Our study shows that high level of isometric endurance capacity of trunk extensor muscles is associated with low levels of depressive symptoms in both males and females. In males, also poor handgrip strength was associated with increased levels of depressive symptoms. Our results are consistent with previous research showing the association between handgrip strength and depressive symptoms [5, 6]. However, these earlier studies have been conducted among middle-aged or elderly population. Therefore, the present study provides new information about the inverse association between muscular fitness and depressive symptoms also among young adults.
We also found that the low level of self-reported physical activity was significantly associated with increased prevalence of depressive symptoms among young adults. The association between depressive symptoms and low level of self-reported physical activity has already been shown in several previous studies [1–3].
Interestingly, there was no significant association between objectively measured cardiorespiratory fitness and depressive symptoms in either males or females. These results are not consistent with the evidence from the first community-based observational study demonstrating the inverse association between maximal cardiorespiratory fitness and depressive symptoms  in which participants completed a maximal exercise treadmill test to estimate cardiorespiratory fitness. However, in our study, we used submaximal step test to measure cardiorespiratory fitness. The main reason we performed submaximal exercise test was that direct measurement of maximum oxygen consumption (VO2max) during a maximal exercise test is time-consuming, requires laboratory equipment and involves health risks especially in large populations. The use of different measurement methods (submaximal vs. maximal exercise test) may explain the differences in the results regarding cardiorespiratory fitness.
On the other hand, in our study, muscular fitness was measured by maximal trunk extension test and maximal handgrip test. It may be that the results from physical fitness tests that need maximal physical effort and at the same time maximal mental motivation to perform the test maximally are more strongly associated with depressive symptoms. This maximal or submaximal physical effort needed during the certain fitness test may explain why results of the submaximal step test were not associated with depressive symptoms.
Major depressive disorder is a highly prevalent condition that worsens functioning and quality of life . Furthermore, the number of people diagnosed as having depressive disorder is rising . The identification of prevention and invention strategies aimed at decreasing depressive symptom, which can be applied to populations inexpensively and without side effects, would be needed. In clinical practice, physical activity and subsequent good physical fitness inevitably have potential benefits with very few obvious risks. Physical activity is a particularly beneficial behaviour due to the combined effects on both physical and mental health [1, 9]. Several mechanisms, how exercise affects depressive symptoms, have been proposed. We speculate that physical inactivity is likely to be both a cause and consequence of depressive symptoms. As depression can be characterized by low energy, it is possible that depression leads to decreased physical activity and physical fitness. People who are depressed may be less likely to engage in physical activities [31, 32] and, on the other hand, good mental health increases likelihood of engaging in physical activity . However, there are also studies showing no effect of exercise on depressive symptoms . According to the recent systematic review, the effectiveness of exercise on treatment of major depression is still unknown . Even causality between depressive symptoms and physical activity/fitness could not be evaluated; our study presents associations with several different measures of physical fitness.
Study limitations and strengths
Limitations of our study are the cross-sectional nature of these data that does not allow inferences about causality. Further, the HSCL-25 does not provide a specific depression diagnosis like structured clinical interviews. The weakness of the study is also that we used self-reported leisure-time physical activity. In addition, cardiorespiratory fitness was measured by using the submaximal step test and not by a maximal exercise test, which is regarded more accurate test to evaluate cardiorespiratory fitness.
The strengths of our study were that it was based on a large representative sample (N = 5497) of young (31 years) people born in the North of Finland. Extensive data based on the general population, describing people of different socioeconomic classes, offers a unique opportunity to study the association between depressive symptoms and physical fitness. Additionally, HSCL-25 has proved to be an acceptable screening scale for obtaining information on symptoms of depression among large population .
Even we used self-reported questionnaire to determine physical activity, objective measurements of muscular fitness by maximal tests can be considered strength of this study.