One of the most consistent ways research has demonstrated to increase connectivity in the brain is to learn new physical skills. Controlling the body in space appears to be particularly good at stimulating the growth of new synapses (and possibly new neurons as well, though the research is not conclusive here outside the hippocampus). Yoga is a good start. Modern dance, breakdancing, capoeira and gymnastics are all e…
Why arent athletes the most intelligent then? Generally in society there is considered a negative correletion between people that exercise/play a ton of sports and intelligence. Whether this is true or not is up to debate, but stereotypes exist for a reason...
Watch Kobe or LeBron on the court. There is an incredible awareness of the shifting context of the game and the available moves. In the greatest athletes it is faster and more fluid than in most athletes.
It's a constantly shifting analytical mode that leans heavily on training and instinct, but is full of novelty. Consider that opponents spend weeks watching tape in hopes of understanding how to anticipate what Kobe will do.
In terms of physical metrics (size, speed, etc.) professional athletes are pretty close to average. I think it is a combination of intellect and general desire to win that makes some people become great athletes.
Because of what I like to call Cartesian Blindness (viewing the world through the mind/body duality) we assume it's simply a matter of great athletes having superior muscle fibers, etc.