Factors influencing physical activity and cardiorespiratory fitness

2.2.3.1. A^{CQUIREDFACTORS} FACTORSPROMOTINGPHYSICALACTIVITY

There is evidence that people who live closer to a variety of recreation facilities are more physically active overall. Of the ten review articles reviewed by Bauman & Bull (10), nine recognized proximity to recreation facilities as a factor associated with physical activity. Further, most reviews concluded the availability of sidewalks was positively associated with physical activity and walking. Overall, the walkability and residential density of neighborhoods were correlates of physical activity of their residents.

Frank et al. (50), for example, used a walkability index based on Geographic Information Systems and measured physical activity with accelerometers.

They found that 37 percent of adults in the highest-walkability neighborhoods met the recommendation of physical activity for thirty minutes per day, while only 18 percent of those in the lowest-walkability neighborhoods met this recommendation.

In addition to the cross-sectional relationships, studies show that relocating to a more walkable neighborhood increases physical activity (60). The effects of community-wide interventions to promote physical activity are, however, inconsistent (8).

Motives and perceived barriers for physical activity were examined in a study among twin pairs discordant for physical activity for 30 years. It was found that the main factors promoting persistent leisure time physical activity were the participants’ wish to improve or maintain their physical skills or techniques, a feeling that exercise would improve their mental and physical health and that they found the activity enjoyable. Main factors reported as reasons for physical inactivity were pain and various health problems. More than half of

the subjects did not, however, mention any specific reasons for being inactive (1). In children and adolescents, home environment and parental support have also been associated with physical activity (98).

EFFECTSOFEXERCISE

Physical training induces a variety of metabolic and physiological changes, which increase aerobic capacity. Endurance training increases use of fatty acids as oxidative fuel during exercise. This is the consequence of enhanced muscle mitochondrial respiratory capacity, greater blood flow within trained muscle, an increased amount of enzymes mobilizing and metabolizing fat and decreased catecholamine release at a given power output (113).

The enhanced mitochondrial respiratory capacity in response to training is characterized by increased mitochondrial content, usually ranging from 30 to 100% within 4 to 6 weeks (77). This results in improved endurance performance that is largely independent of the much smaller 10–20%

training-induced changes in VO2max. Formation of lactic acid at a given VO₂ decreases (77). Further, exercise improves insulin-independent glucose uptake in skeletal muscle (54), as well as whole-body insulin sensitivity (53, 62). The contractile activity during a single bout of exercise increases skeletal muscle glucose uptake due to insulin-independent translocation of GLUT4 glucose transporters to the cell surface (54). Long-term training, on the other hand, improves insulin sensitivity by influencing the expression and activity of proteins involved in insulin signal transduction (53, 62).

In addition to its metabolic effects, training produces a number of cardiovascular adaptations. These include increased maximal cardiac output due to increased stroke volume, optimization of blood flow distribution to working muscles during exercise, increased muscle capillarization and increases in total blood volume (113).

2.2.3.2. G^{ENETICFACTORS} HERITABILITY

Genetic factors are known to account for a considerable part of the variance in physical activity within populations, but previous studies show variation in the degree of its heritability (23, 42, 81, 108, 122, 168). Heritability estimates for exercise participation ranged from 27 to 70%, with a median heritability of 62%, in a large pooled twin sample from seven countries (168). It has been suggested that genetic factors contribute more strongly to physical activity in men than in women, especially during adolescence (13, 23), and that genetic influences increase from childhood to adulthood (166, 169, 184). A recent paper shows that heritability decreases from young adulthood to age 50 (184). Thus, the heritability of physical activity would appear to be highest at the period when physical maturation is complete and physical fitness is at its best.

26 | OBESITY, PHYSICAL ACTIVITY AND CARDIORESPIRATORY FITNESS IN YOUNG ADULTHOOD

In most studies unique environmental effects contribute to the rest of the variance leaving the effect of common environment insignificant (23, 42).

However, in a small study (N=40 twin pairs) by Joosen et al. (81) common and unique environmental factors explained all of the variance in physical activity recorded with a triaxial accelerometer in a respiration chamber for 24 h, and no genetic contribution was found. Energy expenditure was measured simultaneously and presented a similar pattern. In two weeks daily life measurements, however, the additive genetic contribution to physical activity was 78% with unique environmental factors explaining the rest of the variance. Duncan et al. (42) also presented results differing slightly from the majority of observations. In a sample consisting of 1,003 same-sex twin pairs (62% women), with a mean age of 30 years, unique environment provided the strongest influence on physical activity, with genetic factors accounting for only 11% to 45% of the total variance.

There are many possible routes, through which genetics might influence physical activity. Traits such as body composition and body type, as well as aerobic capacity, muscle strength and muscle endurance are, for example, all influenced moderately or highly by genetic factors (13, 166). These characteristics are likely to influence physical activity, as they are related to exercise capacity, which in turn is related to exercise behavior (168). Personality characteristics, such as self-discipline, self-motivation and conscientiousness, also affect activity behavior, and because of their relatively high heritability, they are likely to contribute to the genetic influence on physical activity (166, 168).

For cardiorespiratory fitness, as measured by maximal oxygen uptake (VO2max), familial aggregation studies and twin studies show heritability estimates that vary between 50% and 67% (15, 48, 105).

A^{CTIVITYANDFITNESSGENES}

Only few data are available on specific genes associated with exercise behavior and physical activity. Linkage studies and candidate gene association studies have yielded a number of genes possibly associated with physical activity phenotypes, but the results are far from consistent (32). The first and for the present only genome wide association study on physical activity levels was performed in 2009 in a Dutch-American twin sample (31). Study subjects were divided into ‘exercisers’ and ‘nonexercisers’ based on their answers on questions about leisure-time physical activity. It was found that, while none of the 1.6 million SNP reached the commonly used threshold for genome-wide significance, SNP in three genomic regions had P-values of borderline significance. The strongest evidence of association was observed at the gene locus of PAPSS2, a gene encoding an enzyme involved in sulfation of glycosaminoglycans and other molecules. Previously reported candidate gene associations or linkage findings were not replicated (31).