What’s Actually Happening Inside a Marathon Runner’s Body

You Should Know: Spring marathon season is here. If you’ve ever watched someone cross a finish line looking simultaneously triumphant and completely destroyed after running 26.2 miles, there’s a legit physiological reason for that. Running for that long puts your entire body through an acute stress test and demands more of it than almost any other activity. Your heart rate climbs dramatically, and cardiac output can increase by as much as eight times.

Going Deeper: Things get weirder inside your body as the miles accumulate. Blood flow gets redirected away from the gut by as much as 80%, which explains why GI distress is so common on race day. Around miles 18 to 20, most runners exhaust their glycogen stores and hit the infamous wall, forcing the body to switch to fat as fuel, which is a slower process that tends to affect your pace.

Takeaway: Post-race soreness, which typically peaks in the 24 to 72 hours after finishing, is the body repairing the microscopic muscle damage caused by tens of thousands of steps. So all that soreness is part of the recovery process.

Bottom Line: Running a marathon is a full-body physiological event, and the recovery is part of the experience.