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APPLIED SCIENCES: Physical Fitness and Performance

Occupational physical activity is related to physical fitness in young workers

TAMMELIN, TUIJA; NÄYHÄ, SIMO; RINTAMÄKI, HANNU; ZITTING, PAAVO

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Medicine & Science in Sports & Exercise: January 2002 - Volume 34 - Issue 1 - p 158-165
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Abstract

Although the beneficial effect of leisure-time physical activity on physical fitness is well documented, no similar association has been found between occupational physical activity and fitness. On the contrary, in middle-aged workers, heavy physical work has been associated with poor physical fitness (3,12,20), although findings have varied from a weak positive association (18,20) to no association at all (7,15,18,21). Two previous studies have reported a higher level of muscular strength in young manual workers than in their white-collar counterparts (3), and better cardiovascular fitness in young men who daily sweat visibly at work, compared with others (9). However, both studies mentioned above contain small numbers of young workers and inaccurate measurements of occupational physical activity. We, therefore, tested the hypothesis that in young people heavy physical work is associated with better physical fitness than lighter work. In contrast to previous studies, the confounding effects of leisure-time physical activity, height, weight, and smoking habits were controlled for statistically to permit a more accurate interpretation of the influence of occupational work level on physical fitness.

METHODS

Study Design and Population

The study population comprised the Northern Finland birth cohort, originally 12,231 men and women, who were born in the two northernmost provinces of Finland in 1966 (14). In 1997–1998, all members of the cohort who were still alive and whose mailing addresses were known (N = 11,541) were invited to participate in a follow-up survey. A questionnaire was mailed to all of them and was returned by 8767 (76%). Those 8463 cohort members who were still living in Northern Finland, or had moved to the Helsinki capital area, were also invited to a medical examination including fitness tests, which were performed in local health centers. The invitation was accepted by 6033 (71%), of which 4715 (2188 male and 1987 female) were included in the present analysis. These participants reported that they were employed at the time of the survey and had also provided information on their leisure-time physical activities in the questionnaire. Written informed consent was obtained from all subjects before participation in the study. This study was approved by the Ethics Committee of the Faculty of Medicine at the University of Oulu and the Ethics Committee of the Institute of Occupational Health.

Fitness Tests

Step test.

Cardiorespiratory fitness was measured by a submaximal single-step test, which lasted 4 min (16). A stepping rate of 23 ascents per minute was paced by a metronome. Stepping was performed without shoes on a bench adjusted to a height of 33 cm for women and 40 cm for men. Heart rate was measured immediately after stepping by a heart rate monitor handle (Fitwatch, Polar Electro, Kempele, Finland), which displayed the value within 5 s after setting the handle on the chest.

Handgrip test.

Maximal isometric handgrip strength of the dominant hand was measured with a hand dynamometer (Newtest, Oulu, Finland) based on the strain-gauge technique. Measurements were performed with the subject in a standing position, holding the dynamometer, with the hand beside but not touching the trunk. The wrist and the elbow were extended. The width of the grip in the dynamometer was adjusted to the size of the hand. The highest value in Newtons (N) of the three trials, each lasting from 2–4 s, was accepted as the result.

Trunk extension test.

During the trunk extension test (2), the subject was in a prone position, the lower body lying on the stand and the upper body unsupported from the level of the anterior superior iliac spine upward. The legs were stabilized by the tester sitting on them, and the arms were held beside the trunk. The isometric endurance capacity of the trunk extensor muscles was evaluated by holding the upper part of the body in a horizontal position as long as possible, however, not exceeding 4 min. When the subject was no longer able to maintain the horizontal position, the test ended. The outcome measure of the test was the endurance time in seconds.

Reasons for not performing fitness tests.

Less than 10% of all subjects were excluded from the analysis for various reasons. Employed subjects who declined or were unable to perform the tests were step test 6%, handgrip 2%, and trunk extension 6%. The corresponding figures were 9%, 4%, and 8%, respectively, for those who attended the medical checkup, including those who were unemployed at the time of the survey. The most common reasons for not performing step or trunk extension tests were ill health and pregnancy.

Occupational Physical Activity

In the questionnaire, subjects were asked to indicate the intensity of physical activity in their current work, using the response alternatives given in Table 1. This question has previously been used in the population-based Mini-Finland Health Survey (11), and its test-retest reliability has been stated to be reasonably good (kappa coefficient 0.69) (10).

Table 1
Table 1:
Classification of occupational physical activity and description of work content in the questionnaire.

Confounders

Leisure-time physical activity.

Because leisure-time physical activity is known to improve physical fitness and is likely to vary depending on the level of occupational physical activity, it was considered a potential confounding factor. In the questionnaire, subjects were asked how often they engaged in physical activity during their leisure time to the extent that it caused some sweating and breathlessness. Response alternatives included: 1) once a month or less, 2) two to three times a month, 3) once a week, 4) two to three times a week, and 5) four times a week or more.

Body size and smoking.

Additional factors considered were body height, weight, and smoking. All three may affect cardiorespiratory fitness and muscular endurance. Body height and weight may affect the result of the step test because stepping on a stair of fixed height might be more strenuous for a short, heavy person than a very tall or lightweight person. Body height might also affect the trunk extension test. In addition, body size is likely associated positively with maximal isometric handgrip strength. Body height and weight were measured to an accuracy of 0.1 cm and 0.1 kg. The subjects were classified as smokers and nonsmokers by using information they had provided in the questionnaire. Those smoking less often than once a week were classified as nonsmokers.

Statistical Analyses

The results of the step and handgrip tests were assessed using analysis of variance, with heart rate (beats·min−1) and handgrip strength (N) as response variates and occupational physical activity, leisure-time physical activity, body height and weight, and smoking as explanatory factors. Body weight and height were classified into quintiles and were treated as categorical variables. The results of these analyses were presented graphically in the form of model-predicted heart rates and handgrip strengths, together with 95% confidence intervals of the estimated values. Because many subjects in the trunk extension test exceeded the maximum time of 4 min, endurance time remained unknown for some. To control for this, the probability of failure in the trunk test was analyzed by Cox regression, with the unknown cases being defined as censored observations. The result was expressed as a hazard ratio (HR), i.e., ratio of the probability of failure (hazard) in a given class of the explanatory variable divided by the corresponding hazard in the reference class. The analysis presupposes proportionality of the hazards, which was checked graphically using log/−log functions of cumulative hazards. In all multivariate analyses, occupational physical activity was entered first in the model, with other explanatory variables added one at a time thereafter, keeping an eye on their confounding effect on initial parameter estimates. First-order interactions were also considered. Analyses were conducted using SPSS software (19).

RESULTS

Characteristics of Subjects

Average body height and weight were 178.5 cm (SD 6.4 cm) and 80.5 kg (SD 12.6 kg) in men and 165.0 cm (SD 6.0 cm) and 65.2 kg (SD 12.4 kg) in women. Altogether 27% of men but only 14% of women did heavy or very heavy manual work; light sedentary as well as light standing or moving work was more typical of women (Table 2). Men engaged in heavy or very heavy manual work were more often inactive and less often active during leisure time compared with men engaged in light sedentary work (Table 2 and Fig. 1). In women, the trend was similar but less pronounced (Fig. 1). The occupational physical activity group two has been excluded from Fig. 1, as it also contains heavy manual tasks and is not quite in line with the entire scale, which is basically ordinal.

Table 2
Table 2:
Leisure-time physical activity according to occupational physical activity level.
FIGURE 1
FIGURE 1:
Percentages of physically active and inactive subjects according to their occupational physical activity: 1) light sedentary work, 3) light standing or moving work, 4) medium heavy moving work, 5) heavy manual work, and 6) very heavy manual work. Active = brisk exercise two times a week or more often during leisure time, inactive = brisk exercise less often than once a week during leisure time.

Univariate Analyses

Men.

Results of step and handgrip tests were significantly better in men doing heavy manual work (Table 3). No significant association was seen between occupational physical activity and the trunk extension test, although a better result was suggested for men engaged in very heavy manual work compared with other men. Frequent leisure-time physical activity was associated with better results in the step and trunk extension tests but not in the handgrip test. The tallest men had the greatest handgrip strength, but no other associations existed between body height and fitness. The heaviest men showed the best results in the handgrip test but the worst results in the step and the trunk extension tests. Smokers exhibited a 33% higher probability of failure in the trunk extension test than nonsmokers.

Table 3
Table 3:
Crude means (± SD) for step test and handgrip test, and unadjusted hazard ratios (HR) for failure in the trunk extension test according to explanatory variables in men.

Women.

Occupational physical activity was not significantly associated with any of the fitness tests, although a weak trend was seen in the trunk extension test, with women doing heavy sedentary work having the highest risk of failure (Table 4). Other associations were similar to those observed in men, with the exceptions that tall women had a smaller risk of failure in the trunk test than other women, and smokers achieved a better result in the step test than nonsmokers.

Table 4
Table 4:
Crude means (± SD) for step test and handgrip test, and unadjusted hazard ratios (HR) for failure in the trunk extension test according to explanatory variables in women.

Multivariate Analyses

Step test.

In men, occupational physical activity remained a significant factor, even when the effects of leisure-time activity, body height and weight, and smoking were controlled (Table 5). All the other factors shown in Table 3 were also significant. In women, the inclusion of all factors in the analysis actually turned occupational physical activity into a significant factor. The insignificance of the association between occupational activity and the step test in the crude analysis (Table 4) was mainly due to a 2–5 kg greater weight in women doing heavy manual work compared with women doing lighter work, which caused some flaw in the unadjusted results. Body height also became a significant factor when weight was added to the model, the tallest men and women having the lowest heart rates in the step test. The adjusted results of Figure 2 show a 5% decline in heart rate from lowest to highest occupational activity groups in men, whereas in women a decline in heart rate is recorded mainly in those doing very heavy work.

Table 5
Table 5:
Step test: multivariate analysis of variance.
FIGURE 2
FIGURE 2:
Step Test: Heart rate after step test (beats·min−1) according to the following occupational physical activity levels: 1) light sedentary work, 2) other sedentary work, 3) light standing or moving work, 4) medium heavy moving work, 5) heavy manual work, and 6) very heavy manual work. Mean values and their 95% confidence intervals, • = crude values from univariate analyses, ○ = values adjusted for leisure-time physical activity, height, weight, and smoking.

Handgrip test.

The initial association of greater handgrip strength in men doing heavy physical work persisted after adjusting for the other factors (Table 6, Fig. 3). In women, the association remained insignificant (Table 6), although women who did very heavy work did show a relatively high value (Fig. 3).

Table 6
Table 6:
Handgrip test: multivariate analysis of variance.
FIGURE 3
FIGURE 3:
Handgrip Test: Grip strength (N) of the dominant hand according to the following occupational physical activity levels: 1) light sedentary work, 2) other sedentary work, 3) light standing or moving work, 4) medium heavy moving work, 5) heavy manual work, and 6) very heavy manual work. Mean values and their 95% confidence intervals, • = crude values from univariate analyses, ○ = values adjusted for leisure-time physical activity, height, weight, and smoking.

Trunk extension test.

Altogether 399 men (19%) and 705 women (39%) reached the maximum time of 4 min and were treated as censored in the Cox regression analyses. Plots of log/−log hazards suggested no violation of the assumption of proportional hazards. Occupational physical activity was significantly associated with the trunk extension test in men, independently of other factors (Table 7), with men doing very heavy manual work having a 27% lower risk of failure than men doing the lightest work (Fig. 4). The adjusted analysis revealed no significant association between occupational physical activity and the trunk extension test in women, but an isolated high value was seen in women doing heavy sedentary work (Fig. 4).

Table 7
Table 7:
Trunk extension test: Cox regression analysis.
FIGURE 4
FIGURE 4:
Trunk Extension Test: Hazard ratios (HR) and their confidence intervals (95% CI) representing the probability of failure at the test before maximum time according to the following occupational physical activity levels: 1) light sedentary work, 2) other sedentary work, 3) light standing or moving work, 4) medium heavy moving work, 5) heavy manual work, and 6) very heavy manual work. Mean values and their 95% confidence intervals, • = crude values from univariate analyses, ○ = values adjusted for leisure-time physical activity, height, weight, and smoking.

DISCUSSION

We showed that young men engaged in heavy physical work exhibit higher levels of cardiorespiratory fitness, handgrip strength, and trunk muscle endurance than young men doing lighter work; the same applies to cardiorespiratory fitness in women. This result is similar to that of Jonsson and Åstrand (9), who stated that young men who sweat daily at work possess a better aerobic capacity than their nonperspiring counterparts; however, their finding was restricted only to men 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−1 in submaximal cycle ergometer work—similar in magnitude to the difference of 8 beats·min−1 observed in our study. Our findings on muscular endurance are in line with those reported by Era et al. (3), 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 (3). We further suggest that a positive association may exist between occupational physical activity and cardiorespiratory fitness in young women, although the effect may be restricted to women 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 men and the step test in women. The true relationship between occupational physical activity and heart rate in women was initially hidden by the greater weight of women doing heavy work, which shows the importance of adjusting for confounding factors. Interestingly, although smoking is generally known to impair cardiorespiratory fitness (4), here, smoking women had a lower heart rate after the step test, suggesting better fitness compared with nonsmoking women. In long-term smokers, the increase in heart rate during exercise may be delayed due to decreased activity of beta-receptors (17); thus, a result based on a submaximal exercise would be an inadequate indicator of cardiorespiratory fitness.

Although the optimal amount of leisure-time activity for developing and maintaining cardiorespiratory fitness is well documented in earlier reports (1), the ideal amount of occupational physical activity remains unclear. 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 (5). 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, 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. However, the high fitness of the young men 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.

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 (12). 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. (20), 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 (6,8).

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 (12). With advancing age, work’s beneficial effect may be reversed by decreased fitness related to biological aging and an increase in chronic diseases. In addition, the young men 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 women, a heavy physical load during occupational work may exceed a tolerable level at an earlier age than in men because of their lower level of physical capacity. This would offer some grounds for allocating younger and older workers, as well as men and women, 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 focusing on the effects of leisure-time physical activity.

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 (12) does not reflect this kind of health-based selection (13). 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.

CONCLUSION

We have shown that an association exists between heavy physical work and a high fitness level in young men and to some extent in young women as well. Due to limitations of the study setting, causality cannot be asserted, but a training effect of heavy work is a reasonable assumption. Because earlier studies have suggested a reversal of the positive effect of heavy work on fitness with advancing age, regular monitoring of a worker’s fitness seems justified. Such monitoring would help at an early phase to assess workers’ capacity for heavy jobs and identify any need for fitness-improving activities.

The authors acknowledge Professor Eino Heikkinen from the University of Jyväskylä and Dr. Olli Korhonen from the Finnish Institute of Occupational Health for their valuable contributions.

This study was funded by the Ministry of Education, the Academy of Finland, the Ministry of Health and Social Affairs, the Juho Vainio Foundation, and the Oulu University Hospital.

Address for correspondence: Tuija Tammelin, Oulu Regional Institute of Occupational Health, Aapistie 1, 90220 Oulu, Finland; E-mail: tuija.tammelin@occuphealth.fi.

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Keywords:

CARDIORESPIRATORY FITNESS; MUSCULAR FITNESS; PHYSICAL WORKLOAD

© 2002 Lippincott Williams & Wilkins, Inc.