Study population

The study population consisted of the Northern Finland birth cohort of 1966, originally including all 12 231 males and females whose expected year of birth was 1966 [12]. In 1997–1998, at age 31, all the members of the cohort who were still alive (N = 11 541) were invited to participate in a follow-up survey. A questionnaire including questions about depressive symptoms and physical activity was mailed to this group, and 8767 (76%) persons responded. The subjects who were living in the Northern Finland, or had moved to the Helsinki capital area, were also invited to a medical examination performed in local health centres. The invitation to participate in the medical examination was accepted by 6033 persons (71% of those invited), and 5497 (65% of those invited) males and females completed all fitness tests. Altogether 51% of them were women, 73% were married or cohabiting and 71% were employed.

The Ethics Committee of Oulu University Hospital has approved this study. Written informed consent was obtained from all the subjects before participation in the study.

Variables

Depressive symptoms

Information on depressive symptoms was obtained through Hopkins’ Symptom Checklist-25 (HSCL-25) [13–17] which was included in the above-mentioned postal questionnaires. HSCL-25 is a 25-item shortened version of an originally 90-item questionnaire designed by Derogatis et al. [13]. A depression subscale consists of 13 items [13, 14, 17]. Cohort members recorded their estimates of severity of their depressive symptoms on a scale ranging from 1 (“not at all”) to 4 (“extremely”). Responses were summed and divided by the number of answered items to generate a depressive symptoms mean score ranging from 1.0 to 4.0. There are two commonly used mean scores of 1.55 and 1.75 [13, 14, 17]. These points are cut off points for important depressive symptoms but not for diagnosis of major depression.

Physical fitness

Four teams of trained research nurses, who supervised the fitness tests and performed the anthropometric measurements, carried out the medical examinations. Before the fitness test, the subjects were interviewed to screen out persons with cardiovascular diseases or orthopaedic problems. Less than 10% of all subjects were excluded from the analyses for various reasons. The most common reasons for not performing step or trunk extension tests at this age of 31 years were ill health and pregnancy [18].

Cardiorespiratory fitness

In the medical examination, the subjects performed a submaximal four-minute single step test conducted without shoes on a bench 33 cm high for the females and 40 cm high for the males [19]. We used heart rate after step test as an indicator of cardiorespiratory fitness. A metronome paced a rate of 23 steps per minute. Heart rate (beats min-1) was measured immediately after the test by a heart rate monitor handle on the chest (Fitwatch, Polar Electro, Kempele, Finland). In a laboratory measurements of 123 adults aged 31 years the correlation coefficient between heart rate after step test and peak oxygen uptake during maximal exercise test was 0.53 [19].

Muscular fitness

Muscular fitness was measured by trunk extension test and maximal isometric handgrip test. During the trunk extension test[18, 20] the subject was in a prone position, the lower body lying on the stand and the upper body unsupported from the level of the anterior superior iliac spine upwards. The tester sitting on them stabilized the legs and the arms were held beside the trunk. The isometric endurance capacity of the trunk extensor muscles was evaluated by holding the upper part of the body in a horizontal position as long as possible, however, not exceeding four minutes. When the subject was no longer able to maintain the horizontal position, the test ended. The outcome measure of the test was the endurance time in seconds.

Maximal isometric handgrip strength of the dominant hand was measured with a hand dynamometer (Newtest, Oulu, Finland) based on the strain-gauge technique. Measurements were performed with the subject in a standing position, holding the dynamometer, with the hand beside but not touching the trunk. The wrist and the elbow were extended. The width of the grip in the dynamometer was adjusted to the size of the hand. The highest value in Newton (N) of the three trials, each lasting from two to four seconds, was accepted as the result [18].

Leisure-time physical activity

In the above-mentioned postal questionnaires, subjects were also asked how often they participated in light and brisk physical activities. Response alternatives in this study were daily, four to six times a week, two to three times a week, once a week, two to three times a month, and once a month or less often. The duration of one bout of activity was considered separately for light and brisk activities with the following alternatives: more than 90 minutes, 60–90 minutes, 40–59 minutes, 20–39 minutes, less than 20 minutes, and not at all. In the questionnaire, the term ‘brisk’ was defined as physical activity causing at least some sweating and getting out of breath, and the term ‘light’ as physical activity causing no sweating or getting out of breath. Total volume of leisure-time physical activity was expressed as metabolic equivalent hours/week (MET-hours/week), which was formed by calculating duration and frequency of both brisk and light physical activity. In the calculations, an intensity value of 3 METs was used for light physical activity and 5 METs for brisk physical activity. We formed five equally distributed categories to describe quintiles of physical activity (Q1-Q5) (Table 1).

Table 1 Prevalence of depressive symptoms according to quintiles groups of cardiorespiratory fitness, muscular fitness and physical activity Full size table

Potential confounding variables

Since alcohol intake, obesity, smoking and somatic diseases have been shown to be associated with depressive symptoms [21–23] and physical fitness [24, 25], those were used as potential confounding factors in multivariate regression analyses.

Alcohol intake

Information on the frequencies of beer, wine and other spirit consumptions as well as statements on the usual amounts of each alcoholic drink per one drinking occasion was requested in the questionnaire. For each type of drink (the alcohol percentages of which were turned into amounts of pure alcohol consumed), the frequency of alcohol use was proportioned to 365 days. The average amount of pure alcohol (g/day) was calculated as follows: pure alcohol (g) at any one time x frequency of alcohol use (1/day). Alcohol consumption was categorized as abstainers/light drinkers (< 15 g of pure alcohol/day), moderate drinkers (15–40 g/day), and heavy drinkers (> 40 g/day) [26].

Obesity

Body mass index (BMI) was calculated as weight/height2 (kg.m-2). Body height and weight were measured to an accuracy of 0.1 cm and 0.1 kg, respectively.

Smoking

Cohort member’s regular daily smoking at the age of 31 was classified as follows: regular smokers (i.e. smoking on 7 days a week), occasional smokers, (i.e. smoking less than on 7 days per week) and non-smokers [27].

Somatic diseases

We used data of national Finnish hospital discharge register about the presence of lifetime hospital-treated somatic disease [28].

Statistical methods

The prevalence of depressive symptoms (as defined by the HSCL-25 depression subscale mean score cut off points of 1.55 and 1.75) was compared in quintiles groups of physical activity and fitness (Q1-Q5). In order to adjust the results for potential confounding variables, multiple binary logistic regression analyses were used. Odds ratios (OR) with 95% confidence intervals (95% CI) were calculated for having depressive symptoms by different quintiles of physical activity and fitness. The third quintile group (Q3) was used as reference group.

All statistical analyses were performed using the statistical program Stata (Stata Statistical Software: Release 11. StataCorp. 2009. College Station, TX: StataCorp LP.).