It's interesting to see that. My first hypothesis was probably wrong, but in the interests of open discussion, I thought, "Wait, do the poor people up north not bike or run?" I realized immediately that this was unlikely, just they don't own smartphones to track themselves, or some variant of skewed sampling. If they DON'T bike, that's sad, because biking is a money-printing fountain of youth (http://www.mrmoneymustache.com/2013/06/13/bicycling-the-safe...).
I follow a guy on Strava who is in the process of riding every street in St. Louis City. He's been all over the Northside. But, as this map shows, it's not a popular place to go cycling.
There's a difference between "cycling" and "cycling while using Strava". I suspect that few people who cycle because they can't afford to fix their car, or they lost their license, or their wife commutes in the single family car, etc. belong to the latter group.
http://labs.strava.com/heatmap/#13/-90.28904/38.64610/gray/b...