This study contributes to the knowledge on risk for hypertension from heavy occupational lifting by its aim to verify previous findings  and to perform further analysis accounting for the moderating effects of LTPA. Thus, this study explored associations between heavy occupational lifting and hypertension in the Copenhagen General Population Study. The adjusted cross-sectional analysis indicated a 3% lower risk of hypertension by exposure to heavy occupational lifting (table 2), which was supported by the adjusted linear associations between heavy occupational lifting and SBP (mmHg) indicating a negative association (-0.29 mmHg, 99% CI -0.82 – 0.25 mmHg, table 3). These associations could be explained by the cross-sectional design of this analysis, meaning that this result may be owed to either i) exposure to heavy occupational lifting to lower the risk for hypertension or ii) those exposed to heavy occupational lifting being less frequently hypertensive than those not. Within occupational medicine studies, results are assumed to be prone to healthy worker selection bias, implicating less healthy workers migration into occupational groups less exposed to heavy occupational lifting or other strenuous activities . Thus, one could speculate that the acute peaks in BP, while performing lifting tasks , may give rise to angina , among the workers with poor cardiovascular health. Hence, workers experiencing angina or such would be more likely to migrate into less strenuous occupational groups.
On the contrary, the adjusted prospective analysis indicated an 8% higher risk for being a SBP case by exposure to heavy occupational lifting (table 2), while the linear regressions showed the greatest increase in SBP among those exposed to heavy occupational lifting (table S4). Opposite to the BP effects from resistance training during LTPA [29, 30], these results indicate heavy occupational lifting to have hazardous effects on BP, as previously indicated . The background for increased risk for hypertension by exposure to heavy occupational lifting, may lie within the repeated acute peaks in BP during lifting tasks , occurring due to the occlusion of the vessels induced by static muscle activity leading to increases in total peripheral resistance . During heavy occupational lifting, these BP peaks are repeated, both during the 7-9 hour workday, as well as during the 5-day work-week. Thereby the recovery between BP peaks may be insufficient [14, 31], and could give rise to the increased BP both during working hours as well as across the 24h BP . However, this higher risk of being a SBP case by exposure to heavy occupational lifting is not reflected in the linear regressions, showing no associations between heavy occupational lifting and SBP, DBP, PP, and MAP (table 3 and table S1, S2, and S3). Thus, these findings ought to be interpreted with care.
The age-stratified prospective analysis ´showed exposure to heavy occupational lifting to increase the risk for being a SBP case; 11% higher risk among workers aged ≥ 50 years and 5% higher risk among workers aged < 50 years (table 2), similar to a previous study . Older workers are likely to have been occupationally active throughout a longer time than younger workers, and therefore the effect of the occupational exposures might be more pronounced. Furthermore, a higher strain from heavy occupational lifting will be expected among older than younger workers, due to the combination of age-related declines in aerobic capacity  and arterial compliance [15, 16], giving rise to a greater increase in BP and thus a potentially higher risk of hypertension .
The adjusted OR for being hypertensive by exposure to heavy occupational lifting stratified by level of LTPA showed a minimal numerical tendency of increasing risk for hypertension and for being a SBP case, by an increased level of LTPA (Table S8). These OR indicate positive associations between heavy occupational lifting and BP and higher levels of LTPA than seen among those with lower levels of LTPA. However, these rather weak associations do not support the common assumption of a beneficial effect of a decrease in risk of hypertension by increased levels of LTPA, as presented in general physical activity recommendations and previous literature [19, 25, 26, 32]. Thus, it could be assumed that the volume of physical activity by a combination of leisure-time MVPA with heavy occupational lifting, results in overstrain, and cardiovascular damage, rather than optimized cardiovascular health. This notion is supported by previous findings among both veteran athletes , and workers having both high OPA and high LTPA [34, 35]. However, the lack of a beneficial effect from LTPA on BP was not reflected in the baseline mean BP, showing higher levels of LTPA to relate to a lower BP, regardless of OPA level (table S6). Nevertheless, the mean BP, at follow-up, did not show any effect by the level of LTPA, independently of the level of OPA, except for those reporting a light level of OPA and not using anti-hypertensives (table S7). Thus, these tendencies of effect from the level of LTPA and OPA on BP, only seen in the cross-sectional analysis, could be explained by the fact that in the prospective analysis, the stratification on level of LTPA and OPA are made at baseline values, and thus the effect from the level of LTPA and OPA at baseline seems to have vanished in the 10-year follow-up BP. Conclusively, as causal effects cannot be drawn from the cross-sectional analysis, these presented results do not indicate the level of LTPA to affect BP across OPA strata.
The analysis stratified by the use of anti-hypertensives did not show results indicating users of anti-hypertensives to be especially vulnerable to rises in BP when exposed to heavy occupational lifting, as previously shown . Yet, users of anti-hypertensives did not seem to have beneficial lowering effects on BP by increasing the level of LTPA, in the cross-sectional analysis (table S6). Previously, greater or similar beneficial effects on BP from LTPA have been seen among hypertensives compared to normotensives [19, 36]. However, these previous studies did not take OPA or heavy occupational lifting into consideration. Thus, future investigations on the effects of LTPA on BP among working-age adults should account for the level of OPA. Hence, to develop recommendations for the prevention of hypertension more knowledge is needed, especially targeted to vulnerable groups of older workers. Also, further investigations are needed to uncover the potential for prevention of progression of hypertension among users of anti-hypertensives, as the level of LTPA does not seem to have the assumed BP-lowering effect [19, 36].
The population included in the cross-sectional analysis were younger, less hypertensive, better educated, and more frequently exposed to heavy occupational lifting than those excluded from the analysis. For the prospective analysis, the differences between those in- and excluded from analysis were similar to the cross-sectional analysis. Besides the inclusion criteria of answering the question regarding exposure to OPA and heavy occupational lifting and being aged < 70 years at baseline, these differences may be explained by the frequency of excluded study participants being retired or unemployed (80% in the cross-sectional data and 32% in the prospective data). Moreover, the inclusion criteria for the prospective analysis of being normotensive excluded 52% of the study participants at follow-up. Taken together, these differences between in- and excluded study participants indicate that the population analyzed was overall healthier, but also more frequently exposed to heavy occupational lifting, and thus the results may reflect associations being more conservative than if based on the entire sample of study participants. Also, as the complete follow-up sample is currently being collected, these associations should be repeated for verification in the complete sample.
A limitation of the study is the self-reported exposure measure of occupational lifting, as self-reports of ergonomic work exposures have been found imprecise and at risk of recall [37, 38], and social desirability bias, compared to exposures collected by worn devices as accelerometers . The self-reported exposure to occupational lifting is therefore inadequate for a more detailed description of the frequency and duration of lifting tasks and weight of the lifted burden. Due to the dichotomized response category of the exposure to heavy occupational lifting question (yes/no) the responses might be biased of the individual worker´s perception of what heavy occupational lifting are, as no categories of the weight of the lifted burdens are given as an example. Thus, the evaluated exposure to heavy occupational lifting is a quite crude indication of exposure to occupational lifting or not, and a further investigation by use of more detailed and accurate measures of exposure to occupational lifting is warranted. Another limitation is the single measurement of a casual BP, shown to have a lower prognostic value than ambulatory BP, or BP monitored during sleep [40, 41].
The main strengths of this study include the limited risk of false-negative classification of hypertension due to the determination of hypertension based on both use of anti-hypertensives and the casual BP, in mmHg, and the high number of randomly selected study participants in the study population.
Perspectives of the proposed findings
The Eurofound survey states that 33% of the European workforce is exposed to occupational lifting (6th survey in Eurofound). Knowledge of the impact on cardiovascular health from occupational lifting is sparse, and to be able to develop preventive initiatives and clinical guidelines, investigations of the effect of heavy occupational lifting on precursors of cardiovascular disease should be encouraged. In this paper, the associations indicated workers aged ≥ 50 years to have an increased risk for hypertension (OR 1.11, 99% CI 0.95 – 1.29), when exposed to heavy occupational lifting. Thus this group holds the potential for prevention of hypertension by minimizing exposure to heavy occupational lifting, e.g. by automatization of manual work tasks or use of assistive devices.
Furthermore, future preventive initiatives and clinical guidelines should strive to modify the exposure to heavy occupational lifting, as well as stay informed on the growing knowledge of the effects on BP from the combined LTPA and OPA levels to avoid the risk of cardiovascular overstrain. However, for this development, it is essential to keep in mind the lower effect of LTPA on BP among users of anti-hypertensives. Thus, to develop preventive initiatives and clinical guidelines for heavy occupational lifting in relation to risk for hypertension more knowledge is needed.