Special Factors in Flexibility

    Besides those factors previously discussed, a number of additional factors can affect one's flexibility and suppleness, including age, gender, body build, laterality (handedness), training, and circadian rhythms. All of these factors are discussed in this chapter.

CHILDREN AND FLEXIBILITY DEVELOPMENT

    Data concerning the relationship between age and flexibility are conflicting, especially data on the increase or decrease of flexibility during the growing years. The complexity is compounded because studies often focus on specific joints or specific populations involved in various athletic disciplines. Also, lack of standardized testing procedures makes comparing the various studies difficult. Consequently, the literature must be read carefully and completely. Generally, the research seems to indicate that small children are quite supple and that during the school years, flexibility decreases until about puberty, then increases throughout adolescence. After adolescence, however, flexibility levels off and then decreases. Although flexibility decreases with age, the loss appears to be minimized in individuals who remain active.
 

Flexibility Changes in Young Children
    Gurewitsch and O'Neill (1944) carried out one of the earliest studies on flexibility and found gradual declines in flexibility from ages 6 to 12 years and then increases through age 18. Kendall and Kendall (1948) administered two flexibility tests to some 4,500 children from kindergarten to 12th grade. The tests were toe touching and touching the forehead to the knees in a long-sitting position. They found that at age 5 years, 98% of the boys and 86% of the girls could perform the toe-touch test. Beginning at age 6, these percentages declined sharply, so that by age 12, only 30% of both sexes could perform this test. After about age 13, the percentages that were successful gradually increased each year through age 17. At age 5 years, only 15% of the girls and 5% of the boys could touch their foreheads to their knees. This percentage did not change appreciably in either group through age 17.
    Hupprich and Sigerseth (1950) investigated a group of girls 9 to 15 years of age and reported no significant differences among them in six different flexibility test items. However, shoulder, knee, and hip flexion appeared to decrease from ages 12 through 15 years. Leighton (1956) measured the flexibility characteristics of boys 10 to 18 years of age and found decreases in flexibility during adolescence. Buxton (1957) found decreases in both girls and boys from age 6 to 12 and then increases through age 15. Burley et al. (1961) reported no significant age group differences in several flexibility measures among 7thgrade through 9th-grade girls. Clarke (1975) reported flexibility decreases beginning at age 10 for males and age 12 for females. Milne et al. (1976) found significant decreases in flexibility between kindergarten and 2ndgrade children. Krahenbuhl and Martin (1977) found a decrease in shoulder, knee, and hip flexibility between the ages of 10 and 14 years. A study of shoulder flexibility by Germain and Blair (1983) indicated an increase at 5 to 10 years of age and then a steady decrease with age thereafter. Docherty and Bell (1985) found a significant decrease in trunk and neck extension, shoulder and wrist elevation, and sit-and-reach flexibility between 6 and 15 years of age. Koslow (1987) investigated 320 males and females ranging in age from 9 to 21 years. Shoulder flexion-extension was greater in the 13-year-old males and females than in the 9-year-old males and females. Males and females ages 17 and 21 years were significantly more flexible than 9-year-old and 13 -year-old males and females using the modified sit-and-reach test to evaluate lower-extremity flexibility. A decrease in flexibility was even found between 5-year-olds and 6-year-olds (Gabbard and Tandy 1988).
    A study by Mellin and Poussa (1992) of spinal mobility in 294 male and female subjects 8 to 16 years of age found no age differences in forward flexion of the lumbar spine. This result differed from earlier reports of a decrease between the ages of 10 and 15 years (Moran et al. 1979; Salminen 1984). Lateral flexion increased with age in both genders, with greater mobility among the girls (Moran et al. 1979). Moll et al. (1972) also confirmed that lateral flexion is greater in females than in males, which continued into the eighth decade.
    According to Sermeev (1966), flexibility develops unequally in various age periods and for various movements. Nonetheless, Harris (1969b) believes that one age is as good as another for studying the structure of flexibility as long as the study is kept within the specific age range. However, Corbin and Noble (1980) suggest that when evaluating the flexibility of children and adolescents, growth (especially individual differences in growth) should be considered. Pratt (1989) found that maturational age as measured by Tanner staging was better correlated with strength and flexibility for the lower extremity than was chronological age.
    One's degree of flexibility depends on many interacting factors. In athletics and dance, flexibility relates to the level of preparation and training (Alexander 1991; Chatfield et al. 1990; Klemp et al. 1984; Nelson et al. 1983; Sermeev 1966). The higher the qualification requirements for many sports and events, the greater the mobility of the athlete. For laypeople, the quality and quantity of one's activities, both occupational and avocational, is of chief importance (Salminen et al. 1993). Although flexibility does decrease with age, the loss appears to be minimized in those individuals who remain active.
 

Increased Tightness of Children Growing Into Adolescence
    Several explanations have been offered for the decline in flexibility experienced by children growing into adolescence. One explanation is that during periods of rapid growth, bones grow much faster than the muscles stretch. As a result, muscle-tendon tightness about a joint increases (Bachrach 1987; Kendall and Kendall 1948; Leard 1984; Micheli 1983). According to Feldman et al. (1999), "growth could not cause a decrease in flexibility, but rather is only associated with it" (p. 28). The controversy regarding growth spurts and flexibility is discussed in chapter 2.
    Another explanation is that the decrease in flexibility, especially in the hamstrings, is directly related to the prolonged sitting position in school (Milne and Mierau
1979; Milne et al. 1981; Feldman et al. 1999). The mechanics of sitting have been investigated by Pheasant (1991, 1996). In brief, most people are comfortable when sitting with a backward rotation of the pelvis so that the superior iliac spine lies well behind the pubis. Palpation of the hamstring tendon Gust behind the knee), will reveal that they are slack. Sitting up straight tightens them. Consequently, over an extended period of time, the hamstrings will shorten to take up the slack. Decreased flexibility and increased tightness could be the result of a less physically active population that is instead watching television, talking on the telephone, playing computer games, and working at desks.
 

Critical Periods of Flexibility Development
    Does a critical period exist during which stretching is most effective in developing flexibility? A critical period is the time after the age when one becomes capable of performing a particular function effectively, when changes are most likely to occur at rapid or optimal rates. Flexibility can be developed at any age, given the appropriate training. However, the rate of improvement will not be the same at every age, nor will the potential for improvement.
    Sermeev (1966) studied hip-joint mobility in 1,440 athletes, 10 to 30 years of age, of both sexes, and 3,000 children and adults not participating in sports. He demonstrated that hip-joint mobility is not developed identically at various ages and not equally for various movements. Specifically, the greatest improvement occurs between the ages of 7 and 11 years. By 15 years of age, the indices of mobility in the hip joint are maximal, and in later years, that amount decreases.
    This information does not mean that a stretching program has no benefit after the critical period has passed or that one critical period determines all potential. Can the effects of the lack of stretching and consequent tightness during the critical period (i.e., the growing years) be counteracted by engaging in stretching programs after the critical period has passed? This question is relevant for older adolescents and adults.
    Evidence suggests that even senior adults benefit from exercise programs for developing ROM (Barrett and Smerdely 2002; Bell and Hoshizaki 1981; Dummer et al. 1985; Frekany and Leslie 1975; Germain and Blair 1983; Hong et al. 2000; Hopkins et al. 1990; Lan et al. 1996; Morey et al. 1989; Rider and Daly 1991; Rikli and Busch 1986; Van Deusen and Harlowe 1987). Maintained or increased use of full joint range could help maintain ROM and offset some of its age-related loss (Bassey et al. 1989). In general, however, the longer one waits to start some type of flexibility program after adolescence, the less likelihood of absolute improvement.

GENDER DIFFERENCES IN FLEXIBILITY

    Evidence suggests that, generally, females are more flexible than males (Allander et al. 1974; Gabbard and Tandy 1988; Haley et al. 1986; Jones et al. 1986). Although conclusive evidence is lacking, several factors, including anatomical and physiological differences, may account for the difference in flexibility between the sexes. Other factors could be smaller muscle mass, joint geometry, and gender-specific collagenous muscle structure (McHugh et al. 1992).
 

Anatomical Gender Differences
    The pelvic regions of men and women allow the female human body a greater range of flexibility. Men's pelvic bones are generally heavier and rougher; the brim is not as rounded; the cavity is less spacious; the sacrosciatic notch, pubic arch, and sacrum are narrower; and the acetabula are closer together than women's. Generally, most women have broader and shallower hips than men and therefore a greater ROM in the pelvic region. In particular, the shallowness of the female pelvis permits a greater degree of joint play.
    However, even among women, pelvic types vary, and each has its own influence on ROM. The most commonly used pelvic classification system was developed by Caldwell and Moloy (1933). It describes four main groups based on the shape of the pelvic brim.
    1. The gynecoid pelvis is the most common type, occurring in 50% of all women. This pelvic type permits the easiest vaginal birth and is characterized by a round or slightly oval pelvic inlet. The subpubic angle, or the pubic arch, is almost 90°.
    2. The android pelvis resembles the male pelvis and is found in about 20% of women. It is characterized by a heart-shaped brim, a wedge-shaped pelvic inlet, and a subpubic angle between 60° and 75°. This pelvis shape, also called the "funnel" pelvis, produces difficulty in delivery because the baby's head frequently becomes arrested transversely in the midpelvis.
    3. The platypelloid or flat pelvis is the least common among men and women. It is found in less than 5 % of those examined and has a kidney-shaped brim and a narrow anteroposterior diameter. During labor, rotation of the baby's head may be restricted, and deep transverse flattening of the head may occur.
    4. The anthropoid pelvis is found in about 20% of women. It has an oval brim, a larger anteroposterior diameter, and a smaller transverse diameter compared with the other types of pelvises. Generally, the pelvis is so large that labor is easy.
 

    Women usually have a greater range of extension in the elbow. Hyperextension may sometimes be linked to the presence of a supratrochlear foramen, an aperture linking
the cornoid and olecranon fossae (Amis and Miller 1982). This ability is the result of women having a shorter upper curve of the olecranon process of the elbow than men.
 

Hormonal Effects of Pregnancy on Flexibility
    Pregnancy affects flexibility by increasing joint laxity (Abramson et al. 1934; Bird et al. 1981; Brewer and Hinson 1978). According to McNitt-Gray (1991), the changes in the pelvic joint during late pregnancy may have both local and systemic causes. Local causes include the weight of the uterus on the pelvic brim and biomechanical factors such as modifications in the center of mass and changes on mechanical loading. Systemic causes are presumably circulating hormones the most commonly of which is relaxin. After childbirth, the production of relaxin decreases, and the ligaments tighten up again. Additional research is required to quantify relaxin-induced changes that occur throughout the body.
 

Relaxin
    Relaxin is a polypeptide hormone structurally related to insulin. It is secreted by the corpus luteum. Three main biological actions have been identified with relaxin: inhibition of uterine contraction, elongation of the interpubic ligament, and softening of the cervix. During pregnancy, the cervix undergoes modifications that allow sufficient dilation for the passage of the fetus at birth. Relaxin was thought to cause joint laxity in pregnant women, but some studies found that increased joint laxity in pregnant women is not associated with serum relaxin levels (Blecher and Richmond 1998; Samuel et al. 1996). Relaxin levels were not associated with human cervical ripening (Eppel et al. 1999) nor with symphyseal distention or pelvic pain in pregnancy (Bjk6rklund et al. 2000). The hormonal influences that bring about softening of the cervix are still poorly understood.
 

Hormone Effects on Newborns
    The limited literature on the relationship between estrogen and joints in newborns deals with congenital dislocation of the hip. Andren and Borglin (1961) suggested that congenital dislocation of the hip could be the consequence of abnormal estrogen metabolism in the fetus during the perinatal period. However, Aarskog et al. (1966) criticized the work of Andren and Borglin (1961) and found no supporting data in their study. Thieme et al. (1968) also determined that this hypothesis was not supported.
 

Other Effects of Pregnancy on Flexibility
    The biological changes that occur in pregnant women have significance for various specialized health-care providers, such as podiatrists, orthodontists, chiropractors, osteopaths, medical doctors specializing in orthopedics, and physical therapists. Peripheral joints, such as the feet, fingers, and knees experience increases in joint laxity during pregnancy (Alvarez et al. 1988; Block et al. 1985; Calguneri et al. 1982; Danforth 1967). Ligament laxity in the lower back and pelvis has been linked with sacroiliac dysfunction (Don Tigny 1985) and changes in the pubic symphysis (Don Tigny 1985; Mikawa et al. 1988). Regarding potential cause and treatment, Williams et al. (1995) write

During pregnancy, the pelvic joints and ligaments relax, while movements increase. Relaxation renders the sacroiliac locking mechanism less effective, permitting greater rotation and perhaps allowing alterations in pelvic diameters at childbirth, although the effect is probably small. The impaired locking mechanism diverts the strain of weight-bearing to the ligaments, with frequent sacroiliac strain after pregnancy. After childbirth the ligaments tighten and the locking mechanism improves; but this may occur in a position adopted during pregnancy. Such sacroliac 'subluxation' causes pain by unusual ligamentous tension; reduction by forcible manipulation may be attempted. The most common position in this condition of subluxation is believed to be backward rotation of the innominate bone relative to the sacrum; usually unilateral, it is on occasion bilateral. (p. 678)

Effects of Oral Contraceptives
    Oral contraceptives are administered to female athletes for a variety of therapeutic reasons (Lebrun 1993). Bennell et al. (1999b) identified several benefits from this use of oral contraceptives: It is a reliable and reversible form of contraception. It decreases the risk of iron deficiency anemia by decreasing menstrual blood loss. It allows manipulation of the menstrual cycle for travel, training, and competition commitments. However, several researchers have published papers lending credence to a hormonal influence on ligamentous laxity and anterior cruciate ligament (ACL) injury. Oral contraceptives may induce structural changes in the metabolism of ACL fibroblasts, resulting in structural and compositional changes. These changes, in turn, could reduce strength of the ACL, predisposing female athletes to ligament injury (Liu et al. 1997). Oral contraceptives may also have significant influences on factors such as neuromuscular coordination and muscular strength (Bennell et al. 1999b; Hewett 2000). Moller-Nielsen and Hammar (1989) found that women using oral contraceptives had a lower injury rate than women not using oral contraceptives. The investigators suggested oral contraceptives might "ameliorate some symptoms of the premenstrual and menstrual period which might also affect coordination and hence the risk of injury" (p. 126).
    However, several investigators have raised awareness that oral contraceptives could possibly increase the risk of ligament injury (Baker 1998; Bennell et al. 1999b; Hewett 2000; Liu et al. 1997; Liu et al. 1996). Pokorny et al. (2000) found that "self-reported oral contraceptive use was not associated with peripheral joint laxity with the knee, fifth finger distal interphalangeal joint and second finger of the proximal interphalangeal joint" (p. 687). However, the researchers cautioned, "another possibility is that oral contraceptives do affect joint laxity, although not in the 3 joints examined" (p. 687). Clearly, additional clinical studies are needed to evaluate the effect of the oral contraceptives on joint and muscle structure.

BODY BUILD AND FLEXIBILITY

    Attempts to relate flexibility to factors such as body proportions, body surface area, skin fold thickness (obesity), and weight have yielded inconsistent results. What is almost unanimously agreed upon is that flexibility is specific (American College of Sports Medicine 2000; Dickenson 1968; Harris 1969a, 1969b). Thus, ROM in the shoulder is not correlated with ROM in the hip, and ROM in one hip or shoulder may not be highly related to ROM in the same joint on the opposite side. Furthermore, flexibility is not only specific to the joints but is also specific to individual joint movements because different musculature, bone structure, and connective tissue are involved in different joint movements. Therefore, no evidence confirms that flexibility exists as a single general characteristic of the human body. Thus, no single composite test or joint action measure can give a satisfactory index of the flexibility characteristics of an individual (American College of Sports Medicine 2000; Harris 1969a, 1969b).
 

Body Segment Lengths and Flexibility
    Several investigators have found that body build as determined by segmental length is not significantly correlated with toe-touch flexibility (Broer and Gales 1958; Harvey and Scott 1967; Mathews et al. 1957; Mathews et al. 1959). In contrast, Broer and Gales (1958) and Wear (1963) found that people with a longer trunk-plus-arm measurement and relatively short legs have an advantage in the toe-touch test over those with long legs and relatively short trunk-plus-arm measurements. The ability to touch the toes with the fingertips may be considered normal for young children; however, between the ages of 11 and 14 years, many young adolescents who show no signs of muscle or joint tightness
are unable to complete this movement. Thus, apparently limited flexibility occurs gradually over the same period of years during which the legs become proportionally longer in relation to the trunk (Kendall and Kendall 1948; Kendall et al. 1970; Kendall et al. 1971). However, Harvey and Scott (1967) found no significant difference between means of the best bend-and-reach scores and excess upper body length (trunk-plus-arm length minus leg length) or the ratio of the trunk-plus-arm length to leg length. When prone back extension and supine back extension were compared with trunk length, no significant correlation was found (Wear 1963).
    Questions persist regarding bias for individuals with extreme arm-leg length differences and other extreme body dimensions. Jackson and Baker (1986) investigated the validity of the sit-and-reach test. They found moderate support (r = .64) for the test as a measure of hamstring flexibility and less support (r = .28) for the test as a measure of low-back flexibility. Jackson and Langford (1989) found good support (r = .89 for males and r = .70 for females) for the sit-and-reach test as a measure of hamstring flexibility. In contrast, support (r = .59 for males and r = .12 for females) for the test as a measure of low-back flexibility was less. Liemohn et al. (1994) found the sit-and-reach test "does not have criterion related validity (r = .29 to .40, ns] as a field test of low-back flexion ROM" (p. 93).
    Cornbleet and Woolsey (1996) have criticized the standard sit-and-reach test as not being a valid measure of back motion. They contend, "more attention needs to be given to the final position of the hip joint rather than measuring the final position of the finger tips" (p. 854). Therefore, an inclinometer should be placed vertically on the sacrum to measure the hip-joint angle during the sit-and-reach test. Safety concerns are another reason some have attempted to modify the sit-and-reach test. The back saver sit-and-reach test (BSR) is a modified version that stretches one hamstring at a time while the other leg is flexed. The rationale "emanates from the work of Cailliet (1988) who suggested that simultaneously stretching both hamstrings may result in excessive posterior disc compression due to the anterior portion of the vertebrae being pressed together" (Patterson et al. 1996). Patterson et al. (1996) found the BSR test similar to the sit-and-reach and modified sit-and-reach test of Hoeger et al. (1990) as a test of hamstring flexibility. Another improvisation is the chair sit-and-reach test (CSR). This test was designed for many older people who, because of their medical conditions or functional limitations, cannot get down and up from the floor positions. The CSR required "participants to sit near the front edge of a chair, extending one leg straight out in front of the hip, with the other leg bent and slightly off to the side" a ones et al. 1998, p. 339). The CSR test results for both male and female participants were reasonable accurate (r = .76 and .81, respectively).
    One possible confounding factor is the difference in individual scapular abduction during the sit-and-reach test. Scapular abduction may account for an estimated 3 to 5 cm of variation in the final sit-and-reach score (Hopkins 1981). Consequently, Hopkins (1981) and Hopkins and Hoeger (1986) have proposed a modified sit-and-reach (MSR) test to negate the effects of shoulder girdle mobility and proportional differences between arms and legs. The MSR test establishes a zero point for each individual on the finger-to-box distance (FBD) based on proportional differences in limb lengths (see figure 10.1). Hoeger et al. (1990) and Hoeger and Hopkins (1992) found that the MSR test does help control for disparities. Normative data and flexibility fitness categories for the MSR test have been reported (Hoeger 1991; Hoeger et al. 1991).
    Gatton and Pearcy (1999) suggest that taller subjects have a larger range of spinal flexion than shorter subjects, based on the assumption that ligament strain is the limiting factor in spinal flexion. Because taller people in general are expected to have slightly longer torsos compared with shorter people, taller people are therefore reasonably expected to have longer ligament lengths. Elaborating, they write

We can investigate the effect of subject height change in a mathematical model of the lumbar spine simply by adjusting the initial height of the intervertebral discs. A 10 mm change in subject height approximately represents a 0.45 mm change in height of the lumbar spine. Spreading this change equally over the joints of the lumbar spine results in an increase in trunk rotation of 0.8 degrees. This change in [sic} comparable to the observed relationship between height and range of spinal rotation where a 1 cm height change results in a 0.72 degree increase in spinal flexion.

    The relationship between subject height and range of spinal flexion raises some interesting questions in relation to apparent differences in range of spinal flexion between males and females. This study reported a significantly larger range of spinal flexion for males than females. Taking into account that the mean height of males is 0.11 m larger than that for females, one must ask whether the difference in range of spinal flexion between the sexes is not, at least partially, a remnant of the height difference (p. 381).
 

Body Weight and Somatotype Effects on Flexibility
Weight, somatotype, skin fold thickness, and body surface area have all been investigated for their possible relationships to flexibility. McCue (1963) found very few significant relationships between overweight and underweight body builds and flexibility. Tyrance (1958) found few significant relationships between flexibility and three extremes in body build: thinnest underweight, fattest overweight, and most muscular. Correlations between flexibility and somatotype are also insignificant (Laubach and McConville 1966a, 1966b). A misconception is that very large individuals (in excess 0000 pounds, or 150 kg) have limited flexibility. Size may be a factor where the extra accumulation of either fat or muscle serves as a wedge (e.g., a large midsection impeding a sit-and-reach test). However, the ability of sumo wrestlers weighing 200 kg to master a complete straddle split eliminates any doubt that size is necessarily a limiting factor in flexibility.
    In terms of lean body mass as calculated by skin fold measurements, flexibility differences were again found to be insignificant (Laubach and McConville 1966a). Krahenbuhl and Martin (1977) found that relationship between body surface area and flexibility was significantly inversely related or not related at all, depending upon the body parts tested. Gabbard and Tandy (1988) examined the relationship of body fatness to the performance of 5-year-old and 6-year-old males and females on the sit-and-reach flexibility test. The data suggested that body fat at four measured sites had little to do with flexibility in either sex. Kettunen et al. (2000) found former elite athletes aged 45 to 68 years with a high body mass index (BM!) had lower ROM than subjects with low BM!.
    Passive stiffuess is significantly correlated to body mass and muscle thickness (Kubo, Kanehisa, and Fukunaga 2001a). Furthermore, Magnusson et al. (1997) reported that the cross-sectional area of the lateral hamstrings was positively related to the passive torque offered by the hamstrings during a stretch maneuver. Women have less resistance to stretch than men, which is attributed to their muscle mass (Gajdosik et al. 1990). Women also have less collagen in their connective tissue.
 

RACIAL DIFFERENCES IN FLEXIBILITY

    The literature reveals that the properties of skeletal muscle and connective tissue are related to physical performance, and these also differ among certain racial groups (Suminski et al. 2002). The term race implies membership in a group in which substantial genetic similarities exist among individuals (Malina 1988). Milne et al. (1976) compared 553 black and white children in kindergarten, 1st grade, and 2nd grade. The grade-race interaction effect indicated that white children were generally more flexible than black children, but the only significant difference (p < .01) was at the 2nd-grade level. Jones et al. (1986) tested 2,546 black and white children in grades 2, 4, and 6. Racial differences in flexibility across sexes and grades were not substantiated. Elbow hyperextension has also been reported to be greater in blacks than in whites (Amis and Miller 1982).
    The relationship between genetics, race, and viscoelastic characteristics of muscles has not received much attention. Fukashiro et al. (2002) investigated the viscoelastic properties of the triceps surae muscle group. They compared 44 college athletes (black: n = 22; white: n =22; female: n = 11; male: n = 11). Black athletes were found to have significantly greater muscle viscosity and elasticity than white athletes. Thus, muscle stiffness was greater among black athletes. The researchers speculated that the "greater muscle stiffuess could contribute to greater sprint/jump performance among black athletes, compared with white athletes" (p. 183).
    A decisive conclusion regarding the relationship between racial origin and flexibility or muscle stiffuess cannot be rendered without detailed and controlled studies. The obstacles to such studies are extremely challenging: "This could be a difficult task given the methodological issues facing such studies, for example, confounding factors (e.g., influence of environmental factors), genetic heterogeneity, and the need to use a longitudinal study design" (Suminski et al. 2002, p. 671).
 

GENETICS AND FLEXIBILITY

    Researchers have documented a genetic factor among some contortionist families. Inherited connective tissue syndromes such as EDS clearly have a genetic component. What does the research indicate regarding flexibility values for biological relatives? Unfortunately, research in this area is limited. A review of the literature was carried out by Bouchard et al. (1997).

Estimated heritability for lower-back flexibility in a sample of male twins 11 to 15 years old was 0.69 (Kovar 1981a), while heritabilities in a combined sample of male and female twins 12 to 17 years old were .84, .70, and .91 for trunk, hip, and shoulder flexibility, respectively (Kovar 1981 b). In contrast, estimated heritability of the sit-and-reach in Indian twins of both sexes 10 to 27 years of age was only .18; controlling for age and several anthropometric indicators of body size raised the estimated heritability to.50 (Chatterjee and Das 1995).

    Correlations for lower-back flexibility in biological siblings and parent-offspring pairs were, respectively, .43 and .29 in a Mennonite community (Devor and Crawford 1984) and .36 and .26 in a nationally represented sample of the Canadian population (Perusse, Leblanc, and Bouchard 1988). Interestingly, among the Mennonite community, grandparent-grandchild and uncle/aunt-nephew/niece correlations were of similar magnitude, .3 7 and .30, respectively (Devor and Crawford 1984). Although the data are limited, the preceding findings may suggest somewhat more genetic influence in flexibility than in strength and motor tasks. In addition, spouse resemblance in lower-back flexibility is quite low, -.99 in the Mennonite (Devor and Crawford 1984) and..10 in the Canadian samples (Perusse, Leblanc, and Bouchard 1988).

DOMINANT LATERALITY AND FLEXIBILITY

    Human handedness has been a source of curiosity for centuries. One of the earliest mentions of handedness is found in the Bible (Judges 20:16), which states that the 26,700-man army of Benjamin had 700 left-handed soldiers. Many people tend to favor a dominant side in their chosen sports or activities. Consequently, they often possess greater strength, coordination, balance, and proprioceptive awareness on one side. Reasons for unilateral development are unknown, although theories exist.
 

Lateralization Versus Mixed Dominance
    Sometimes dominance may be mixed (i.e., no clear hand preference exists). For example, a baseball player may bat left-handed and throw right-handed, or a right-handed diver may twist to the right side (right-handed people normally twist to the left side because the dominant right arm is used to thrust or wrap across the body). Occasionally, some athletes exhibit bilateral skills, despite the unilateral nature of their activities. A baseball switch-hitter, for example, can bat from either side of the plate. In some disciplines, bilateral skills are possible (e.g., dribbling a basketball, leg kicks in the martial arts, and dribbling and kicking in soccer), and in others they are required (e.g., swimming and power lifting).
    Studies have provided evidence of the effect of dominant laterality on the normal musculoskeletal system. Hand grips are stronger on the dominant side (Haywood 1980; Lunde et al. 1972), and bone density is reportedly greater on the dominant side in the lower radius (Ekman et al. 1970) and in the os calcis (Webber and Garnett 1976). Dobeln (personal communication, cited in Allander et al. 1974) found that the radioulnar width in 434 males aged 16 to 27 years (including 307 subjects aged 19 to 21 years) was greater in the right side (p = .001). Muscles of the leg and forearm on the dominant side tend to be larger, more dense on computer tomographic scans, and stronger than on the nondominant side of normal people (Merletti et al. 1986; Murray and Sepic 1968). Furthermore, bone density and muscle mass are also greater in the dominant arm of tennis players (Chinn et al. 1974).
    Mysorekar and Nandedkar (1986) observed that human beings have a tendency to incline their heads predominantly to one side or the other. They investigated
whether dominance in the atlantooccipital articulations would account for this phenomenon. They found that the right side has a tendency to have larger facets or condyles. Because the difference was not statistically significant, a clear right-side dominance could not be identified.
    The relationship between dominant laterality and ROM has received attention, but only a few studies have involved general populations. Allander et al. (1974) found reduced mobility in the right wrist in comparison with the left in both sexes. The researchers believed that this observation was "in accordance with the higher level of exposure to trauma of the right hand in a predominantly right-handed population" (p. 259). Their study also found restriction of movement in the rotation of the left hip joint compared with the right (p = .001 for males; p = .05 for females). This observation might be relevant to the position of the body at work. Kronberg et al. (1990) determined that the average angle for humeral head retroversion was 33° on the dominant side and 29° on the nondominant side in 50 healthy subjects, regardless of gender. A larger retroversion angle was consistent with an increased range of shoulder external rotation. Nonetheless, the study found only slight ROM differences between the dominant and nondominant shoulders. Moseley et al. (2001) measured passive ankle plantarflexion-dorsiflexion flexibility obtained from 300 able-bodied volunteers aged 15 to 34 years. Flexibility variables did not differ between the left and right ankles nor between the dominant and nondominant legs. The symmetry "may be a response to the relatively equal demands placed on both lower limbs during locomotion" (p. 517).
 

Effect of Lateralized Athletic Skills on Flexibility
    Most of the research regarding ROM and laterality pertains to athletes. Chandler et al. (1990) found that tennis players' internal shoulder rotation was significantly tighter on the dominant side than on the nondominant side, and that their range of external shoulder rotation was significantly greater on the dominant side than on the nondominant side. Chinn et al. (1974) substantiated that both male and female tennis players displayed significant decreases in flexibility in internal shoulder rotation of the playing arm. Both sexes also had significant decreases in radioulnar pronation and supination of the playing arm.
    Gurry et al. (1985) found no significant differences in the flexibility of the right and left sides among baseball players. In contrast, Tippett (1986) found a significantly greater hip flexion on the kick leg than on the stance leg and a greater internal hip rotation of the stance leg than of the kick leg of baseball pitchers. According to Tippett (1986), "the results appear to be products of the pitching mechanism or the pitcher himself, just as specific upper extremity motion, strength, and anatomical characteristics have been found specific to pitchers" (p. 14).
    Koslow (1987) tested 320 male and female students of specific ages (i.e., 9, 13, 17, and 21 years) for bilateral flexibility of shoulder flexion-extension and of the lower extremity (by a modified sit-and-reach test). Little difference was found between the dominant and nondominant shoulders of the 13-year-old, 17 -year-old, and 21-year-old females. In contrast, shoulder range measurements for the males significantly decreased for the same age groups. Dominant shoulder joint flexibility measures of the 17 -year-old and 21-year-old females were significantly greater than those of the 17 -year-old and 21-year-old males. Males' decrease in dominant shoulder flexibility with age may relate to their activity patterns in such a way as to inhibit increases in flexibility. Specifically, males may exhibit a more forceful and mature (effective) throwing pattern as compared with females across all ages. Flexibility measures of the nondominant lower extremity of the 17 -year-old and 21-year-old males were significantly greater than the same measures of the dominant lower extremity. For the females, a very small and insignificant increase of flexibility in the nondominant leg Was found for all the age groups except the 9-year-olds.
    In another study (Bonci et al. 1986) of static and dynamic range of the glenohumeral joint of male and female athletes, the dominant arm had approximately 5% more motion compared with the nondominant arm in both sexes. Dynamic ROM averaged 25° more than static motion. An analysis of the effect of a modified Bristow surgical procedure for recurrent dislocation or subluxation of the shoulder demonstrated that static and dynamic ROM were significantly reduced by surgery. Henry (1986, p. 17), commenting on the paper of Bonci et al. (1986), stated that the main point is "postoperative range of motion in the dominant shoulder compared to the range of motion in the nondominant side can be misleading due to the increased range of motion of the dominant shoulder prior to surgery."
 

WARMING UP AND COOLING DOWN

    Warm-up is a group of exercises performed immediately before an activity to provide a period of adjustment between rest and exercise. Warm-up improves performance and reduces the chance of injury by mobilizing the individual mentally as well as physically (Sweet 2001). Analogous to warm-up is cool-down (also called warmdown). Cool-down is a group of exercises performed immediately after an activity to provide a period of adjustment between exercise and rest. Stretching is often utilized as an adjunct to warm-up or cool-down. However, stretching is not a warm-up activity. Stretching before warm-up increases the risk of injury.
 

Warming Up

    Warm-up is either passive or active. Passive warm-up incorporates the use of an outside agent or modality (e.g., hot baths, infrared light, or ultrasound). Active warm-up is self-initiated and can be further divided into formal and general warm-up. Formal warm-up includes movements that either mimic or are employed in the actual performance activity (e.g., a baseball player will throw a ball or swing a bat to warm up). General warm-up consists of movements not directly related to those employed in the activity itself (e.g., light calisthenics, jogging, or stationary bicycling). The nature of the warm-up depends on the individual's needs. It should be intense enough to increase the body core temperature and cause some sweating but not so intense as to cause fatigue (Hagerman 2001; Karvonen 1992; Kulund and Tottossy 1983; Shellock and Prentice 1985; Stewart and Sleivert 1998). The effects of warm-up will ultimately wear off (Whelan et al. 1999), but how soon depends on a number of factors such as clothing, exercise intensity, and specificity of the warm-up. Hardy et al. (1983) found that passive warm-up was significantly more effective in increasing hip flexion than active warmup. Whelan et al. (1999) also found warm-up significantly increased flexibility as measured by the sit-and-reach test in downhill skiers. Stewart and Sleivert (1998) found that warm-up improved ROM in ankle dorsiflexion and hip extension, but knee flexion did not change.
    The benefits of warm-up are possibly more psychological than physiological (Harmer 1991; Karvonen 1992; Kulund and Tott6ssy 1983; Miller 2002; Shellock and Prentice 1985; Sweet 2001; Tiidus and Shoemaker 1995). Explanations depend upon specific circumstances and methodologies. Conventional warm-up may help athletes become more mentally prepared, "if they use a specific method of warm-up which provides them with a rehearsal of the event" (Shellock and Prentice 1985, p. 271). The time before the athletic competition may be a time of frustration for certain athletes. Warm-up routines may provide a suitable constructive outlet channel for athletes to vent their anxieties. An athlete's level of arousal influences performance. As explained by Karvonen (1992), "complex performances are enhanced if arousal can be alleviated, while simple performances are improved when arousal is increased. Perhaps warm-up could be used either to alleviate or to enhance arousal depending on the type of performance to follow" (p. 197). Consequently, performance is enhanced.
    A clear distinction should be made between warm-up exercises and flexibility exercises. Flexibility exercises are used to increase the ROM of a joint or set of joints progressively and permanently. Flexibility exercises should always be preceded by a set of mild warm-up exercises because the increase in the tissue temperature produced by the warm-up exercises makes the flexibility exercises both safer and more productive (Sapega et al. 1981).

    However, an increase in temperature causes a reduction in tensile strength of connective tissue, and thus more ruptures might be expected after warming up, but increased temperature seems to cause an increase in extensibility, which may be the reason warming up does indeed prevent ruptures (Troels 1973). Despite the widely held belief that warm-up reduces the risk of injury and improves performance, compliance of athletes and non-athletes has been found inadequate among golfers (Fradkin et al. 2001) and college students (Simon 1992). Furthermore, contrary to popular belief, warm-up performed "without" stretching does not increase ROM (Shrier and Gossal 2000).
Benefits associated with warming up include the following (Bishop 2003; Goats 1994; Hemmings et al. 2000; Karvonen 1992; Whelan et al. 1999; Verkhoshansky and Siff 1993):
    • Increased body and tissue temperature
    • Increased blood flow through active muscles by reducing vascular bed resistance
    • Increased heart rate, which will prepare the cardiovascular system for work
    • Increased metabolic rate
    • Increases in the Bohr effect, which facilitates the exchange of oxygen from hemoglobin
    • Increased speed at which nerve impulses travel, and thereby facilitation of body movements
    • Increased efficiency of reciprocal innervation (thus allowing opposing muscles to contract and relax faster and more efficiently)
    • Increased physical working capacity
    • Decreased viscosity (or resistance) of connective tissue and muscle
    • Decreased muscular tension (improved muscle relaxation)
    • Enhanced connective tissue and muscular extensibility
    • Enhanced psychological performance
 

    Kopell (1962) believes that some fatalities associated with exercise may have been avoided if adequate warm-up had occurred. Barnard et al. (1973a, 1973b) have suggested that warm-up also prevents STsegment depression (an electrocardiographic abnormality). This abnormality is sometimes seen in healthy people at the beginning of fast running performances.
 

Viscosity Effects
    Viscosity is resistance to flow, or an apparent force that prevents fluids from flowing easily. Connective tissue and muscular viscosity might be partially responsible for restricting movement. Viscosity has no long-term effect on the improvement of one's flexibility. Rather, its effects relate to various physiological factors that exist at the moment stretch is developed. Temperature has an inverse effect on viscosity; that is, as the temperature increases, fluid viscosity decreases, and vice versa. Reduced viscosity facilitates relaxation of collagenous tissues (Sapega et al. 1981). The mechanism behind this thermal transition is still unknown. However, the collagen intermolecular bonding possibly becomes partially destabilized, enhancing the flow properties of collagenous tissue (Mason and Rigby 1963; Rigby et al. 1959). This reduced viscosity in turn decreases resistance to movement and increases flexibility.
    The most common method of elevating body temperature and reducing tissue viscosity is warm-up exercise. Other methods include superficial heat (heat packs, hot showers) and deep heat (diathermy and ultrasound). The effectiveness of a heat pad applied to the back of the thigh did not affect the ROM in the hip joint. However, when combined with stretching, hip flexion increased further but not significantly (Henricson et al. 1984). Heating pads raise temperature in superficial muscles only a few degrees. The subcutaneous fat and natural vascular cooling system possibly prevent further increases in temperature of the muscles and the connecting tissues of the hip joint (Lehmann et al. 1966; Prentice 1982).
Continuous ultrasound can effectively increase temperatures in human muscle and tendon (Draper et al. 1995; Draper et al. 1991) to therapeutic levels. However, Draper et al. (1998), Draper and Ricard (1995) and Rose et al. (1996) found tissue temperatures remain at therapeutic levels for only 2 to 4 minutes. Therefore, stretch must immediately follow treatment to take advantage of these higher temperatures (Draper et al. 1998). Draper et al. (1998) found the increased ROM associated with ultrasound heat "is not maintained over the long term and is not more than the range of motion gained from stretching alone".
 

Effect of Warm-Up on Injury Rates
    Several studies have raised questions about the ability of warm-up exercise to increase flexibility and reduce injury. Williford et al. (1986) investigated the effects of warming up the joints by jogging and then stretching on increasing joint flexibility. Their results did not support the claim that warming up the muscle by jogging before stretching results in significant increases for all the joint motion angles evaluated. The Ontario cohort study of 1,680 runners found that runners who say they never warm up have less risk of injury than those who do, and runners who use stretching "sometimes" are at apparently higher risk of injury than those who usually or never use stretching (Walter et al. 1989). However, Grana cautioned in an interview by Finkelstein and Roos (1990), that the study's findings probably reflect "the terrible number of variables that you can't control" (p. 49) in such a study. van Mechelen et al. (1993) conducted a 16-week study of 316 subjects randomly split into an intervention group (159 subjects) and a control group (167 subjects). Injury incidences for control and intervention subjects were 4.9 and 5.5 running injuries, respectively, per 1,000 hours of running exposure. Therefore, warm-up, cool-down, and stretching exercises did not reduce the running injury incidence.
    Further confounding the controversy regarding the benefits of warm-up was a study by Strickler et al. (1990) investigating the effects of passive warming on biomechanical properties of the musculotendinous unit of rabbit hindlimbs heated to 35° C (95° F) and 39° C (102° F) and then subjected to controlled strain injury. The force at failure was greater at 35° C than at 39° C, and the difference in energy absorbed by the muscles before rupture was not statistically significant. Obviously, the relationship among warm-up, stretching, flexibility, and injury is extremely complex, and additional research is needed to resolve these uncertainties.
    Murphy (1986) points out a dangerous misconception about the order of stretching and warm-up in an exercise program:

Some health clubs and fitness instructors have encouraged athletes to stretch before warming up. Their reasoning: Cold muscles, they claim, are like plastic, and stretching them results in a more permanent stretch, as opposed to stretching the muscles when they are warm and pliable like a rubber band.

This method is not supported by any research. It is an invitation to probable injury. Stretching should always be preceded by warm-up.
 

Cooling Down
    Cool-down is a group of exercises performed immediately after an activity to provide a period of adjustment between exercise and rest. Although cool-down may serve as an additional effort to improve flexibility, its main objective is to facilitate muscular relaxation, promote the removal of muscular waste products by the blood, reduce muscular soreness, and allow the cardiovascular system to adjust to lowered demand. Stretching should be incorporated immediately after the main part of a workout and cooldown period, because tissue temperatures are highest (Sapega et al. 1981).
    Karvonen (1992) has suggested that cool-down is also important in reaching an emotional balance after the possible disappointment of a poor performance. In particular, when the next competition or performance soon follows, preparation for it can begin during the cool-down from the initial performance. Furthermore, the cool-down period may be the most beneficial time for the coach to give feedback.
 

FROM: Science of Flexibility by Michael J. Alter