1994; De Zwart et al. 1995; Bemben 1998; Hunter et al. 2005). Cross-sectionally, we found optima of static endurance time of the back muscles at the age of 36 years, However, for the neck and shoulder muscles, static muscle endurance time at the age of 59 years was between 2.0 and 1.5 times higher than at the age of 19 years. In contrast, longitudinally, we found MLN2238 concentration that muscle endurance decreased for all age groups. The direction of the aging effect was opposite when comparing the cross-sectional with the longitudinal results. With regard to performance by sports participation, the

results of this study suggest that younger workers who participated in sports for 3 hours per week or more had the highest isokinetic lifting strength and the longest static muscle endurance time. This is in-line with results

from previous studies (Rantanen et al. 1993; De Zwart et al. 1995; Ilmarinen 2001; Brach et al. 2004; Macaluso and De Vito 2004). As expected, we found that isokinetic lifting strength was lower at older ages than BI 6727 chemical structure at younger ages due to the aging Momelotinib in vitro process. The differences by age were the largest in the group participating in sports for 3 h per week or more, i.e. the plotted lines crossed over between the ages of 30 and 40. Furthermore, the results suggest that older workers who participated in sports between 0 and 3 h per week had better performance in tests of physical capacity than those who were inactive or participated in sports for 3 h per week or more, which was not in-line with our expectation that the age-related differences would be smallest among the most active workers. Possible explanations for the differences between the cross-sectional and longitudinal results The

differences between the cross-sectional and longitudinal analyses were contrary to our expectations. Owing to a potential healthy worker effect, most we expected to find equal or fewer age-related differences in within-worker comparisons compared with between-worker comparisons. However, the results suggest that there was no healthy worker effect. Several factors can explain this finding. First, there could have been a period or measurement time effect (Twisk 2003) due to different test circumstances at follow-up compared with baseline. Possible differences in test circumstances may have been the result of less motivation of the workers during the tests, to other physiotherapists who conducted the tests or to seasonal effects. In pilot studies, reproducibility was found to be high for the isokinetic neck/shoulder lifting test and the trunk muscle endurance test and moderate for the other tests of muscular capacity (Hamberg-van Reenen et al. 2006).