In the last article we looked at how your Ground Contact Time affects your running performance. It turns out that GCT can have a very large impact on your running, but how do you make the changes and, more importantly, how do you verify those changes are making you a better runner? Simply focusing on spending less time in contact with the ground and ignoring everything else is most likely not going to make you a faster or better runner.
Fortunately, the data you create can be utilized to monitor changes in your running that are positively correlated with your running performance.One useful way to look at the data in the context of becoming a better runner is to look at Running Effectiveness (RE). Since Running Economy and Running Efficiency cannot be calculated solely from the data a running power meter provides (they require sophisticated laboratory equipment), RE becomes our stand in. RE is calculated by dividing your Speed (in m/s) by your Power (in W/Kg).
For RE to increase, running speed must increase faster than power and that implies an increase in your efficiency in converting power into speed. Not surprisingly, RE generally correlates positively with speed as well as power, but not always.
Most runners tend to become more inefficient as they speed up, especially after a certain point. This is the result of numerous factors including the inherent inefficiency of the human body to translate effort into forward motion and the increased fatigue building within the body leading to form deterioration.
That is why RE is often most useful when taken within the context of a specific speed or pace. Doing this allows an athlete or coach to quantitatively measure the improvements for a specific speed or pace, such as the speed that will be required on race day. This metric can be used either over the course of a training cycle or within a single run.
Within a single run the trend of RE provides valuable insight, such as highlighting a lack of Stamina, alerting you or your coach to the need to address this weakness. Over the course of a training cycle the goal is to see a positive trend for your RE at goal race pace to confirm that improvement is being made.
It is still important to train at different speeds and engage the full range of energy systems since certain stimuli that are very helpful to running effectiveness at goal marathon race pace are found in workouts targeting very different speeds. It should be the goal of the training plan to build and compound the adaptations to culminate in the athlete’s best performance.
Let us look at how RE breaks down and what factors have the greatest impact.
Speed and Power increase as Ground Contact Time decreases. It seems logical that as this happens you should become more efficient in transforming power into speed. After all, you are spending less time on the ground and are pushing harder off the ground in the direction of travel, so you should be travelling farther and faster with each step.
In theory YES, but this is where the clean-cut theory does not match the reality of running. To increase your Running Effectiveness your speed would have to increase faster than your power. What actually happens is that since your cadence has increased you spend less time in the air with each step, and that reduces the distance you travel per unit of power you generate. In other words, although you are pushing off harder, you don’t travel as far because you have less time between steps. We all have a “natural” or default preferred cadence and we tend to adopt it when running. As we run faster we eventually must increase our cadence to achieve further speed and that is when we start reducing our Stride Length.
You can see the data is showing a high correlation between Stride Length (SL) and Cadence. Although it is positively correlated, the data shows two distinct “populations” of datapoints. This makes sense since this was a Tempo Run with the Main Set being at Functional Threshold Power. Within each population, however, it is pretty clear the correlation disappears.
The next chart shows the “distance per Watt” the athlete is achieving for each stride while running. We divided Stride Length by Power (in this case Non-Form Power, or Power used to propel yourself forward rather than “wasted”). It is clearly a negative correlation, proving that the athlete is not moving farther with each step per unit of Power as cadence increases.
I initially found these results very counterintuitive. Upon further thought it occurred to me that humans may not be a very efficient running machine. With each step, depending on your landing point and stride mechanics you are “braking” upon impact thus dissipating a potentially large percentage of energy. This would be magnified with a higher cadence, making you progressively less Effective as you try to run faster. This goes to the heart of your stride mechanics. You may become extremely fit and capable of sustaining a very high power but that may not translate into your speed, confirming the age-old “fitter or better” dilemma – being able to generate more power will not by itself make you a better runner.
In the past the focus of training has largely been only on becoming fitter. There simply wasn’t the visibility into the data to determine if improvements a runner made were the result of being fitter, or an improvement in the form and efficiency. Today, with the availability of this data, you can attribute performance improvement among the factors that contribute to improvement. In doing so you can identify those factors that will be most advantageous to work on and thus make training both more effective and time efficient.
So, what are the factors that most affect running Effectiveness?
Remember that RE is defined as Speed / Power, where speed is in meters per second and power is normalized for weight to make it comparable among runners. By its very definition speed and power affect RE, but what affects them? Power is simple; it’s just your ability to perform work. Speed on the other hand is a complex construct that will vary among runners both in absolute terms as well as in the individual factors affecting it.
The biggest factor affecting Speed is Power, so it should be no surprise the two are highly positively correlated.
We mentioned earlier that speed is correlated with Ground Contact Time as well as Cadence, and it also happens that Ground Contact Time and Cadence are also highly correlated.
We also talked about the fact that Cadence has no real effect on Stride Length, but it does affect the distance travelled per unit of power during each step a runner takes. It would follow that the distance per unit of power would be a strong determinant of RE, since you are covering more distance per unit of power generated during the same period of time.
It is interesting to note that Ground Contact Time does not correlate positively to RE, at least for this athlete.
Furthermore, Power correlates negatively.
While Speed has a slight positive correlation to Running Effectiveness.
The reasons could be numerous ranging from form flaws to fatigue or biomechanics.
In this particular case it appears the athlete is fit but has trouble translating that fitness into velocity, most prominently by not being able to channel the extra power into distance covered during the flight phase of the stride. The chart below shows the relationship between the distance per unit of power and actual Horizontal Power produced. As you can see distance traveled per step declines as power increases at a faster rate than the effect of Cadence alone would suggest.
Armed with this data this athlete can now train specifically to counteract this tendency and over time, track and confirm improvement.
What I believe is the most important takeaway message is that Running Effectiveness is comprised of many underlying factors, each potentially being the “weak link” holding you back as a runner. You must be willing to leave no stone unturned as you search for what your weak link is. As in every other aspect of training, what is true for someone else may very well not work for you.
By using the data, you will be able to identify the unique characteristics that make you the runner you are and create a truly personalized training program that will provide the most benefit to your performance.