Physical activity is extremely beneficial for brain health via a number of the mechanisms.
Exercise increases the volume of grey matter in the hippocampus, the area of the brain responsible for memory formation, and the prefrontal cortex, involved in working memory, attentional control, decision making and behavioural inhibition.
It should be noted that the vast majority of studies looking at the effect of exercise on brain health consider aerobic exercise. The effects of resistance training are much less well documented. However, resistance exercise has been seen to improve executive cognitive function (Liu-Ambrose et al., 2012; Nagamatsu et al., 2012).