6.4. Association between occupational physical activity and fitness in young adults (IV)

Young males engaged in heavy physical work exhibit higher levels of cardiorespiratory fitness, handgrip strength and trunk muscle endurance than young males doing lighter work; the same applies to cardiorespiratory fitness in young females. The associations were independent of leisure-time physical activity, body weight, height and smoking. Causality cannot be asserted, but a training effect of heavy work is a reasonable assumption. Self-selection of the fittest to physically demanding jobs, and of the least fit or sickly individuals to lighter work, may also partly explain our findings.

The finding of the positive association between heavy physical work and good cardiorespiratory fitness is similar to that of Jonsson & Åstrand (1979), who stated that young men who sweat daily at work possess better aerobic capacity than their non-perspiring counterparts; however, their finding was restricted only to males who were inactive during leisure time. In their study, the difference in heart rate between the heaviest and lightest work-load groups was 10 beats/min in submaximal cycle ergometer work – similar in magnitude to the difference of eight beats/min observed in our study. Our findings on muscular endurance are in line with those reported by Era et al. (1992), who showed greater muscular strength in young male manual laborers than in young white-collar workers. The large sample size, more accurate assessment of occupational physical activity and adequate control of relevant confounding factors may partly explain why significant associations were observed here, but not so clearly elsewhere. We further suggest that a positive association may exist between occupational physical activity and cardiorespiratory fitness in young females, although the effect may be restricted to females doing very heavy physical work.

Although leisure-time physical activity, height, weight and smoking were all strongly related to different measures of physical fitness, their confounding effect was only a minor one, except for the trunk extension test in males and the step test in females. The true relationship between occupational physical activity and heart rate in females was initially hidden by the greater weight of females doing heavy work, which shows the importance of confounders to be considered.

These results, involving young workers engaged in heavy physical work, differ from those involving middle-aged workers, who were found to have lower fitness than workers engaged in lighter work (Nygård et al. 1987). Most earlier studies also fail to report any positive effect of heavy work on muscular fitness. An exception is the study by Torgen et al. (1999), who found some evidence that heavy work, especially lifting of heavy objects, may have a training effect on the muscular fitness of the upper extremities of middle-aged workers. Two intervention studies have also suggested that increased frequency of stair climbing during the workday may enhance the cardiovascular fitness of subjects with poor physical fitness (Ilmarinen et al. 1978, Ilmarinen et al. 1979).

However, the high fitness of the young males engaged in heavy physical work in this study raises the issue of an optimal level of occupational physical activity for improving and maintaining physical fitness. In some occupations, the frequency, intensity and duration of exercise at work may be very close to optimal, especially if its rhythm, manner and pauses can be freely modified by the worker.

Our findings also raise the question of why physical work would be advantageous in younger but not in older workers, as reported in the literature (Nygård et al. 1987). With advancing age, the beneficial effect of work may be reversed by decreased fitness related to biological aging and an increase in chronic diseases. In addition, the young males engaged in heavy physical work in this study were often relatively inactive during leisure time, and this, too, could render their fitness relatively poor later in life. In females, a heavy physical load during occupational work may exceed a tolerable level at an earlier age than in males because of their lower level of physical capacity. This would offer some grounds for allocating younger and older workers, as well as males and females, to work tasks with different demands on physical capacity.

Our results also emphasize the importance of taking occupational physical activity into account in epidemiological studies evaluating the relationship between leisure-time physical activity and fitness. Occupational physical activity is associated with both leisure-time physical activity and physical fitness, and may therefore counfound the relationship between these two variables.

Self-selection of the fittest to physically demanding jobs, and of the least fit or sickly individuals to lighter work, might partly explain our findings. However, in an earlier study involving middle-aged workers, the poorer fitness of workers in heavy jobs (Nygård et al. 1987) did not reflect this kind of health-based selection (Östlin 1988). One reason for this phenomenon may be that a change of occupation or work may be more difficult in later life than at some earlier stage of the career. The cross-sectional setting of the present study does not enable us to assert causality between occupational physical activity and fitness; this remains to be assessed in subsequent follow-up studies. In addition, there is a need to determine the critical age beyond which the beneficial effect of occupational activity becomes deteriorating, and whether it could be shifted by appropriate leisure-time activity.

Occupational physical activity is said to be too one-sided and static, typically having an overloading rather than a training effect on the cardiorespiratory system (Ilmarinen 1989). This may be because in industry the rhythm of movements is often determined by machines rather than by the individual, and other adverse circumstances, such as high or low environmental temperature, an awkward posture or heavy loading of small muscle groups, may be present. During heavy work, the frequency and intensity of exercise may be adequate, but its excessive duration provides insufficient recovery time between workdays, particularly in persons with poor physical fitness.