With the long-anticipated release of the book Born To Walk, there is more reflection and second-guessing of the prevailing approach to endurance running that often leads to injury, exhaustion, and burnout. For the vast majority of fitness enthusiasts, running–even for just a few miles at a modest pace–is far too stressful to promote fitness benefits or weight loss. The higher degree of difficulty of running causes the heart rate to elevate beyond the aerobic “fat burning” zone into slightly or significantly anaerobic “sugar burning” heart rate zones. Aerobic workouts are comfortable and minimally stressful, and emphasize fat burning. When you exceed your aerobic limit, also known as “fat max” heart rate, you start to emphasize glucose burning, produce more stress hormones, and incur more impact trauma, muscle fatigue, and glycogen depletion.
Of course, pushing your body to a peak performance effort at higher heart rates once in a while is a great way to boost fitness and feel invigorated, but when you drift above your fat max heart rate on a regular basis, this is a recipe for trouble. Amidst a hectic, high-stress modern life, a chronically stressful training regimen will be highly ineffective—hampering weight loss efforts, dysregulating appetite, suppressing immune function, and plunging into the common pattern of recurring running overuse injuries.
We don’t notice the adverse impact of an overly-stressful workout right away due to the endorphin effect. When we exceed aerobic limit and cross over into mild-to-significant anaerobic stimulation, we prompt the release of pain-killing endorphin chemicals into the bloodstream. We feel “buzzed” and euphoric after a vigorous workout. It’s nice to stimulate a natural “runners high” in an appropriate manner from time to time, but a pattern of endorphin-promoting workouts will exhaust delicate fight or flight mechanisms—especially when combined with many other forms of chronic stress in life (traffic jams, credit card debt, mean boyfriends, toxic workplace dynamics, etc.)
Doing the vast majority of your endurance training at or below your fat max heart rate is a great way to promote continued aerobic development and avoid exhaustion. Your fat max heart rate can be identified by the Maffetone formula of "180 minus age" in beats per minute. For example, a 40-year-old runner would have a fat max of 140 bpm (180 minus 40.)
Emphasizing comfortably-paced aerobic training is a strategy followed by the world’s greatest endurance athletes in every sport for the past sixty years. The vast majority of their training is conducted at a comfortable pace–at or below fat max. A detailed analysis of the training diary of legendary Kenyan marathoner Eliud Kipchoge revealed that he runs 82-84 percent of his weekly mileage (usually 120-130 miles per week) at a pace he calls “easy/50% capacity.” This equates to "zone 1" in the popular five-zone vernacular that categorizes training intensity by percentage of max heart rate. It's great to see the popularity of "zone 2 cardio" these days, but we mustn't forget the tremendous benefits offered by zone 1 exercise.
See the article, Slowing Down To Fat Max, for details about how to identify and monitor your fat max training pace during workouts, and track how your fat max pace improves over time when you do it correctly.