Learn About Lactate Threshold from I Love Lucy

October 15 is National Lucille Ball Day. In honor of that day, here is a reprint of an old post from my former blog, first written in the early 2000s.

The I Love Lucy show gave us one of the funniest—and most accurate—analogies for lactate threshold I’ve ever seen. In honor of Lucille Ball, I’m sharing this classic clip that I’ve been using since my early days as a Master Instructor for Spinning®.

Every so often, another Master Trainer tags me on social media after using this analogy with their riders, and it makes me smile. If this helps you explain LT to your peeps, please use it! (And yes, tag me so I can see the chocolate love making the rounds.)

So… what does I Love Lucy have to do with lactate threshold?

Chocolates. Obviously.

When we talk about exercise intensity, we often refer to lactate threshold (LT). It can sound technical and intimidating, but this famous 1952 clip from I Love Lucy provides an analogy that explains it better than most textbooks. Watch this before moving to the next section…get ready to giggle! 

What is lactate threshold?

Lactate is a byproduct of anaerobic glycolysis. Your body produces it all the time—even at rest. As lactate is formed, your body clears it from the blood and reuses it as fuel. As long as production and clearance are balanced, you’re operating primarily aerobically, and the effort is sustainable.

As intensity increases, lactate production increases.

Eventually, production can outpace clearance. When that happens, lactate—and the accompanying hydrogen ions (H+)—begin to accumulate in the blood. The drop in pH (more acidity due to increasing H+) is believed to contribute to the familiar burning sensation you feel in the muscles during high-intensity efforts. (This is still the prevailing theory, even though science continues to refine exactly how much of the sensation comes from acidity versus other metabolic factors.)

There is a point where you are working at the highest intensity you can sustain while still clearing lactate at the same rate you are producing it. Lactate levels remain stable. Nothing is accumulating. 

That point is lactate threshold, and the heart rate at which it occurs is known as the lactate threshold heart rate (LTHR). This point is sometimes referred to as Maximal Lactate Steady State (MLSS) — the maximal workload at which blood lactate concentration remains steady.

Below LTHR? Predominantly aerobic. Sustainable.
At LTHR? Hard, focused, “comfortably uncomfortable,” but steady.
Above LTHR? Lactate and H+ begin accumulating, and the effort becomes progressively less sustainable.

The farther above LT you go, the shorter your survival time.

You can train around this threshold and raise it. That doesn’t necessarily mean your threshold heart rate climbs dramatically—but with focused training, you can produce more watts at that same heart rate. In other words, you’re doing more work for the same physiological cost.

Translation: You’re fitter.

Enter Lucy and Ethel

Now let’s bring our intrepid chocolate wrappers into the story.  Think of the chocolates on the conveyor belt as lactate. Wrapping them represents your body clearing it.

At first, the belt moves at a manageable speed. Lucy and Ethel wrap efficiently. Every chocolate is handled. Nothing piles up. That’s your aerobic zone—production and clearance are balanced.

Then the mean boss lady speeds the belt up.

Lucy and Ethel are working hard now. Focused. But still keeping up. Every chocolate gets wrapped; none are slipping past.

This is lactate threshold.

The belt is moving at the fastest speed they can sustain while still maintaining balance. Production equals clearance. No accumulation. This is that strong, steady, “I can hold this, but I wouldn’t want to chat much” intensity.

Now…

The conveyor belt accelerates just a little more.

Lucy and Ethel begin to panic—chocolates begin to get past them and start accumulating. Production has exceeded clearance. The system is no longer steady.

That’s working above lactate threshold.

And when Lucy and Ethel start stuffing chocolates into their hats and mouths? That’s not slightly above LT—that’s a full anaerobic chaos. Completely unsustainable. You can only survive there briefly before you’re forced to back off.

Different people can handle different “belt speeds.” Highly trained athletes can keep up with a much faster conveyor before they hit their wrapping limit. Less trained individuals reach that point sooner.

But everyone has a limit.

Knowing your LT (or even a close estimate of it) is one of the most powerful tools for improving performance. It tells you where steady ends and accumulation begins. Train wisely around that line, and you move it.

So thank you, Lucille Ball, for decades of laughter—and for giving endurance coaches everywhere the most delicious physiology lesson of all time.