The Physiological Benefits
If your long run is easy – i.e., not a specific workout — then what is the optimal pace? Let’s look at some of the physiological benefits of the long run and see how pace affects the intended benefit.
Capillaries are the smallest of the body’s blood vessels and they help deliver oxygen and nutrients to the muscle tissues. The greater the number of capillaries you have surrounding each muscle fiber, the faster you can shuttle oxygen and carbohydrate into your muscles.
Various studies have shown that capillary development appears to peak at between 60 and 75 percent of 5K pace.
This isn’t to say that running really slowly (or much faster) on occasion doesn’t have any benefit. However, running much faster or slower than this pace doesn’t significantly increase or decrease capillary development.
Increased Myoglobin Content
Myoglobin is a special protein in your muscles that binds the oxygen that enters the muscle fiber. When oxygen becomes limited during exercise, myoglobin releases the oxygen to the mitochondria. Simply speaking, the more myoglobin you have in your muscle fibers, the more oxygen you can sequester to the muscle under aerobic duress – like in a race!
While all muscle fibers contain myoglobin, the ones we’re most concerned with targeting during the long run are the Type-I (slow twitch) muscle fibers. Research has shown that maximum stimulation of Type I muscle fiber occurs at about 63-77 percent of VO2max. This is about 55-75 percent of 5K race pace.
Increasing Glycogen Storage
The body stores carbohydrates in the muscles for usable energy in the form of glycogen. While this isn’t important for races that last under 90 minutes, when racing the marathon, the more glycogen you can store in your muscles, the longer you can prevent the dreaded bonk.
The goal with easy long runs is to deplete the muscles of their stored glycogen. The body responds to this stimulus by learning to store more glycogen to prevent future depletion.
The faster you run, the greater the percentage of your energy will come from carbohydrates. While there isn’t any scientific research on the optimal pace that burns significant carbohydrate while still providing enough energy to get through a long run, my experience and studying the training of elite runners has shown that a pace of about 65-75 percent of 5K pace is optimal.
Mitochondria are the microscopic organelle found in your muscle cells that contribute to the production of ATP (energy). In the presence of oxygen, mitochondria break down carbohydrate, fat, and protein into usable energy. Therefore, the more mitochondria you have, and the greater their density, the more energy you can generate during exercise, which will enable you to run faster and longer.
Two researchers, Holloszy (1967) and Dudley (1982) published some of the defining research on optimal distance and pace for mitochondrial development. In short, Holloszy found that maximum mitochondrial development occurred at about 2 hours of running at 50-75 percent of V02max. Likewise, Dudley found that the best strategy for slow-twitch, mitochondria enhancement was running for 90 minutes at 70 to 75 percent V02 max. So what does this mean in real-world terminology? Let’s sum it up on the next page.