Post-pubertal (post PHV)
STAGE OF LONG-TERM ATHLETE DEVELOPMENT
FUNdamentals Learning to train Training to train Training to compete
LONG-TERM DEVELOPMENT OF MUSCULAR FITNESS (STRENGTH, POWER, ENDURANCE)
- Coordination training
- Agility training
- Balance training
- Muscular endurance training with
own body mass/training tools (e.g.,
medicine ball) with a focus on
exercise technique
- Balance training
- Plyometric training as part of
deliberate play (e.g., rope skipping)
with a focus on correct jumping and
landing mechanics
- Core strength training
- Muscular endurance training with
own body mass/training tools (e.g.,
medicine ball)
- Free weight training with a focus on
exercise technique
- Balance training
- Plyometric training (depth jumps
from low drop heights)
- Core strength training
- Free weight training at light to
moderate loads
- Heavy resistance strength training
(hypertrophy)
- Eccentric resistance training
- Sport-specific resistance training
- Balance training
- Plyometric training (depth jumps
from moderate drop heights)
- Core strength training
- Free weight training at moderate to
high loads
- Heavy resistance strength training
(neuromuscular activation +
hypertrophy)
- Sport-specific resistance training
TRAINING-INDUCED ADAPTATIONS
Neuronal adaptations Hormonal/Neuronal/Muscular/Tendinous adaptations
RT programs were allocated to LTAD stages based on expert opinion and according to Lesinski et al. (2016),Faigenbaum et al. (2016),Lloyd et al. (2011, 2015),Balyi et al. (2013), as
well as Kraemer and Fleck (2005).
Legend: PHV, peak height velocity.
TABLE 3 | Identified research gaps in the literature and recommendations for future studies.
Identified problems from scoping review Implications for future research
Lack of studies that examined RT effects in child athletes Particularly examine the effects of RT in child athletes
Lack of studies that reported measures of biological age Always determine and report a measure of biological age (e.g., peak-height
velocity)
Lack of studies that examined sex-specific effects of RT Particularly examine the effects of RT in female youth athletes
Lack of studies that examined physiological adaptive processes following RT in child
and adolescent athletes
Elucidate neuromuscular and tendomuscular mechanisms following RT in
youth athletes according to sex and biological age
Insufficient reporting and inclusion of stress and strain-based parameters in RT
studies
Describe RT protocols in more detail (report stress and strain-based
parameters)
Insufficient matching of RT protocols when comparing different protocols When comparing different RT protocols make sure that protocols are
matched for strain-based parameters (e.g., time under tension) or
mechanical work (i.e., lifted overall load)
Legend: RT, resistance training.
that only studies examining youth athletes were included while
previous work reported findings from athletic and non-athletic
populations.
Of note and in accordance with Behm et al. (2008), we define
RT as a specialized method of conditioning that involves the
progressive use of a wide range of resistive loads, including
body mass, and a variety of training modalities (e.g., machine-
based training, free weight training, plyometric training, complex
training, functional training) designed to enhance muscular
fitness and athletic performance.
METHODS
This article is presented in the form of a scoping review
(Armstrong et al., 2011). According to the principles of a
Frontiers in Physiology | www.frontiersin.org 6May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
scoping review the research question was broad. For instance,
“Can RT increase muscular fitness and athletic performance in
youth athletes?” or “What are appropriate and evidence-based
RT methods to be implemented during the stages of LTAD?.”
Typically, a scoping review can be used to identify research
gaps in the existing literature (Table 3) as well as to summarize
research findings (Table 1) without necessarily performing a
study quality assessment or an extensive data synthesis in the
form of a systematic review and meta-analysis (Armstrong et al.,
2011). We identified relevant studies listed in review articles
(Lloyd et al., 2014, 2015; Faigenbaum et al., 2016; Lesinski et al.,
2016) with the key inclusion criterion that the subjects were
youth sub-elite, elite, and/or competitive athletes. According to
Araujo and Scharhag (2016), the term “athletes” was defined as a
person to be training in sports aiming to improve performance.
We additionally checked the reference lists of each included
article in an effort to identify additional suitable studies for
inclusion. Studies were not included if they failed to specify the
athletic level. Finally, all included studies had to examine the
effects of RT (≥6 training sessions) on at least one measure of
muscular fitness (i.e., strength, power, endurance) and/or athletic
performance (i.e., proxies of performance in specific sport
disciplines). Whenever possible, effect sizes (ES) were reported
for the included studies to illustrate the practical relevance of the
respective study outcomes. Between-subject standardized mean
differences (corresponds to ES) were computed according to the
following formula: standardized mean differences =mean post
value intervention group-mean post value control group)/pooled
standard deviation). We adjusted the standardized mean
differences for the respective sample size by using the
term (1−(3/(4N-9))). According to Rhea’s (2004) scale for
determining the magnitude of ES in strength training research
for individuals who have been consistently training for 1–
5 years (i.e., youth athletes), we interpreted ES as trivial
(<0.35), small (0.35–0.79), moderate (0.80–1.50), or large
(≥1.50).
EFFECTS OF RESISTANCE TRAINING ON
MUSCULAR FITNESS IN YOUTH
ATHLETES
To the authors’ knowledge, there are three meta-analyses
available in the literature which examined the effects of RT on
measures of muscular fitness in healthy (non-athletic) youth.
These three studies (Falk and Tenenbaum, 1996; Payne et al.,
1997; Behringer et al., 2010) clearly illustrate the general
effectiveness of RT to improve muscular fitness in youth. The
reported ES ranged from trivial (ES =0.20) to large effects (ES =
1.91) depending on biological/chronological age, sex, and mode
of muscle action (i.e., isometric, isotonic, isokinetic protocols).
However, findings from these meta-analyses cannot directly
be transferred to youth athletes because athletes differ from
healthy but untrained youth with regards to training capacity,
adherence, physical demands of activities, physical condition,
and injury risk (Bergeron et al., 2015). Thus, it is imperative to
report cohort specific results when discussing and interpreting
the effects of RT in youth athletes which will be done in the
following.
Effects of Resistance Training on Muscular
Strength in Youth Athletes
Muscular strength can be defined as the maximal force or
tension a muscle or a group of muscles can generate at a
specified velocity (Knuttgen and Kraemer, 1987). Many sport-
specific situations demand high accelerations of an external
resistance (e.g., own body mass, body mass of opponent, mass
of object) for athletic success in competition. According to
Newton’s second law of motion (Force =mass •acceleration),
the acceleration of an external resistance is determined by the
magnitude of an acting force. This clearly illustrates that the
ability to voluntarily produce a maximal force or torque is
important for sports performance. In addition, Schmidtbleicher
(2004) considered maximal muscular strength as the basic
dimension of muscular fitness. In other words, proficiency
level of maximal muscular strength influences performance in
muscular power and muscular endurance. This implies that
training-induced improvements in maximal muscular strength
result in concomitant enhancements of muscular power and
muscular endurance (Schmidtbleicher, 2004) which is why
conditioning programs should focus on the development of
maximal muscular strength.
A number of studies investigated the effects of RT on measures
of maximal muscular strength in youth athletes (Chelly et al.,
2009; Klusemann et al., 2012; Sander et al., 2013) (Table 1). A
study that is of particular interest in this context was conducted
by Sander et al. (2013) because it examined the effects of long-
term strength training (i.e., 2 years) on measures of maximal
strength of the lower extremities in three age groups (i.e., under
13, 15, and 17 years) of male elite soccer players. For this purpose,
participants from each age group were allocated to a RT or an
active control group. The RT group conducted four regular soccer
training and two additional RT sessions per week. During RT,
subjects performed parallel front and back squat, bench press,
neck press, deadlift, and core exercises. A periodization model
was applied for the squat exercises with a focus on technical
skill training during the first 4 weeks. For the next 8 weeks, the
focus emphasized inducing muscle hypertrophy with a training
protocol consisting of five sets, 10 repetition maximum (RM),
and 3 min rest between sets. During the next 4 weeks, training
intensity was increased to five sets, six RM, and a 3 min rest
between sets. The last 4 week cycle consisted of five sets, four
RM, and a 5 min rest between sets. The RT protocol for the
upper extremities and the core muscles comprised three to five
sets, 10 RM, and a 3 min rest between sets over the course
of the training period. This 20-week periodization model was
conducted twice during one soccer season and four times over the
2 years intervention period. The active control group conducted
regular soccer training over the course of the study with four
soccer sessions per week and no specific RT. One RM front
and back squat tests were performed on three occasions, prior
to the start of the study, after 1 and 2 years. Findings revealed
significant group by time interactions for the front and back squat
Frontiers in Physiology | www.frontiersin.org 7May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
1 RM in favor of the three age-specific RT groups. The largest
ES were observed for the youngest age group with ES =1.9 for
the front squat and ES =2.0 for the back squat with tremendous
relative percentage increases (% RT group minus % active control
group) of 230 to 250% (front/back squat) over the 2 years training
period. This relative increase was lower in the older two cohorts
and ranged between 56 and 80% indicating that the RT induced
adaptive potential was largest in those subjects that were under
13 years of age at the beginning of the study.
Lesinski et al. (2016) recently conducted a systematic review
and meta-analysis on the effects of RT on measures of muscular
fitness and physical performance in youth athletes aged 6 to 18
years. The systematic search revealed 43 studies that were eligible
for inclusion in the meta-analysis. Based on findings from 16
studies, the analysis indicated moderate effects (ES =1.09) of
RT on measures of muscular strength (Figure 2). In addition, an
age-group specific sub-analysis showed larger effects for children
(boys ≤13 years; girls ≤11 years; ES =1.35) as compared to
adolescents (boys 14–18 years; girls 12–18 years; ES =0.91),
which strengthens the findings of Sander et al. (2013) (Figure 3).
The larger adaptive potential following RT in child athletes can
most likely be explained by greater relative (percentage) strength
gains and increased neural plasticity in children as compared
to adolescent athletes (Pfeiffer and Francis, 1986; Ramsay et al.,
1990). Finally, the meta-analysis of Lesinski et al. (2016) revealed
that free weight training produced the largest effects (ES =2.97)
on measures of muscular strength in youth athletes followed
by the combination of machine-based and free weight training
(ES =1.16), functional training (ES =0.62), plyometric training
(ES =0.39), and machine-based training (ES =0.36) (Figure 5).
The superiority of free weight training either in isolation or
in combination can be attributed to the additional muscular
stabilization of the trunk and limb joints needed to control
the greater degrees of freedom associated with the multi-planar
movements (Behm et al., 2010).
In summary, RT is an effective means to improve muscular
strength in youth athletes of all ages, with the introduction of
free weights RT from the late childhood LTAD stage and beyond
(Table 2). It appears that trainability in terms of relative strength
gains is higher in child athletes as compared to adolescent
athletes and that free weight training is particularly effective.
Further, it is recommended that researchers consistently report
information on the maturation status of the investigated cohort
(e.g., peak height velocity, Tanner stages) because chronological
and biological age can largely differ in children and adolescents
(Table 3).
Effects of Resistance Training on Muscular
Power in Youth Athletes
Muscular power refers to the rate at which muscles perform
work (Power =work/time) (Smith et al., 2014). The importance
of muscular power for the athletic context becomes clearly
manifested when inserting the term work =force •distance in
the equation Power =work/time resulting in the deduced term
Power =force •velocity. In other words, dynamic muscle actions
at high forces, and movement velocities are essential components
FIGURE 2 | Effects of resistance training on measures of muscular
strength (n=16 studies), muscular power (n=33 studies), muscular
endurance (n=3 studies), and athletic performance (n=20 studies) in
youth athletes. Of note, only studies with an active control group were
included if they investigated the effects of resistance training in youth athletes
(6–18 years) and tested at least one measure of muscular fitness and athletic
performance. Legend: SMD =standard mean difference (effect size). Modified
from Lesinski et al. (2016).
FIGURE 3 | Effects of resistance training on measures of muscular
strength (children: n=3 studies; adolescents: n=13 studies),
muscular power (children: n=10 studies; adolescents: n=22 studies),
muscular endurance (adolescents: n=2 studies), and athletic
performance (children: n=6 studies; adolescents: n=13 studies) in
youth athletes depending on chronological age. Of note, only studies
with an active control group were included if they investigated the effects of
resistance training in youth athletes (6–18 years) and tested at least one
measure of muscular fitness and athletic performance. Legend: p=p-value
refers to the respective subgroup analysis; SMD =standard mean difference
(effect size). Modified from Lesinski et al. (2016).
of power production. Furthermore, the concept of the velocity
specificity of RT indicates that the greatest increases in strength
or power occur at or near the velocity of the training exercises
Frontiers in Physiology | www.frontiersin.org 8May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
(Behm and Sale, 1993). This implies that both components of
the power equation (i.e., force and velocity) have to be trained
if the goal is to maximize muscular power. In this context,
we explicitly defined “power” in Newtonian mechanics and we
comply with Winter et al. (2016) that the use of the term “power”
in the exercise scientific literature should be limited to the true
definition of mechanical power. Nevertheless, when conducting
this literature review we were confronted with several studies that
used a rather colloquial meaning of “power” as a surrogate for
muscular performance that includes extremes of force or speed.
We decided not to exclude those studies to get a comprehensive
picture of the scientific literature. However, we recommend that
future studies stick to the mechanical definition of the term
“power.”
There is evidence in the literature that plyometric training
is well-suited to enhance muscular power (de Villarreal et al.,
2009) and/or proxies or surrogates of muscular power because
plyometrics (e.g., hops, jumps) refer to exercises that link strength
with speed of movement (Faigenbaum and Chu, 2001). More
specifically, plyometric exercises start with a rapid stretch of a
muscle or muscle group during the breaking or eccentric phase
resulting in high forces at ground contact. During the subsequent
push-off or concentric phase, the same muscle or muscle group
shortens rapidly to accelerate the body in vertical direction.
Thus, both components of the power equation are stimulated
during plyometric training. Consequently, plyometric training is
an often applied method during the stages of LTAD (Table 2)
particularly in sports like soccer, basketball, and handball (Santos
and Janeira, 2011; Ramírez-Campillo et al., 2014a) (Table 1).
The general effects of in-season plyometric training on proxies
of muscular power (i.e., countermovement jump [CMJ] height,
hurdle jump height) were shown by Meylan and Malatesta (2009)
for early pubertal soccer players with a mean age of 13 years.
Following 8 weeks of training with two training sessions per
week, significant improvements were found in the intervention
as compared to the control group in CMJ (+7.9%) and hurdle
jump height (+10.9%). These performance enhancements were
substantiated by the already presented meta-analysis of Lesinski
et al. (2016) who reported moderate effects (ES =0.80) of RT
on proxies of muscular power in youth athletes (Figure 2). This
is in accordance with a previously conducted meta-analysis by
Harries et al. (2012) who aggregated findings from 14 studies
and reported similar effects of RT on proxies of muscular
power in adolescent athletes. Lesinski et al. (2016) additionally
examined age- and sex-specific effects of RT on muscular power
and observed slightly smaller effect sizes for child (ES =0.78)
compared to adolescent athletes (ES =0.85) (Figure 3) and larger
effect sizes for boys (ES =0.85) compared to girls (ES =0.61)
(Figure 4). The sub-analysis regarding training type revealed
that complex training which combines weight and plyometric
exercises during the same training session produced the largest
effects (ES =1.66) on proxies of muscular power in youth
athletes followed by machine-based training (ES =1.45), free
weight training (ES =0.90), plyometric training (ES =0.81),
the combination of machine-based, and free weight training
(ES =0.77), and functional training (ES =0.39) (Lesinski et al.,
2016) (Figure 5). The inability of plyometric training alone to
FIGURE 4 | Effects of resistance training on measures of muscular
strength (boys: n=12 studies), muscular power (girls: n=3 studies;
boys: n=27 studies), muscular endurance (boys: n=2 studies), and
athletic performance (girls: n=2 studies; boys: n=15 studies) in
youth athletes depending on sex. Of note, only studies with an active
control group were included if they investigated the effects of resistance
training in youth athletes (6–18 years) and tested at least one measure of
muscular fitness and athletic performance. Legend: p=p-value refers to the
respective subgroup analysis; SMD =standard mean difference (effect size).
Modified from Lesinski et al. (2016).
provide the greatest magnitude of change may be ascribed to
the greater balance challenges with this activity. Behm et al.
(2010) in their review reported that unstable environments (bases
and implements) tend to decrease force output compared to
more stable environments. Hence, since balance capabilities are
immature and not fully developed in youth (Behm et al., 2010),
the Lesinski et al. (2016) findings emphasize the need for RT to be
included with plyometric training for optimal power gains with
youth.
Taken together, these findings clearly illustrate that both
components of the power equation (i.e., force, velocity) have
to be emphasized to induce large training-induced effects on
muscle power. While the findings of the general effects of RT
on muscular power in youth athletes are robust (Lesinski et al.,
2016), the results of the age- and sex-specific RT effects (Lesinski
et al., 2016) have to be interpreted cautiously due to a limited
number of studies that were included in the meta-analysis (see
captions of Figures 3,4). Nevertheless, the larger effects in
adolescent compared to child athletes and boys compared to
girls could be explained by biological maturation (i.e. children
vs. adolescents) and maturational differences (i.e., boys vs. girls).
There is in fact evidence from magnetic resonance imaging (MRI)
studies that muscle cross-sectional area increases with age from
childhood through adolescence and more in boys compared to
girls (De Ste Croix et al., 2002). Besides changes in muscle mass
with maturation, there is knowledge of sex-specific fiber type
growth and distribution particularly during adolescence. While
percentage of type I fibers is equal in boys and girls during
Frontiers in Physiology | www.frontiersin.org 9May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
FIGURE 5 | Effects of resistance training on measures of muscular
strength (machine-based training: n=3 studies; free weight training:
n=2 studies; machine-based and free weight training: n=4 studies;
functional training: n=2 studies; plyometric training: n=4 studies),
muscular power (machine-based training: n=3 studies; free weight
training: n=3 studies; machine-based and free weight training: n=3
studies; functional training: n=2 studies; plyometric training: n=16
studies; complex training: n=4 studies), muscular endurance
(machine-based and free weight training: n=2 studies), and athletic
performance (machine-based training: n=3 studies; functional
training: n=5 studies; plyometric training: n=10 studies; complex
training: n=2 studies) in youth athletes depending on type of
resistance training. Of note, only studies with an active control group were
included if they investigated the effects of resistance training in youth athletes
(6–18 years) and tested at least one measure of muscular fitness and athletic
performance. Legend: p=p-value refers to the respective subgroup analysis;
SMD =standard mean difference (effect size). Modified from Lesinski et al.
(2016).
childhood, there are apparent differences during adolescence
with females having a lower percentage of type I fibers than males
and male type II fibers being bigger than their type I fibers which
is not the case in females (Vogler and Bove, 1985; Glenmark et al.,
1992). Given that muscle mass and fiber type distribution are
important prerequisites for the generation of muscular power, the
described age- and sex-specific physiological characteristics may
partially explain the reported outcomes of Lesinski et al. (2016).
In summary, RT is an effective means to improve proxies of
muscular power in youth athletes of all ages, with free weight
RT and plyometric training best introduced at the late childhood
stage (Table 2). Training types that emphasize both components
of the power equation appear to produce the largest effects (i.e.,
complex training). The reported age- and sex-specific effects
might be caused by methodological limitations (i.e., lack of
studies in girls and child athletes) and/or maturational and
sex-specific physiological characteristics (Table 3). Finally, more
research is needed to elucidate the underlying neuromuscular
and tendomuscular adaptations following RT in youth athletes
and to separate training-induced adaptations from growth and
maturation.
Effects of Resistance Training on Muscular
Endurance in Youth Athletes
Local muscular endurance is the ability to voluntarily generate
consistent or repeated submaximal force or torque output (>30%
1 RM) utilizing a single muscle or a group of muscles for an
extended period of time while resisting fatigue (Moir, 2012). If
for instance the goal is to maintain high movement speed during
an 800-m running competition or to minimize the fatigue-related
performance decrements, the impulse-momentum theorem
(force •time =mass •1velocity) illustrates that if body mass
remains constant, the magnitude of the impulse (force •time)
over the competition/race time is directly related to the achieved
momentum (mass •1velocity), i.e., step velocity and ultimately
race pace. Thus, adequate RT programs are needed in specific
sports disciplines like rowing, running, swimming, kayaking and
others, which promote the ability of the active musculature to
resist fatigue and to optimize performance.
So far, only a few studies examined the specific effects of
RT on measures of local muscular endurance in youth athletes
(Granacher et al., 2014; Weston et al., 2015). A recently published
randomized controlled trial (Granacher et al., 2014) studied the
effects of a 6 week core RT using stable vs. unstable surfaces
(e.g., Thera-band©stability trainer) on local muscular endurance
of the trunk in male and female youth athletes aged 14 ±1
years. Subjects trained two times per week performing static
and dynamic frontal, dorsal, and lateral core exercises (i.e., curl-
up, side bridge, quadruped position) for three sets per exercise
with progressively increasing contraction time (static condition:
40–50 s) and/or number of repetitions (dynamic condition: 20–
25 repetitions). Following training, both intervention groups
significantly improved local muscular endurance of the ventral
and lateral trunk muscles in a range of 8–41%, independent of
surface conditions. In addition, Weston et al. (2015) determined
the effects of a 12-week core RT on local muscular endurance
of the trunk (i.e., timed prone-bridge test) in national-level
adolescent swimmers (16 ±1 years) compared to an active
control. The intervention group performed three training
sessions per week, incorporating static and dynamic exercises
(e.g., prone bridge, side bridge, bird dog etc.) with two to three
sets per exercise and a progressively increasing contraction time
(static condition: 30-90 s) and/or number of repetitions (dynamic
condition: 10–30 repetitions). After training, the intervention
group showed small improvements on the timed prone-bridge
test compared to the active control (+9.0%).
To the authors’ knowledge, there are only three studies
(DeRenne et al., 1996; Christou et al., 2006; Weston et al.,
2015) available that examined the effects of RT on measures
of local muscular endurance in youth athletes compared to an
active control (Table 1). Findings from these three studies were
aggregated and presented in Figure 2. An ES of 0.57 is indicative
of a small effect of RT on local muscular endurance measures
with youth athletes (Figure 2). Because of the limited number of
studies available in the literature, age-, sex-, and RT- type-specific
sub-analyses were not applicable (Figures 3–5).
In summary, results from four studies indicate that youth
athletes can improve their local muscular endurance following
Frontiers in Physiology | www.frontiersin.org 10 May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
RT. However, the aggregated effects of RT on measures of local
muscular endurance were small compared to the previously
reported moderate effects of RT on measures of muscular
strength and power (Figure 2). Considering the small number of
available studies concerning this topic (Table 1), more research
is needed to elucidate and substantiate the effects of RT on local
muscular endurance in youth athletes.
EFFECTS OF RESISTANCE TRAINING ON
ATHLETIC PERFORMANCE IN YOUTH
ATHLETES
There is evidence that RT has the potential to improve muscular
strength, muscular power, muscular endurance, agility, balance
and stability, coordination, and speed of movement in youth
athletes (Harries et al., 2012; Lesinski et al., 2016). These
training-induced gains in health and skill-related physical fitness
parameters may support young athletes during the acquisition
phase of complex movements, for mastering sport tactics,
and to withstand the demands of long-term athletic training
and competition (Faigenbaum et al., 2016). Given this broad
spectrum of RT efficacy, it is plausible to argue that the previously
reported effects of RT in youth athletes translate to their athletic
performance (Faigenbaum et al., 2016). In this context, the
term “athletic performance” refers to proxies of performance
in specific sport disciplines for instance, soccer (e.g., kicking
velocity), handball (e.g., throwing velocity), baseball (e.g., hitting
and throwing velocity), tennis (e.g., velocity of tennis-serve), and
swimming (e.g., start and swimming performance).
Saeterbakken et al. (2011) examined the effects of a 6-week,
two times per week sling exercise RT on maximal throwing
velocity among female handball players with a mean age of 16.6 ±
0.3 years (Table 1). After training, maximal throwing velocity
significantly increased 4.9% in the intervention group but was
unchanged in the control group. These results suggest that core
stability training using unstable, closed kinetic chain movements
can significantly improve maximal throwing velocity in youth
female handball players. In another study, Ramírez-Campillo
et al. (2014b) scrutinized the effectiveness of low-volume and
high-intensity plyometric training on maximal kicking distance
in youth soccer players with a mean age of 13.2 ±1.8 years.
After 7 weeks of training with two training sessions per week,
maximal kicking distance significantly improved by 14% in the
intervention compared to the control group. Further, Behringer
et al. (2013) evaluated the transferability of two different RT
protocols (i.e., upper/lower extremity plyometric training vs.
upper/lower extremity machine-based RT) on service velocity
and its precision consistency in junior tennis players with a
mean age of 15.0 ±1.6 years (Table 1). Following a training
period of 8 weeks with two training sessions per week in
addition to the regular tennis training, mean service velocity
over 20 maximum-velocity serves increased significantly more in
the plyometrics group (+3.8%) when compared with a control
group. No significant changes were found in the machine-based
RT group (+1.2%). Service precision (i.e., 20 targeted maximum-
velocity serves from the baseline to the intercept point of the
service line and the center service line) did not significantly
change from pre- to post-test in all three experimental groups.
In their meta-analysis, Lesinski et al. (2016) aggregated results
from 20 studies and found overall small effects of RT on proxies
of athletic performance in youth athletes (ES =0.75) (Figure 2).
Age- and sex-specific sub-analyses revealed a tendency toward
significantly larger effects in adolescent athletes (ES =1.03)
compared to children (ES =0.50) (Figure 3) and significantly
larger effects for girls (ES =1.81) compared to boys (ES =0.72)
(Figure 4). In terms of training type, larger effects were found
for complex training (ES =1.85) followed by functional training
(ES =0.79), plyometric training (ES =0.74), and machine-based
training (ES =0.30) (Figure 5). As most athletic performances
demand a power component, it is consistent that similar to
the findings on the effect of training type on muscular power
that the complex training involving both plyometrics and RT
demonstrated the greatest training gains.
Again, it is highly speculative to interpret age- and sex-
specific effects of RT on proxies of athletic performance in
youth athletes. Lesinski et al. (2016) aggregated six studies
to illustrate the specific effects of RT in children while 13
studies were summarized in adolescents. Only two studies
examined sex-specific RT effects in girls while 15 studies
scrutinized this subject in boys (Lesinski et al., 2016). Therefore,
methodological issues might account for the observed findings.
In addition, physiological reasons may also explain the larger
adaptive potential in adolescents compared to children because
adolescents have larger muscle mass and type II fibers compared
to children (Vogler and Bove, 1985; Glenmark et al., 1992), which
may account for the higher trainability of athletic performance
in adolescents. Finally, female athletes may possess larger
adaptive reserves than their male counterparts to enhance athletic
performance due to a larger potential to respond to neural stimuli
(Streckis et al., 2007).
In summary, RT produces only small effects on proxies of
athletic performance in youth athletes. Among the examined
training types, complex training appears to be the best-suited
agent to improve athletic performance. The reported age- and
sex-specific effects could be due to methodological limitations
(i.e., lack of studies in girls and children) and/or maturational and
sex-specific physiological characteristics (Table 3).
PRACTICAL IMPLICATIONS FOR
COACHES AND FITNESS PROFESSIONALS
It has been established for years by a number of national
associations (i.e., American, Australian, British, Canadian,
German and others) that RT is an acceptable and safe mode
of training for children and adolescents with positive effects on
health, psycho-social skills, well-being, and a reduction in the
severity and incidence of injuries (Behm et al., 2008; Lloyd et al.,
2014). In regards to older myths, misinformation regarding the
potential negative effects of RT for children has been refuted, and
thus coaches, fitness professionals, and young athletes can focus
on the optimal training regimens to enhance muscular fitness and
athletic performance.
Frontiers in Physiology | www.frontiersin.org 11 May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
If a primary characteristic of the sport necessitates the
development of muscular strength, the present findings show
that it is most optimally improved with the use of free weight
resistance exercises. Free weights (e.g., dumbbells, barbells)
permit multi-planar movements around the three axes through
a spectrum of velocities, which allow a greater diversity of
more task and velocity specific resisted actions. This freedom
of movement also places greater stress on the maintenance of
balance and stability, which are requisites for almost all athletic
endeavors. Since, balance and coordination are not fully matured
in children (Payne and Isaacs, 2008), balance training would be
an important preliminary training phase for enhancing latter
strength and power training progressions and reducing the risk
of athletic and RT injury (McGuine and Keene, 2006). Balance is
essential for optimal athletic performance (McGuine and Keene,
2006). Hence, balance training should be incorporated prior to
and during strength and power training (Table 2). In accordance
with this recommendation, Hammami et al. (2016) trained young
elite soccer players aged 12–13 years twice per week for 8 weeks
either with an initial 4 weeks of balance training followed by 4
weeks of plyometric training or 4 weeks of plyometric training
proceeded by 4 weeks of balance training. Balance training prior
to plyometric training initiated greater training improvements
in reactive strength, leg stiffness, triple hop test, and the Y
balance test. Furthermore, Chaouachi et al. (2014) trained 12–
15 years old boys over an 8-week training period with either
plyometric exercises only or with a combination of balance and
plyometric training. There were no differences in 8 of the 11
balance and power measures. However, with only half the volume
of plyometric training, the combined training enhanced leg
stiffness, 10-m sprints and shuttle runs to a greater degree. Thus,
balance exercises should play a significant prior and concurrent
role in the strength and power training of child and adolescent
athletes, which was accounted for in Table 2. In other words,
balance training represents an essential part of the conditioning
program at all stages of LTAD.
While RT on unstable surfaces is quite popular, Behm et al.
(2015) concluded from their meta-analysis that the performance
of unstable RT compared with stable RT has limited extra effects
on muscular strength, power, and balance performance in healthy
adolescents. Hence, the balance training exercises can be applied
alone or in conjunction with the free weight RT.
Although there is not a single optimal training plan for all
participants, a reasonable approach is to begin RT with one
or two sets of 8–15 repetitions with a light to moderate load
(30–60% 1 RM) with 8–12 exercises and a training frequency
of at least two non-consecutive days per week (Behm et al.,
2008). Youth with RT experience can progress to more intense
or extensive training sessions (i.e., heavy resistance strength
training) to achieve their training objectives (i.e., muscular
strength, power, and endurance). In their meta-analysis, Lesinski
et al. (2016) extracted dose-response relationships following
RT in youth athletes for single RT parameters (e.g., training
period, training frequency, training volume) independently and
revealed that a training period of more than 23 weeks, five
sets per exercise, 6 to 8 repetitions per set, a training intensity
of 80–89% of the 1 RM, and 3–4 min rest between sets
were most effective for conventional RT programs to improve
muscular strength in youth athletes. Thus, it appears that heavy
resistance strength training should be applied if the goal is to
enhance muscular strength in young athletes (Table 2). However,
these evidence-based findings should be adapted athlete specific
considering individual abilities, skills, and goals (Lesinski et al.,
2016).
The present review also demonstrates that for muscular
power and athletic performance development, complex training
involving RT and plyometric exercises should provide the largest
magnitude of change. According to the Behm et al. (2008)
review, improvements in athletic performance are typically only
shown when RT was combined with specific sports training.
Consequently, plyometric training should not be the only
component of an exercise program, with the most desirable
approach to incorporate other types of strength, conditioning
and sport-specific practice into a well-rounded program (Behm
et al., 2008). It is recommended that youth should start
plyometric training with less-intense drills (e.g., double-leg
jumps) and gradually progress to more advanced drills (e.g.,
single-leg hops) as balance, competence and confidence improves
(Behm et al., 2008). Relatively few repetitions (i.e., <10) are
needed to bring about significant performance gains (Lephart
et al., 2005). Plyometric training should occur on yielding
surfaces (e.g., gymnasium floor or playing field) and the focus
of early training should be on proper athletic positioning and
landing (Behm et al., 2008).
While there is evidence that core strength training is effective
in improving trunk muscle endurance (Granacher et al., 2014), it
has limited effects on proxies of athletic performance in youth
athletes (Prieske et al., 2016b). In fact, Prieske et al. (2016b)
recently conducted a systematic review and meta-analysis on
the effects of core strength training on proxies of trunk muscle
fitness (e.g., time in plank test) and athletic performance (e.g.,
5000-m run time) in trained individuals aged 16–44 years. The
authors observed that irrespective of the athletes’ expertise level
(i.e., recreational, sub-elite, elite athletes), core strength training
is an effective means to increase measures of trunk muscle fitness
(ES =1.07) but has no effects on proxies of athletic performance
(ES =0). Nevertheless, core strength training should be an
integral part of conditioning programs during all stages of LTAD
(Table 2) because it may enable youth athletes to withstand
the demands of long-term athletic training and competition
(Faigenbaum et al., 2016).
The exertion of eccentric training should be emphasized
during later LTAD stages (i.e., training to train) to lower the risk
of sustaining injuries in the muscle-tendon unit (Table 2). Of
note, adolescence may be regarded as a critical phase of tissue
plasticity in young growing athletes, as the adaptation process
of the muscle-tendon unit is affected by both, environmental
mechanical stimuli and maturational processes (Mersmann et al.,
2014). In fact, non-uniform development of muscle strength
and tendon mechanical and morphological properties have been
found in adolescent athletes which increase the risk of sustaining
overuse injuries (Mersmann et al., 2014). There is evidence that
specific eccentric stimuli have a positive effect on the balanced
development of muscle and tendon.
Frontiers in Physiology | www.frontiersin.org 12 May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
Table 2 contains a summary of the practical implications
of this scoping review for coaches and fitness professionals
as it illustrates a conceptual model containing adequate RT
types for the application of conditioning programs during the
stages of LTAD based on expert opinion and according to
Lesinski et al. (2016),Faigenbaum et al. (2016),Lloyd et al.
(2011, 2015),Balyi et al. (2013), and Kraemer and Fleck (2005).
In this context, balance training appears to be an important
preparatory (facilitating) training program during all stages of
LTAD but particularly during the early stages. As youth athletes
become more mature, specificity and intensity of RT methods
increase. During the last years, increased research efforts have
been accomplished to elucidate the effects of RT on muscular
fitness and athletic performance in youth athletes. Nevertheless,
with this scoping review we were able to identify research gaps
in the literature that should be addressed in future studies
(Table 3).
AUTHOR CONTRIBUTIONS
All authors listed, have made substantial, direct and
intellectual contribution to the work, and approved it for
publication.
FUNDING
This study is part of the research project “Resistance Training
in Youth Athletes” (http://www.uni-potsdam.de/kraftprojekt/
english.php) that was funded by the German Federal Institute of
Sport Science (ZMVI1-081901 14-18).
ACKNOWLEDGMENTS
The authors would like to thank Dr. Andrea Horn for her support
during the course of the research project.
REFERENCES
Araujo, C. G., and Scharhag, J. (2016). Athlete: a working definition for
medical and health sciences research. Scand. J. Med. Sci. Sports. 26, 4–7. doi:
10.1111/sms.12632
Armstrong, N., and McManus, A. M. (2011). “Physiology of elite young male
athletes,” in The Elite Young Athlete, eds N. Armstrong and A. M. McManus
(Basel: Karger), 1–22.
Armstrong, R., Hall, B. J., Doyle, J., and Waters, E. (2011). Cochrane update.
‘Scoping the scope’ of a cochrane review. J. Public Health 33, 147–150. doi:
10.1093/pubmed/fdr015
Balyi, I., Way, R., and Higgs, C. (2013). Long-Term Athlete Development.
Champaign: Human Kinetics.
Behm, D. G., Drinkwater, E. J., Willardson, J. M., and Cowley, P. M. (2010). The
use of instability to train the core musculature. Appl. Physiol. Nutr. Metab. 35,
91–108. doi: 10.1139/H09-127
Behm, D. G., Faigenbaum, A. D., Falk, B., and Klentrou, P. (2008). Canadian
society for exercise physiology position paper: resistance training in children
and adolescents. Appl. Physiol. Nutr. Metab. 33, 547–561. doi: 10.1139/H08-020
Behm, D. G., Muehlbauer, T., Kibele, A., and Granacher, U. (2015). Effects of
strength training using unstable surfaces on strength, power and balance
performance across the lifespan: a systematic review and meta-analysis. Sports
Med. 45, 1645–1669. doi: 10.1007/s40279-015-0384-x
Behm, D. G., and Sale, D. G. (1993). Velocity specificity of resistance training.
Sports Med. 15, 374–388. doi: 10.2165/00007256-199315060-00003
Behringer, M., Neuerburg, S., Matthews, M., and Mester, J. (2013). Effects of
two different resistance-training programs on mean tennis-serve velocity in
adolescents. Pediatr. Exerc. Sci. 25, 370–384.
Behringer, M., Vom, Heede, A., Yue, Z., and Mester, J. (2010). Effects of resistance
training in children and adolescents: a meta-analysis. Pediatrics 126, e1199–
e1210. doi: 10.1542/peds.2010-0445
Benson, A. C., Torode, M. E., and Fiatarone Singh, M. A. (2008). Effects
of resistance training on metabolic fitness in children and adolescents: a
systematic review. Obes. Rev. 9, 43–66. doi: 10.1136/bjsports-2011-090170
Bergeron, M. F., Mountjoy, M., Armstrong, N., Chia, M., Côté, J., Emery, C. A.,
et al. (2015). International olympic committee consensus statement on youth
athletic development. Br. J. Sports Med. 49, 843–851. doi: 10.1136/bjsports-
2015-094962
Chaouachi, A., Othman, A. B., Hammami, R., Drinkwater, E. J., and Behm,
D. G. (2014). The combination of plyometric and balance training
improves sprint and shuttle run performances more often than plyometric-
only training with children. J. Strength Cond. Res. 28, 401–412. doi:
10.1519/JSC.0b013e3182987059
Chelly, M. S., Fathloun, M., Cherif, N., Ben Amar, M., Tabka, Z., and Van Praagh, E.
(2009). Effects of a back squat training program on leg power, jump, and sprint
performances in junior soccer players. J. Strength Cond. Res. 23, 2241–2249. doi:
10.1519/JSC.0b013e3181b86c40
Christou, M., Smilios, I., Sotiropoulos, K., Volaklis, K., Pilianidis, T., and
Tokmakidis, S. P. (2006). Effects of resistance training on the physical
capacities of adolescent soccer players. J. Strength Cond. Res. 20, 783–791. doi:
10.1519/00124278-200611000-00010
Cohen, D. D., Voss, C., Taylor, M. J., Delextrat, A., Ogunleye, A. A., and
Sandercock, G. R. (2011). Ten-year secular changes in muscular fitness
in English children. Acta Paediatr. 100, e175–e177. doi: 10.1111/j.1651-
2227.2011.02318.x
DeRenne, C., Hetzler, R. K., Buxton, B. P., and Ho, K. W. (1996). Effects of training
frequency on strength maintenance in pubescent baseball players. J. Strength
Cond. Res. 10, 8–14. doi: 10.1519/00124278-199602000-00002
De Ste Croix, M. B., Armstrong, N., Welsman, J. R., and Sharpe, P. (2002).
Longitudinal changes in isokinetic leg strength in 10-14-year-olds. Ann. Hum.
Biol. 29, 50–62. doi: 10.1080/03014460110057981
de Villarreal, E. S., Kellis, E., Kraemer, W. J., and Izquierdo, M. (2009).
Determining variables of plyometric training for improving vertical jump
height performance: a meta-analysis. J. Strength Cond. Res. 23, 495–506. doi:
10.1519/JSC.0b013e318196b7c6
Faigenbaum, A. D., and Chu, D. A. (2001). Plyometric Training for Children and
Adolescents. Indianapolis: American College of Sports Medicine (ACSM) -
Current Comment.
Faigenbaum, A. D., Lloyd, R. S., MacDonald, J., and Myer, G. D. (2016). Citius,
Altius, Fortius: beneficial effects of resistance training for young athletes. Br. J.
Sports Med. 50, 3–7. doi: 10.1136/bjsports-2015-094621
Falk, B., and Tenenbaum, G. (1996). The effectiveness of resistance training in
children: a meta-analysis. Sports Med. 22, 176–186. doi: 10.2165/00007256-
199622030-00004
Ford, P., De Ste Croix, M., Lloyd, R., Meyers, R., Moosavi, M., Oliver, J., et al.
(2011). The long-term athlete development model: physiological evidence and
application. J. Sports Sci. 29, 389–402. doi: 10.1080/02640414.2010.536849
Glenmark, B., Hedberg, G., and Jansson, E. (1992). Changes in muscle fibre type
from adolescence to adulthood in women and men. Acta Physiol. Scand. 146,
251–259. doi: 10.1111/j.1748-1716.1992.tb09414.x
Granacher, U., Schellbach, J., Klein, K., Prieske, O., Baeyens, J. P., and Muehlbauer,
T. (2014). Effects of core strength training using stable versus unstable surfaces
on physical fitness in adolescents: a randomized controlled trial. BMC Sports
Sci. Med. Rehabil. 6:40. doi: 10.1186/2052-1847-6-40
Hammami, R., Granacher, U., Makhlouf, I., Behm, D. G., and Chaouachi, A.
(2016). Sequencing effects of balance and plyometric training on physical
Frontiers in Physiology | www.frontiersin.org 13 May 2016 | Volume 7 | Article 164

Granacher et al. Resistance Training in Youth Athletes
performance in youth soccer athletes. J. Strength Cond. Res. doi: 10.1519/jsc.
0000000000001425
Harries, S. K., Lubans, D. R., and Callister, R. (2012). Resistance training to improve
power and sports performance in adolescent athletes: a systematic review
and meta-analysis. J. Sci. Med. Sport 15, 532–540. doi: 10.1016/j.jsams.2012.
02.005
Klusemann, M. J., Pyne, D. B., Fay, T. S., and Drinkwater, E. J. (2012).
Online video-based resistance training improves the physical capacity of
junior basketball athletes. J. Strength Cond. Res. 26, 2677–2684. doi:
10.1519/JSC.0b013e318241b021
Knuttgen, H. G., and Kraemer, W. J. (1987). Terminology and measurement in
exercise performance. J. Appl. Sport Sci. Res. 1, 1–10. doi: 10.1519/00124278-
198702000-00001
Kraemer, W. J., and Fleck, S. J. (2005). Strength Training for Young Athletes.
Champaign: Human Kinetics.
Lephart, S. M., Abt, J. P., Ferris, C. M., Sell, T. C., Nagai, T., Myers, J. B.,
et al. (2005). Neuromuscular and biomechanical characteristic changes in high
school athletes: a plyometric versus basic resistance program. Br. J. Sports Med.
39, 932–938. doi: 10.1136/bjsm.2005.019083
Lesinski, M., Prieske, O., and Granacher, U. (2016). Effects and dose–response
relationships of resistance training on physical performance in youth athletes:
a systematic review and meta-analysis. Br. J. Sports Med. doi: 10.1136/bjsports-
2015-095497. [Epub ahead of print].
Lloyd, R. S., Faigenbaum, A. D., Stone, M. H., Oliver, J. L., Jeffreys, I., Moody,
J. A., et al. (2014). Position statement on youth resistance training: the 2014
International Consensus. Br. J. Sports Med. 48, 498–505. doi: 10.1136/bjsports-
2013-092952
Lloyd, R. S., Meyers, R. W., and Oliver, J. L. (2011). The natural development and
trainability of plyometric ability during childhood. Strength Cond. J. 33, 23–32.
doi: 10.1519/SSC.0b013e3182093a27
Lloyd, R. S., Oliver, J. L., Faigenbaum, A. D., Howard, R., De Ste Croix, M.
B., Williams, C. A., et al. (2015). Long-term athletic development- part
1: a pathway for all youth. J. Strength Cond. Res. 29, 1439–1450. doi:
10.1519/JSC.0000000000000756
McGuine, T. A., and Keene, J. S. (2006). The effect of a balance training program
on the risk of ankle sprains in high school athletes. Am. J. Sports Med. 34,
1103–1111. doi: 10.1177/0363546505284191
Mersmann, F., Bohm, S., Schroll, A., Boeth, H., Duda, G., and Arampatzis,
A. (2014). Evidence of imbalanced adaptation between muscle and tendon
in adolescent athletes. Scand. J. Med. Sci. Sports 24, e283–289. doi:
10.1111/sms.12166
Meylan, C., and Malatesta, D. (2009). Effects of in-season plyometric training
within soccer practice on explosive actions of young players. J. Strength Cond.
Res. 23, 2605–2613. doi: 10.1519/JSC.0b013e3181b1f330
Moir, G. L. (2012). “Muscular endurance,” in NSCA’s Guide to Tests & Assessments,
ed T. Miller (Champaign: Human Kinetics), 193–216.
Payne, V. G., and Isaacs, L. D. (2008). Human Motor Development.A Lifespan
Approach. New York, NY: McGraw-Hill Companies, Inc.
Payne, V. G., Morrow, J. R. Jr., Johnson, L., and Dalton, S. N. (1997). Resistance
training in children and youth: a meta-analysis. Res. Q. Exerc. Sport. 68, 80–88.
doi: 10.1080/02701367.1997.10608869
Pfeiffer, R., and Francis, R. (1986). Effects of strength training on muscle
development in prepubescent, pubescent and postpubescent boys. Phys. Sports
Med. 14, 134–143.
Prieske, O., Muehlbauer, T., Borde, R., Gube, M., Bruhn, S., Behm, D. G., et al.
(2016a). Neuromuscular and athletic performance following core strength
training in elite youth soccer: role of instability. Scand. J. Med. Sci. Sports. 26,
48–56. doi: 10.1111/sms.12403
Prieske, O., Muehlbauer, T., and Granacher, U. (2016b). The role of trunk
muscle strength for physical fitness and athletic performance in trained
individuals: a systematic review and meta-analysis. Sports Med. 46, 401–419.
doi: 10.1007/s40279-015-0426-4
Ramírez-Campillo, R., Andrade, D. C., Alvarez, C., Henriquez-Olguin, C.,
Martinez, C., Báez-Sanmartin, E., et al. (2014a). The effects of interset rest on
adaptation to 7 weeks of explosive training in young soccer players. J. Sports Sci.
Med. 13, 287–296.
Ramírez-Campillo, R., Meylan, C., Alvarez, C., Henriquez-Olguin, C., Martinez,
C., Canas-Jamett, R., et al. (2014b). Effects of in-season low-volume
high-intensity plyometric training on explosive actions and endurance
of young soccer players. J. Strength Cond. Res. 28, 1335–1342. doi:
10.1519/JSC.0000000000000284
Ramsay, J. A., Blimkie, C. J., Smith, K., Garner, S., MacDougall, J. D., and Sale, D.
G. (1990). Strength training effects in prepubescent boys. Med. Sci. Sports Exerc.
22, 605–614. doi: 10.1249/00005768-199010000-00011
Rhea, M. R. (2004). Determining the magnitude of treatment effects in strength
training research through the use of the effect size. J. Strength Cond. Res. 18,
918–920. doi: 10.1519/00124278-200411000-00040
Runhaar, J., Collard, D. C., Singh, A. S., Kemper, H. C., van Mechelen,
W., and Chinapaw, M. (2010). Motor fitness in Dutch youth: differences
over a 26-year period (1980-2006). J. Sci. Med. Sport. 13, 323–328. doi:
10.1016/j.jsams.2009.04.006
Saeterbakken, A. H., van den Tillaar, R., and Seiler, S. (2011). Effect of core stability
training on throwing velocity in female handball players. J. Strength Cond. Res.
25, 712–718. doi: 10.1519/JSC.0b013e3181cc227e
Sander, A., Keiner, M., Wirth, K., and Schmidtbleicher, D. (2013). Influence of a 2-
year strength training programme on power performance in elite youth soccer
players. Eur. J. Sport Sci. 13, 445–451. doi: 10.1080/17461391.2012.742572
Santos, E. J., and Janeira, M. A. (2011). The effects of plyometric training
followed by detraining and reduced training periods on explosive strength in
adolescent male basketball players. J. Strength Cond. Res. 25, 441–452. doi:
10.1519/JSC.0b013e3181b62be3
Schmidtbleicher, D. (2004). “Training for power events,” in Strength and Power in
Sport, ed P. V. Komi (Oxford: Blackwell Science Ltd.), 381–395.
Smith, J. J., Eather, N., Morgan, P. J., Plotnikoff, R. C., Faigenbaum, A. D., and
Lubans, D. R. (2014). The health benefits of muscular fitness for children and
adolescents: a systematic review and meta-analysis. Sports Med. 44, 1209–1223.
doi: 10.1007/s40279-014-0196-4
German Federal Statistical Office (2015). Population of Germany According to Age
Groups. (Wiesbaden: Federal Statistical Office).
Streckis, V., Skurvydas, A., and Ratkevicius, A. (2007). Children are more
susceptible to central fatigue than adults. Muscle Nerve 36, 357–363. doi:
10.1002/mus.20816
Tomkinson, G. R., Léger, L. A., Olds, T. S., and Cazorla, G. (2003). Secular trends
in the performance of children and adolescents (1980-2000): an analysis of 55
studies of the 20m shuttle run test in 11 countries. Sports Med. 33, 285–300. doi:
10.2165/00007256-200333040-00003
Vogler, C., and Bove, K. E. (1985). Morphology of skeletal muscle in children. an
assessment of normal growth and differentiation. Arch. Pathol. Lab. Med. 109,
238–242.
Weston, M., Hibbs, A. E., Thompson, K. G., and Spears, L. R. (2015). Isolated core
training improves sprint performance in national-level junior dwimmers. Int.
J. Sports Physiol. Perform. 10, 204–210. doi: 10.1123/ijspp.2013-0488
Winter, E. M., Abt, G., Brookes, F. B., Challis, J. H., Fowler, N. E., Knudson,
D. V., et al. (2016). Misuse of “power” and other mechanical terms in
sport and exercise science research. J. Strength Cond. Res. 30, 292–300. doi:
10.1519/JSC.0000000000001101
Conflict of Interest Statement: The authors declare that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2016 Granacher, Lesinski, Büsch, Muehlbauer, Prieske, Puta, Gollhofer
and Behm. This is an open-access article distributed under the terms of the Creative
Commons Attribution License (CC BY). The use, distribution or reproduction in
other forums is permitted, provided the original author(s) or licensor are credited
and that the original publication in this journal is cited, in accordance with accepted
academic practice. No use, distribution or reproduction is permitted which does not
comply with these terms.
Frontiers in Physiology | www.frontiersin.org 14 May 2016 | Volume 7 | Article 164