A self-determination approach to the understanding of the impact of physical activity on depressive symptoms.

The purpose was to test a new motivational sequence. It was hypothesized that more autonomous forms of motivation would predict the intensity of physical activity (PA), which in turn, would predict depressive symptoms. In order to evaluate self-determined motivation, the Self-Determination Index (SDI) was used. Because the reasons that can lead a person to engage in walking, moderate PA, or vigorous PA may be different, 3 independent self-determination indexes were measured (SDIWalking, SDIModerate, and SDIVigorous). It was also measured the metabolic equivalent of task values (METs) for walking, moderate, and vigorous PA, as well as the depressive symptoms. The sample consisted of 357 college students whose ages ranged from 18 to 29 years. Structural equation modeling was used to test the hypothesized model. The indices of fit showed that the revised model fits the data reasonably well: S-Bχ2 (11)  = 14.83, p = .190; χ2 /df = 1.35; *comparative fit index = .99; *root mean square error of approximation = .03, 90% CI [.000, .068]; standardised root mean square residual = .03. It was found that vigorous PA is the only intensity that predicts depressive symptoms. In other words, SDIV-predicted vigorous PA (measured as METS V), which subsequently predicted less depressive symptoms (SDIV → METS V → Depressive symptoms). Further research should investigate the effects of vigorous PA on depressive symptoms.