Second, there could have been a cohort effect (Twisk 2003), because the population in the longitudinal analyses was different from the population at baseline in the cross-sectional analyses due to loss to follow-up. The loss to follow-up rates were 15% for the low back tests, 31% for the
neck tests and 18% for the shoulder tests, respectively. The main reasons for loss to follow-up were general reasons, such as discharge, lack of motivation, et cetera. We investigated if this loss to follow-up could have been selective by comparing the total mean performance at baseline among workers who became lost to follow-up to those who did not become lost to follow-up. The static endurance time of the shoulder muscles at baseline was significantly shorter among those who became lost to follow-up, although the mean difference was only 3 s (256 compared to 259 s). In contrast, we found a significantly longer static endurance Autophagy Compound Library screening time of the neck muscles for that group (305 compared to 274 s). This means that there was selective loss to follow-up, but the difference
for the shoulder muscles was very small, and the difference for the neck muscles was not in the expected direction. Therefore, it seems unlikely that a cohort effect on muscular capacity could have played a role in the differences between the cross-sectional and the longitudinal results. Third, the statistical analyzing techniques were different, i.e. cross-sectionally, regression analyses were used, and longitudinal, a description of repeated means was presented for 5-year age groups. However, if we had described means in the cross-sectional analyses as well, the results LY294002 mouse would have been quite the same compared with the estimated regression functions (data not shown). This means that HSP90 it is unlikely that differences in statistical
analyzing techniques have contributed to the differences between the cross-sectional and longitudinal results. Finally, a comment should be made on the longitudinal results, since we had only data at two measurements with a three-year interval. Owing to this short interval, in particular compared to the duration of a general working lifetime, conclusions on the longitudinal results have to be taken with caution. In conclusion, other factors than differences in test circumstances, selectiveness of loss to follow-up, or differences in statistical analyzing techniques have to be sought to explain the difference between cross-sectional and longitudinal results regarding the static muscles endurance. Conclusions The results of this study suggest age-related differences of isokinetic lifting strength, and static muscle endurance of the back and neck/shoulder muscles. For isokinetic lifting strength and static endurance of the back muscles, the performance was higher among younger workers than among older workers, but for static endurance of the neck and shoulder muscles, the age-related differences were opposite.
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