The Efficiency Factor in Running

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By Joe Friel

High-performance runners are typically very efficient. One can observe this just by watching them run: there is no apparent wasted energy. They look very graceful. Their efficiency can actually be determined in a physiology lab by measuring how much oxygen they use to run. Oxygen consumption is a good indicator of how much stored energy, in the forms of fat and carbohydrates, is used to produce a running speed. As oxygen consumption rises, more energy is burned. A middle-of-the-pack runner will typically require much more energy to produce the same speed.

Efficiency is therefore a very good way of gauging a runner’s aerobic fitness. The problem is that measuring efficiency in a lab is not only inconvenient, it’s also expensive. Fortunately, there’s another way of measuring efficiency that doesn’t require a lab and can be done with common, everyday training technology. On the TrainingPeaks website it’s called the Efficiency Factor (EF). By measuring your EF for sub-maximal, aerobic runs, you can gauge your fitness by measuring your efficiency on a daily basis and comparing trends over time. If your EF is improving, then you are becoming more efficient and therefore more aerobically fit. TrainingPeaks automatically does this analysis for you after you post a run.

All it takes is a speed-and-distance device—a runner’s GPS—and a heart rate monitor. The GPS device tells you how fast you were running—your performance. The monitor reports what your average heart rate was for the run. While heart rate doesn’t tell us anything about performance, it is a good indicator of effort. Effort is another way of saying the cost of the run. Knowing both performance and cost, you essentially know your efficiency for a run. With these two devices, and with TrainingPeaks as a post-workout analysis tool, you have your own “lab.”

So how is heart rate an indicator of cost? We know that as the speed of your run increases, more energy is required, and your heart rate rises accordingly to supply oxygen to the working muscles in order to produce the energy. The energy you burn and heart rate during a run therefore follow the same trend. When the energy required increases, the heart rate also increases. As you become more efficient (we call this “fitness”) the energy cost of running decreases along with heart rate.

TrainingPeaks calculates EF by dividing your speed of running by the heart rate required to produce that speed. If the run was done on a flat course or a track, this is a pretty simple process. But if there were hills, the changes in speed must be taken into consideration. That brings us to something TrainingPeaks calls Normalized Graded Pace (NGP). The GPS knows when you are on a hill. It also knows how steep the hill is. Therefore, it can calculate what your speed would have been had you been running on flat terrain. That adjustment is the NGP for your run. It will typically show up in the post-run analysis as being faster than your average pace for the run.

Following the workout, and once data is uploaded, TrainingPeaks knows both your NGP and average heart rate. It converts NGP to a Normalized Graded Speed (NGS—yards or meters per minute) and then divides that result by average heart rate. That produces EF. Here’s an example to show you what’s going on inside of TrainingPeaks after you’ve uploaded a run workout in which the NGP was an average 7 minutes and 30 seconds per mile and the average heart rate was 150:

NGP = 7.5 min per mile
NGS = 60 ÷ 7.5
NGS = 8 mph
Yards/Minute = 234.7 (1760 yards x 8 ÷ 60)
Avg HR = 150
EF = 234.7 ÷ 150
EF = 1.56

By comparing the EF for similar workouts over time, you can gauge how your aerobic fitness is changing. As the EF rises, aerobic fitness is improving. As it falls, aerobic fitness is decreasing. Of course, the workouts you’re comparing should be similar, meaning the course and terrain are about the same, as well as the weather, your effort and a number of other elements that typically affect heart rate such as caffeine, lifestyle stress, altitude and more.

Paying close attention to your EF over time is an easy way to measure your fitness improvement with field tests. It’s also much less expensive and easier than going to a physiology lab every time you want to know.

Editor’s note: Fast After 50 is a well-written and insightful book that includes useful information and guidelines for aging ultrarunners who want to continue in the sport at their highest potential. Check it out!

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Joe Friel has written 14 books on training, including his most recent Fast After 50. To learn more about his training methods, go to joefrielsblog.com.