Fatigue and Running Form

My Garmin FR920 watch keeps track of running form related measurements: Cadence, Stride, Ground Contact Time and Vertical Oscillation. Given that I collected a lot of data during my last Marathon training, I decided to have a look at how running form changes as I get more tired during a run.

During my last Marathon training season, my watch recorded 440179 data points from 97 running sessions for a total distance of 1347 km or 122 hours of running. Some of these are long and easy runs and some are interval training runs — there is a large variety of running paces at different fatigue levels. I wrote another blog post about some statistics from this data, but this post focuses on fatigue and how it affects Cadence, Stride, Ground Contact Time and Vertical Oscillation.

I wrote a separate post about how I decided to quantify fatigue accumulated during a run workout. The plots in this post use that method to group data points into five fatigue levels.

Cadence + Stride vs Fatigue

Running pace (how fast you run), is composed of two elements: cadence — how many steps one takes each minute and stride — how long is each step. The plot below plots the cadence vs. stride for each data point, the points being colored by fatigue. Each point also represents a running pace value, The lines on the plot represent the “constant pace” lines: pace is “stride” (the length of each step) multiplied by “cadence” (how many steps each minute). For example, to maintain a 4:00 min/km pace at a cadence of 90 steps/minute, the stride would have to be 1.38 meters. To maintain the same pace at a cadence of 80 steps/minute, the stride would have to increase to 1.56 meters.

A note on cadence values: Originally, running cadence was measured by counting number of steps taken with one leg, either left or right. A few years ago, Garmin switched to measuring cadence by counting steps from both legs. I prefer the old style, where a cadence of 90 means “90 steps taken with the right (or left) leg only”. If you prefer the newer style, just multiply everything by 2. For example, an “old style” 90 SPM becomes 180 SPM when counting each leg.

Density plots for each running form metric

The plots below show how the density profile (a histogram, really) for each of the running metrics. Ground Contact Time, or GCT, is the time the foot is in contact with the ground at each step. Vertical Oscillation, or VOSC, is the amount of up-down bouncing the body makes while running. It is measured in millimeters.

The cadence, GCT and VOSC density profiles do not seem change much during the course of a run. There are some humps in the plot mainly because I did intervals at hard and severe fatigue levels and I maintain a higher cadence during these.

Stride (the length of each step) is the only measurement that shows a clear shift towards lower values as fatigue increases.

Conclusions

I’m not sure what conclusions to draw from this data, apart from the fact that the plots look pretty. The only running form metric that seems to change as I get more tired is the stride, which gets shorter.

The script used to generate the plots in this blog post is available here