At BL, V̇O2max values were within what is considered normal for a comparable Norwegian healthy population [11] or even exceeding that of other Europeans [12]. In the present population, the annual reduction in V̇O2max was 4.6%, which is not solely attributable to aging, as the general decline in V̇O2max, suggests an annual loss of only 0.5- 1% [13, 14]. The dramatic reduction in V̇O2max is indicative of decreased physical activity, since reductions in training volume seems to positively correlate with decline in V̇O2max [15].
We observed an increase in RHR during the 3-year FU. Increased RHR can result in increased blood pressure (BP), dyslipidemia and obesity, insulin resistance, all in all increasing the risk for atherosclerosis [16]. This might explain why individuals with high RHR have an increased risk for cardiovascular disease (CVD) compared to those with lower or stable RHR [17, 18]. During the 3-year FU we noted an increase of 3 beats per minute (bpm). This suggests an elevated risk for CVD since a 5-bpm increase is associated with an 11,3% higher relative risk for fatal and non-fatal myocardial infarction in clinical settings [19]. Low RHR, on the other hand, is beneficial for the heart, reflecting less hemodynamic and sympathetic stress and diminished mechanical load. Intense training also decreases arterial stiffness and thus the strain on myocardium [17, 18, 20].
Self-reported MVPA levels decreased during FU in our study. A decrease in physical activity could affect energy expenditure and potentially increase CVD risk factors, including increased weight. However, BMI and fat mass remained consistent over the three years of follow-up. This stability in BMI and fat mass does not, however, negate the possibility of adverse effects on the fat metabolism. A reduction in energy expenditure can lead to increased intra-abdominal fat without a corresponding rise in BMI [21]. Such an increase in obesity is associated with increased inflammation, disturbance of lipid metabolism, increased blood pressure, decreased insulin sensitivity and, consequently, a higher risk for diabetes 2, and CVD events [6, 21, 22].
Throughout the study, workers continued their regular duties at the plant. This suggests that their daily physical activity remained relatively consistent from BL to FU. Physical activity in an occupational setting, however, does not appear to offer the same protective benefits against CVD as leisure-time activity [23] which, in our study, was reported to decrease during FU. Therefore, the reasons for our observed reduction in self-reported MVPA during FU likely lie in the external circumstances faced by workers during FU.
The latter part of the FU, spanning 17–18 months, coincided with the COVID-19 pandemic. Initially, there were some restrictions of movement outdoors in Norway. Later, fitness centers remained closed for many months and organized outdoor physical activity in groups was prohibited. Compared to fitness centers in the local community, the ones in the plants were blocked for even longer periods; workers had restricted or no access from March 2020 to March 2022 (Xander Nordli-Bergsholm, personal communication).
The COVID-19 pandemic has negatively impacted physical activity [24, 25], and a shift towards sedentary behavior is known to dramatically reduce V̇O2max [26]. It is plausible that the results of the present study is a consequence of lack of leisure-time MVPA during the COVID-19 pandemic. A significant decline in physical fitness due to the COVID-19 pandemic has also been reported among children and adolescents [27].
A strength of the present study is its prospective design with information on MVPA and examination of V̇O2max and RHR during FU. Using the same technicians throughout the study period ensured consistency, and measurements were taken on the same time of the day to avoid diurnal variation. In addition to BL and FU, we measured V̇O2max in late autumn 2018 and there were minimal differences when these results were compared to those at BL [9]. Therefore, it is likely that the reduction in V̇O2max during the FU is valid and not a result of methodological or technical challenges.
A limitation of the study is the relatively small number of workers performing the V̇O2max test at FU. However, regarding BL-measures, these workers did not differ significantly from the group of workers who performed the test. The group of workers in the present study may not represent the general Norwegian population. However, V̇O2max at BL was in line with what is considered normal for a corresponding general population sample [11]. Furthermore, leisure-time MVPA was not measured objectively, which is a limitation since self-reported physical activity can introduce response bias or even misclassification and seems to be overestimated [28]. Lastly, there was a lack of registration of low activity. However, we believe that this type of activity remained consistent from BL to FU since the workers were working during the entire FU period.