Abstract
<title>Abstract</title> <p> Introduction: Cerebellar Crus I/II gray matter volume (GMV) is a structural marker of cognitive aging, and motor performance (grip strength and manual dexterity) predicts age-related cognitive decline. Whether these motor measures dissociate in their cerebellar and cognitive correlates is unknown. Methods We examined Pearson partial correlations (age, sex, and race covaried) between grip strength, Nine-Hole Peg Test (9HPT) dexterity, cerebellar Crus I/II GMV, and Montreal Cognitive Assessment (MoCA) subscores in 304 healthy adults aged 20–79 from the University of South Carolina Aging Brain Cohort. GMV was derived from T1-weighted magnetic resonance imaging using the Computational Anatomy Toolbox (CAT12) with the Automated Anatomical Labeling 3 (AAL3) atlas. Results After false discovery rate (FDR) correction, dominant 9HPT time correlated with MoCA total score ( <italic>r</italic> =-.240, <italic>p</italic> <sub>FDR</sub> <.001) and memory index ( <italic>r</italic> =-.220, <italic>p</italic> <sub>FDR</sub> <.001); no other MoCA subscores survived. Grip strength was not associated with any MoCA measure but correlated bilaterally with Crus II GMV ( <italic>r</italic> =.156–.224, all <italic>p</italic> <sub>FDR</sub> ≤.038). Bootstrapped mediation (5,000 iterations) revealed that dexterity partially mediated age effects on cognition (28.8% mediated; 95% confidence interval [-0.022,-0.006]), and Crus II volume partially mediated age effects on grip (34.4–41.4%). Conclusion A double dissociation emerged: Crus II GMV, but not dexterity, independently predicted grip strength, whereas dexterity, but not grip, predicted cognition. Grip strength and dexterity are therefore not interchangeable indices of motor function but map onto distinct substrates: posterior cerebellar integrity tracks maximal-force capacity, whereas fine motor performance indexes memory-related cognitive status independently of cerebellar volume. </p>