Friday, 21 February 2014


Coll. Antropol. 37 (2013) Suppl. 2: 219–225
Strength Training for Children and Adolescents:
Benefits and Risks
Davide Barbieri and Luciana Zaccagni
University of Ferrara, Department of Biomedical and Specialty Surgical Sciences, Ferrara, Italy
Physical activity has proved to be an effective means of preventing several diseases and improving general health. In
most cases, though, light to moderate efforts are suggested, for both youngsters and adults. Common sense advices call
for late inception of intense, strength training-related activities, like weight lifting and plyometrics, which are usually
postponed at the end of the growth age, even among sport practitioners. However, such advices seem to have a mainly anecdotal
nature. The purpose of this review is to evaluate risks and benefits of early inception of strength training, at adolescence
or even earlier, and to verify whether concerns can be grounded scientifically. Current literature does not seem to
have any particular aversion against the practice of strength training by children and adolescents, provided that some
safety rules are followed, like medical clearance, proper instruction from a qualified professional and progressive overload.
At the same time, several studies provide consistent findings supporting the benefits of repeated, intense physical efforts
in young subjects. Improved motor skills and body composition, in terms of increased fat free mass, reduced fat
mass and enhanced bone health, have been extensively documented, especially if sport practice began early, when the subjects
were pubescent. It can be therefore concluded that strength training is a relatively safe and healthy practice for children
and adolescents.
Key words: strength training, weight lifting, adolescents, growth, body composition
Modern Western societies imply increasingly sedentary
life styles and reduced physical exercise. Technological
progress, limited outdoor activities and economic improvement
have modified dietary habits and reduced the
amount of exercise performed by children and adolescents1.
It is well known that regular moderate intensity
physical activity – such as walking, cycling, or participating
in sports – has significant benefits for health. According
to the 2008 guidelines of the European Commission2,
school-aged youth should participate in moderate to vigorous
daily physical activity for 60 minutes or more. Obesity,
sedentary lifestyle and poor cardio-respiratory fitness
in childhood and adolescence may increase the risk
of health problems later in life. The teenage years bring
many physical, social and psychological changes for the
individual. From infancy to adulthood, growth, maturation
and development occur simultaneously and interact:
growth consists in the increase of the size of the body as a
whole and of its parts, maturation refers to progress towards
the biologically mature state and development refers
to the acquisition of behavioral competence3.
Changes in body dimensions and composition during
growth and maturation are factors affecting strength
and motor performance4. Some changes in anthropometric
traits and strength in a sample of Italian adolescents
studied by Gualdi-Russo and Toselli5 are reported in Figures
The strength and motor performance varies during
childhood and adolescence in relation to biological and
environmental factors. Among biological factors the specific
contribution of maturity status is apparent: the
strength advantage of early-maturing subjects is related
to their larger body size in comparison to late-maturing
ones. These differences are more marked in boys than in
girls. Regular physical activity is an important factor
during growth and maturation, regulating body weight
and, particularly, fatness.
Received for publication October 15, 2011

Strength Training: Concepts and Objectives
Strength training is a form of physical activity, usually
structured and planned, involving intense efforts
against a resistance. Its main aim is to increase muscular
strength, in order to improve performance, at least in
case a sport is practiced. It is extensively adopted in
power-oriented sports, like sprinting6 and soccer7, even if
its benefits are recognized also in endurance sports, like
long distance running8,9 and cross country skiing10,11. In
a non-competitive environment instead, strength training
is adopted for many different purposes. For example,
strength training may be used to improve overall fitness,
increasing muscle hypertrophy and reducing body fatness
at the same time. In fact, strength training can be
an effective means to improve body composition12. In
other cases, some individuals may adopt it in order to accomplish
some professional goal, like achieving the degree
of physical fitness which is required in the military
or to join the fire brigade.
To train strength, muscular force is applied against
some kind of resistance. In most cases, especially when
the individual is healthy, resistance is provided by free
weights, like barbells, dumbbells or the athlete’s own
body weight, or by weight machines, like the leg press,
the lat machine etc. This kind of training is usually
adopted in sport conditioning, because the load can be increased
progressively according to the athlete’s strength,
which can be considerable. Athletes employ gravity also
in other ways in order to improve their performances,
like in plyometrics or high-impact training, where body
mass is accelerated dropping from a pre-determined
height, according to the athlete’s ability and conditioning
level. This kind of strength training is usually considered
the most dangerous, because the real impact forces applied
to the athlete’s body (bones, muscles, tendons, ligaments
etc.) are not easily measured, as in weight lifting.
Since force is defined as mass times acceleration, we can
say that weights mainly focus on the first factor, while
plyometrics relies on the second to increase force. Nonetheless,
also weights can be accelerated, in order to increase
force production without adding kilograms, and
advanced plyometrics may imply added weight by means
of weighted belts or vests.
It must be considered, though, that similar strength
training effects can be found in sport practices other
than weight lifting or plyometrics, like sprinting, gymnastics
and other kinds of power-oriented sports, or team
sports involving leaping and bouncing, like volleyball and
basketball. These types of physical efforts produce great
acceleration, which, applied to the athlete’s body mass,
produce great force. Nonetheless, these intense efforts
are usually practiced by children, even outside a sport
environment, simply while playing with their peers.
Strength training has in important role in rehabilitation
after injuries, especially those which involve surgery
and/or a long period of immobilization, in order to re-gain
the physiological muscle hypertrophy and joint range of
motion13–18. In case of injuries to lower limbs, when the
patient is still lying in bed, body weight can be excessive
and not suitable for post-surgery rehabilitation. Therefore,
non-bodyweight bearing exercises can be used, by
means of cables and/or small weights, attached to the ankles
of the patient, like in leg raises and knee extensions.
Body weight can be excessive also for healthy individuals
who have a low relative strength, that is a low
strength-to-body weight ratio. A push up, a pull up or
even a body weight squat can be a demanding task for
people who are too young, elderly, overweight or out of
shape. Free weights or machines can provide a controlled
and adjustable source of resistance. For example, a push
up can be effectively substituted by a bench press, a pull
up by a lat pull down using a lat machine, a body weight
squat by a leg press, involving more or less the same
muscle groups. Weights can be adapted to the individuals’
actual strength, which may be relatively low compared
to their own body weight.
Other kinds of resistance than weights may be applied
in order to increase muscular strength, like elastic
bands, or friction, as in water or on a steady bike. In fact,
gravity is not necessarily present (e.g. astronauts during
space missions are at risk of losing considerable amounts
of muscle mass19,20) or not fully applicable. Orthopedics
patients may have access to a swimming pool, where the
weight-bearing effort of an injured knee, ankle or hip can
be reduced. At the same time, also competitive swimmers
may use swim paddles to increase the resistance provided
by water.
Exercises are usually performed in sets of several repetitions
(i.e. consecutive lifts). If heavy loads are employed,
providing stimulus for maximal strength, then
repetitions are necessarily low in numbers. When the
load is moderate, in order to improve body composition
and cardiovascular fitness, then the overall number of
repetitions can be considerably high. The main training
parameters are intensity and volume. Intensity is given
as percentage of the maximal load which can be lifted for
the prescribed number of repetitions: 1 repetition-maximum
(RM) is the load which can be lifted just once, 10
RM is the load which can be lifted 10 times within one
set. Strength training implies relatively heavy loads, between
60% and 100% of 1 RM, the so-called »strength
training zone«21. For example, the 90% of 1 RM is a quasi
maximal load, allowing for small volume (i.e. low repetitions).
Volume is the total number of repetitions per exercise.
For example, performing 3 sets of 10 repetitions in
one given exercise determines a volume of 30 repetitions.
D. Barbieri and L. Zaccagni: Resistance Training before and during Adolescence, Coll. Antropol. 37 (2013) Suppl. 2: 219–225
The most common strength training exercises are
listed in Table 1, with the discipline in which they are
usually practiced, even if in most cases athletes involved
in different sports may use a blend of them. This is especially
true in body building, where the overall balanced
development of muscle mass is of great importance.
Therefore, body builders use most of the listed exercises
(and even more than those), while strength training for
athletes usually comprises a small set of exercises, like
the clean, the squat and the bench press, involving most
skeletal muscles in a coordinated fashion.
Benefits and Risks of Strength Training
for Children and Adolescents
For reasons which have been mainly reported anecdotally,
strength training, especially if involving weight
lifting, has been considered dangerous for children and
adolescents, and at risk of limiting their growth. However,
the American College of Sports Medicine highlights
that there is no current scientific evidence of the fact
that strength training and weight lifting are inherently
dangerous or can restrain the growth of children and adolescents.
Like any other kind of sport practice, there are
some risks which can be considerably diminished following
a small set of suggestions: proper supervision form
an expert adult, warm up and stretching before lifting,
focus on proper form rather than load, gradual resistance
increases as technique, strength and control improve22.
The American Academy of Pediatrics gives comparable
guidelines, implying that strength training can be
safe and effective for children and adolescents, provided
that medical clearance is granted. At the same time, it
discourages them from practicing sports, like Olympic
weightlifting and powerlifting, which involve maximal
A similar position has been taken by the National
Strength and Conditioning Association, which is in favor
of supervised and appropriately prescribed strength
training for both pre-adolescents and adolescents26.
In strength training, the gains in muscular strength
are often associated with improvements in body composition.
In a study by Faigenbuam et al.27 a group of boys
and girls aged between 8 and 12 followed a twice-a-week
resistance training program for 8 weeks. After warm up
and stretching, the training group performed the following
5 exercises: leg extension, leg curl, bench press, overhead
press and biceps curl. Both training and control
groups continued physical education at school. As expected,
strength gains in the training group were significant
compared to both pre-training and control. Also improvements
in body composition were significant:
skinfold thickness decreased of 2.3% on average, compared
to an increase of 1.7% in control group. It is interesting
to note that upper arm, chest and hip girths did
not change significantly. The only exception was the
thigh girth, which anyway increased relatively less than
control (+2.4% vs. +3.9%).
The volume-intensity schema adopted was the popular
Delorme method: 3 sets of 10 repetitions each, the
first one with 50% of 10 RM, the second one with 75% of
10 RM and the third one with 100% of 10 RM. Delorme
was among the first physicians who realized the importance
of strength training – and weight lifting in particular
– in rehabilitation after injuries28.
A similar pyramiding method was adopted in a study
by Schwingshandl et al.29. Obese children and adolescents
were prescribed a diet with caloric restriction. Unfortunately,
diet alone may reduce both fat and fat free
mass. Subjects were therefore divided into 2 groups:
training and control. After some light aerobics and stretching
as a warm up, the training group performed 3 to 4
sets, 10 repetitions each, of the prescribed exercises,
which were chosen to involve all major muscle groups.
The first set was performed using the 50% of 10 RM.
Load was increased progressively in each set, until muscle
failure because of fatigue. When the child was able to
complete more than the prescribed 10 repetitions in the
last set, the load was increased in the following training
session. After 12 weeks, weight change was not significant
in both groups, while the increase in fat free mass
was significantly higher in the training group than in
control, implying that resistance training may have a
positive effect on body composition in fat reduction programs
for obese children and adolescents.
Supervised strength training, involving weight lifting
(bench press, leg extension, lat pull down etc.) and stretching,
after an adequate warm up, has proved to be effective
in a group of children, males and females, increasing
strength, reducing skinfold thickness, improving
body composition, motor skills and flexibility30.
In a study by Watts et al.31 obese adolescents were involved
for 8 weeks in a strength training program consisting
in 1 hour of circuit training, 3 times per week, including
both cycle ergometer and resistance training.
Since the program was primarily designed to treat obesity
rather than improving strength, exercise intensity
was kept between 55–70% of pre-training 1 RM. Training
reduced abdominal and trunk fat, thus diminishing cardiovascular
and metabolic risks, and increased strength,
body composition and overall fitness at the same time.
Even if the main purpose of strength training is to increase
muscle strength, it seems to have a positive carry
D. Barbieri and L. Zaccagni: Resistance Training before and during Adolescence, Coll. Antropol. 37 (2013) Suppl. 2: 219–225
Powerlifting Body
Body weight
Clean and
Bench press
Biceps curls
Leg extensions
Leg curls
Push ups
Pull ups
Parallel dips
Body weight
One-leg squats
Sit ups
over also in bone density and therefore it qualifies as an
interesting means for preventing and reducing osteoporosis.
This is particularly true for children: if strength
training is adopted early, bone mass gains last longer.
Skeletal exposure to mechanical loading during growth
seems to be an effective strategy to increase bone mass
and density, according to Khan et al.32. In a study by
Fuchs et al.33, high impact training is used to verify its efficacy
in improving skeletal mass in a group of elementary
school children. Bone mineral content, bone area
and bone mineral density were adopted as indices of bone
health. The training protocol consisted in 100 drop
jumps form a 61 cm box, 3 times per week for 7 months,
implying ground reaction forces up to 8 times body
weight. However, the adopted method proved to be safe
and effective in improving the above mentioned parameters
at the femoral neck and lumbar spine. Actually, in a
popular sport like gymnastics, impact forces in drop
landings range from 8.2 to 11.6 times body weight, according
to a study by Ozguven and Berme34.
Even if the authors say that the program could be introduced
in physical education classes, its main limitation
may be in the fact that high-impact training may result
in an excessive effort for overweight children. Still,
in the training group no injuries occurred during the
whole duration of the study. Actually, selected children
had to be within the 20% of the recommended weight for
height and age. The benefits at the femoral neck persisted
even after several months of detraining, when the
same bone health parameters were re-assessed in both
exercise and control group35.
Significant positive effects of impact training on bone
mineral content at the hip was also found by Gunter et
al.36 in a longitudinal study. The benefits of 7 months of
impact training on a group of school children were partially
maintained up to 8 years later.
Osteoporosis is a major problem especially for adult
women. Even if considerable improvements in terms of
bone health can be assessed in adults engaging in some
form of strength training, the benefits do not seem to
persist as long as in children or adolescents, suggesting
that early inception of intense physical exercise may be
prescribed for long-lasting improvements. A study by
Winters and Snow37 assessed bone mineral density in a
group of females aged 30–45, before and after a 12 month
training period. The training program included both
high impact and resistance training (squats, lunges and
calf raises). Drop jumps off a box generated ground reaction
forces of 4 to 5 times body weight. Intensity was
gradually increased using weighted vests. After the training
period, exercisers improved their bone mineral density
and strength significantly, with respect to both baseline
(pre-training) and control values. Unfortunately
though, after 6 months of detraining, values decreased
significantly towards baseline values.
A study by Kannus et al.38 evaluated the effects of
playing starting age on bone mineral content of the dominant
arm in a group of female tennis players. Athletes
had a significantly higher difference in bone mineral content
between dominant and non-dominant arm compared
to control. The difference was 2 to 4 times greater
in individuals who had started playing tennis before or at
menarche, compared to those who had started 15 years
after menarche. Tennis resembles strength training and
may carry over similar effects on the bones since it consists
of ballistic and explosive movements, handling a
light implement. Even if the involved masses are small
(ball and racquet), the acceleration produced during the
impact may be very large, producing great force against
the dominant arm.
Similar positive effects on bone mineral density of female
gymnasts were found by Proctor et al.39 in the
whole body and in particular in the upper limbs, without
any significant bilateral differences, which is a major advantage
compared to tennis. Gymnastics exercises, like
pull ups and ring or parallel dips, are often employed in
body weight strength training, for their carry over to upper
body muscle strength.
Swimming and cycling are among the most popular
sports and bring several health benefits. However, bones
seem to be less directly addressed by these activities, because
of their non-weight-bearing nature, which limits
the loading on the skeleton. A group of well trained adolescent
females (track and field athletes, gymnasts and
water polo players) were assessed by Greene et al.40. Although
all the selected sports require intense physical
work, gymnastics involves weight-bearing in both the upper
and lower body, track-and-field (sprints and jumps)
only in the lower body, and water polo has no weight-
-bearing component. Water polo players did not show
greater bone strength or muscle size in the lower leg
compared to controls. On the contrary, gymnasts showed
significantly greater bone strength than non active females.
Also track-and-field athletes displayed greater
bone strength in the lower leg, compared to controls. The
gymnasts showed the greatest musculoskeletal benefits
in the upper body. Despite intense training, water polo
players showed no significant benefits in musculoskeletal
health in the lower body and only limited benefits in
the upper body when compared with non active girls.
Ferry et al.41 investigated bone mineral density in female
adolescent soccer players, swimmers and control
group. Bone mineral density was significantly higher in
soccer players compared with swimmers. In contrast,
swimmers had weaker bones than controls, despite the
fact that female swimmers cannot be considered sedentary
Effects of strength training on connective tissues (ligaments
and tendons in particular) have not been as
widely assessed as those on bones. However, a recent
study42 has found a positive correlation between resistance
training (in particular Olympic weightlifting) and
cruciate ligaments’ cross sectional areas. The authors
conclusions are that the benefits were induced by early
inception of heavy training at the age of puberty.
D. Barbieri and L. Zaccagni: Resistance Training before and during Adolescence, Coll. Antropol. 37 (2013) Suppl. 2: 219–225
Discussion and Conclusions
An meaningful distinction should be made between
weight lifting for strength training and Olympic weightlifting.
The latter implies competitions in which maximal
or even supra-maximal (when the lift fails) loads are employed,
as in powerlifting. In strength training instead,
sub-maximal weights, which can be lifted more than
once, are used. This distinction may account for a different
risk factor between the aforementioned disciplines.
In general, whenever a maximal effort is required, as in
competitive sport, it is believed that risks tend to be present
in a higher percentage than in recreational activities.
More specifically, even if strength training may be strenuous
and intense, if no maximal loads are employed, than
it can be considered a safe and effective form of physical
activity for most individuals, including children and adolescents,
provided that proper instruction and supervision
are given.
However, a study by Hamill43 questions the common
belief that resistance training is safer than Olympic
weightlifting, since both appear to be relatively safe according
to his findings, especially if compared to other
sports. The surveyed subjects were UK students, aged 13
to 16. Practicing both Olympic weightlifting and weight
training had an injury rate of only 0.0012 per 100 participation
hours. Individually, both disciplines scored well
below other popular British sports, like soccer, rugby or
even athletics.
In a study by Risser et al.44 muscle strain, a non-disabling
injury, was reported to be the most common accident
among high school American football players practicing
weight lifting as a form of strength training. The
cumulative percentage of injuries among all athletes was
a reasonable 7.6%, corresponding to 0.082 injuries per
person/year. Much higher rates can be found in adolescent45
or amateur46 soccer players. However, the study
did not specify whether injuries were caused by maximal
lifts (i.e. excessive load) or poor form, as it may happen in
a competitive environment, where fatigue and strive for
performance may lead to an excessive demand on the
athlete's physical capabilities.
The topic of growth and strength training could be
further assessed from an endocrine point of view, considering
the relationship between exercise and hormonal responses.
A review by Kraemer and Ratamess47 highlights
the well established finding that resistance training and
growth hormone are positively correlated, but further research
is needed in order to verify whether strength
training could induce positive endocrine responses in adolescents.
In conclusion, early inception of strength training, at
adolescence or even earlier, does not seem to imply higher
risks than other popular sport disciplines, provided
that the young athletes follow the aforementioned guidelines.
In particular, supervision by an expert instructor,
focus on proper technique and cautious progression in increasing
loads are the most common advices which must
be adhered to. On the positive side, resistance training
has proved to increase basic motor skills, like muscle
strength, coordination and flexibility, but also body composition,
in terms of improved fat free to fat mass ratio
and increased bone health.


Published: 20 February 2014

Abstract (provisional)


A plasma glucose value >=155 mg/dl for 1-hour post-load plasma glucose during an oral glucose tolerance test (OGTT) is able to identify subjects with normal glucose tolerance (NGT) at high-risk for type-2 diabetes and with subclinical organ damage. We designed this study to address if 25-hydroxyvitamin D [25(OH)D] circulating levels are associated with glucose tolerance status, and in particular with 1-hour post-load plasma glucose levels.


We enrolled 300 consecutive Caucasian hypertensive never-treated outpatients (160 men and 140 women, aged 52.9 +/- 9.2 years) Subjects underwent OGTT and measurements of 25(OH)D and standard laboratory tests. Estimated glomerular filtration rate (e-GFR) was calculated by CKD-Epi formula and insulin sensitivity was assessed by Matsuda-index.


Among participants, 230 were NGT, 44 had impaired glucose tolerance (IGT) and 26 had type-2 diabetes. According to 1-h post-load plasma glucose cut-off point of 155 mg/dL, we divided NGT subjects into: NGT < 155 (n = 156) and NGT > 155 mg/dL (n = 74).
NGT >= 155 had higher significant fasting and post-load glucose and insulin, parathyroid hormone and hs-CRP levels than NGT < 155. On the contrary, Matsuda-index, e-GFR, and 25(OH)D were significantly lower in NGT > 155 than NGT < 155 subjects. In the multiple regression analysis, 25(OH)D levels resulted the major determinant of 1-h post-load plasma glucose in all population and in the four groups of glucose tolerance status. In the whole population, Matsuda-index, hs-CRP and e-GFR explained another 12.2%, 6.7% and 1.7% of its variation.


Our data demonstrate a significant and inverse relationship between 25(OH)D levels and glucose tolerance status, particularly with 1-h post-load glucose.