Health-Related Fitness Measures for Youth: Flexibility
Flexibility has been defined as the range of motion of muscle and connective tissues at a joint or group of joints. In contrast to other, more general or systemic fitness components, flexibility is highly specific to each of the joints of the body. For this reason, although flexibility has been included in national fitness test batteries, linking it to one or more health outcomes is difficult, and few data support such an association. Future efforts to study the relationship of flexibility to health will require a multivariate approach.
The principal health outcomes hypothesized to be associated with flexibility are prevention of and relief from low-back pain, prevention of musculoskeletal injury, and improved posture. These associations have been studied most frequently in adults, and the strength of any associations for specific flexibility tests in youth is minimal. Various reasons may explain the difficulty of establishing a link between flexibility and health. First, in contrast with other fitness components, no large-scale studies have been specifically designed to assess the relationship between flexibility and health. Second, flexibility may be associated with health in combination with other musculoskeletal variables. Finally, studies addressing flexibility have varied substantially in the tests used, the study designs, and the characteristics of participants (e.g., age, gender, weight).
Although the evidence is not yet clear, flexibility in youth may in fact be linked to various health outcomes, such as back pain, injury preven-
tion, and posture, and appropriate studies are needed to explore such associations. The limitations described above led the committee not to recommend a flexibility test for a national youth fitness survey. Instead, the committee recommends conducting further research on this fitness component, as well as considering the use of flexibility tests in schools and other educational settings for educational purposes.
Until the relationship to health is confirmed and national normative data and health data are collected for youth, the comparatively relative position method should be used for setting cut-points (cutoff scores) for performance on flexibility tests. With this method, percentiles established for other fitness measures are used to establish interim cut-points for the measure of interest. For example, interim cut-points corresponding to the 20th percentile should be used for flexibility tests, analogous to the cut-points for cardiorespiratory endurance tests.
Flexibility as a component of fitness first gained prominence in the early 1900s as the field of physical therapy emerged (Linker, 2011). Later in that century, circumstances (i.e., two world wars and a polio epidemic) provided further impetus for growth in the professions of occupational and physical therapy and a rise in schools for preparing therapists. In 1980 the first health-related physical fitness test was published (AAHPERD, 1980), and it included a test of flexibility (sit-and-reach). Subsequent U.S. and international health-related test batteries—including the President’s Council on Fitness, Sports and Nutrition (PCFSN) and Fitnessgram® batteries—have included items to measure flexibility.
This chapter reviews existing data on the relationship between flexibility and health outcomes in youth. The focus is on the extent to which flexibility is associated with better health and function, excluding those outcomes related to athletic performance. The chapter begins by defining flexibility and describing the relevant physiology as a basis for explaining the challenges involved in identifying an association between a single flexibility test and a health outcome. The most frequently used flexibility tests are then described. Next, the chapter presents findings from the literature on what is known about the relationship between flexibility and health in adults and in youth, which serve as the basis for the committee’s guidance for interpreting results of flexibility tests, as well as for its conclusions about the associations between flexibility tests and health outcomes in youth. The validity and reliability of these tests are also examined. The process for selecting the studies included here is described briefly in this chapter and in more detail in Chapter 3. Based on its conclusions about the
relationship between flexibility tests and health, the committee makes no recommendation for including a flexibility test in national fitness surveys, but only recommendations regarding the use of specific flexibility tests with educational value in schools and other educational settings (see Chapter 9). These recommendations are based on the validity and reliability of the tests and on additional factors that should be considered when implementing fitness tests in schools (also described in Chapter 9). Future research needs related to this fitness component are addressed in Chapter 10.
Flexibility has been defined in many different ways, although the focus has consistently been on the characteristics and functioning of muscle. Kraus and Hirschland (1954), whose research in the 1950s precipitated the U.S. national youth fitness testing movement, referred to flexibility as a muscle fitness component associated with “muscle stiffness” and “tension.” Kraus and Raab (1961) referred to muscle “tension” and “tightness” when discussing flexibility in their classic book Hypokinetic Disease. Fleischman (1964) identified two flexibility components using factor analysis: extent flexibility and dynamic flexibility. Extent flexibility was defined as “the ability to flex or stretch the trunk and back muscles as far as possible” (p. 77) (e.g., twist and touch tests). Dynamic flexibility was defined as “the ability to make repeated, rapid, flexing movements” (p. 79) (e.g., rapid bending, twisting, and touching movements).
According to Cureton, an early fitness pioneer, “Flexibility indicates that joints are not muscle bound or stiff for some other reason” (Cureton, 1965, p. 42). It is important that his definition included reference to joints and not just muscles, consistent with clinical definitions that evolved from the development of the field of physical therapy and focused on “range of joint motion” as the key component of flexibility. Textbooks on physical fitness also focused on joint range of motion. For example, Johnson and colleagues (1966, p. 23) defined flexibility as “the functional capacity of the joints to move through a
full range of motion.” A review of issues related to flexibility (Knudson et al., 2000) uses the definition of Holt and colleagues (1996, p. 172). Flexibility is defined as “the intrinsic property of body tissues which determines the range of motion achievable without injury at a joint or group of joints.”
The definition of flexibility used in this report is an adaptation of that of Holt and colleagues (1996). In this report, flexibility is operationally defined as “the intrinsic property of body tissues, including muscle and connective tissues that determines the range of motion achievable without injury at a joint or group of joints.” Flexibility is highly specific to each joint.
Fitness is considered to be a “state of being” (Corbin et al., 2000), which is different from the behavior that produces that state. In the case of flexibility, stretching is a physical activity behavior or exercise typically performed to increase muscle-tendon unit (MTU) length and to allow improved joint range of motion. Common forms of stretching include static stretch (passive and active), proprioceptive neuromuscular facilitation, ballistic stretch, and dynamic stretch (see ACSM, 2010; Garber et al., 2011). Other forms of physical activity that require stretching of the MTU (e.g., gymnastics, dance) can also result in improved flexibility.
Flexibility tests measure joint range of motion and can in general be classified into two categories: laboratory tests and field tests. Laboratory tests are those often used in controlled settings and are administered to patients or study participants on a one-to-one basis with specifically designed devices. As a result, the administration of laboratory tests can be expensive and time-consuming. Field tests, in contrast, are used in schools, fitness clubs, or similar practical group settings and can be administered to more participants at a relatively lower cost and in a relatively shorter time. Characteristics of laboratory and field tests are briefly described below.
Most clinical assessments of flexibility fall within the category of “goniometry,” which is derived from the Greek words “gonia” (i.e., angle) and “netron” (measure) (Eston and Reilly, 1966; Norkin and White, 2003). Thus, measuring flexibility can simply be viewed as measuring the angle of joints or their range of motion (ROM). The devices used for the assessments are called goniometers. Although they vary in size, shape, and material used, goniometers usually consist of three parts—the body and two thin extensions called “arms.” The body resembles a protractor that forms a half
(0 to 180 degrees) or full (0 to 360 degrees) circle. One arm is called the “stationary arm” and other the “moving arm.” During the assessment, the examiner determines the range of motion by placing the goniometer along the bone immediately proximal and distal to the joint being measured.
Field tests for flexibility have been used in a number of fitness test batteries. In the United States, the shoulder stretch (sometimes called the zipper), trunk lift (assesses both flexibility and muscle fitness), and sit-and-reach (assesses low-back and hamstring flexibility) have been used, as have modifications of these tests. There are also several other tests not used in national batteries, such as the Schober test, the modified Schober test, and the straight leg raise (see also Table 2-6 in Chapter 2).
In the shoulder stretch, the person being tested reaches over the shoulder and down the back with one hand, and reaches behind the back and upward with the other hand, trying to touch the fingers of the hands together. The distance between the hands or distance of overlap is measured on both sides of the body (Meredith and Welk, 2010, pp. 59-60).
The trunk lift is presumed to measure both muscle strength and flexibility. In this test, the person being tested lies prone on the floor, lifts the upper body (trunk) off the floor, and holds the position while the height of the chin from the floor is measured (Meredith and Welk, 2010, pp. 49-50).
Sit-and-reach and other similar tests that require a person to flex the hip to touch the toes are the most common field tests of flexibility. Such tests are designed to assess low-back and upper hamstring (complex of three posterior thigh muscles) flexibility.
The first U.S. health-related fitness battery used a bilateral sit-and-reach test (AAHPERD, 1980). Sitting on the floor or a mat, legs straight and feet 8-12 inches apart, the person being tested reaches forward with the arms (hands overlapping). The distance of reach is measured in inches using a measuring line marked on the floor (PCPFS, 2012).
An alternative to the bilateral sit-and-reach test is the unilateral test called the backsaver sit-and-reach (Meredith and Welk, 2010, pp. 57-59). The Fitnessgram test manual (Meredith and Welk, 2010) outlines the reasons for including this test. A flexibility box with a ruler extension is used. The person being tested sits on the floor or a mat with one leg straight. The other leg is bent to the side, foot near the knee of the straight leg. The person being tested reaches forward with the arms (hands overlapping). The distance reached in centimeters or inches (on the flexibility box ruler) determines the person’s score. The test is then repeated with the other leg extended.
Flexibility is associated with length of muscle and connective tissue, joint structure, age, disease state, and gender. MTU length is typically the prime focus of flexibility testing in the field setting. Factors such as MTU stiffness/compliance, elasticity, and viscoelasticity relate to flexibility and MTU function (Alter, 2004; Knudson et al., 2000). The utility of flexibility as a component of physical fitness has its roots in sports performance, and considerable research has investigated the association between acute stretch and muscle cramps (DeVries, 1967), injury (McHugh and Cosgrave, 2010), performance (Kay and Blazevich, 2012; McHugh and Nesse, 2008), postural stability (Nelson et al., 2011), and delayed muscle soreness (Henschke and Lin, 2011; Herbert et al., 2011). This chapter, however, focuses on outcomes related to better general function and health, not athletic performance.
In contrast to other fitness components that are general or systemic in nature, flexibility is highly specific to each of the joints of the body. For example, a person can be very flexible with a good range of motion in and around the shoulder joint but tight and lacking range of motion in the hip. The specificity of flexibility to joints of the body makes it difficult to isolate a single flexibility-related factor contributing to a health outcome. The ability to touch the toes in a sit-and-reach test, for example, involves many joints and MTUs. MTU length in one area of the body may contribute to poor performance on the test but not account for a large amount of variance in total test performance. As a result, establishing a relationship between flexibility and health outcomes is likely to require a multisite, multivariate approach specific to each health outcome. Accordingly, establishing a link to one or more health outcomes for one specific flexibility test item is difficult.
An added complication is that field tests used to assess flexibility may not have the specificity to isolate particular joints of interest to health-related outcomes. For example, although low-back pain has been hypothesized to be associated with flexibility, the sit-and-reach test that is commonly used to assess low-back and hip flexibility has been shown to measure hip flexibility rather than low-back flexibility (Chillon et al., 2010). The extent to which range of motion around the hip joints is a better predictor of low-back pain than range of motion around the lumbar region is not known. Results of a study by Cornbleet and Woolsey (1996) indicate that the sit-and-reach test is correlated with hamstring length. However, attention must be paid to the final position of the hip joint rather than the final position of the fingertips and any mobility in the spine in assessment of hamstring length. Of interest, this study also suggests that hamstring length differs between boys and girls.
Although flexibility may be associated with health outcomes, strong evidence of a health link to an individual field test is not apparent. Flex-
ibility is not necessarily linearly related to health outcomes. Excess range of motion around a joint or joints (e.g., joint hypermobility syndrome [JHS]) is characterized by excessive movement and wear of joints that can lead to injury and disability (Wolf et al., 2011). JHS affects children more than adults and females more than males (Remvig et al., 2007). Accordingly, care must be taken in interpreting an individual’s range of flexibility in terms of health outcomes.
Flexibility and Health in Adults
The evidence relating flexibility to health outcomes among adults is equivocal. The American College of Sports Medicine’s (ACSM’s) position statement (Garber et al., 2011) indicates that flexibility exercises may enhance postural stability and balance (see also Bird et al., 2011). Plowman (2008) reports that some studies show an association between flexibility and low-back pain, while others do not. Recent studies using Functional Movement Screening (FMS), a multi-item musculoskeletal screening battery, have shown promise for predicting injuries among military personnel (O’Connor et al., 2011), firefighters (Peate et al., 2007), and professional athletes (Kiesel et al., 2007, 2011). These preliminary studies suggest that batteries of musculoskeletal test items may prove to be better predictors of injury than single musculoskeletal test items (including items designed to test flexibility), at least in people for whom high-intensity exercise and vigorous-intensity physical activity are important job features.
The association between flexibility and functional capacity among adults is unclear, although several recent studies have investigated exercise training and functional capacity. Studies in cancer survivors (Eyigor et al., 2010) and people with Parkinson’s disease (Reuter et al., 2011), fibromyalgia (Carbonell-Baeza et al., 2012), and other conditions have sought to determine the effect of multimodal exercise on various aspects of functional capacity. These studies are rooted largely in the physical therapy literature, where a goal of patient care is increasing or returning musculoskeletal function. Reuter and colleagues (2011) compared a stretching and relaxation treatment (ostensibly a control condition) with a walking or gym-based exercise treatment in a randomized study of 90 Parkinson’s patients. After 6 months, the control patients showed improvements in their reported pain, balance, and health-related quality-of-life measures equal to those of the exercise treatment groups. As with the bulk of the literature on flexibility and health outcomes, few studies have focused specifically on stretching (and changes in flexibility) as the key exposure as it may relate to functional capacity. Moreover, the heterogeneity of populations and conditions studied makes general conclusions tenuous.
Stretching as Part of a Regular Exercise Program
There is some evidence that stretching, if included as part of a regular program of exercise, results in improved flexibility. The ACSM (Garber et al., 2011) found there were limited randomized controlled trials showing the effect of frequency, type, volume, and pattern, and only observational or nonrandomized trials showing the effect of intensity and time (length of stretch). However, the ACSM notes that “no consistent link has been shown between regular flexibility exercise and reduction of musculotendinous injuries, prevention of low back pain, or DOMS [delayed onset muscle soreness]” (Garber et al., 2011, p. 1344). Yet, it is important to note that stretching has been used in physical therapy for injury rehabilitation, treatment of neuromuscular symptoms of disease, and restoration of functional capacity for daily living, although the need for solid scientific support continues (Reurink et al., 2012). Stretching also has been used for improving/correcting posture (Nelson et al., 2011) and for treating neck, back, and other types of pain (Franca et al., 2012). Stretching is useful in relieving muscle cramps (Schwellnus et al., 2008) associated with muscle pain.
Other activities that involve stretching (i.e., Tai Chi, Qigong, yoga) have been associated with health outcomes as well. But because they also rely on strength, muscular endurance, balance, and other neuromuscular factors, it is impossible to quantify the independent effect of stretching (and resultant flexibility). Three different literature reviews (Chang et al., 2010; Jahnke et al., 2010; Wang et al., 2004) indicate that Tai Chi and Qigong have a variety of associated health benefits (e.g., bone health, cardiopulmonary fitness, some aspects of physical function, quality of life, self-efficacy, and factors related to prevention of falls [Jahnke et al., 2010, p. 22]), especially among older adults. Yoga has been associated with benefits in treating low-back pain (Sherman et al., 2005, 2011; Tilbrook et al., 2011) and with psychological health benefits among cancer survivors (Lin et al., 2011).
The stretching warm-up (acute static stretch) has long been considered important in preparing for vigorous-intensity physical activity, including sports, dance, and various forms of fitness training. Recent research, however, has questioned some of the purported performance and health benefits, including prevention of soreness and injury. In a recent systematic review, Kay and Blazevich (2012) cite 18 studies and indicate that static stretching can reduce strength, power, and speed. However, they also note that strength, power, and speed are not compromised after short-duration
stretches (45 seconds or less). Another recent meta-analysis (Simic et al., 2012) that includes 104 studies published from 1966 to 2010 suggests that static stretching should be avoided as the sole warm-up routine for strength, power, and explosive strength performance, but notes that negative effects are greatest for stretches lasting more than 45 seconds. After reviewing 12 relevant studies, Herbert and colleagues (2011, p. 2) found that “there was little or no effect on muscle soreness experienced in the week after physical activity.” There is evidence, however, that acute static stretching decreases musculoskeletal stiffness (Kay and Blazevich, 2009).
Witvrouw and colleagues (2004) and Thacker and colleagues (2004) report no association between acute static stretching and injury reduction. A recent review (McHugh and Cosgrave, 2010) indicates that acute stretching can reduce the risk of acute muscle strain injuries, but also reports no reduction in overuse injuries after a static stretch warm-up. Pereles and colleagues (2010) note that there were no differences in injury risk between prerun stretching and nonstretching groups of teens and adults and suggest that an immediate shift in a regimen (i.e., from stretching to no stretching) may be more important than the regimen itself.
Flexibility and Health in Youth
Literature Review Process
The majority of the studies cited come from a literature review by the Centers for Disease Control and Prevention (CDC). This literature search screened a total of 6,016 studies addressing flexibility. As mentioned in Chapter 3, the CDC did not abstract these articles because of time and resource limitations. However, when flexibility was measured in studies that were identified from the aerobic, muscular endurance, and muscular strength search libraries, that information was coded and extracted into a central database. Of these studies, seven were classified as experimental, five as quasi-experimental, and four as longitudinal. In addition, the committee reviewed studies provided through a public information gathering session. Because there were so few relevant studies, the committee also examined cross-sectional studies to gain further insight; however, these studies yielded no findings relevant to the committee’s task. The criteria used to select high-quality studies are discussed in Chapter 3. Given the paucity of studies and the lack of evidence, this section presents findings from all the studies reviewed regardless of the quality of the evidence in support of a relationship of flexibility to health, as a basis for the committee’s conclusions on flexibility.
Review of the Scientific Literature
A variety of forms of stretching (e.g., static stretch, active stretch, passive stretch, proprioceptive neuromuscular facilitation [PNF]) produce increases in flexibility. Results of studies included in this report indicate that programs of physical activity for youth, even those not designed primarily to improve flexibility (Cheung and Ng, 2003; Dorgo et al., 2009; Faude et al., 2010; Katz et al., 2010; Serbescu et al., 2006), result in improved flexibility (Ahlqwist et al., 2008; Jones et al., 2007).
It should be noted, however, that there are differences in flexibility based on gender and ethnicity. Alter’s (2004) text the Science of Flexibility indicates that in general, girls are more flexible than boys, younger youth are more flexible than older youth, and youth are more flexible than adults. More recently, Tremblay and colleagues (2010) found that girls were more flexible than boys across all age groups during the school years, but found no differences across age groups for either boys or girls. In a large cross-sectional study of youth fitness in Texas, Welk and colleagues (2010) found higher sit-and-reach scores for girls than boys at the high school level but not at lower school levels. The study also found that boys had better sit-and-reach scores in high school than in elementary or middle school, and that girls had lower sit-and-reach scores in high school than in elementary or middle school (Welk et al., 2010). Results of the most recent California physical fitness test indicate that the percentage of students meeting sit-and-reach standards is higher among girls than boys and that for both sexes, more youth meet the standards at upper than at lower grades.1 Finally, results of a statewide fitness survey of students in fifth and seventh grades in Georgia suggest that 21 percent of students failed to meet flexibility standards (as measured by the sit-and-reach test) (The Philanthropic Collaborative for a Healthy Georgia, 2008). No gender differences were noted among the younger (fifth-grade) students, but the percentage of older girls meeting the standards was higher than that of older boys (25 percent versus 20 percent). Results of the Georgia survey also suggest differences by race/ethnicity, with Hispanic students being less likely to reach flexibility standards than their white or African American peers.
In terms of secular changes, a longitudinal study of the fitness of Canadian youth compared fitness scores (cardiorespiratory endurance, body composition, flexibility, muscle fitness) collected between 2007 and 2009 with scores from 1981. Sit-and-reach scores for boys and girls in all age groups were lower in 2007-2009 than in 1981 (Tremblay et al., 2010). In a study by McMillan and Erdmann (2010), girls improved in sit-and-reach performance over a 6-year period, but performance among boys decreased.
1Available at http://www.cde.ca.gov/ta/tg/pf/pftresults.asp (accessed June 18, 2012).
Pain and injury Of seven experimental studies in the CDC review dealing with flexibility, only one (Ahlqwist et al., 2008) looked directly at health outcomes commonly associated with flexibility (e.g., pain, injury). Its results suggest that back pain scores in teens improved as flexibility (as measured by the sit-and-reach test) improved. Improvements in flexibility were greater in the physical therapy group than in the home exercise and educational materials groups. In some of the studies, the intervention did not result in the desired change in flexibility. For example, Faude and colleagues (2010) compared children in a soccer intervention group with controls. Both groups improved in sit-and-reach performance, as well as in body mass.
Of the five studies in the CDC review classified as quasi-experimental, one focused specifically on a dependent variable associated with flexibility. Jones and colleagues (2007) studied a small group of teens with back pain who were exposed to 8 weeks of rehabilitation versus no-exercise controls. Side bending, hip flexion (sit-and-reach), and sit-up performance increased in the rehabilitation group but not the controls. Pain intensity decreased in the intervention group.
In an observational study reviewed, Feldman and colleagues (2001) tracked adolescents over 1 year and found tight quadriceps and tight hamstrings to be associated with back pain. An initial study by Kujala and colleagues (1992) found that flexibility measures were not associated with back pain. However, a 3-year follow-up found that poor lumbar flexion was part of a multivariate profile that predicted pain for boys, and that decreased range of motion in the lower lumbar segments, low maximal lumbar extension, and high body weight at baseline predicted low-back pain for the following 3 years (Kujala et al., 1997). A retrospective study of 1,025 men and women for whom sit-and-reach and sit-up performance was measured as teens found that good flexibility (sit-and-reach) in boys and good endurance strength (sit-up) in girls were associated with decreased risk of neck tension (Mikkelsson et al., 2006). Neither sit-and-reach nor sit-up performance was associated with back pain. A high body mass index (BMI) was associated with increased neck tension, and the authors speculate that it may be related to poor hamstring length and back stiffness. In a study with 402 subjects (6-18 years old), Miereau and colleagues (1989) found that adolescent males with a history of low-back pain also had decreased hamstring length; the same relationship was not found in girls. Lower straight leg raise scores were found among older teens. Salminen and colleagues (1992) studied 15-year-olds with and without back pain and found lumber extension and hamstring length to be associated with back pain, but no relationship was found between back pain and trunk flexion. A later study by Salminen and colleagues (1995) found no association between low-back pain and flexibility measures, but
showed low activity levels to be a risk factor for low-back pain. Bloemers and colleagues (2012) also found an increased risk of injury among inactive youth, but no direct link to flexibility or other fitness parameters was established. Finally, Burton and colleagues (1996) tracked 11-year-olds over 4 years (to age 15) and found that flexibility measures were not predictive of back pain. Lower flexibility was reported between ages 11 and 15, and girls were more flexible than boys.
Body composition and cardiometabolic health Two experimental studies (Manios et al., 2002; Serbescu et al., 2006) found that after an exercise training intervention, improvements were seen in body composition or lipids and lipoproteins that were measured as health outcomes, which in theory are not physiologically linked to flexibility. It should be noted, however, that in one of the studies (Manios et al., 2002), the exercise intervention did not change the flexibility of the participants. Five prospective studies provide information relevant to flexibility and health outcomes. Inconsistent results were found with regard to the association between flexibility (as measured by sit-and-reach) and body composition. Two studies showed an association between decreases in flexibility and higher skinfold measurements (Matton et al., 2006) or BMI (Kim et al., 2005). Others (Aires et al., 2010; Chen et al., 2007), however, found no association between performance on the sit-and-reach test and BMI. These inconsistencies could be due to differences in study designs, such as the length of the studies, the ages of the children, or the appropriateness of the health outcome itself (body composition).
Limitations of the Scientific Literature
The committee notes that the quality of the research reviewed was less than optimal for several reasons. In some cases, there were problems with the design of the study (e.g., no controls). There have been no large trials with adequate statistical power to demonstrate a relationship between flexibility and any health outcome or marker. Moreover, studies typically were not designed to test hypotheses central to flexibility. For example, flexibility measures often were included as one of the fitness components measured, but the health outcomes assessed were chosen because of their hypothesized association with fitness variables other than flexibility, such as BMI. Early studies that influenced eventual large-scale fitness testing of youth focused on the importance of flexibility to back health. The six-item Kraus-Weber test, which was clinically derived, was thought to predict future back pain in adults and was subsequently used as a fitness test for youth (Kraus and Hirschland, 1954). Flexibility has not been theoretically linked to metabolic markers as have cardiorespiratory endurance and body composition, nor has it typically been linked to bone density, as has musculoskeletal fitness.
As noted earlier, the principal health outcomes thought to be associated with flexibility have been relief from back pain symptoms, as well as prevention of injury and posture problems.
VALIDITY, RELIABILITY, AND FEASIBILITY OF SELECTED FLEXIBILITY TEST ITEMS
Evidence on the validity and reliability of the commonly used field tests of flexibility discussed here has been reported (see, e.g., Castro-Piñero et al., 2010; España-Romero et al., 2010; Freedson et al., 2000; Plowman, 2008; Safrit, 1990). In general, the test-retest reliability of the tests is consistently high. Validity, on the other hand, ranges from low to moderate depending on the criterion used. Using the sit-and-reach test as an example, a reliability of 0.99 was reported for 13- to 15-year-old girls, of 0.94-0.97 for 11- to 14-year-old boys, and of 0.80-0.96 for 11- to 14-year-old girls. However, validity was moderate (0.60-0.73) when hamstring flexibility testing was used as the criterion, and was only 0.27-0.30 when goniometer-measured low-back flexibility was used (Plowman, 2008; Safrit, 1990). The finding of moderate validity with hamstring and lumber flexibility tests was recently affirmed in a systematic literature review (Castro-Piñero et al., 2009).
A list of questions to be addressed in assessing the feasibility of a test is presented in Box 3-2 in Chapter 3. While a compelling link between health and flexibility measures has not been established, the widely used sit-and-reach test has been the most frequently studied. The backsaver sit-and-reach is also widely used and has acceptable feasibility based on the criteria in Box 3-2. Additional factors to consider when implementing fitness tests in schools are described in Chapter 9.
GUIDANCE FOR INTERPRETATION OF TEST RESULTS
This report provides guidance to assist those interpreting health-fitness relationships in youth (Chapter 3). Ideally, once there is enough evidence of an association between a test and a health outcome or health marker in youth, cut-points (cutoff scores) for a specific test can be established by mining data on that association collected from a broad population of youth. However, national normative data from flexibility tests for U.S. youth and concurrent data on possible associated health outcomes or health markers are not available. Further, cut-points for adults have not been established for flexibility tests. Until the relationship to health is confirmed and population-based data are collected, the comparatively relative position method should be used in interpreting the results of flexibility tests. With this method, percentiles established for other fitness measures are used to establish interim cut-points for the measure of interest.
Exercises designed to produce changes in flexibility have been shown to be effective in increasing flexibility, and youth who participate in active sports generally have better flexibility than those who do not. There has been a decrease in flexibility among youth in the past 20 years, at a time when body weight has increased dramatically.
Flexibility is specific to joints, and relationships to general systemic health outcomes or health markers are therefore less likely to exist than is the case for other fitness components, such as cardiorespiratory endurance. Clinical theory suggests that complex interaction among multiple musculoskeletal factors (e.g., flexibility, strength, muscular endurance, and neuromuscular factors), rather than any individual variable, is most likely to show a relationship to health. Therefore, establishing an association with health outcomes (e.g., back pain, risk of injury, posture problems) and a single flexibility test item is challenging. Further, possible associations are complicated by the fact that the relationship between flexibility and health outcomes is not linear; that is, risk may be higher for both those with low flexibility and those with exceptionally high flexibility than for those in the middle ranges.
The strength of any association between specific flexibility tests and health outcomes in youth is minimal. There may be various reasons for this. First, in contrast with other fitness variables, there have been no large-scale studies of flexibility and health. Second, flexibility may be associated with health when other musculoskeletal variables are taken into account. Finally, the tests used to measure flexibility, the study designs, and the characteristics of the subjects (e.g., age, gender, weight) have varied substantially, making it difficult to establish any possible link between flexibility and various health outcomes. Data were insufficient to permit assessment of the influence of several potential modifiers, such as age, gender, race/ethnicity, body composition, and maturation status, on performance on the various flexibility tests.
The validity and reliability of some of the flexibility tests used in youth fitness test batteries in the United States and abroad have been confirmed. Among the tests reviewed, the various forms of the sit-and-reach have reasonable validity and reliability when used in both survey and school settings. The degree to which the sit-and-reach test is an indicator of overall systemic flexibility is unclear, however.
Based on the lack of evidence for an association between flexibility tests and health outcomes in youth, the committee does not recommend including such tests in a national survey at this time. At the same time, the committee recognizes that, although the evidence is not yet clear, flexibility in youth may in fact be linked to various health outcomes, such as back
pain, injury risk, and posture problems. Further, the committee found no evidence of adverse events observed in studies of flexibility, nor is there reported evidence of adverse events in field testing of flexibility using popular test items. With this in mind, the committee suggests that in schools and other educational settings, flexibility test items may be included to educate youth and their parents about flexibility as a component of overall musculoskeletal fitness, function, and performance. The selection of such a test should be based on its validity, reliability, and feasibility. To establish interim cut-points for such tests, the guidance provided in Chapter 3 of this report should be followed. Full recommendations on the use of these tests in schools and other educational settings are presented in Chapter 9.
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