Effects of six month personalized endurance training on work ability in middle-aged sedentary women: a secondary analysis of a randomized controlled trial

Work ability is defined as the balance between individual resources and the specific demands of a work task [1]. The demographic changes in most Western societies have been accompanied by the challenge of longer employment. German employees between the ages of 45 and 65 have significantly more inability to work than younger workers [2]. Moreover female workers have more frequent and longer days of illness than men [2]. In 2015, 47,249 million Euros in medical expenses were caused by female employees between 45 and 65 years old, which is a substantially higher amount than that of their younger colleagues (15–45 years 33,582 million Euros) [3].

To assess an individual’s subjective work ability, the Finnish Institute of Occupational Health developed a questionnaire called the Work Ability Index (WAI) [4]. This questionnaire is often used in workplace interventions to measure the effects of interventions [5]. The questionnaire is a kind of early warning system that enables the assessment of the current ability to work and the adoption of measures to maintain and increase the ability to work [6].

A possibility to improve work ability is through regular activity, as shown in previous studies [7, 8]. However, a review [9] concluded that there is insufficient and limited evidence on the effects of interventions for work-related components of aging workers. An even more recent review [5] found that only one-third of randomized studies showed an improvement in work ability through exercise programs. To maintain and possibly increase the ability to work through physical activity in this context, new approaches have to be established. One way is activity promotion during working hours and the other is special focus on eligible employees with a limited ability to work.

The context of female middle-aged employees working in university hospitals may involve unique problems, such as an above average absenteeism rate, working conditions, shift work, time pressure, unsafe employment and a high proportion of physical and psychological pressure [10‐13]. Not surprisingly this group of individual’s is prone to exhibit a decreased ability to work and more sick leaves with corresponding costs for the health care system [2, 3]. Therefore, we tested the hypothesis that individualized moderate endurance training partly performed at the workplace improves work ability. We conducted a prospective and randomized controlled study to investigate the effects of a 6-month endurance exercise intervention on work ability in middle-aged sedentary women working at a German university hospital.

One hundred forty-six women were randomized to the EG and 145 to the CG (Fig. 1). The two study groups were well matched for age, body weight, BMI and cardiorespiratory fitness (Table 1). The largest group of employees was the medical and technical workers (33%), followed by administration (27%), nursing (19%) and physician/scientist (12%). For 9% information is missing, based on the fact that the question about the “job title” was voluntary. Adherence to the goal of 210 min/week of endurance activity per week in the EG was 207 ± 86 min/week (range 10 to 512 min/week). Participants that did not reach the goal of 210 min/week (23%) were also included in the statistical analysis. The analysis of variance showed no significant interaction effect between menstrual status and changes in the WAI total score over time (p = 0.067). There was no significant correlation between age and the WAI score at baseline (r = − 0.07, p = 0.276).

The hypothesis that a 6-month individualized moderate endurance training partly performed at the workplace during working hours would improve work ability has been confirmed. An important finding is that female employees with low baseline work ability seem to particularly benefit from the intervention. This finding supports the classification of individuals according to their work ability before starting an intervention to identify which employees might benefit most from such programs.

Our observations were in line with other studies that have increased the ability to work with exercise training [25‐28]. Previous results of a comparable study showed that the control group had a threefold faster reduction of work ability than the intervention group [29]. Therefore, not only have we been able to stop the natural decline but also improve work ability.

We conducted an intervention that offered the possibility to train during the working hours. It has been previously shown that a supervised physical exercise intervention with middle-aged female health care workers is more successful during working hours than during leisure time at home [30]. In addition, physical exercise during work hours is considered to be the most effective form of primary prevention [31]. Given the weekly working time of 39.4 h per week in Germany [32] in the health sector, the workplace should receive special attention. This view is supported by the WHO, which has established the workplace as a priority setting for health promotion [33]. This approach by the WHO has provided a basis for intervention programs performed during the working hours [34]. Not all previous studies have shown positive effects of interventions during working hours on work ability [35, 36]. Reasons for this might be that participants already work physically (e.g., in the case of cleaners, construction workers, and home care workers) or have a high work ability score at baseline, resulting in a ceiling effect. We focused on physically inactive women and observed a high adherence of 98.8%, which was possibly due to their ability to exercise during their working hours and use the workplace health club. This likely facilitated their exercise by decreasing the amount of time and travels required and thus contributed to their incorporation of exercise into the workplace [37]. The personal guidance and regular contact between participants and the supervisor were also likely helpful for strong compliance. Furthermore, problems could be detected early on and solved, thereby maintaining motivation. This approach made it possible to eliminate the two most important barriers for exercise training: lack of time and motivation [38].

For the present study group, it seems reasonable to conclude that an increase in work ability can be slowed by hormonal changes [39, 40]. Therefore, we also investigated the influence of hormonal status and, in contrast to other studies [41, 42], were unable to determine an association between hormonal status and work ability.

In times of demographic change with increasing aging of the working population, maintaining working capacity is of particular interest. In the general population there is a relationship between poor work ability and higher age, as well as resulting earlier retirement [17, 43]. In our group of inactive women we did not observe a significant relation between age and the WAI. However, in that age-group of female employees there might be a particularly increase in physical inactivity and sedentary time [44] which could impact overall health, health-related quality of life and eventually work ability. All three WAI subgroups improved their cardiorespiratory fitness and body composition, with no significant differences between the subgroups. Considering work ability, the poor/moderate (WAI 1) subgroup improved their WAI total score significantly compared to those of the good and excellent subgroups. The good and excellent groups were unable to benefit from the present intervention in terms of the total work ability score. This might indicate the need to determine participants’ current work ability if the goal of endurance training is an improvement in work ability and not fitness alone. Since work ability is easy to estimate it could be a valuable instrument beside established measures like cardiorespiratory fitness or adiposity, in particular when the goal is to increase or maintain an individual’s productivity in relation to job demands or impairment due to diseases. This consideration has already been mentioned in previous studies examining more intensive programs in persons with lower baseline WAI scores [45]. A previous comparable but uncontrolled study observed similar improvements as our intervention after a 12-month program [46]. In this study, the group with a poor/moderate WAI (n = 43) increased their scores by approximately 2.9 points after 4 months and by approximately 3.5 points after 8 months, which was consistent with the increase of 3.6 points in our study indicating that 6 months might be sufficient to observe meaningful benefits on work ability.

Considering the individual items, the second WAI item (workability in relation to the demands of the job) and third WAI item (number of current diseases diagnosed by a physician) scores increased significantly in the EG but not in the CG. The women in the EG found themselves more able to cope physically and psychologically with the work requirements and reduces their diseases. This finding is possibly attributable to the positive effect of the increase in performance and body composition throughout the intervention. The women with the lowest baseline work ability again experienced the greatest benefits for these subitems.

The short-term increase in work ability resulting from the intervention is only an intermediate step. For long-term positive outcomes, an intervention should affect all levels of work ability [47]. The basic level (health resources) and the fourth level (work ability) represent the most important effects. However, to be comprehensive, an intervention should include the other levels (competence, values and work) as well. In addition, Lidegaard et al. emphasized that longer interventions may be needed to induce sustained effects on work ability [26]. The investigated age-group is of special interest due to their high labour force participation rate and the related contribution to the gross national product [48]. Furthermore, the demanding work in hospitals is mainly performed by female employees [49]. This situation, together with the decline in physical activity and hormonal changes in women at midlife [39, 44] implicate the need for targeted efforts to promote physical activity across female hospital employees. This indicates the need for further studies with overarching, interdisciplinary and holistic approaches to evaluate long-term effects.

A 6-month exercise intervention with individualized endurance activities and group fitness courses in middle-aged female hospital workers resulted in positive effects on work ability. For the best outcomes, employees should first be categorized into WAI subgroups to focus on the group with the lowest WAI scores for individualized endurance training. In this context, the support of employees with the weakest work ability could be a beneficial strategy for long-term work ability.