Exercise is known to be beneficial for brain health and performance; however, it is not known if changes in cortical microstructure underlie this effect. To investigate this, R1 maps acquired on cognitively healthy older adults (n=24, 65-90 years old) were analyzed before and after a 12-week exercise intervention. R1 prolongation indicating increased myelin levels were significant in the right (p=0.033) and trending in the left (p=0.052) leg motor regions with respect to a control group (n=22). ΔR1 correlated with aerobic cycling performance improvements (left: p=0.012, right: p=0.011). This study demonstrates that exercise promotes myelination in cortical motor regions.