John Davis

Written by John Davis


The Effects of Fatigue on Your Running Form: How Your Form Breaks Down (and what injuries occur) During Short Races and Speed Workouts

In several previous articles, we have seen how important it is to run with good form.  Any number of running form deficits can put you at a higher risk for a range of injuries.  While it’s easy to think about running smoothly and efficiently when you are feeling fresh, it can be a different story when you are tired at the end of a run or a workout.

This week, we’ll be looking at the effects of fatigue on your form: what happens when you get tired? And what type of injuries might occur if you run yourself to exhaustion on a regular basis?

Two types of fatigue

It’s important to recognize that there are at least two different types of fatigue to consider when we investigate this topic. The type of fatigue you feel after a hard interval workout or a 5k race is different than what you feel at the end of a long training run at an easy pace:

  • Metabolic fatigue – after a hard, anaerobic effort, your muscles are overloaded with acid from the oxygen debt you’ve accumulated.
  • Exertion fatigue – at the end of a long training run, your muscles and nervous system are wearing out and you are running low on carbohydrates to fuel your body.

Today, we’ll be specifically looking at the effects of metabolic fatigue on form and mechanics.

Effect of fatigue on mechanics

Fortunately, there is a fairly large body of research to look at—it’s pretty easy to round up some moderately experienced runners and have them run to exhaustion on a treadmill.

Shin injuries and tibial shock

One such study was published in 2000 by Mizrahi, Verbitsky, and Isakov.  In their experiment, 14 runners had their running mechanics checked throughout the duration of a 30-min treadmill run at five percent above their anaerobic threshold—this would be, roughly speaking, about 10-km race pace for most runners, so this treadmill workout was extremely tough!

By using accelerometers strapped to the shins of the test subjects, the researchers were able to track the impact shock traveling up the runners’ legs from overstriding.

As the run progressed, the impact shock increased markedly, and muscle activation patterns, as measured by an EMG machine, changed as well.  The authors warned that this could lead to an increased risk of shin injuries, as they’ve been linked to increased tibial shock.

Impact force and loading rates

A similar study by a group of researchers led by Kristen Gerlach at SUNY-Buffalo also evaluated changes in mechanics during a simulated interval workout on a treadmill.  Ninety women participated in the study and had their impact force and loading rates measured before and after the interval workout via a force plate in the treadmill.

Though we’d expect to see higher impact forces in the runners when they were fatigued given the results of Mizrahi, Verbitsky, and Isakov, Gerlach et al. actually found that impact forces were lower when the runners were fatigued.  This was accompanied by a longer stride length and a lower cadence, which might have helped dampen impacts.

Interestingly, subjects in the study who had previously been injured did not see as much of a decrease in impact forces, leading Gerlach et al. to hypothesize that suffering an injury can leave a runner less able to manage impact forces during an exhaustive run.

Cadence and knee flexion

A third group of researchers published a paper in 2001 that tackled the apparent paradox of impact forces decreasing but tibial acceleration and shock increasing as runners become fatigued.  Timothy Derrick, Darren Dereu and Scott McLean had 10 male runners complete a continuous treadmill run to exhaustion at their two-mile race pace.

With a comprehensive gait lab, Derrick, Dereu, and McLean measured impact forces, leg acceleration and shock, and knee and foot mechanics during the run.

Though they found that the impact forces decreased as the subjects became fatigued, they also found that the shock going up the shin increased. 

This was explained by changes in the knee and ankle: because the leg had a lower “effective mass” when the runners were fatigued, it was able to experience both lower impact forces and higher acceleration.Because the leg effectively weighed less because the runners’ knees and ankles were flexed more, the leg could accelerate more with a lower force applied overall.

This is good news from an injury perspective but it has some performance implications.  Derrick, Dereu, and McLean point out that the increased knee flexion that is required to lower the leg’s effective mass has a substantial degree of increased oxygen cost associated with it, presumably because more knee flexion means you get a less efficient “bounce” off the ground while you run.

In their conclusion, the authors suggest that running-specific plyometric or other leg strength exercises could prolong the onset of a slower cadence and increased knee flexion and thus increase your performance.

Conclusion and recommendations

From the scientific studies we’ve reviewed, it appears that healthy runners are fairly good at adapting to metabolic fatigue from an injury perspective during hard, anaerobic workouts.

While we don’t want to read too much into these studies, as they all used relatively untrained runners and different protocols when running to exhaustion, we can infer some helpful advice to keep in mind:

  • It does appear that when you get tired, your cadence slows and your stride length increases, raising the oxygen cost of running and further amplifying your fatigue.
  • These changes, however, allow you to decrease your impact forces as well.  Though the shock going up your shin increases, researchers predict that this will not increase injury risk because the magnitude of the force itself is lower.
  • There is one caveat, though: runners who have a history of injuries do not seem to adapt to metabolic fatigue as well as runners who’ve been healthy. If you have a history of injuries, you should be aware that you’ll be experiencing relatively greater impact when you run hard intervals or a race, which might increase your risk of future injuries.

One of the easiest ways to prevent these form breakdowns as you get tired is to work to improve your overall mechanics and increase the strength and endurance of the elements in the kinetic chain most responsible for these form breakdowns, such as posture, hip extension, and cadence.

By first identifying and fixing flaws in your mechanics and then improving fatigue resistance through strength work, you can mitigate some of these injury risks.

Our Improve Running Form Course can help you with this.

First, we conduct a detailed, in-depth video analysis of your current form (all done remotely, you just need to shoot the video) and provide you with personalized recommendations and exercises.

Then, we have you compete a 6-week form-specific strengthening and stretching course designed to teach you the science of running biomechanics while providing you with a simple-to-follow, progressive set of exercises, drills and mental cues to help you make lasting changes to your form that withstand the fatigue associated with training and racing hard.

You can start improving your running form with a personalized video analysis and 6-week course for just $149

Click here to get started now!

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1. Mizrahi, J.; Verbitsky, O.; Isakov, E., Fatigue-Related Loading Imbalance on the Shank in Running- A Possible Factor in Stress Fractures. Annals of Biomedical Engineering 2000, 28 (463-469).
2. Gerlach, K. E.; White, S. C.; Burton, H. W.; Dorn, J. M.; Leddy, J. J.; Horvath, P. J., Kinetic Changes with Fatigue and Relationship to Injury in Female Runners. Medicine & Science in Sports & Exercise 2005, 37 (4), 657-663.
3. Derrick, T. R.; Dereu, D.; McLean, S. P., Impacts and kinematic adjustments during an exhaustive run. Medicine & Science in Sports & Exercise 2001, 34 (6), 998-1002.

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