Why do you run faster during a 5K than a marathon? While a seemingly stupid question to ask as it appears to be common sense, the fact is that there are multiple factors at play. In respect to the mind, you realize that there is minimal downside to running fast in a 5K as the chance for hitting the wall and not making it to the finish line is much less than in a marathon.
From a physiological standpoint, you’re burning mostly carbohydrates during a 5K, which are oxidized at a much faster rate than fat and therefore is a much more immediate energy source and more specifically, you won’t run out of carbohydrates during a 5K. Lastly, muscle fatigue is likely much less during a 5K than during a marathon.
MODELS OF FATIGUE
There are two primary models of fatigue: Peripheral and Central.
The peripheral model states that the body is the reason for fatigue. For example, when running uphill, there is build up of muscle acidity in the quadriceps, thus causing a runner to slow down or stop.
The central model states that the mind is the reason for fatigue. More specifically, the mind is always looking to ensure that we don’t hurt ourselves and thus deviate too far from homeostasis. Using the above example of running uphill, the mind is what reduces the speed of a runner to ensure that the runner does not hurt themselves by pushing too hard.
CENTRAL GOVERNOR MODEL (CGM)
The CGM is most like the central model of fatigue. The CGM was originally proposed by physiologist, AV Hill in 1924. Hill proposed that the heart was protected by some sort of a governor, likely either from the heart itself or the nervous system.
The CGM is akin to a governor in a go-kart motor that limits how fast it can go, thereby ensuring that little Bobby doesn’t go careening into a wall at 50 mph!
More recently, the existence of a central governor was theorized by physiologist Dr. Tim Noakes. Dr. Noakes states, “Exercise performance is regulated by the central nervous system specifically to ensure that catastrophic physiological failure does not occur during normal exercise in humans.”
It should be noted that not all researchers believe that the CGM exists.
RISK vs. REWARD
Going back to the 5K versus marathon comparison, at the most basic level, it’s a risk management issue. Run too fast, too early and pay for it later with a slow time or worse, a DNF. Run too slow and you likely won’t hit your goal time.
Hazard Score (HS)
The hazard score was developed by a group of researchers (de Koning et al). The hazard score correlates the intensity at which an individual is performing to the distance or time remaining in the exercise bout and thus looks to determine the likelihood that an individual will change pace. The formula is as follows:
Hazard = Momentary RPE * Fraction of the distance remaining
In other words, the HS infers that the intensity one runs at is highly correlated to the distance left to run. The farther from the finish, the higher the risk – and the closer to the finish, the lower the risk. Additionally, de Koning’s research found that subjects also changed pace based on sensations during a race (ex: “I feel good, so I’ll increase my pace).
This is one reason why you run a 5K at a faster pace than a marathon, and also why a lot of runners sprint to the finish line… aside from possibly tripping and falling, there is little to no risk to upping the pace substantially with just 100 meters left to go in a race.
In summation, the variables involved with pacing is likely a combination of both the mind and body.
This perspective is backed by research from Weir et al,. Their research represents the most extensive critical review of the CGM in respect to what his team feel are not valid in regard to the CGM. Weir et al. conclude that it is likely that both the central and peripheral fatigue models work together to influence and regulate human performance.
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