2.2. Relationships between physical activity, fitness and health
The relationships between physical activity, health-related fitness and health in adult population can be examined and understood by a model presented in Figure 1 (Bouchard & Shephard 1994). Physical activity may influence fitness, which in turn may modify the level of physical activity. With increasing fitness, people tend to become more active, and the fittest persons tend to be the most active. The association between fitness and health is also reciprocal. Fitness influences health, but health status also influences both physical activity and fitness. (Bouchard & Shephard 1994.)
Figure 1. A model describing the relationships between physical activity, health-related fitness and health (Bouchard & Shephard 1994).
There are also other factors that influence physical activity, fitness and health: lifestyle factors other than physical activity, personal attributes, physical environment and social environment (Fig. 1). Lifestyle behaviors include for instance smoking, diet, alcohol consumption and sleeping patterns. Several personal attributes, such as age, gender, socioeconomic status, personality, motivation and attitude toward physical activity and other health habits may shape a person’s lifestyle pattern. Social environment combines social, cultural, political, and economic conditions that affect physical activity, fitness and health. Environmental conditions such as temperature, humidity, air quality, altitude and climatic changes, may influence physical activity, health-related fitness and health. (Bouchard & Shephard 1994.)
Heredity has an impact on all three components of the model: physical activity, fitness and health. There are inherited differences in the levels of physical activity and in the components of health-related fitness. Interaction between the genes and the environment is largely responsible for the variability in the health-related phenotypes in response to physical activity. Different genotypes may be at different risk for diseases associated with physical inactivity and a low level of health-related fitness. (Bouchard & Pérusse 1994.) There are marked individual differences in responsiveness to a certain dose of physical activity. Although the results of physical activity intervention studies are usually presented as the average effects of the observed groups, individual responses to a certain training program may vary between no change to 100% increase in VO2max among sedentary persons. (Bouchard & Rankinen 2001.)
2.2.1. Leisure-time and occupational physical activity in relation to fitness
Increase in the volume and intensity of leisure-time physical activity is associated with increase in physical fitness in adults (Oja 2001). Exercise recommendations to improve and maintain cardiorespiratory fitness suggest exercise that uses large muscle groups, is performed three to five times a week, at intensity of 60–90% of maximum heart rate and for 20–60 minutes at a time (ACSM 1998). These latest fitness recommendations (ACSM 1998) also include guidelines for enhancing muscular fitness and flexibility.
Associations between occupational physical activity and fitness are not so clear in the light of earlier studies. Studies on the association between occupational physical activity and physical fitness are summarized in Appendix 3. In middle-aged workers, heavy physical work has been related to poor physical fitness (Era et al. 1992, Nygård et al. 1987, Torgen et al. 1999), although the findings have varied from a weak positive association (Sobolski et al. 1988, Torgen et al. 1999) to no association at all (Ilmarinen et al. 1991, Rantanen et al. 1993, Sobolski et al. 1988, Tuxworth et al. 1986). Two previous studies have reported a higher level of muscular strength in young manual workers than in their white-collar counterparts (Era et al. 1992), and better cardiorespiratory fitness in young men who daily sweat visibly at work, compared with others (Jonsson & Åstrand 1979). However, both studies mentioned above contained only small numbers of young subjects and used rather inaccurate measurements of occupational physical activity.
2.2.2. Physical inactivity, low level of cardiorespiratory fitness and obesity as risk factors
The favorable effects of regular physical activity on health are nowadays well recognized. The dose-response relationships between physical activity and health varies for different health outcomes (Kesäniemi et al. 2001). The general physical activity recommendation to enhance health suggests 30 minutes of moderate-intensity physical activity on most days of the week (Pate et al. 1995). This can be interpreted as a minimal dose of physical activity to guarantee most of the health benefits. However, the prevalence of physically inactive persons is relatively high in most western countries, emphasizing the significance of inactivity as a public health hazard (U.S. Department of Health and Human Services 1996).
A low level of physical activity is known to be associated with an increased rate of all-cause mortality (Lee & Skerrett 2001), increased incidence of cardiovascular diseases (Kohl 2001), obesity (Ross & Janssen 2001), type-2 diabetes (Kelley & Goodpaster 2001), colon cancer (Thune & Furberg 2001), osteoporosis (Vuori 2001) and depression symptoms (Dunn et al. 2001). Physical inactivity is also associated with an unfavorable profile of cardiovascular risk factors, such as high level of blood pressure (Fagard 2001) and blood lipids (Leon & Sanchez 2001). Among the oldest adults long-term physical activity is related to postponed disability and independent living (Spirduso & Cronin 2001).
A low level of cardiorespiratory fitness is associated with an increased risk of cardiovascular diseases and mortality (Blair et al. 2001, Laukkanen et al. 2001, Talbot et al. 2002), and the least fit 20% of the population is reported to be at special risk compared with moderately (the next 40%) or highly fit groups (the highest 40%) (Blair et al. 1989, Blair et al. 2001). Overall obesity is associated with an increased risk of cardiovascular diseases and type-2 diabetes (WHO 1998). The major health risks of obesity are more related to the abdominal distribution of body fat than to its total amount (Samaras & Campbell 1997). Obesity is related to physical activity and cardiorespiratroy fitness. Physical activity is a means of controlling weight by increased energy expenditure, but obesity may also influence motivation to participate in physical activities (DiPietro 1995). An increase in body fat decreases cardiorespiratory fitness, especially when cardiorespiratory fitness is expressed in relation to body weight.