Maintaining sufficient physical activity (PA) is crucial for physical and mental health, yet a significant portion of adults fail to meet recommended levels. Understanding the factors influencing PA adoption and maintenance is essential for developing effective interventions. While self-reported questionnaires are common, they have limitations in validity and reliability compared to objective measures like accelerometers. Previous research has shown PA can induce structural brain changes, particularly in older adults, with increases in gray matter (GM) volume in various regions. However, research on young adults is limited and often focuses on the hippocampus, neglecting other reward-related areas like the orbitofrontal cortex (OFC), anterior cingulate cortex (ACC), and nucleus accumbens (NAcc). A motivational approach suggests that the reward system, particularly mesolimbic dopaminergic pathways, plays a crucial role in PA engagement. The ACC, in particular, is thought to be involved in the effortful control of actions leading to rewards. Personality traits, specifically reward sensitivity (SR) as measured by the Behavioral Activation System (BAS) scale, are also relevant, as they influence motivated behaviors. Although the relationship between SR and PA is complex, with potential links to exercise addiction, the overall effect remains unclear. Existing studies on PA's effects on the reward system show positive correlations between volume or perfusion measures and physical condition, but the findings are inconsistent and often lack objective PA assessments in young adults. Therefore, this study aims to investigate the relationship between GM volume in reward-related areas (OFC, ACC, NAcc), objectively and subjectively measured PA levels, and personality traits in a sample of healthy young adults. The study hypothesizes a positive association between GM volume in reward-related structures and PA, as well as a mediating role for reward sensitivity.

The literature review extensively covers the benefits of physical activity (PA) on physical and mental health, highlighting its positive impact on cognitive function. It then addresses the limitations of self-reported PA measures and discusses existing research on the relationship between PA and brain structure, specifically focusing on changes in gray matter (GM) volume in various brain regions. The review then delves into the motivational approach to understanding PA, emphasizing the role of the reward system and mesolimbic dopaminergic pathways. Key brain regions involved in reward processing, including the OFC, ACC, and NAcc are detailed. The review also explores the reinforcement sensitivity theory (RST) and its connection to personality traits, particularly focusing on the Behavioral Activation System (BAS) and its implications for motivated behaviors. The complex relationship between reward sensitivity, as assessed through the BAS scale, and PA is discussed, highlighting the inconclusive findings from previous studies.

Sixty-six right-handed healthy young adults (33 females; mean age 22.7 years) participated. Physical activity (PA) was assessed using a GENEActiv accelerometer worn on the non-dominant wrist for at least four days, providing objective measures of sedentary time, light, moderate, and vigorous PA. Participants also completed the Global Physical Activity Questionnaire (GPAQ) for subjective PA assessment. Personality traits were assessed using the Sensitivity to Reward Scale (SPSRQ) and the BAS scale. Brain imaging involved high-resolution T1-weighted MRI scans acquired using a 3T GE Signa Architect scanner. Voxel-based morphometry (VBM) analysis using CAT12 and SPM12 software was used to analyze gray matter (GM) volume in six regions of interest (ROIs): left and right OFC, NAcc, and ACC. Statistical analyses included Spearman's and Pearson's correlations (controlling for age and total GM volume) and multiple regression analyses to examine the relationship between PA measures (objective and subjective), ROI GM volumes, and personality traits. The stepwise regression method was used to extract only significant predictors.

Significant correlations were found: a negative correlation between objectively measured sedentary time and right ACC GM volume (r = -0.44, p < 0.001), and a positive correlation between self-reported moderate-to-vigorous PA (MVPA) and both right (r = 0.32, p < 0.05) and left (r = 0.31, p < 0.05) ACC GM volume. Multiple regression analysis revealed a significant model predicting sedentary time, with right ACC volume and BAS activity negatively related to sedentary time (adjusted R² = 0.23, F(4,61) = 5.90, p < 0.001). Objective and subjective PA measures showed moderate correlations, with stronger correlations for vigorous activity compared to light or moderate activity. Correlations between personality traits and PA were less consistent, with only significant correlations between Sensitivity to Punishment and self-reported vigorous PA.

The findings demonstrate that the ACC volume is significantly associated with both objective and subjective PA measures in healthy young adults. The negative correlation between right ACC volume and sedentary behavior suggests that greater ACC volume may be associated with a higher propensity for engaging in effortful activities, which can be related to PA behaviors that promote long-term gains. The study also highlighted the independent contribution of both personality (BAS activity) and brain structure (ACC volume) in explaining sedentary behavior. The discrepancy between objective and subjective PA measures is consistent with previous meta-analyses and is potentially influenced by factors like registration period and social desirability bias. Future research could explore the causal relationship between ACC volume and PA, potentially examining longitudinal data to track changes in both measures.

This study provides evidence that the volume of the right ACC is associated with physical activity levels and sedentary time in young adults. The findings highlight the importance of considering both brain structure and personality traits when designing interventions to promote healthy lifestyles. Future studies should investigate the causal relationship between ACC volume and PA, explore the effects of interventions targeting ACC function and reward sensitivity, and investigate whether these findings generalize across different age groups and populations.

The cross-sectional nature of the study limits causal inferences. The sample, while relatively large for neuroimaging studies, might not fully represent the diversity of the population. The reliance on self-report questionnaires for personality and subjective PA introduces potential biases. The study focused on young adults, limiting the generalizability of the findings to other age groups. Future research could address these limitations by adopting longitudinal designs, expanding the sample size, and using more objective assessment tools.