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Zone 2: Basic Endurance Training

Aerobic threshold is the intensity at which almost all of the endurance fibers are being used, but none of the speed-endurance or sprint fibers.  Basic endurance training is best accomplished at or slightly below aerobic threshold intensity.  Basic endurance training forms the backbone of any good endurance run training program, with the majority of training volume being at a very easy intensity.  This training should comprise a higher percentage of most athletes’ training programs.  Most athletes spend too much time running medium-hard and would do better to slow down for most of their training

Running at a basic endurance pace, in heart rate zone 2, trains the endurance fibers.  These fibers have tremendous capacity for endurance, but not much capacity for speed.  They do most of the work in endurance sports.

These muscle fibers can withstand and benefit from enormous training volume if intensity is appropriate.  At higher intensities, muscle fibers that lack the endurance capabilities of the slow twitch fibers are recruited.  These fibers cannot withstand training volumes appropriate for the slow twitch fibers.

At basic endurance intensity, the muscles burn a relatively even mixture of fat and carbohydrate for fuel (individual differences in genetics and training affect this somewhat).  Since fat is such a concentrated energy source, containing about 2.25 times as much energy per gram as carbohydrate, even the leanest athlete has enough fat stored to travel many, many miles. 

When intensity increases above basic endurance pace, the muscle shifts over to burning more carbohydrate and less fat.  This happens for two reasons.  First, the body must begin recruiting some of the speed-endurance muscle fibers, which do not burn fat effectively.  These fibers compete with the endurance fibers for available oxygen, so the endurance fibers lack adequate oxygen to burn fat and begin to burn more carbohydrate.  This depletes muscle and liver glycogen storage unnecessarily.  Many athletes run their basic endurance workouts just slightly too fast, depleting resources and delaying recovery for tomorrow’s workout.  Doing basic endurance workouts too fast won’t make you faster.

Most athletes perform basic endurance workouts too hard.  Above aerobic threshold, the body is recruiting all of the endurance fibers and some if the speed-endurance fibers (F.O.G.).  At this intensity fuel use has shifted significantly away from fat and the muscle burns mostly sugar.  At this intensity you aren’t really going hard enough to make yourself faster, but you are going fast enough to deplete yourself for tomorrow’s workout that is designed to make you faster.  The speed-endurance (F.O.G.) muscle fibers cannot handle as much volume as the endurance (slow twitch) fibers, and require much greater recovery time.  Exercising slightly above aerobic threshold is generally very inefficient, limiting volume and negatively affecting the quality of high intensity workouts.