Why Your PR Plateaus – Newtonian Salubrity

Why does weather continue to get hotter even while the days get shorter? Why does your fitness progress slow even while it becomes a more and more common part of your daily routine?

Think back to your ninth grade physics courses and history of Cambridge lore – Sir Isaac Newton.


For those of you who have difficulty casting your mind back, here is a refresher:

  1. There is a reality that exists at a point in time. Call it the temperature outside or how fast you can run a mile.
  2. Then you have the derivative of that which is namely the change in temperature over a series of days (is it getting warmer or cooler, in general?) or the change in your mile PR over the course of a month. Clearly, knowing that your mile time has come down by 15 seconds is a different piece of information than knowing that your best time is 8:00/mile.
  3. Where it gets a little more complicated – and this is where people can get lost – is when you take the derivative of the change. The change in the change, if you will, also called acceleration. In the weather example, we would say, “the weather is really starting to warm up quickly.” And in the running example, we might say, “it took me longer to shave off the last 15 seconds from my PR than the first 15 seconds.”

So, knowing this, we can try to answer the questions posed at the beginning of the post: Why does weather continue to get hotter even while the days get shorter? Well, this is because that while the change in the change is falling, the change itself is actually increasing. To illustrate let’s look at the average low temperature in Eugene, OR. The acceleration is in parentheses:

  • January 34F (+0)
  • February 35F (+1)
  • March 37F (+2)
  • April 40F (+3)
  • May 44F (+4)
  • June 48F (+4)
  • July 51F (+3)
  • August 51F (+0)
  • September 47F (-4)
  • October 41F (-6)
  • November 38F (-3)
  • December 34F (-4)

We can see that the acceleration maxes out when the days are longest even though the average temperature continues to increase.

The same is true in fitness. Once you have achieved a certain level of progress, you must do exponentially more in order to achieve the same change in the change of results. Doubling your routine may not even be enough. You may have to triple it. And you can see that when you start to think this way it simply becomes impossible to shave off 15 seconds from your time faster than the previous 15 seconds.


So next time you check your watch and shrug that you just can’t bring down your time like you used to, don’t slink off and get frustrated. Just blame it on physics, or rather your Newtonian Salubrity.

(Image Credit: Newton,  Plateau)

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