Optimizing Endurance Workouts

What is the best way to get the most out of your endurance workouts?

For years, endurance athletes were taught that long, slow distance was the correct context for how to think about improving endurance. But, as you shift to thinking about priming and adapting, then you should begin to consider hard and how long should your workout be for sufficient priming, and how do you maximize the time spent adapting to each endurance workout?

To answer that question, let’s first look at what changes you’re aiming to encourage during your workout. Muscle cells produce energy in two different locations: the mitochondria, and the cytosol. The mitochondria are specialized parts of the cell that solely act to make energy. The cytosol is the fluid contained within the walls of the cell. The mitochondria can use either fatty acids or carbohydrates as a fuel source that they aerobically oxidize, or burn. The cytosol only uses carbohydrates as fuel. Your goal during endurance workouts is to prime the mitochondria to improve. Since carbohydrates can be burned in either the cytosol or the mitochondria, but fatty acids can only be burned in the mitochondria, the target effort should be close to where you are using fatty acids at the maximum possible rate.

Fortunately, researchers have recognized over the past decade that there is a point - termed fat max - where your mitochondria use fatty acids at the maximum possible rate. As you increase the effort for a recovery effort, initially most of your muscular energy production is from fatty acids. As the effort increases, you eventually reach your fat max peak, and the body must begin to shift to using carbohydrates to meet the demand for energy you require from your muscles. At this point, fatty acid usage declines as the body requires more energy, at a faster rate than can be produced using fatty acids. But, since carbohydrates can be burned in either the mitochondria or the cytosol, there is no guarantee that you stimulate your mitochondria more by exercising harder than the fat max point.

Exercising at Fat Max provides the optimal endurance stimulus

The effort level to maximally prime your body to improve endurance is this fat max point. Harder efforts may not provide any benefit because part of the effort might be powered by energy from the cytosol. But, you also don’t want to workout at an effort too far below this level either if you want to maximize the fitness you gain in the time your have to workout. Fat max corresponds to the threshold point between your endurance workout zone and your tempo or sub threshold zone. For athletes who use numeric zones, this is the boundary point between zone 2 and zone 3. It is roughly 85% of your lactate threshold heart rate, or approximately 75% of your maximum heart rate.

How long should these workout be? There is little concrete data to support a particular minimum or maximum endurance workout length, but there is a basic principle to keep in mind: a shorter workout will provide extra time before the next workout in the adapting phase. Choosing a workout that is just 15 minutes shorter than another provides you with 1% more time to adapt before you start a new workout on the following day. The gains from the extra time to adapt might seem small, or even tiny, but over months and years the compounding from these gains multiply.

Given a choice between a long, slow distance workout, and a shorter, slightly more intense workout near fat max, the shorter workout will provide greater priming stimulus, and greater time for adapting.

Short, moderate optimize endurance priming stimulus, while maximizing time for adapting.