2.3. Physical activity and health-related fitness – pathways from adolescence into adulthood
Physical activity may influence health-related fitness in youth and throughout life. Figure 2 illustrates potential relationships between physical activity and health-related fitness during youth and adulthood. The potential pathways from youth to adulthood are numbered as 1–6. In this chapter the pathways 1 and 5 are discussed more in depth than the others because these two potential pathways are tested in the articles I, II and III. Summary of the longitudinal studies on physical activity from youth and from youth to adulthood is presented in Appendix 1. In these summarized studies, age of the subject at baseline was less than 25 years and the follow-up time was at least two years.
Figure 2. A model describing the relationships between physical activity and health-related fitness in youth and adulthood and potential pathways from youth to adulthood (modified after Blair et al. 1989, Malina 2001a and Twisk et al. 2002a).
2.3.1. Pathway 1: Youth physical activity and adult physical activity
The potential relationship between youth and adult physical activity assumes that physical activity tracks from youth to adulthood (Malina 2001b). Tracking or stability refers to the tendency of individuals to maintain their rank or position within a group over time. Interage correlations between the repeated measurements are most often used to estimate tracking. The age at first observation and the time span between the measurements influences the correlations. Correlations < 0.30 are considered low, 0.30–0.60 moderate and > 0.60 high. (Malina 2001b). Correlations indicate only the association between the two measurements, but nothing about causality or determining factors. Stability of physical activity can be described by the maintenance of a given rank (e.g. active vs. inactive) over time. The association between youth and adult physical activity can also be approached by percentile or risk analyses. The prevalence (%) or probability (risk or odds) of being physically active or inactive in adulthood can be presented for different groups of adolescent physical activity or related variables.
The amount of physical activity declines with inreasing age from youth to adulthood (Campbell et al. 2001, Kemper et al. 2001b, Kimm et al. 2000, Telama et al. 1997). The decline in physical activity is steeper in males than in females after age 12 and the steepest at age 12–15 in males and at age 15–18 in females in Finland (Telama & Yang 2000). Decline in physical activity is observed especially in participation in vigorous physical activity (Barnekow-Bergkvist et al. 1996, Engström 1986, van Mechelen et al. 2000).
2.3.1.1. Youth physical activity as predictor of adult physical activity
A high level of physical activity in childhood and adolescence has been reported to be associated with a high level of physical activity in adulthood (Barnekow-Bergkvist et al. 1998, Engström 1986, Pietilä et al. 1995, Telama et al. 1997). Of the different physical activity variables in adolescence, frequent participation in sports (Barnekow-Bergkvist et al. 1996, Pietilä et al. 1996, Telama et al. 1997), participation in organized sports after school hours (Dennison et al. 1988), membership in a sports club (Barnekow-Bergkvist et al. 1996), participation in sport club training (Telama et al. 1997), playing sport in a school team (Dovey et al. 1988) and participation in competitive sports (Hirvensalo et al. 2000, Telama et al. 1997) have been associated with a high level of adult physical activity.
Being a top-level athlete in young adulthood was associated with a high physical activity level 20 years later (Fogelholm et al. 1994). All different athlete groups, endurance, mixed and power athletes, were more active than the controls, and the former endurance athletes were physically the most active 20 years later (Kujala et al. 2000). In a retrospective study of middle-aged males there was no significant difference in physical activity between former athletes and nonathletes (p > 0.05), although former athletes were shown to be slightly more active in adulthood (Dishman et al. 1988). In a retrospective study of Taylor et al. (1999) the type of sports (team, individual, both or none) in childhood was not associated with males’ physical activity level at 45 years.
Only few studies have evaluated physical inactivity as an outcome. Frequent participation in sports at age 16 was associated with a decreased risk of being inactive at age 34 (Barnekow-Bergkvist et al. 1996). Males who were inactive at age 17 were more likely to be inactive at age 30 than their active counterparts (Vanreusel et al. 1997).
There are several other factors related to adolescent physical activity which are associated with adult physical activity. High marks in physical education at school (Barnekow-Bergkvist et al. 1996, Glenmark et al. 1994, Kuh & Cooper 1992, Telama et al. 1997), a high level of self-rated skills in physical activity (Taylor et al. 1999), being satisfied with ones’s own sports perfomance (Barnekow-Bergkvist et al. 1996) and a positive attitude towards aerobic training in males (Barnekow-Bergkvist et al. 1996) in adolescence have been associated with a high level of physical activity in adulthood. Being forced to exercise in childhood was negatively (Taylor et al. 1999), and parental and spousal encouragement of exercise in childhood was positively (Dennison et al. 1988, Taylor et al. 1999) associated with physical activity level in adulthood. Parental physical activity and fitness at age 13 did not markedly predict physical activity level at age 25 (Campbell et al. 2001).
2.3.1.2. Tracking of physical activity from youth to adulthood
Tracking of physical activity during adolescence and from youth to adulthood have varied from low to moderate. The closer the time span between the measurements, the higher the correlation. Over a span of 3 to 4 years, the tracking of physical activity has been moderate (Aarnio et al. 2002b, Janz et al. 2000, Raitakari et al. 1994, Telama et al. 1996, van Mechelen & Kemper 1995, Vanreusel et al. (1997), but over longer spans of 5 years or more, the tracking correlations have generally been lower (Andersen et al. 1993, Campbell et al. 2001, Fortier et al. 2001, Raitakari et al. 1994, Telama et al. 1996, van Mechelen & Kemper 1995, Vanreusel et al. 1997).
The tracking correlations are higher for males than for females between 9 to 21 years of age (Andersen et al. 1993, Raitakari et al. 1994, Telama et al. 1996) and tend to increase with age at first observation (Raitakari et al. 1994, Telama et al. 1996). Tracking for the frequency of participation in overall physical activity was lower than tracking of physical activity index or intensity of physical activity (Telama et al. 1996). Among these different physical activity variables, the frequency of participation in sports club training had the highest tracking correlations (Telama et al. 1996). In the study of Kemper et al. (2001a) the tracking for heavy physical activity was higher than for light physical activity. Rather intensive participation in sport seems to track better than less intensive physical activity, but an exception is a study of Vanreusel et al. (1997) in which the males who were very active in adolescence did not have a better chance of being active at 30 years than their moderately active counterparts, suggesting that a very active physical activity pattern is not necessary for guaranteeing an active adult life.
Sedentary behavior seems to have higher tracking in males than in females in some studies. In the study of Campbell et al. (2001), the tracking of physical inactivity between ages 13 and 25 years was higher in males than in females (0.25 vs. 0.06). In the study of Janz et al. (2002), 75% of males, but only 21% of females who were in the lowest physical activity tertile at 11 years, remained so four years later. In the study of Andersen et al. (1993), 53% of males but only 8% of females were persistently inactive between ages 17 and 25 years.
2.3.1.3. Participation in different sports in youth and in adulthood
The decline in the total amount of physical activity with age has been reported to be related to the decline in the number of activities (Aaron et al. 2002, Dovey et al. 1988). Tracking correlations for participation in different types of sports have not been described earlier. Such an approach would demand a rather large study population. Aarnio et al. (2002b) reported that the proportion of those who were persistently active at all three annual surveys between 16 and 18 years was highest among those who at 17 participated in cross-country skiing, gym-training and ball games in both sexes, and jogging in males. This may indicate quite an active and stable physical activity pattern of the adolescents who are involved in these sports. Aaron et al. (2002) reported that the probability of maintaining participation in a specific activity between 14 and 18 years was low to moderate (4–71%), and the probability of not participating in a specific activity was high (70–100%), indicating that if one has not participated in some sport at 14 years, one most probably will not get involved in that sport by age 18 either.
2.3.1.4. Youth social and behavioral factors as predictors of adult physical activity
A high level of parental education (Kuh & Cooper 1992) and social class (Kuh & Cooper 1992, Pietilä et al. 1995) in youth have been associated with a high level of adult physical activity. As an exception for this, Barnekow-Bergkvist et al. 1998 reported that father’s occupation as a manual worker at 16 years was associated with a high level of females’ physical activity at 34 years. When own social class in adulthood was taken into account in multivariate analyses, the parental social class was not significantly associated with physical activity in adulthood (Blane et al. 1996). Being extrovert or extremely energetic in childhood was associated with a high level of physical activity at 34 years (Kuh & Cooper 1992).
2.3.2. Pathway 2: Youth physical activity and youth health-related fitness
The Toronto model on physical activity, fitness and health (Fig. 1) has mainly been developed based on research carried out in adult population. There is only weak evidence of the relationship between habitual physical activity and health-related fitness in youth, in contrast to a strong relationship in adulthood. Earlier results suggest that also other factors besides physical activity have a strong impact on health-related fitness in youth. Several components of health-related fitness that respond to physical activity also change with normal growth, maturation and development from childhood to adulthood. It is thus difficult to separate the changes that are induced by enhanced physical activity from those that accompany normal maturation. More active youths have been found to be more fit in the tasks that demand good cardiorespiratory fitness, but the associations with the other components of health-related fitness are inconsistent. And reciprocally, youths classified as fit in health-related fitness tests are, on average, physically more active, but there is a lot of variability. (Malina 2001a.)
Although the health effects of physical activity in youth are less obvious than in adulthood, physical activity can have multiple beneficial health outcomes in youth as well. Physical activity can enhance psychological well-being, self-esteem, and if appropriately structured, physical activity may enhance social and moral development. Physical activity can reduce symptoms of depression and anxiety. Additionally, physical activity has small but beneficial effects on reducing body fat and beneficial associations with serum lipids, blood pressure and skeletal health in young people. (Biddle et al. 1998.)
2.3.3. Pathway 3: Youth health-related fitness and adult health-related fitness
Longitudinal data for different components of health-related fitness suggest generally better tracking for fitness than for physical activity. Data on tracking of fitness from youth to adulthood are limited but indicate higher interage correlations for flexibility and strength compared to cardiorespiratory fitness. Correlations for cardiorespiratory fitness and muscular fitness from youth to adulthood are moderate. Growth and maturity are linked to the tracking of physical performance. For instance, as males progress through adolescence, correlations between the measures of strength and flexibility in adolescence and at age 30 tend to increase with age. (Malina 1996, Malina 2001a.)
Obesity has been shown to track from youth to adulthood (Parsons et al. 1999, Laitinen et al. 2001). Fatter children are more likely to be obese later in life, although the prediction of adult obesity from child and adolescent adiposity measures is only moderate (Power et al. 1997). The evidence also suggests that obesity in adolescence is associated with chronic diseases that develop in adulthood, independently of adult obesity (Must & Strauss 1999).
2.3.4. Pathway 4: Youth health-related fitness and adult physical activity
Health and health-related fitness during childhood may be predictive of adult physical activity. Very good self-assessed health at 15 years predicted a high level of physical activity at 18 years (Dovey et al. 1998), and fewer health problems in childhood was associated with a high level of physical activity at 34 years (Kuh & Cooper 1992). Some data suggest that those who are more fit in youth, especially in terms of cardiorespiratory fitness, tend to be more active in adulthood. In a study of Dennison et al. (1988), males who were active at 24 years had had better scores in youth fitness tests than their inactive counterparts, and the result of the 548.6 m run was the best predictor of later physical activity. The proportion of inactive adults was higher among those who scored in the lowest quintile of the 548.6 m run, compared to those who scored higher (Dennison et al. 1988). In a study of Barnekow-Bergkvist et al. (1998), males’ good results in a 9-minute run and females’ good results in a two-hand lift at age 16 were associated with a high level of physical activity at age 34. Great aerobic potential at age 16, including good running performance, high VO2max and high proportion of type I muscle fibers, was associated with a high level of physical activity at age 27 (Glenmark et al. 1994). In a study of Kemper et al. (2001a) cardiorespiratory fitness at 13 years predicted a high level of physical activity at 33 years, but this was true only in females, and muscular fitness tests did not have any predictive value. In a study of Beunen et al. (2001) good muscular fitness at 13, 15 and 18 years predicted a high level of physical activity in adulthood. However, some measures (arm pull and bent arm hang) were inversely related to adult physical activity level. Judgement of own fitness to be better than that of peers at 15 years predicted a high level of physical activity at 18 years (Dovey et al. 1998). Kemper et al. 1997 reported that those who maturated later in youth had a slightly higher activity pattern at 17–22 years than those who maturated early, and speculated that this may be due to an earlier change of the early maturers to physically inactive adults.
2.3.5. Pathway 5: Youth physical activity and adult health-related fitness
Evidence about the relationship between physical activity in youth and health or health-related fitness in adulthood is meager, except for the evidence for physical activity in youth and skeletal health in adulthood. Physical activity in youth can contribute to increased peak bone mass. Weight-bearing activities produce high-impact loading that stimulates bone formation effectively. Peak bone mass that is accumulated in youth and the subsequent rate of bone loss are thought to be equally important in determining bone mass in the elderly. Limited information suggests that much of the achieved peak bone mass may be lost during adult years, but this diminution can be deterred by substantially less physical activity than what was needed to gain it. (Vuori 2001.)
Twisk et al. (2002c) summarized the results of six longitudinal studies which evaluated the relationship between physical activity and fitness in youth and cardiovascular disease risk factors later in life. They concluded that a high level of physical activity in adolescence and young adulthood did not seem to be predictive, but high physical fitness did seem to be predictive of a healthy cardiovascular disease risk profile later in life (Twisk et al. 2002c). The authors (Twisk et al. 2002c) also discussed the difficulties in measuring physical activity in large epidemiological studies and the possibility that the relationship between physical activity and cardiovascular diseases risk profile might be underestimated. Relatively small sample sizes of the reviewed studies was another problem under discussion. The relationship between physical activity in youth and obesity in adulthood, one of the five areas of the present study, is discussed more in depth in the following chapter.
2.3.5.1. Physical activity and obesity from youth to adulthood
Longitudinal studies on the association between physical activity and obesity from youth to adulthood are summarized in Appendix 2. The follow-up time in these studies has been at least two years. Previous follow-up studies have given inconsistent results regarding the association between youth physical activity and adult obesity, which may be explained by relatively small samples and methodological differences in these studies (Parsons et al. 1999, Raitakari et al. 1994, Twisk et al. 1997, Lefevre et al. 2002). Some studies have suggested a protective effect of adolescent physical activity on adult obesity defined by skinfold thickness (Parsons et al. 1999, Raitakari et al. 1994, Twisk et al. 1997). However, this protective effect was not confirmed when adult obesity was defined by body mass index (Kemper et al. 1999) or by measures of abdominal obesity (Twisk et al. 1997, Hasselstrom et al. 2002, Twisk et al. 2002b), and some studies have reported no association between adolescent physical activity and adult obesity (Lefevre et al. 2002, Twisk et al. 2002b). Only one of these studies (Hasselstrom et al. 2002) took into account the change in the level of physical activity between adolescence and adulthood and suggested that the change in physical activity from 17 to 25 years was negatively associated with males’ waist circumference and fatness at 25 years.
Among adult population, Williamson et al. (1993) showed that the change in the level of physical activity, whether a decrease or an increase, was associated with greater weight gain compared with those who had been persistently active or inactive, and Haapanen et al. (1997) reported that the decrease in physical activity was more strongly associated with significant weight gain than being persistently inactive. Abdominal obesity was not evaluated in these two follow-up studies but in the study of Wing et al. (1991) the change in physical activity was negatively associated with the change in the waist to hip ratio after adjustment for body mass index in middle-aged females.