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5 Physical Activity and Special Considerations for Children, Adolescents, and Pregnant and Postpartum Women A number of population groups merit special attention with regard to the relationship between physical activity and health. This chapter pre- sents evidence of the effects of physical activity on the health of school- age children and adolescents and of pregnant and postpartum women. For convenience, the term youth was sometimes used by the presenters to refer to school-aged children and adolescents. The discussion section presents promising evidence relating physical activity to specific out- comes in children and adolescents, information on physical activity and skeletal growth in children and adolescents, and a brief summary of points raised during the group discussion. PHYSICAL ACTIVITY AND CHILDREN AND ADOLESCENTS Presenter: Robert M. Malina This presentation included background information, evidence related to the benefits of physical activity for children and adolescents, and con- sideration of the amount of physical activity needed by youth. Background It is generally presumed that regular physical activity is essential to support the normal growth and maturation, health, and fitness of children and adolescents. Fairly recently, an expert panel was formed to review 95
96 PHYSICAL ACTIVITY WORKSHOP the effects of physical activity on health and behavioral outcomes in school-aged children and adolescents ages 6 to 18 years. The panel also was charged with developing a recommendation for physical activity for children and adolescents. The effects of physical activity on indicators of growth and maturation were not considered. In particular, the panelists addressed the following questions: â¢ What are the health, fitness, and behavioral benefits of regular physical activity for school-aged children and adolescents? â¢ In terms of frequency, intensity, and time (duration), what amount and type of activity is needed to bring about beneficial effects on the selected indicators? â¢ What amount of physical activity should be recommended? The indicators considered included measures of health, fitness, behavior, and injury risk. Panelists used a standardized evaluation format to exam- ine the evidence. The panel report is published in the Journal of Pediat- rics (Strong et al., 2005). Evidence of Specific Outcomes Adiposity Normal weight youth Enhanced physical activity programs appear to have a minimal effect on adiposity among youth of normal weight. Cross-sectional and longitudinal comparisons of active and less active youth give equivocal results regarding skinfold measurements. Two ex- perimental studies suggest that youth need a considerable amount of physical activity to maintain a healthy weight. Obese youth Experimental studies of obese youth indicate a reduction in total body and visceral adiposity with regular activity. In this case, regu- lar activity is defined as a variety of moderate to vigorous activity for 30 to 40 minutes per day on 3 to 5 days per week.
97 CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN Lipids and Lipoproteins In a variety of intervention studies of youth, weak beneficial effects of physical activity are reported for reducing the level of high-density lipoprotein cholesterol and triglycerides. The amount of physical activity needed to achieve beneficial effects appears to be a minimum of 40 min- utes per day of moderate to vigorous activity for 5 days per week over a 4-month period. A sustained volume of physical activity may be a key factor. School-based programs generally were not effective in improving the lipid and lipoprotein profiles of youth. Blood Pressure Normotensive youth No clear association was found between physical activity and blood pressure in normotensive youth. Hypertensive youth Experimental aerobic training programs have dem- onstrated beneficial effects on the blood pressure of hypertensive youth. Thirty-minute sessions of sufficient intensity to improve aerobic fitness on 3 days per week for 12 to 32 weeks provided benefit. Two studies showed no change in blood pressure with resistance training. Among youth with mild essential hypertension, some evidence suggests that aerobic programs may reduce blood pressure. Other Indicators of Cardiovascular Health In general, the data that physical activity may improve other indica- tors of cardiovascular health among youth are very limited. However, data suggest that aerobic training increases resting vagal tone as reflected in higher heart rate variability, which is a marker of cardiac parasympa- thetic activity. The implications of this change for the development of cardiovascular disease are uncertain. Metabolic Syndrome and Type 2 Diabetes Two studies indicate that insulin and triglyceride concentrations and adiposity are favorably influenced by physical activity in obese youth.
98 PHYSICAL ACTIVITY WORKSHOP The physical activity level was moderate to vigorous three times per week. Essentially no useful data are available regarding associations of physical activity and type 2 diabetes in children and adolescents. Skeletal Health Evidence from a variety of studies suggests a beneficial effect of regular physical activity on bone mineral content and bone mineral den- sity. Most of the studies have been conducted on prepubertal children or adolescents in the early stages of puberty. Typically, the studies involve moderate- to high-strain physical activity for 10 to 60 minutes per day at least 2 to 3 times per week. Some have used 10 minutes of impact activ- ity and 45 to 60 minutes of general weight-bearing activity. Limited evi- dence suggests little or no positive effect after puberty. Aerobic Fitness In both cross-sectional and longitudinal studies, data suggest higher aerobic fitness in active youth than in less active youth. In experimental studies of children ages 8 years and older, the evidence consistently shows a favorable effect of physical activity on aerobic fitnessânamely, about a 10 percent increase in peak VO2. The programs that produced benefits involved continuous vigorous aerobic activity of various types for 30 to 45 minutes per day at least 3 days per week. Muscular Strength and Endurance Although the cross-sectional and longitudinal data are equivocal re- garding the association of physical activity with muscular strength and endurance among youth, experimental data show significant gains with weight training 2 to 3 days per week with a rest day between sessions. The key appears to be a variety of progressive resistance activities, adult supervision, and the involvement of both reciprocal and large muscle groups.
99 CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN Behavioral Outcomes Examining evidence relating physical activity to behavioral out- comes poses a number of challenges, including a large number of out- come measures, problems with sampling, quasi-experimental studies, and various modes and combinations of activity. Physical activity is fairly strongly associated both with global self-esteem and physical self- concept. Aerobic activity generally has a small positive effect on symp- toms of depression and anxiety, but the addition of cognitive behavioral modification produces a stronger positive effect. The data on effects of physical activity on academic performance are limited, with very mixed results that tend toward the positive. Notably, quasi-experimental data indicate that the allocation of more curricular time to physical education or activity programs did not negatively influ- ence academic achievementâeven when less time was allocated to other subjects. Risk of Injury As discussed earlier by Drs. Jones and Hootman (see Chapter 4), physical activity increases the risk of injury. Data are limited, however, regarding associations of school, recreational, and free-time activities with injury. Comparability among studies is limited because of the chal- lenges of accurate surveillance and reporting, the definition of injury, and exposure statistics, among other reasons. Evidence from six studies of physical education indicates very low risk of injury in programs involving moderate to vigorous activity for 10 to 40 minutes on 3 days per week. In a study of self-reported injuries in recreational and sporting activities among Australian adolescents, about one in four students reported an injury, and the rate of injury was one per three instances of participation (Grimmer et al., 2000). Most (72 percent) of the injuries were classified as minor (e.g., bruises, aches, strains) and 5 percent as major (e.g., fracture, dislocation, concussion). Summary of the Evidence on Physical Activity and Youth The expert panel that addressed the benefits of physical activity in children and adolescents (Strong et al., 2005) determined that doseâ
100 PHYSICAL ACTIVITY WORKSHOP response data are limited. The evidence base is strongest for skeletal health, aerobic fitness, muscular strength and endurance, adiposity in the obese, and blood pressure in the hypertensive. Evidence is suggestive for adiposity and lipids in nonobese youth; for blood pressure in normoten- sive youth; and for self-concept, anxiety and depression symptoms, and academic performance. Data on associations of physical activity with metabolic syndrome, type 2 diabetes, and some indicators of cardiovas- cular health currently are very limited for youth. Most of the experimental intervention protocols used a continuous activity program, but the activities of children, especially young children, are primarily intermittent. Figure 5-1 illustrates how activity needs vary with age during childhood and adolescence. This suggests the need to develop activity protocols that examine the effects of high-intensity, in- termittent activity on indicators of health, fitness, and behaviors. A difficulty in evaluating the effects of activity on indicators of health and fitness in children and adolescents relates to the fact that the outcome variables change with normal growth, maturation, and devel- opment, whether or not the individual is physically active on a regular basis (Malina et al., 2004; Malina and Katzmarzyk, 2006). Available data do not consider interindividual differences in maturity status and pro- gress, especially during the adolescent years. Data also are needed on the persistence of activity-induced beneficial effects and on the amount of activity needed to maintain these beneficial effects.
101 CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN Dashed line: general physical 100 activity, emphasis on motor skills 90 80 Solid line: prescriptive physical Relative emphasis, % 70 activity, emphasis on health, fitness, behavioral outcomes 60 50 Childhood: activities largely 40 intermittent Adolescence: 30 increased capacity for continuous activities 20 10 0 2 4 6 8 10 12 14 16 18 Age (years) FIGURE 5-1 Changes in types of activity needs with increasing age of children and youth. SOURCE: From R.M. Malina, 1991, Fitness and Performance: Adult Health and the Culture of Youth. In New Possibilities, New Paradigms? American Academy of Physical Education No. 24, edited by R.J. Park and H. M. Eckert, pages 30- 38. Â© 1991 by American Academy of Physical Education. Adapted with per- mission from the American Academy of Kinesiology and Physical Education and Human Kinetics (Champaign, IL). Concluding Remarks In closing, Dr. Malina highlighted the importance of investigating how to prevent unhealthy weight gain in children and adolescents, con- sidering physical inactivity as well as physical activity. Inactivity and activity have different meanings and contexts in children and youth, and they are generally independent of each other.
102 PHYSICAL ACTIVITY WORKSHOP PHYSICAL ACTIVITY DURING PREGNANCY AND THE POSTPARTUM PERIOD Presenter: James M. Pivarnik This presentation addressed the historical concern with potential harm from maternal physical activity, short-term benefits from maternal physical activity for the mother and fetus, and the role of maternal physi- cal activity on chronic disease risk of the mother later in life. Historical Concerns and Guidelines Early studies on physical activity and pregnancy were concerned with harm rather than benefit. Most studies used animal models. Human studies examined cardiorespiratory responses and thermoregulation in the mother, heart rate of the fetus, and outcomes of pregnancy such as birth weight, gestational length, and adverse events. The American College of Obstetricians and Gynecologists (ACOG) developed the first exercise guidelines for pregnant women in 1985 (ACOG, 1985). Those guidelines were based on the early studies and were conservative. They included a maximum heart rate of 140 beats per minute and time limits for physical activity. The potential need for indi- vidualization of the recommendations was noted. Between 1985 and 1994, nearly 600 relevant studies were published, most of which focused mainly on doing no harm. Many of these were laboratory studies with small sample sizes, and most involved acute ma- ternal responses to exercise. The data suggested (1) no detrimental ef- fects of the exercise to the mother or fetus, (2) possible reduced length of labor, (3) possible improvement in gestational diabetes mellitus (GDM), and (4) relatively little loss of fitness by chronic exercisers. The use of a target heart rate was found to be somewhat problematic because of in- consistent influences of pregnancy on heart rate among women. ACOG updated its guidance for exercise during pregnancy in 1994 (ACOG, 1994) and again 8 years later (ACOG, 2002).
103 CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN Current Evidence Relating Physical Activity to Outcomes More recently, a number of studies have examined whether maternal physical activity affects birth weight. Perkins and colleagues (2005) per- formed a meta-analysis of the effect of maternal physical activity on birth weight, stratified by exercise intensity. They found a slightly lighter (but not low) mean birth weight for infants born to women who took part in vigorous activity (see Figure 5-2). The topic merits attention because in the year 2000 more than two-thirds of women in the childbearing years took part in some type of physical activity (Evenson, 2004), and many women want to continue these activities during pregnancy. Information on vigorous activity during pregnancy is very limited. In 2005, an expert panel was assembled to examine the impact of physical activity during pregnancy and the postpartum period on chronic disease risk (Pivarnik et al., 2006). That panel addressed a number of topics, in- cluding the association of physical activity with the risk of preeclampsia and with the risk of GDM. Regular physical activity in early pregnancy is associated with a reduced risk of preeclampsia in two caseâcontrol studies (Marcoux et al., 1989; Sorenson et al., 2003) and one cohort study (Saftlas et al., 2004). The evidence is not strong, but it is in the same direction. Although pilot data from Mottola and colleagues (2005) show a slightly reduced rate of GDM in overweight pregnant women who began a program of physical activity, data are insufficient to de- velop specific optimal physical activity guidelines for GDM prevention. Some evidence is available regarding the role of physical activity in postpartum weight reduction. Larson-Meyer (2002) reviewed approxi- mately 60 cross-sectional and randomized trials on this topic, looking at postpartum exercise but not activity performed during pregnancy. When compared to sedentary women, those who performed moderate physical activity without caloric restriction did not appear to show greater weight or fat loss. In a later report, Rooney et al. (2005) evaluated nearly 800 women early in pregnancy and followed them for 15 years. Disease and risk factor development (i.e., diabetes, heart disease, dyslipidemia, and hypertension) were positively related to weight gain over the 15 years. The women who continued to perform aerobic exercise postpartum were less likely to become obese than those who did not. Among factors that can create barriers to postpartum physical activity are lack of time and lack of child care (Beilock et al., 2001).
104 PHYSICAL ACTIVITY WORKSHOP Moderate intensity PA Vigorous intensity PA Pooled Effect Size 0.05; Heterogeneity P = .49 Pooled Effect Size -0.27; Heterogeneity P < 0.01 -2.00 0 2.00 Standardized mean difference (birth weight) FIGURE 5-2 Effect of maternal physical activity on birth weight, stratified by exercise intensity. The sizes of the boxes reflect the relative sample sizes. NOTE: PA = physical activity. The studies represented by the boxes in the fig- ure are listed in descending order as follows: Beckmann and Beckmann, 1990; Bell et al., 1995; Botkin and Driscoll, 1991; Clapp, 1990, 1996; Clapp and Dick- stein, 1984; Clapp et al., 1998, 1999, 2000; Collings et al., 1983; Homs et al., 1996; Lewis et al., 1998; Magann et al., 1996, 2002; Marquez-Sterling et al., 2000; Pivarnik et al., 1994; Rice and Fort, 1991; Stemfeld, 1997. SOURCE: Perkins et al. (2005).
105 CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN Concluding Remarks Dr. Pivarnik identified the effects of maternal recreational exercise on the health, growth, and development of the offspring as an exciting new topic of research. Dr. Jim Clapp is a lead investigator in this area, but few studies are available and sample sizes are small. Initial studies indicate that beginning or continuing recreational exercise during preg- nancy has no identifiable acute or chronic adverse effects on the off- spring and may have some positive effects. There is a great need for prospective, randomized exercise-intervention studies in diverse popula- tions. DISCUSSION Promising Lines of Evidence Relating Physical Activity to Outcomes in Children and Youth Discussant: Russell R. Pate There is much less evidence supporting the development of physical activity guidelines for children and youth than there is for adults. The young age group poses enormous research design challenges for two ma- jor reasons: (1) most young persons are healthy and remain so during childhood and adolescence, and (2) young people are the most active segment of our society. These two factors make it very difficult to dem- onstrate positive health effects. In view of these factors and the data pre- sented by Dr. Malina, Dr. Pate suggested directing attention to the following three topics: 1. The amount of activity sustained over time that minimizes the risk of excessive weight gain over that period. This focus could be valuable in part because the increased prevalence of obesity is of concern to society at large. A few U.S. studies and more European studies have addressed activity and weight gain over time in children and youth of normal weight. 2. Activity related to fitness. Children and youth with low fitness levels tend to have low fitness during adulthood, and evidence shows a strong association of low adult fitness with a range of chronic disease outcomes (see Chapter 2). Using data from the
106 PHYSICAL ACTIVITY WORKSHOP National Health and Nutrition Examination Survey, Dr. Pateâs group recently found that about one-third of U.S. children and youth fail to meet aerobic fitness standards. Moreover, Dr. Malina reported that the literature on this topic is fairly extensive. 3. Activity and the multiple metabolic syndrome. This is a rapidly developing area, and it is possible that evidence will be available soon regarding protective levels of physical activity. Physical Activity and the Skeleton in Children and Youth Discussant: Heather McKay Topics addressed in this discussion include the optimal period for a response of bone to exercise, bone geometry related to bone strength, modes and intensity of exercise, and the benefits of muscular strength for the skeleton. Timing Childhood is an essential time to introduce physical activity in terms of healthy growth of the skeleton. The key time appears to be early pu- berty. Bone mineral content velocity lags behind peak growth and peak linear growth by about 8 months. Approximately 26 percent of oneâs adult skeleton is accrued in the 2 years around the age of peak skeletal growth (age 11.8 years for girls and 13.4 years for boys). This amount of accrual represents as much bone as most people will lose in their entire adult lives (Arlot et al., 1997). Mechanical Loading and Bone Geometry Stress causes strain on bones. Bone is accrued primarily on the pe- riosteal (outer) surface, meaning that the bone is getting larger (Turner and Robling, 2003). An increase of as little as one millimeter in the outer surface of bone increases strength substantially. Bone may alter its geo- metric properties independently of changes in bone mass (Jarvinen et al., 1999). Adding bone to the outside increases strength, and adding bone to
107 CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN the endosteal (inner) surface increases strength also. Dual-energy X-ray absorptiometry (DEXA) does not detect these responses. Bone mineral density by DEXA may be the same across different configurations of a cross section of bone, even though strengths may differ by four or eight times. Thus there is a huge gap between knowing the amount of bone mineral in a given bone and knowing its structural capacity. The whole bone structure substantiates the mechanical competence of bone (Jarvinen et al., 2005). Modes and Intensity of Exercise Data are lacking on what mode or intensity of physical activity is os- teogenic (builds bone). Studies with rats indicate that 5 jumps per day were as effective as 40 jumps per day in increasing bone strength (Umemura et al., 1997). Preliminary data from Dr. McKayâs group suggest that taking five jumps each of three times per day during school increases distal tibia muscular strength in boys but not in girls. Muscular Strength and the Skeleton In both males and females, peak lean mass accrual occurs about 4 to 6 months before peak bone mass accrual (Rauch et al., 2004). Data show a very close relationship between a surrogate for muscle strength, total lean body mass, and bone strength. Similarly there is a close asso- ciation between an increase in muscle mass and an increase in bone mass. Thus interventions that increase muscular strength also may bene- fit the skeleton. Concluding Remarks Dr. McKay highlighted the following points: â¢ Early puberty is the period when the response of bone to exercise is optimized. â¢ Small changes in bone geometry result in substantial increases in bone strength.
108 PHYSICAL ACTIVITY WORKSHOP â¢ Frequent episodes of short exercise bouts may be as effective as sustained exercise. â¢ Further studies are needed. â¢ Schools may be the key avenue to having an impact on chil- drenâs health. â¢ The benefits of exercise do not persist if exercise is withdrawn. GROUP DISCUSSION Moderator: Patty S. Freedson Among the points raised during discussion were the following: â¢ The quantification of intermittent bouts of physical activity may pose a challenge. Accelerometry may be helpful, as may other new technological approaches to assess behavior objectively. â¢ Questions were raised about the appropriateness of an exercise prescription model for children. Considering a diversity of ac- tivities may be helpful, along with setting appropriate targets for the duration of moderate to vigorous intensity activity. â¢ More data are needed about the risk of physical activity among children and youth. â¢ Work in animals suggests that dividing loads over several ses- sions per day substantially increases the osteogenic response. Evidence regarding this relationship in humans is needed. REFERENCES ACOG (American College of Obstetricians and Gynecologists). 1985. Technical Bulletin: Exercise During Pregnancy and the Postnatal Period. Washington, DC: ACOG. ACOG. 1994. Technical Bulletin No. 173, Women and Exercise. Washington, DC: ACOG. ACOG. 2002. Exercise during pregnancy and the postpartum period. ACOG Committee Opinion 267. Obstet Gynecol 99(1):171â173.
109 CHILDREN, ADOLESCENTS, AND PREGNANT AND POSTPARTUM WOMEN Arlot ME, Sornay-Rendu E, Garnero P, Vey-Marty B, Delmas PD. 1997. Ap- parent pre- and postmenopausal bone loss evaluated by DXA at different skeletal sites in women: The OFELY cohort. J Bone Miner Res 12(4): 683â690. Beilock, SL, Feltz DL, Pivarnik JM. 2001. Training patterns of athletes during pregnancy and postpartum. Res Q Exerc Sport 72(1):39â46. Evenson KR, Savitz DA, Huston SL. 2004. Leisure-time physical activity among pregnant women in the US. Paediatr Perinat Epidemiol 18(6): 400â407. Grimmer KA, Jones D, Williams J. 2000. Prevalence of adolescent injury from recreational exercise: An Australian perspective. Adolesc Health 27(4): 266â272. Jarvinen TL, Kannus P, Sievanen H. 1999. Have the DXA-based exercise stud- ies seriously underestimated the effects of mechanical loading on bone? J Bone Miner Res 14(9):634â1635. Jarvinen TL, Sievanen H, Jokihaara J, Einhorn TA. 2005. Revival of bone strength: The bottom line. J Bone Miner Res 20(5):717â720. Larson-Meyer DE. 2002. Effect of postpartum exercise on mothers and their offspring: A review of the literature. Obes Res 10(8):841â853. Malina RM. 1991. Fitness and performance: Adult health and the culture of youth. In: Park RJ, Eckert HM. Eds. New Possibilities, New Paradigms? American Academy of Physical Education Papers No. 24. Champaign, IL: Human Kinetics. Pp. 30â38. Malina RM, Bouchard C, Bar-Or O. 2004. Growth, Maturation, and Physical Activity, 2nd edition. Champaign, IL: Human Kinetics. Malina RM, Katzmarzyk PT. 2006. Physical activity and fitness in an interna- tional growth standard for preadolescent and adolescent children. Food and Nutrition Bulletin 27(Suppl 4):S295âS313. Marcoux S, Brisson J, Fabia J. 1989. The effect of leisure time physical activity on the risk of preeclampsia and gestational hypertension. J Epidemiol Com- munity Health 43:147â152. Mottola MF, Sopper MM, Vanderspank D, Charlesworth S, Hanley A. 2005. Insulin sensitivity is maintained in late pregnancy among overweight women at risk for gestational diabetes participating in a Nutrition and Exercise Life- style Intervention Program (NELIP). Can Federation Biological Societies Proceedings. Pp. 62. Perkins CD, Pivarnik JM, Reeves MJ, Feltz DL, Womack CJ. 2005. Maternal physical activity and birth-weight: A meta-analysis. Med Sci Sports Exerc 37(Suppl 5):S177. Pivarnik JM, Chambliss HO, Clapp JF, Dugan SA, Hatch MC, Lovelady CA, Mottola MF, Williams MA. 2006. Impact of physical activity during pregnancy and postpartum on chronic disease risk. Med Sci Sports Exerc 38(5):989â1006. Rauch F, Bailey DA, Baxter-Jones A, Mirwald R, Faulkner R. 2004. The 'mus- cle-bone unit' during the pubertal growth spurt. Bone 34(5):771â775.
110 PHYSICAL ACTIVITY WORKSHOP Rooney BL, Schauberger CW, Mathiason MA. 2005. Impact of perinatal weight change on long-term obesity and obesity-related illnesses. Obstet Gynecol 106(6):1349â1356. Saftlas AF, Logsden-Sackett N, Wang W, Woolson R, Bracken MB. 2004. Work, leisure-time physical activity, and risk of preeclampsia and gestational hyperten- sion. Am J Epidemiol 160(8):758â765. Sorensen TK, Williams MA, Lee I-M, Dashow EE, Thompson ML, Luthy DA. 2003. Recreational physical activity during pregnancy and risk of preeclamp- sia. Hypertension 41(6):1273â1280. Strong WB, Malina RM, Blimkie CJR, Daniels SR, Dishman RK, Gutin B, Hergenroeder AC, Must A, Nixon PA, Pivarnik JM, Rowland T, Trost S, Trudeau F. 2005. Evidence-based physical activity for school-age youth. J Pediatr 146(6):732â737. Turner CH, Robling AG. 2003. Designing exercise regimens to increase bone strength. Exerc Sport Sci Rev 31(1):45â50. Umemura Y, Ishiko T, Yamauchi T, Kurono M, Mashiko S. 1997. Five jumps per day increase bone mass and breaking force in rats. J Bone Miner Res 12(9):1480â1485.