While most runners and coaches believe that more running equals greater success, Daniels cautions about its potential to dissuade potential distance runners. "We may be going overboard with the mileage thing in running, especially for youngsters. We may lose too many potential runners if we start off stressing mileage when they are middle-school or even high-school aged," he says.
Effect of Training Volume on Physiology and Performance
As runners, we all know that the better we get, the harder it is to improve. Unfortunately, none of the adaptations associated with training continue indefinitely. Much of the research on biochemical adaptations to endurance training has been done on animals.
For example, the mitochondrial enzyme content of rats has been shown to reach its maximum adaptation with running 60 minutes per day, five days per week. A study published in European Journal of Physiology in 1998 on horses training for 34 weeks found that increases in muscle fiber area and the number of capillaries per fiber plateaued after 16 weeks of training. After the first 16 weeks, the horses were divided into two groups: a control group and an overload training group, which trained with higher mileage. Both groups increased mitochondrial volume and VO2 max with the increased mileage over the next 18 weeks, but there was no difference in those variables or in muscle fiber area and capillarization after 34 weeks despite the two-fold difference in training volume between groups over the final 18 weeks. Clearly, there is a limit to muscles' adaptive response to training.
Obviously, the more untrained you are, the more you can expect to improve by increasing your mileage. For example, a study published in Journal of Applied Physiology in 1992 found that weekly mileage ranging from five to 75 miles per week explained 86.5 percent of the difference in VO2 max between runners. Another study published in European Journal of Applied Physiology and Occupational Physiology in 1986 found that runners training more than 62 miles per week ran significantly faster in races from 10K to 90K compared to those who ran less than 62 miles per week.
While it is likely, and even probable, that running more mileage leads to a higher VO2 max and faster race times due to all of the previously described adaptations, we cannot conclude cause and effect from cross-sectional studies comparing separate groups of runners. It's likely that genetically gifted runners who have high a VO2 max are capable of running more miles, and run faster races.
According to David Costill, Ph.D., professor emeritus of exercise science at Ball State University and former director of its Human Performance Laboratory, physiological changes plateau at a modest amount of mileage.
"When you go from an untrained state to a trained state, running 30 to 40 miles per week, VO2 max and the measurements commonly taken from muscle biopsies increase, but as you move up to about 60 miles per week, things start to plateau," Costill says. "The exact mileage at which this plateau occurs depends on the individual, but beyond about 60 to 70 miles per week, there's not much change taking place." So, if VO2 max and muscle cellular adaptations plateau at about 70 miles per week, why do people run much more than that? "I really have no idea," says Costill. "People who run 5Ks and 10Ks still need a lot of speed, and when you run 120 or 130 miles per week, you can't do much quality."
How Much Do Elite Athletes Run?
In 2004, I conducted a study on the training characteristics of the U.S. Olympic marathon trials qualifiers. My findings, which were published in International Journal of Sports Physiology and Performance in March 2007, revealed that the men averaged 90 miles per week with a peak mileage of 120, while the women averaged 72 miles per week with a peak mileage of 95 for the year of training leading up to the trials. However, the elite male marathoners (sub 2:15) didn't run statistically more than the national-class marathoners (2:15-2:22). The elite men averaged 97 miles per week with a peak mileage of 126, while their national-class counterparts averaged 90 miles per week with a peak mileage of 119.