How Fatigue Affects Running Form and Injuries

Many running injury studies look at differences between injured and healthy runners using gait analysis, but relatively few also examine how running form changes during a prolonged bout of running due to the effects of fatigue. Runners who continue running past the point of fatigue may be susceptible to injury due to alterations in their form caused by exhaustion. 

Running Form and Fatigue

Fatigued RunnerChanges in running form are suspected as likely contributors  to many common running injuries including patellofemoral pain syndrome, iliotibial band syndrome, and plantar fasciitis. A decrease in muscle activity and recruitment due to fatigue reduces the body’s ability to control joint motion, putting additional strain on muscles, tendons, and ligaments that may ultimately lead to injury. A recent study published in the Journal of Biomechanics suggests that as runners become fatigued during a long run, their form changes, with an increase in motion at the knees and hips.  

Altered Joint Mechanics

Increased knee and hip motion are two likely contributors to the development of painful problems for runners. It’s been suggested that weakness in the muscles of the hip and thigh can lead to excessive joint motion, but the effects of fatigue on muscle endurance (the ability of a muscle to repeatedly contract) can also be a factor.

Alteration of gait mechanics as the result of fatigue might help explain why some injuries are not painful at the start of a run but surface after a certain distance or time. Measuring a runner’s strength or evaluating their form prior to a run does not provide insight into what happens once the runner becomes tired. Assessing running form after fatigue has set it may be needed to determine which runners are more at risk for injury. 

Evaluating Risk Factors

A runner might display good hip abduction strength and motor control during a single-leg squat test, but 7 miles into a run their form starts to break down and their hip adduction or ankle pronation increases. Stride rate is another variable that might change or become less consistent as a runner grows tired.  A slower cadence (taking less steps per minute) can lead to overstriding and a resultant increase in forces traveling through the leg.  

Understanding and Measuring Fatigue

Fatigue was once believed to be directly caused by a depletion of the energy stores or the build up of chemical by-products in muscles which then in turn causes the muscles to shut down to protect the body from further injury. Professor Tim Noakes, an exercise physiologist at the University of Cape Town, proposed an alternate theory stating that fatigue actually occurs in the brain, not the muscles. According to Noakes’ theory, the brain will subconsciously decrease muscle activation based on the perceived level of exertion in order to protect the body from damage caused by excessively hard work. The idea that the brain acts as a thermostat, responding to various signals coming from the muscles, is called the Central Governor Theory. 

Central Governor Theory

In the central governor model of fatigue, the brain adjust muscle recruitment and regulates exercise performance in order to prevent the body’s systems from failing. Several scientists have pointed out that the central governor theory does not completely explain all the observations associated with fatigue, and there is some evidence that factors in the muscles themselves or elsewhere in the body other than the nervous system may still play a part in the sensation of fatigue. 

Regardless of the exact nature of fatigue, a more practical matter for runners and athletes is how to account for fatigue during their training sessions to minimize the risk of injury. Many runners may adhere to training schedules that do not account for their level or feeling of exertion, meaning during long runs they could likely be running with a form that increases their chance of injury. 


Endurance Athletes and Fatigue

Running by itself is a good way to increase muscular endurance. When you account for the changes in form that come with fatigue though, there needs to be a balance between the amount of work being performed and the ability of the runner to maintain good form and efficiency during their training. Above all else, most running injuries are a result of training error. Knowing how to adjust your training in relation to your muscle endurance helps ward off injuries. 

Rating of Perceived Exertion

A simple tool to determine how hard you are working out is the Rating of Perceived Exertion (RPE) scale. The RPE scale, also known as the Borg scale, allows you to match how hard you feel you are exerting yourself with numbers 6 through 20. A RPE of 15 is usually associated with the onset of fatigue for runners. 

Tips to Adjust Training in Response to Fatigue

1. Stop running at or just past the rating of perceived exertion of 15. A RPE of 15-17 is likely when a runner’s form will begin to change so continuing to train beyond this point might bring a higher risk of injury. Stopping at the onset of fatigue can still yield many of the benefits of training with less downside. Using the RPE scale will allow an athlete to increase their training time or distance in relation to improvements in muscular endurance.

2. Pay extra attention to form at the end of a run. Focusing on good form at the onset of fatigue will help build muscular endurance in the muscle groups that act as a buffer against injuries. Everyone has a different opinion for what is considered optimal running form, but simple cues like maintaining a set cadence or watching out for crossover strides can go a long way toward maintaining consistent mechanics throughout the duration of a run. 

3. Mix strength training exercises aimed at increasing muscular endurance in with your runs. There is some evidence that performing high repetition strengthening exercises or long duration isometric holds will result in improved muscular endurance with running. An increase in muscle strength may not directly improve running form, but the way muscles perform when fatigued can help safeguard against a breakdown in form at the end of a run. 


 Photo Credit: Rennet Stowe


  1. says

    Awesome post. Really interesting about the brain controlling fatigue. All of this new pain research and the brain’s role in therapy and fitness is flipping all of our old concepts on its head.

  2. James Speck, PT says

    Thanks Dan. I’m continually surprised and amazed at how complex the body is. It’s easy to discuss things in terms of our old biomechanical theories, and I think in many ways they are still helpful, but all the newer research and theories on the functions of the brain and interactions between the different systems makes things a lot less certain.

  3. s simfukwe says

    Awesome posts. I ve long suspected it starts in the brain and cns. the body and the brain once overexerted try and protect us from injury. I noticed this in 9th grade the first time i ever lifted weights. The load “felt” heavier and heavier even though it was the same exact weight. I reasoned it was a trick the brain plays on you. Fatigue also occurs in the body due to lactic acid build-up, toxins and atp use but this threshold is much higher than we perceive. Under hypnosis your muscular endurance and max all seem to increase. Exercise occurs more in the mind but also in the body. Its akin to saying there is no time when obviously you get older. Pain is another clue this is called the Reactive Mind. Your body associates injury and pain as a disease and sends pain signals even if there is no pain (phantom limbs, knee injuries, back spasms). If u got hurt running your body will try and stop u from running. If u got sick from an apple u can get sick everytime u eat an apple.

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