Let's take a look at this (almost) perfectly executed workout by Rebecca Rusch! We'll break it down by the red numbers first, because this is the work she did, the stress she is applying to her body.
Rebecca gets off to an okay start. If you'll look closely at the yellow line (cadence), you can see that she isn't really raising her cadence as targeted in the early couple of surges. My guess is she was texting and figuring out talk to text. 🙂 But like the great athlete she is, by the second area you can see she is really hitting her rhythm, and the clear increases in cadence are right on target. We also see this impact her heart rate (the red line) as she begins to hit the higher cadence numbers. She loses focus for one -- the next to last one -- but...she gets RIGHT BACK ON IT! Rebecca is great at this. She doesn't look back, just resets and gets it right. Awesome! Finished strong.
Okay, now let's look at the purple letters and what happens to her body when she applies the above stress. We'll call that STRAIN.
A. You can see her heart rate is pretty flat early on. I hovered over her peak HR in the first surge or two, and the highest number I could find was 135 bpm. When she applied the peak stress early in the workout, her body responded to the the strain of the effort by raising her heart rate to 135 bpm to help fuel the effort.
B. Look as she really starts getting the cadence right. The higher cadence (think about it as more muscle contractions) is a higher stress applied, so as you would expect, her body goes under more strain and responds by increasing HR, now to 140 bpm for the same power output as A. The stress of increased cadence is increasing the strain her body is going under, even though the power is the same.
C. Now look the last one. Same stress as area B, but now the strain response is even higher; her peak HR is 145 bpm here, as she is growing more tired and having to use more muscle fiber to fight off fatigue (see my post on progression), which means, you guessed it, more STRAIN.
So what was your peak heart rate on the first one? What about the last one? What was the difference? Rebecca had a 10 bpm increase for basically the same stress (well, stress was higher later as she increased her cadence), which resulted in more strain. What does this suggest? It tells us this ratio is a tad high, and we need to keep working on that aerobic engine.
And yes, this was a bit of a language lesson. Stress is the external load we apply to the body, and strain is what happens to the body under stress. More on this next week!