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Muscle strength and endurance Muscle strength is defined as the highest
force generated during a single maximum voluntary contraction, whereas
muscle endurance is the ability to perform repeated muscular contraction
and force development over a period of time. Muscle strength and endur-
ance are correlated, especially at higher levels of force production. Muscle
strength is proportional to the cross-sectional area of skeletal muscle; con-
sequently, strength growth curves parallel growth curves for body weight
and skeletal muscle mass (Malina et al., 2004).
Both males and females show impressive increases in muscle strength
from childhood to adolescence. Strength in children increases linearly, with
boys having a slight advantage over girls. However, these sex differences
are magnified during the adolescent years as a result of maturation (Malina
and Roche, 1983). Differences in muscle strength between boys and girls
become more apparent after puberty, primarily as a result of the production
of sex steroid hormones. In boys the increase in strength during adolescence
lags behind the growth spurt by at least a year (peak height velocity), which
may explain why some boys experience a brief period of clumsiness or awk-
wardness during puberty, as they have not yet acquired the muscle strength
necessary to handle the changes associated with their larger bodies. Muscle
strength increases at its greatest rate approximately 1 year after peak height
velocity in boys, whereas for girls the strength spurt generally occurs during
the same year as peak height velocity (Bar-Or, 1983).
A compelling body of evidence indicates that with resistance training
children and adolescents can significantly increase their strength above that
expected as a result of normal growth and maturation, provided that the
training program is of sufficient intensity, volume, and duration (Committee
on Sports Medicine Fitness, 2001). Both boys and girls can benefit, and
strength gains in children as young as 5-6 have been reported (Faigenbaum
et al., 2009), although most studies are of older children and adolescents.
Gains in muscle strength of about 30 percent are typical, although con-
siderably larger gains have been reported. Adolescents make greater gains
than preadolescents in absolute strength, whereas reported relative (per-
cent above initial strength) gains in strength during pre adolescence and
adolescence are similar. A variety of programs and modalities have proved
Copyright © National Academy of Sciences. All rights reserved.
Educating the Student Body: Taking Physical Activity and Physical Education to School
Relationship to Growth, Development, and Health
121
efficacious (Council on Sports Medicine Fitness, 2008), as long as load
(~10-15 repetitions maximum) and duration (~8-20 weeks) are adequate.
As in adults, training adaptations in youth are specific to the muscle action
or muscle groups that are trained, and gains are transient if training is not
maintained (Faigenbaum et al., 2009).
Youth resistance training, as with most physical activities, does carry
some degree of risk of musculoskeletal injury, yet the risk is no greater than
that associated with other sports and activities in which children and ado-
lescents participate (Council on Sports Medicine Fitness, 2008; Faigenbaum
et al., 2009) as long as age-appropriate training guidelines are followed.
A traditional area of concern has been the potential for training-induced
damage to growth cartilage, which could result in growth disturbances.
However, a recent review found no reports of injury to growth cartilage
in any prospective study of resistance training in youth and no evidence
to suggest that resistance training negatively impacts growth and matura-
tion during childhood and adolescence (Faigenbaum et al., 2009). Injuries
typically occur in unsupervised settings and when inappropriate loads and
progressions are imposed.
In addition to the obvious goal of gaining strength, resistance training
may be undertaken to improve sports performance and prevent injuries,
rehabilitate injuries, and enhance health. Appropriately supervised pro-
grams emphasizing strengthening of trunk muscles in children theoretically
benefit sport-specific skill acquisition and postural control, although these
benefits are difficult to study and thus are supported by little empirical
evidence (Council on Sports Medicine Fitness, 2008). Similarly, results are
inconsistent regarding the translation of increased strength to enhanced
athletic performance in youth. Limited evidence suggests that strength-
training programs that address common overuse injuries may help reduce
injuries in adolescents, but whether the same is true in preadolescents is
unclear (Council on Sports Medicine Fitness, 2008). Increasing evidence
suggests that strength training, like other forms of physical activity, has
a beneficial effect on measurable health indices in youth, such as cardio-
vascular fitness, body composition, blood lipid profiles and insulin sensi-
tivity (Faigenbaum, 2007; Benson et al., 2008), bone mineral density and
bone geometry (Morris et al., 1997; MacKelvie et al., 2004), and mental
health (Holloway et al., 1988; Faigenbaum et al., 1997; Annesi et al., 2005;
Faigenbaum, 2007). Some work has shown that muscle fitness, reflected in
a composite index combining measures of muscle strength and endurance,
and cardiorespiratory fitness are independently and negatively associated
with clustered metabolic risk (Steene-Johannessen et al., 2009). Moreover,
children with low muscle strength may be at increased risk of fracture with
exercise (Clark et al., 2011). Finally, muscle hypertrophy, which adds to
fat-free mass, contributes to resting metabolic rate and therefore total daily
Copyright © National Academy of Sciences. All rights reserved.
Educating the Student Body: Taking Physical Activity and Physical Education to School
122
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