An Efficient Machine: Part I

Part I: The physiology of running economy

June 1, 2004

Stride length: Running speed is determined by stride length times stride frequency. Every runner has an optimal stride length for each running speed, based on a complex combination of leg length and muscular characteristics. If you overstride or run with a shorter than optimal stride length you will use more oxygen. Fortunately, the vast majority of runners inherently select the optimal stride length for each running speed. It is not known exactly how we do it, but presumably running with the most economical stride length leads to less perceived exertion, and we naturally seek to run as comfortably as possible. Over time, it is possible to increase your optimal stride length at a given pace, which may lead to further improvements in economy.

Other aspects of running technique: A common misconception is that coaches can look at a runner and tell whether she is efficient or economical. Most of the differences in running economy, however, occur inside the muscles and are not obvious to even the most experienced coach. Actively correcting seemingly sloppy technique—such as arms crossing the body—has not been found to improve running economy, likely because the apparently sloppy technique is actually compensating for other variables. Over the course of months and years however, subtle changes in running technique do seem to lead to subtle improvements in running economy as your running "skill" improves. This may be due to improved ability to recruit the most efficient muscles’ fibers for the task at hand. With increased experience, you may also improve your ability to relax opposing muscles and reduce the use of stabilizing muscles, reducing your consumption of oxygen.

Flexibility: Interestingly, runners who are more economical tend to be relatively inflexible. Reduced flexibility may increase the ability of the muscles to store and return energy, or increase leg stability (so stabilizing muscles are used less). Before you quit stretching, however, keep in mind that the relationship between flexibility and running economy has only been shown between runners, and does not prove that a given individual will improve her running economy by becoming less flexible.

Fatigue level: As you get tired, you use more oxygen to run at a given pace. This may be due to reduced energy return and increased use of additional muscles’ fibers as the muscles fatigue, among other factors. One of the major benefits of tapering training before a race is an improvement in running economy.

Two-time Olympian Pete Pfitzinger is an exercise physiologist.