Background <p>This study develops and tests an extended model to examine exercise behavior during the COVID-19 pandemic. In addition to exercise participation goals and regulatory styles, the model incorporates affective states and stress-related constructs, which are particularly relevant in the context of the pandemic.</p> Methods <p>Data were collected from a sample of 283 participants during the pandemic, a period marked by heightened stress and disrupted routines. Structural equation modeling was employed to test the proposed relationships among exercise participation goals, behavioral regulation, affective states, stress, and exercise behavior.</p> Results <p>The results reveal that physical and psychological goals significantly predict exercise behavior. Regarding behavioral regulation, intrinsic and introjected regulation are positively associated with exercise behavior. Furthermore, negative affect and the ability to cope with stress show significant relationships with exercise behavior. The findings also indicate significant mediating effects of amotivation, introjected regulation, and intrinsic regulation on the relationship between participation goals and exercise behavior. Additionally, exercise participation goals mediate the effects of positive and negative affect, as well as the ability to cope with stress, on exercise behavior.</p> Conclusions <p>These findings highlight the interplay of motivational, affective, and stress-related factors in shaping exercise behavior during the pandemic. The study provides practical implications for designing exercise promotion and intervention programs, particularly in contexts of public health crises.</p>