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6
Health-Related Fitness Measures for
Youth: Musculoskeletal Fitness
KEY MESSAGES
Musculoskeletal fitness is a multidimensional construct comprising the
integrated function of muscle strength, muscle endurance, and muscle
power. The link between musculoskeletal fitness and health in adults has
extended beyond low-back health to other outcomes, such as personal
independence and quality of life, cardiovascular disease, risk of fracture,
and cognitive and functional ability. Although the relationship between
musculoskeletal fitness and these health outcomes in youth is not as
extensively or specifically studied as that in adults, this chapter summa-
rizes what is currently known about this relationship in youth.
A thorough review of the literature revealed a lack of high-quality stud-
ies supporting a strong link between any specific musculoskeletal fitness
test item and health outcomes in youth. This lack of evidence precluded
the identification of any specific musculoskeletal fitness test items for
inclusion in a national fitness survey for the general population of youth.
Nonetheless, based predominantly on evidence indicating a relationship
between musculoskeletal fitness and health outcomes in adults, the com-
mittee concluded that musculoskeletal fitness should be assessed in a
national youth fitness survey. The handgrip strength and standing long
jump tests (to measure upper- and lower-body musculoskeletal strength,
respectively) should be included in such a survey based on their limited
link to health and their acceptable validity, reliability, and feasibility. These
tests should not, however, be interpreted in a health context until their
153
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154 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH
relationships with health outcomes have been established more firmly in
youth.
Limitations of the studies reviewed include that studies were not
designed to answer questions about the relationship between the fit-
ness tests studied and health, that interventions were inadequate, or
that confounders were not considered. Although effects of age, gender,
body composition, maturation status, and ethnicity on performance on
the various tests have been suggested in the past, this review provided
insufficient data for assessing the influence of such modifiers.
For school and other educational settings, administrators should con-
sider the handgrip strength and standing long jump tests, as well as
alternative tests that have not yet been shown to be related to health
but are valid, reliable, and feasible. The modified pull-up and the push-
up are possible alternatives for measuring upper-body musculoskeletal
strength and power. The curl-up could also be considered for measuring
an additional construct, core strength.
In the absence of criterion-referenced cut-points (cutoff scores) for
youth or adults, interim cut-points corresponding to the lower percentile
limit (20th percentile) should be used for tests of musculoskeletal fitness,
analogous to the cut-points for cardiorespiratory endurance, until better
evidence for criterion-referenced health-related cut-points is established
by further research.
T
he functions and capacities of the neuromuscular and musculoskele-
tal systems play important roles in defining the physical fitness status
of individuals and populations. Assessment of musculoskeletal fitness
has traditionally included assessment of muscle strength, muscle endurance,
flexibility, and bone health (Bouchard et al., 2007). With increasing inter-
est in and study of the role of muscle power in the elderly, it is likely that
muscle power will emerge as another important characteristic of musculo-
skeletal fitness worthy of inclusion in future youth fitness assessments (Ashe
et al., 2008; Bonnefoy et al., 2007; Reid and Fielding, 2012).
This chapter addresses musculoskeletal fitness (muscle strength, endur-
ance, and power) as it relates to health markers in youth; the flexibil-
ity component of musculoskeletal fitness is considered in Chapter 7. The
committee's recommendations for selection of musculoskeletal fitness tests
are based primarily on an extensive review of the literature provided by
the Centers for Disease Control and Prevention (CDC). The CDC search
strategy and data extraction procedures are described in Chapter 3. To
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MUSCULOSKELETAL FITNESS 155
make its recommendations on this fitness component, in addition to pro-
viding evidence for a relationship to health, the committee considered the
scientific integrity (reliability and validity) of putative health-related test
items, as well as the administrative feasibility of implementing these items.
The committee also offers recommendations for setting cut-points (cutoff
scores) for interpretation of performance on musculoskeletal fitness tests.
Recommendations regarding specific tests for measuring musculoskeletal
fitness for national surveys and in schools and other educational settings
are presented in Chapter 8 and 9, respectively. Future research needs are
addressed in Chapter 10.
DEFINITIONS
Musculoskeletal fitness is a multidimensional construct comprising
the integrated function of muscle strength, muscle endurance, and muscle
power to enable the performance of work against one's own body weight
or an external resistance. No single measure of any of these dimensions
adequately describes an individual's overall level of musculoskeletal fitness;
rather, each of these dimensions must be assessed individually, compared
with appropriate performance or health standards, and then interpreted
in an integrated and unified assessment of overall musculoskeletal fitness.
Muscle strength is the ability of skeletal muscle (single or group) to
produce measurable force, torque, or moment about a single or multiple
joints, typically during a single maximal voluntary contraction and under
a defined set of controlled conditions, which include specificity of move-
ment pattern, muscle contraction type (concentric, isometric, or eccentric),
and contraction velocity (Farpour-Lambert and Blimkie, 2008; Kell et al.,
2001; Sale and Norman, 1982). In youth fitness assessments, this definition
usually applies to the production of maximal muscle force during a single
maximal voluntary effort on a particular strength test. Some musculo
skeletal fitness tests, however (e.g., the pull-up test), allow the completion
of more than one near-maximal effort (e.g., two to three repetitions) and
have traditionally also been considered tests of muscle strength. Strength
is typically measured as force registered by a dynamometer (e.g., handgrip
dynamometer) or a measurable external load resisted or moved against
(e.g., weight machine or external weight).
Muscle endurance is the ability of a muscle or group of muscles to per-
form repeated contractions against a constant external load for an extended
period of time (Kell et al., 2001). The constant load can be either an abso-
lute external resistance, which provides a measure of absolute endurance, or
a relative load based on an individual's maximal strength, which provides
a measure of relative endurance. In youth fitness assessments, this defini-
tion applies to voluntary submaximal efforts of variable force production
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156 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH
and speed by a muscle or group of muscles during performance on a wide
variety of tests. Muscle endurance is typically measured as elapsed time or
number of paced or nonpaced repetitions of the muscle action within either
a specified or unrestricted time period.
Muscle power is a physiological construct reflecting the rate at which
work is performed (Knuttgen and Kraemer, 1987). It is derived from the
product of the force production of a muscle or group of muscles and the
velocity of the muscle contraction during a single- or multijoint action (Sale
and Norman, 1982). Muscle power is a complex construct consisting of
several subdomains, including average, peak, instantaneous, and contractile
power (Moffroid and Kusiak, 1975). In youth fitness testing, different field
tests probably assess different subdomains of muscle power, although the
specific associations between individual fitness tests and the power sub
domains are poorly defined. Peak muscle power is dependent on the velocity
of the action and is inversely related to the external resistance against the
action. Peak power is typically generated within the range of 40-90 percent
of peak external resistance, or approximately 70 percent of an individual's
one repetition maximum (1RM) (Reid and Fielding, 2012), and at submaxi-
mal velocity. Muscle power then can be defined as the product of force and
velocity during execution of a maximal voluntary effort against a submaxi-
mal external resistance, and it can be measured directly in two ways: by set-
ting a series of constant-velocity efforts and measuring muscle force at each
velocity, or by setting a series of constant loads and measuring the velocity at
each load, with power expressed in watts (W) being the product of force and
velocity for each series effort. In practice, in youth fitness testing the veloc-
ity is either controlled or uncontrolled, and the external resistance is either
the body weight or a resistance that is set below the peak force-producing
capacity of the muscles involved in the action.
Field tests of muscle power typically involve assessment of upper-body
(throwing distance) or lower-body (vertical squat jumps, vertical counter
movement jumps, or long jump) muscle function, and usually measure
height or distance covered. Performance on these tests is directly related to
the attained velocity, which is proportional to the force generated during
the action and provides an indirect measure of muscle power. Field tests
of muscle power have been included as a surrogate measure of muscle
strength even though physiologically, this extrapolation is valid only if
the action is performed at a constant velocity, which is rarely the case in
the field. For this review, the committee considered only power tests that
incorporate a single maximal effort at a submaximal velocity and load (e.g.,
vertical or horizontal jumping tests). These tests require a high degree of
neuromechanical coordination and are less dependent on the biochemical
endurance capacities of the muscles compared with one of the most com-
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MUSCULOSKELETAL FITNESS 157
mon measures of anaerobic power, the Wingate Anaerobic Test. Because of
its unique physiological and neuromechanical characteristics, muscle power
is considered one of three dimensions of musculoskeletal fitness in youth
fitness assessments.
MUSCULOSKELETAL FITNESS TESTS
A plethora of fitness test batteries and items have been used over the
past 55 years to assess musculoskeletal fitness in youth (see Table 2-6 in
Chapter 2) (Castro-Piñero et al., 2010). The tests vary in their specific
protocols, some purportedly assessing the muscle fitness of specific body
regions (upper and lower body, trunk, abdomen, lower back) and some
measuring isolated muscular function (e.g., muscle strength, endurance, or
power) or combined strength and endurance function.
Since the mid-1970s, there has been growing interest in and develop-
ment of health-related musculoskeletal fitness test batteries that have been
based largely on theoretical construct validity and on health data from the
adult population (AAHPERD, 1984; Jackson, 2006; Morrow et al., 2009;
Plowman, 2008). The American Alliance for Health, Physical Education,
Recreation and Dance (AAHPERD) Health Related Physical Fitness Test,
the first of many subsequent international fitness test batteries to claim
assessment of health-related fitness in youth, included the modified, timed
(1-minute) sit-up as the sole measure of musculoskeletal fitness.
More than 11 different classes of fitness test items have since been used
to assess the muscle strength, endurance, or power dimensions of muscu-
loskeletal fitness, many of them evaluating similar dimensions (Table 6-1).
For example, there are several variations on the pull-up test of differing
durations (no time limit, 30- or 60-second limit), with different anatomical
alignment of the body (full arm extension or right-angled pull-up), and with
varying interpretations of what the test items actually measure (upper-body
strength, upper-body endurance, combined upper-body strength and endur-
ance, athletic ability, relative strength).
It is apparent that many of these test items do not satisfy the physiologi-
cal definitions of the three dimensions of musculoskeletal fitness. Muscle
endurance fitness test items arguably may be considered the most physi-
ologically valid field tests in youth as opposed to those measuring muscle
strength and power, which are more subject to velocity control, loads, and
number of repetitions. Additionally, several of the currently used field-based
fitness tests (e.g., curl-up and pull-up) purport to measure more than one
musculoskeletal dimension concurrently. Because of their lower construct
validity, results of muscle strength and power tests must be interpreted
cautiously in youth.
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158 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH
TABLE 6-1 Summary of Muscle Strength, Endurance, and Power
Fitness Test Items Used Historically to Assess Musculoskeletal Fitness in
International Youth Fitness Test Batteries
Fitness Test
Item Fitness Component Evaluated Variant Approaches
Flexed/Bent · Arm and shoulder endurance
Arm Hang · Functional strength
Pull-up · Upper-body strength and · Regular with full arm extension
endurance · Modified with right-angle limited
· Upper-body endurance extension
· Athletic ability · Untimed performance
· Relative strength · Timed performance (30 and 60
seconds)
Push-up ·Upper-body strength and · Regular with full arm flexion/
endurance extension
· Modified with limited arm flexion/
extension to 90 degrees
· Isometric hold position
Dip · Upper-body endurance
Sit-up · Abdominal strength and · Straight-legged
endurance · Bent-legged
· Abdominal endurance · Timed performance (30 and 60
seconds)
Curl-up · Abdominal strength and · Full flexion/extension
endurance · Partial flexion/extension
· Trunk strength · Untimed
· Timed (30 seconds)
· Cadence based
Trunk Lift · Back extensor strength
· Back flexibility
Handgrip · Static/isometric strength
Strength Test
Standing · Explosive power · Levels
Broad/Long · Lower-body strength
Jump · Athletic ability
Vertical · Explosive power · Jump with no countermovement
Jump · Lower-body strength · Countermovement jump
· Athletic ability
Throwing · Upper-body explosive · Softball throw
strength · Handball throw
· Strength and endurance · Basketball throw
· Athletic ability · Medicine ball throw
· Shotput (variable weights)
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MUSCULOSKELETAL FITNESS 159
MUSCULOSKELETAL FITNESS AND HEALTH IN ADULTS
There is increasing evidence of the importance of musculoskeletal fit-
ness as a determinant of health outcomes both in healthy young, middle-
aged, and elderly adults and in adults with disability or chronic disease. A
review of the relationship of early test batteries to health outcomes revealed
that the evidence was limited, even though there was sound anatomical
logical validity for a link between abdominal and back health and muscu-
loskeletal fitness (Plowman, 1992). More recent evidence lends additional
support to the idea that tests of abdominal and back extensor muscle endur-
ance relate to back health status, as assessed by history of low-back pain,
in adults (Payne et al., 2000).
In recent years, the link between musculoskeletal fitness and health in
adults has extended beyond the initial focus on low-back health. Recent
reviews have established positive associations between muscle strength and
personal independence and quality of life, and inverse associations with
cardiometabolic risk factors, frequency of cardiovascular disease events,
risk of general morbidity for nonfatal diseases (e.g., fracture risk and cog-
nitive decline), and all-cause mortality in middle-aged and elderly adults
(Bohannon, 2008; Cooper et al., 2011; Garber et al., 2011; Warburton et
al., 2001; Williams et al., 2007). Likewise, muscle endurance has been posi-
tively associated with overall quality of life and negatively associated with
likelihood of falling and associated skeletal and soft tissue injuries (War-
burton et al., 2001). Muscle power appears to decline more rapidly than
muscle strength with aging, and loss of muscle power is strongly associated
with decreases in functional ability (e.g., reduced ability to stand from sit-
ting in a chair), and it may be predictive of decreased mobility and prema-
ture mortality in adults (Reid and Fielding, 2012; Warburton et al., 2001).
Skeletal muscle and its functional capacities may also be related to
more health-related outcomes than has previously been appreciated. Reduc-
tions in skeletal muscle mass associated with acute or chronic illness may
negatively impact musculoskeletal fitness as assessed by muscle strength,
endurance, and power tests. Reduced muscle strength and function with
accompanying loss of muscle mass in acute or chronic illness are related
to increased recovery times, impaired patient quality of life, and likelihood
of institutionalization (Wolfe, 2006). Further, skeletal muscle is a major
regulator of glucose and fat metabolism and may play an important role
in the development of the metabolic syndrome and perhaps even obesity
(Jurca et al., 2005). The degree to which musculoskeletal fitness tests are
predictive of the development of these conditions and their responsiveness
to clinical management in adults remains an interesting yet untested ques-
tion. Lastly, skeletal muscle may be an important determinant of bone and
joint health in middle-aged and older adults as a result of direct muscle
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160 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH
forces imparted to the skeleton during movement, as well as the effect of
increased muscle mass on skeletal loading. While it is difficult to separate
those two effects (Beck, 2009), suggestive evidence points to a positive
association between measures of musculoskeletal fitness (especially muscle
strength and power) and bone health in adults (Ashe et al., 2008; Cooper
et al., 2011; von Stengel et al., 2005, 2007). Positive associations also have
been reported between muscle strength and power and better quality of
life, lower risk of falls and fractures, and reduced morbidity and mortal-
ity (Cooper et al., 2011; von Stengel et al., 2005, 2007). Likewise, muscle
weakness has been identified as a risk factor for osteoarthritis in this popu-
lation (Garber et al., 2011).
The validity of the relationships described above is further corrobo-
rated by evidence for the effect of resistance training programs on muscle
strength, endurance, and power, along with changes in various health out-
comes. Resistance training programs now are generally accepted as being
effective at improving muscle strength, endurance, and power in both sexes,
across all ages during adulthood, and for both healthy adults and those
with chronic disease or disability (McCartney and Phillips, 2007; Reid and
Fielding, 2012; Williams et al., 2007). These programs also have resulted in
a multitude of adaptations that foster better health among adults, such as
improved body composition, blood glucose and insulin regulation, systemic
arterial blood pressure in prehypertensives, blood lipid and lipoprotein
profiles, bone health and management of arthritic pain and disability, and
prevention or improved management of the metabolic syndrome (Garber et
al., 2011; McCartney and Phillips, 2007; Williams et al., 2007). Similarly,
resistance training has resulted in enhanced exercise and functional capac-
ity, improved balance, and decreased falls (Garber et al., 2011; McCartney
and Phillips, 2007). Resistance training may also improve quality of life and
self-efficacy and moderate levels of depression and anxiety among adults
(Garber et al., 2011; McCartney and Phillips, 2007; Williams et al., 2007).
MUSCULOSKELETAL FITNESS AND HEALTH IN YOUTH
Literature Review Process
The CDC's systematic review of the literature included muscle strength
and muscle endurance, but not muscle power, as components of fitness
because they are the dimensions of musculoskeletal fitness that have been
used most frequently in fitness test batteries. The muscle strength search
screened 2,642 reports, only 63 of which satisfied the CDC search criteria
for further consideration and were abstracted. Of this subset of 63 studies,
23 were classified as experimental, 22 as experimental with no control, 12
as quasi-experimental, and 6 as longitudinal.
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MUSCULOSKELETAL FITNESS 161
The muscle endurance search screened 6,563 reports, 38 of which were
retained for further consideration and were abstracted. Of this subset,
12 studies were experimental, 15 experimental with no control, 6 quasi-
experimental, and 5 longitudinal. The committee chose to review only the
experimental (including those with no control and quasi-experimental)
and longitudinal prospective studies in making its recommendations. In
addition to the CDC search strategy, the committee reviewed the reference
lists in the selected articles and relevant studies published before 2000 or
after 2010.
The committee developed a set of criteria with which to assess the
scientific quality of the studies (see Chapter 3). Each study was evaluated
against those criteria and categorized as of low, moderate, or high quality.
Only those of high quality were reviewed further; they are described in
Table 6-2. The evidence for a link between a test item and a health marker
in the top high-quality studies was categorized as direct or associational
based on the strength of the study design and the rigor of the statistical
analysis. The strength of the evidence was categorized as sufficient or insuf-
ficient based on the number of studies with direct or indirect evidence, the
study designs, and the statistical significance of the association.
Review of the Scientific Literature
Chronic, hypokinetic-related diseases are manifestations of latent pro-
gressive poor health over a protracted period of time. Because these diseases
are relatively less prevalent in youth, there is substantially less scientific
evidence supporting the association of musculoskeletal fitness with health
outcomes in youth than in adults.
The relationship between health and musculoskeletal fitness in youth has
been reviewed recently in relation to the development of the Fitnessgram®/
Activitygram® (Welk and Blair, 2008) and a new health-related physical fit-
ness test battery for European youth--the Assessing Levels of Physical Activ-
ity (ALPHA) study (Castro-Piñero et al., 2010; Ortega et al., 2008b; Ruiz et
al., 2009). In a recent review, Ortega and colleagues (2008b) report signifi-
cant inverse associations of lower-limb explosive strength (i.e., power) and
abdominal endurance with lower-abdominal obesity in youth (e.g., p < 0.001
between performance on the standing long jump and waist circumference in
8-year-old males) (Brunet et al., 2007). The same review also highlights inverse
associations (p = 0.048) between a composite muscle fitness index score
and a standardized composite measure of cardiovascular risk among adoles-
cent girls (Garcia-Artero et al., 2007), and between putative cardiovascular
inflammatory markers and muscle strength in normal-weight and overweight
adolescents (for C-reactive protein, p = 0.02 and p = 0.09, respectively) (Ruiz
et al., 2008). Additionally, positive associations were found between muscle
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TABLE 6-2 Summary of Top-Quality Studies Providing Best Evidence for Muscular Strength/Power
162
Cardio- Musculo- Mental and Age, Gender,
Reference and Body Metabolic respiratory skeletal Cognitive Maturity, Weight Study Summary, Quality,
Study Type Fitness Test(s) Composition Health Health Health Health Status, Population and Level of Evidence
Benson et al., Bench press Waist Homeostatic Ages 11-19, An 8-week strength-
2008 (BP), leg press circumference model male and female training intervention
Experimental (LP) (WC), fat mass assessment-- (M and F), program resulted in
(FM), lean insulin overweight, significant gains in both
mass (LM), % resistance obese, New bench press and leg press
body fat (BF), (HOMA-IR), Zealand strength and differential
body mass glucose,b positive training effects
index (BMI) insulin,b on WC, % BF, FM, and
triglycerides BMI but not on any of the
(TG),b metabolic health markers.
cholesterol and Level of evidence (LE):
subfractionsb Direct
Janz et al., 2002 Handgrip % BF, TG,b lipids,b Systolic blood Ages 10.5-15.5, A 5-year prospective
Longitudinal strength test abdominal fat cholesterolb pressure (SBP), M and F, normal nonintervention study
(AF)a diastolic blood weight found significant negative
pressure (DBP) correlations between
changes in handgrip
strength and changes in
SBP, BF, and AF between
10 and 15 years of age,
with those with high
handgrip strength scores at
the outset having the best
health marker profiles 4-5
years later.
LE: Associational
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Ingle et al., BP/LP, squat, % BF,a LMa Ages 11-12, M, A 12-week intervention
2006 standing long normal weight program consisting of
Experimental jump (SLJ), combined strength and
vertical jump plyometric training
(VJ) resulted in significant
differential increases in
BP, squat, and VJ, with a
significant reduction in %
BF and increase in LM,
and then slight regression
of these changes during
detraining, while control
values remained stable.
LE: Direct
Heinonen SLJ/VJ/LJ, Tibial bone F, pre- and A 9-month intervention
et al., 2000 isometric leg mineral postmenarcheal, study of high-impact
Experimental extensor-- content normal weight exercises found significant
90 degrees (BMC),b increases in LJ in both
femoral pre-and postmenarcheal
neck (FN) girls, with no changes
BMC,a in leg extension strength
lumbar in either age group;
spine (LS) concurrent significant
BMC,a increases in FN and LS
trochanter BMC in premenarcheal
BMC,a girls; and a differential
tibia bone positive effect in
mineral premenarcheal versus
density postmenarcheal girls for
163
continued
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176 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH
In summary, there is an insufficient body of high-quality literature to
support a strong link between performance on any specific musculoskel-
etal fitness test by youth of either gender and across all ages and stages of
development and any health outcomes or markers. The current literature in
this area is too fragmented to permit identification of any specific musculo
skeletal fitness test item that is unequivocally linked to health in the general
population of healthy youth.
VALIDITY AND RELIABILITY OF SELECTED TEST ITEMS
Despite the limitations of the literature discussed above, the growing evi-
dence in youth and stronger evidence in adults is suggestive of a fundamental
relationship between musculoskeletal fitness and health outcomes across the
life span. The committee finds that handgrip strength test and the standing
long jump are two tests that globally represent musculoskeletal strength and
power in youth and demonstrate adequate validity, reliability, and feasibil-
ity of administration for inclusion in fitness test batteries for all youth. This
section reviews the validity and reliability of these two tests, for which there
is some, albeit limited, evidence for a relationship to health in the literature
reviewed. It also looks at the integrity of the modified pull-up and isometric
leg extension tests, which also may be useful for assessing musculoskeletal
fitness; however, the literature review provided very limited high-quality
evidence for a link to health outcomes in youth for these two tests.
While numerous fitness tests purportedly measure muscle strength,
endurance, and power in youth, information about their validity and reli-
ability is limited. Nevertheless, an increasing body of literature pertaining
to the validity and reliability of a few musculoskeletal fitness tests provides
reasonable justification for including these tests in a test battery for assess-
ment of musculoskeletal fitness in youth. As mentioned above, the commit-
tee's systematic literature review included muscle strength and endurance,
but not muscle power, as components of fitness. Some of the tests reported,
however, such as throwing and jumping tests, purportedly assess some
aspects of muscle power (e.g., average, peak, instantaneous, and contractile
power). Although the specific associations between individual fitness tests
and aspects of muscle power are poorly defined, the committee's discussion
of the validity of the tests takes account of the fact that the selected tests
of musculoskeletal fitness could measure either muscle strength, endurance,
or power.
The handgrip strength test is used extensively in European youth fit-
ness testing. Based on the available literature, the handgrip strength test
has moderate to strong construct validity (r = 0.52-0.84) with established
upper-body (i.e., 1RM bench press) and lower-body (i.e., leg press and iso-
kinetic knee extensor torque) strength tests (Holm et al., 2008; Milliken et
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MUSCULOSKELETAL FITNESS 177
al., 2008) and strong reliability (r = 0.71-0.90) in children and adolescents
(Benefice et al., 1999; Brunet et al., 2007; Ruiz et al., 2006). It also has
minimal test-retest learning and fatigue effects (Ortega et al., 2008a). Given
differences in hand sizes among youth, optimal grip span adjustment, elbow
angle, and device calibration are important for valid testing.
The standing long jump has been used extensively as a test of lower-
body muscular strength, power, and explosive strength (see Table 2-6 in
Chapter 2). Although not strictly a measure of power as that subdomain is
defined, the standing long jump is the most widely used field-based test of
muscle power/explosive strength. It demonstrates moderate to strong cor-
relations with 1RM leg press/body weight (r = 0.39) (Milliken et al., 2008),
isokinetic quadriceps torque (r = 0.50) (Holm et al., 2008), and total-body
isometric strength (r = 0.77) (Castro-Piñero et al., 2010) in youth. In addi-
tion, the standing long jump correlates strongly (r = 0.70-0.91) with other
lower- and upper-extremity field-based power tests (i.e., vertical jump,
countermovement vertical jump, upper-body explosive throw) in youth
(ages 6-17), controlling for age, gender, and BMI and/or weight (Castro-
Piñero et al., 2010; Milliken et al., 2008). The standing long jump also has
been found to have acceptable reliability in youth (r = 0.52-0.99) (Benefice
et al., 1999; España-Romero et al., 2010; Malina et al., 2004; Pena Reyes
et al., 2003; Safrit, 1995; Simons et al., 1990). In addition, the reliability
of this test appears not to be affected by either systematic bias or sex dif-
ferences among adolescents (Ortega et al., 2008a), although reliability esti-
mates generally increase with age. Differences in gross motor coordination
and experience with jumping across developmental time may influence the
degree of test-retest reliability for the standing long jump. Controlling indi-
vidually for anthropometric variables (i.e., height and body mass) provides
a more valid assessment of lower-body strength and power for this test
across ages (Castro-Piñero et al., 2010; Milliken et al., 2008).
The modified pull-up and isometric knee extension tests also are valid
and reliable tests of upper- and lower-body musculoskeletal fitness, respec-
tively; however, insufficient scientific evidence supports the link between
these two tests and health outcomes in youth. The modified pull-up has
demonstrated moderate to strong construct validity (r = 0.60-0.79) with
other upper-body criterion strength measures (i.e., 1RM bench press, pull-
down, arm curl) in boys and girls when measured per unit body weight
(Pate et al., 1993). The highest correlation demonstrated (r = 0.75) com-
prises the sum of the multiple upper-extremity strength tests, which dem-
onstrates strong construct validity for a composite measure of upper-body
strength. The modified pull-up also is moderately to strongly correlated
(r = 0.64-0.79) with push-ups, thus demonstrating a crossover effect with
muscle strength and endurance. Moderate to high test-retest reliability (r =
0.52-0.99) (Engelman and Morrow, 1991; Erbaugh, 1990; Kollath et al.,
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178 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH
1991; Saint Romain and Mahar, 2001) and modified Kappa coefficients
(0.87-0.94) (Saint Romain and Mahar, 2001) have been demonstrated for
the modified pull-up.
The isometric knee extension test is a criterion measure of lower-
extremity quadriceps strength (and a general measure of lower-extremity
leg extension strength) used primarily in clinical or laboratory settings.
Many different methods (e.g., supine and upright sitting) and instruments
(e.g., hand-held dynomometer, Cybex isokinetic equipment) have been
used to test knee extension strength, and an optimal knee angle is an
important consideration for adequately measuring maximum torque at the
knee. Optimal knee angle remains relatively untested in children; however,
knee angles of 80-90 degrees may produce the highest torque levels in this
population (Marginson and Eston, 2001). Validation of isometric knee
extension tests with other criterion lower-body strength measures (e.g.,
leg press and bilateral squat exercises) in youth is limited. The strength of
the relationship between isometric and isokinetic knee extension has been
shown to decrease with increasing isokinetic angular speeds (Hill et al.,
1996). The isometric knee extension test demonstrates strong reliability
(r = 0.76-0.97) for single- and double-leg tests (Escolar et al., 2001; Hill
et al., 1996; Mercer and Lewis, 2001; Teeple et al., 1975) in both normal
and disabled children. As with other fitness tests, familiarization with the
testing procedures is advisable to optimize the validity of the test results
(Farpour-Lambert and Blimkie, 2008).
ADMINISTRATIVE FEASIBILITY
In addition to validity and reliability, the selection of musculoskeletal
fitness test items for inclusion in a youth fitness survey will depend on their
administrative feasibility and practicality in the field. Principles relating to
administrative feasibility for fitness testing for all fitness components are
discussed in general in Chapter 3 and more specifically for application in
school settings in Chapter 9. Developers and administrators of fitness sur-
veys should carefully consider the issues outlined in Box 3-2 in Chapter 3
and in Chapter 9 when selecting specific musculoskeletal fitness test items
for inclusion in a youth fitness test battery. The two specific musculoskel-
etal fitness tests discussed in the previous section and highlighted for their
potential relationship to health in youth (i.e., handgrip strength and stand-
ing long jump tests) are among the most practical and feasible of a plethora
of muscle strength, endurance, and power tests for field-based physical
fitness assessment in this population. These tests can be taught effectively
and administered safely to most school-aged youth, with consistency and
reliability likely improving with increasing age and maturity from age 5
until the onset of puberty.
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MUSCULOSKELETAL FITNESS 179
Administrative considerations for musculoskeletal fitness tests appli-
cable to schools and other educational settings are described in Chapter 9.
These tests (modified pull-up, push-up, and curl-up) generally require more
skill and coordination than the handgrip and standing long jump tests and
are perhaps more susceptible to learning and other effects. Thus these tests
may be taught to all school-aged youth; however, performance may be less
reliable than is the case for the handgrip strength and standing long jump
tests for younger ages, but as with those tests, may improve with advancing
age and maturity.
GUIDANCE FOR INTERPRETATION OF TEST RESULTS
Chapter 3 recommends several strategies that developers of fitness test
batteries can employ to ensure accurate interpretation of health-fitness
relationships in youth. The most robust approach requires establishing a
strong link between some fitness parameter and one or several putative
health markers in a broad population of youth and identifying health-
related cut-points. However, the literature contains no recent (within the
past 10 years) national normative data for the muscle strength, endurance,
and power tests discussed in this chapter for U.S. youth, and there is scant
evidence of any link between these tests and possible health markers for
this population. At present, therefore, empirically determined health-related
cut-points cannot be established for these tests. In the absence of criterion-
referenced cut-points in youth or adults, interim cut-points corresponding
to the 20th percentile should be used for tests of musculoskeletal fitness,
analogous to the cut-points for cardiorespiratory endurance, until better
evidence for criterion-referenced health-related cut-points is established by
further research. Experts who will establish cut-points for musculoskeletal
fitness tests in youth should follow the guidance in this report (Chapter
3) and base the cut-points on the unique purposes of the testing (e.g., cut-
points for special populations such as athletes or people with disabilities).
CONCLUSIONS
Based on the CDC literature and the supplemental literature reviewed
for this report, the committee concludes that there currently is sufficient evi-
dence affirming that musculoskeletal fitness is related to health in humans.
This conclusion is based mainly on increasing evidence for the importance
of musculoskeletal fitness, especially muscle strength and power, to health
outcomes in adults. There is some, albeit much more limited, support for
this link among youth. At this time, however, there is insufficient high-quality
evidence supporting an association between any single musculoskeletal fitness
test item and health markers in youth. Studies reviewed also provide insuf-
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180 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH
ficient data for assessing the influence of several potential modifiers--age,
gender, race/ethnicity, body composition, maturation status--on performance
on musculoskeletal fitness tests.
The committee found growing evidence supporting the handgrip and
standing long jump tests as putative health-related (i.e., bone health and
body composition) musculoskeletal fitness test items in youth. The handgrip
strength test demonstrates moderate to strong validity with both upper- and
lower-body criterion strength measures. The standing long jump, although
not strictly a measure of pure muscle strength, demonstrates acceptable con-
current validity with lower- and upper-body criterion strength measures and
lower-body power measures in youth. The handgrip strength and standing
long jump tests demonstrate strong and moderate reliability, respectively.
Both are applicable across a broad age range, in both sexes, and in both nor-
mal and special pediatric subpopulations. These two tests also are currently
included in the ALPHA test battery for musculoskeletal fitness assessment
in European youth. Test administrators may wish to include these tests in a
national youth fitness survey based on their integrity and feasibility; how-
ever, the results of these tests should not be interpreted in a health context
until such relationships are more firmly established. The committee found
no evidence of adverse events associated with the administration of these
tests in the studies reviewed.
Other tests, such as the modified pull-up and isometric knee extension,
also are being used as measures of muscular strength in current fitness
test batteries in the United States but are linked only weakly with health
markers in youth at this time. Therefore, despite their acceptable validity,
reliability, and feasibility, the committee does not recommend these tests
for a national youth fitness survey until such health links are more firmly
established. In addition, although the bench press and leg press tests are
viewed as standard criterion measures of strength or endurance (based
on the number of repetitions demanded) in adults, they cannot be recom-
mended for inclusion in a national youth fitness survey at this time because
of the limited quality and level of the scientific evidence for the relationship
of these tests to health outcomes in youth; the paucity of information on
their reliability across childhood; and concerns regarding their administra-
tive feasibility, practicality, and safety.
For schools and other educational settings, administrators should con-
sider the hand grip strength and standing long jump tests as well as alterna-
tive tests that have not yet been shown to be related to health, but are valid,
reliable, and feasible. The modified pull-up and push-up tests are possible
alternatives for measuring upper-body musculoskeletal strength. The curl-
up could also be considered for measuring an additional construct, core
strength. The committee found no evidence of adverse events associated
with the administration of these tests in the studies reviewed. The com-
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MUSCULOSKELETAL FITNESS 181
mittee's full recommendations on musculoskeletal fitness tests for use in
national youth fitness surveys and in schools and other educational settings
are presented in Chapters 8 and 9, respectively.
Moderate to strong tracking of selected measures of muscle strength and
power both during adolescence (Maia et al., 2001; Malina, 1996; Pate et al.,
1999) and from adolescence into adulthood (Beunen et al., 1992; Malina,
1996; Mikkelsson et al., 2006; Twisk et al., 2000) suggests that measures
of musculoskeletal fitness in youth may prove to be useful predictors of
future adult health. Tracking relationships appear to be weaker during the
preadolescent years and more stable for lower- versus upper-body strength/
power measures (Malina et al., 2004). Tracking variability in youth may
be explained by age-related differences in the development of inter- and
intramuscular coordination and differing levels of experience with specific
fitness tests. Further, there is increasing evidence of moderate tracking of bio-
logic health markers, especially for coronary heart disease, from childhood/
adolescence into adulthood that in the future may be shown to be related to
musculoskeletal fitness in youth (Bao et al., 1995; Froberg and Andersen,
2005; Malina et al., 2004; Twisk et al., 1995, 1997). Whether changes in
muscle strength, endurance, and power during youth are predictive of adult
health outcomes in later life, however, remains to be determined.
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