Development of the Multidimensional Inventory of Physical Activity Identity.

Identity is among the most robust psychological constructs for predicting whether individuals translate physical activity (PA) intentions into action. However, existing identity measures in the PA domain focus narrowly on exercise and largely adopt limited unidimensional conceptualizations. This study aimed to develop and validate the Multidimensional Inventory of Physical Activity Identity (MIPAI-25), a novel instrument grounded in a multidimensional, theoretically integrated framework. Across two samples of university students (Study 1: n = 580, 61.6% women; Study 2: n = 619, 66.4% women), we used classical test theory, Mokken scale analysis, and item response theory to construct and refine the content relevancy of the items for each scale. In Study 1, the final 25-item instrument captures five distinct dimensions of PA identity: Acknowledgment, Compatibility, Centrality, Commitment, and Exploration. Each subscale score demonstrated strong internal consistency, unidimensionality, and measurement invariance across gender groups. In Study 2, all five dimensions were positively associated with moderate-to-vigorous PA behavior and PA-related constructs from the Multi-Process Action Control framework, with the strongest associations observed for habit and behavioral regulation - key post-intentional processes that facilitate action control. The MIPAI-25 offers a more ecologically valid and dynamic representation of PA identity, extending beyond exercise-specific and unidimensional measures. Findings support its use as both a comprehensive tool and a set of five stand-alone subscales, depending on research or practical needs. Future longitudinal studies are needed to explore how these identity dimensions evolve across life transitions and to inform more targeted, identity-based interventions for promoting sustained physical activity.