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 movement 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 musculoskeletal 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 perform repeated contractions against a constant external load for an extended period of time (Kell et al., 2001). The constant load can be either an absolute 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 definition applies to voluntary submaximal efforts of variable force production 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 subdomains 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 submaximal velocity. Muscle power then can be defined as the product of force and velocity during execution of a maximal voluntary effort against a submaximal external resistance, and it can be measured directly in two ways: by setting 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 velocity 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 countermovement 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 common 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 development 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 musculoskeletal 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 endurance, athletic ability, relative strength).

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.

It is apparent that many of these test items do not satisfy the physiological definitions of the three dimensions of musculoskeletal fitness. Muscle endurance fitness test items arguably may be considered the most physiologically 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.

MUSCULOSKELETAL FITNESS AND HEALTH IN ADULTS

There is increasing evidence of the importance of musculoskeletal fitness 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 musculoskeletal fitness (Plowman, 1992). More recent evidence lends additional support to the idea that tests of abdominal and back extensor muscle endurance 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 cognitive 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 positively associated with overall quality of life and negatively associated with likelihood of falling and associated skeletal and soft tissue injuries (Warburton 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 sitting in a chair), and it may be predictive of decreased mobility and premature 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. Reductions 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 question. 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 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 mortality (Cooper et al., 2011; von Stengel et al., 2005, 2007). Likewise, muscle weakness has been identified as a risk factor for osteoarthritis in this population (Garber et al., 2011).

The validity of the relationships described above is further corroborated by evidence for the effect of resistance training programs on muscle strength, endurance, and power, along with changes in various health outcomes. 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 capacity, 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.

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 insufficient based on the number of studies with direct or indirect evidence, the study designs, and the statistical significance of the association.

TABLE 6-2

Summary of Top-Quality Studies Providing Best Evidence for Muscular Strength/Power.

VALIDITY AND RELIABILITY OF SELECTED TEST ITEMS

Despite the limitations of the literature discussed above, the growing evidence 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 feasibility 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 reliability 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 assessment of musculoskeletal fitness in youth. As mentioned above, the committee'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 fitness 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 isokinetic knee extensor torque) strength tests (Holm et al., 2008; Milliken et 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 correlations 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 addition, 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 differences among adolescents (Ortega et al., 2008a), although reliability estimates 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 individually 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, respectively; 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) comprises the sum of the multiple upper-extremity strength tests, which demonstrates 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., 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 surveys 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 musculoskeletal fitness tests discussed in the previous section and highlighted for their potential relationship to health in youth (i.e., handgrip strength and standing 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.

Administrative considerations for musculoskeletal fitness tests applicable 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 evidence 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 insufficient 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 concurrent 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 normal 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; however, 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 recommended 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 administrative feasibility, practicality, and safety.

For schools and other educational settings, administrators should consider the hand grip 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 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 committee'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 biologic 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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