A recent study in Brain Research studied forty-nine, 9 and 10 year old children. MRI measurements were taken comparing the brains of fit and unfit children. The results indicated increased volume of the hippocampus by 12% in the fit children. In addition, the fit children performed better on relational memory (the ability to remember and integrate various types of information).
Reference: Yates, Diana. Child's Brain Development is Linked to Physical Fitness. Retrieved from the web on 10/5/2010 at http://news.illinois.edu/news/10/0915_brain_development_and_fitness_art_kramer.html