What Happens In Your Body When You Run To Exhaustion


Discover the physiological changes that occur during intense excercise. Our data come from over 900 Graded Exercise Tests. Participants ran on a treadmill that steadily increased in speed until they could no longer continue. A respiratory mask measured their oxygen consumption, carbon dioxide production, total respiratory volume, and other biometrics.

In this interactive website, you will learn how to identify the ventilatory thresholds that occur during exercise, and explore how these thresholds vary between individuals.


Terms to Know

  • VO2 - The amount of oxygen consumed by the body during exercise measured in mL/min.
  • VCO2 - The amount of carbon dioxide produced by the body during exercise measured in mL/min.
  • VE - Minute Ventilation: The volume of air breathed measured in L/min.

  • VCO₂ / VO₂ - Respiratory Exchange Ratio (RER): The ratio of carbon dioxide produced to oxygen consumed.
  • VE / VCO₂ - Ventilatory Equivalent for CO₂: The relationship between amount of air breathed and carbon dioxide produced.

First, lets explore a single runner's treadmill test. We will track his Respiratory Exchange Ratio (RER) and Ventilatory Equivalent for CO₂ (VE/VCO₂) as he runs faster and faster.

Why Are We Showing These Two Plots?

These graphs show how your breathing and gas exchange change as you run harder.

As exercise intensity increases, your body shifts how it produces and clears energy.

In the top graph (VCO₂ / VO₂), look for the sharp rise near the green vertical line. This inflection point marks the first ventilatory threshold (VT1), when your body starts relying more on anaerobic metabolism.

In the bottom graph (VE / VCO₂), look for the sudden increase near the red vertical line. This indicates the second ventilatory threshold (VT2), when your breathing rate sharply increases to remove excess CO₂ as acid builds up too quickly.

During the simulation, we’ll pause at each threshold to explain what’s happening inside your body.

Runner Info

ID: --

Age: --

Sex: --

Weight: -- kg

Height: -- cm

Time Elapsed: 00:00

FIRST THRESHOLD

SECOND THRESHOLD

Now, let's explore how to identify the ventilatory thresholds by interpreting the RER and VE/VCO₂ curves.

How to Identify the Thresholds

Identifying the Aerobic Threshold

  1. Use the shape of the Respiratory Exchange Ratio (RER) (VCO2 / O2) curve.
  2. Before the threshold, CO2 production matches O2 consumption resulting in a constant ratio.
  3. After the threshold, excess CO2 is produced in the blood from bicarbonate buffering lactic acid, increasing the ratio.

Identifying the Anaerobic Threshold

  1. Use the shape of the Ventilatory Equivalent for CO2 (VE/VCO2) curve.
  2. Before the threshold, VE/VCO2 remains constant or decreases.
  3. After the threshold, the body attempts to raise it's blood pH by increasing ventilation, causing VE/VCO2 to rise.

Identify the thresholds below by clicking on the plots:

Explore the data on your own! Find runners similar to you and see how their ventilatory thresholds compare.

Explore Runners Similar to You

Instructions

  1. Select a runner by clicking on their progress bar.
  2. Mark their ventilatory thresholds by clicking on the plots below.
  3. Try a few different runners and compare the results.

Aerobic Threshold Tips

  • Start your search in the first half of the test.
  • Look for the point where RER starts to increase.
  • Often occurs 0.8 - 0.9 RER.

Anaerobic Threshold Tips

  • Start your search in the last third of the test.
  • Look for the point where VE/VCO2 starts to increase after decreasing or remaining constant

What did you find?

  • You probably saw a lot of variation in the threshold times between runners.
  • Even very similar runners can have very different RER and VE/VCO2 curves.
  • This suggests that even within demographic group and fitness level, there is still significant variation in respiratory physiology in exercise.

In the next section, we’ll explore how different demographic groups differ, or sometimes overlap, in the average time they take to reach their aerobic and anaerobic thresholds. We’ll also walk through how their respiration rates and O2 intake vary across these groups.

Physiological Metrics by Demographic & Performance

Highlights from Selected Demographic Groups

Age Group

Threshold

From the graph, there is no clear trend that one age group takes more time to reach the first ventilatory threshold than the others.

Threshold Gap

Gap is calculated as time of threshold 2 − time of threshold 1. The longer the gap, the healthier the running (between these two thresholds the body is burning fats). People aged 19–30 tend to have the highest gap across performance levels, while 0–18 and 31+ fall behind.

Heart Rate

The older the participants get, the lower their average heart rate. Across all performance bins, 0–18 year-olds have the highest HR, 19–30 are in the middle, and 31+ are the lowest.

O₂ Volume

Older groups consistently consume more oxygen. 0–18 year-olds always stay below ~61.5 mL/min, whereas 19+ groups reach ~70 mL/min in most performance bins.

BMI Group

Threshold

From the graph, underweight runners tend to take longer to hit the first threshold at higher performance levels, while lower performers reach it at roughly the same time.

Respiration Rate

Underweight participants always show the highest respiration rate (≥ 43 breaths/min). Normal and overweight groups are similar, but never exceed ~40 breaths/min.

CO₂ Volume

Heavier groups produce more CO₂. Overweight individuals top the chart, followed by normal weight, then underweight.

Sex

Threshold

VT2 occurs at roughly the same time for both sexes, indicating similar endurance capacity before anaerobic metabolism kicks in.

O₂ Volume

Males consistently take in more oxygen (~72–73 mL/min) than females (< 60 mL/min) across all performance bins.

Summary

Throughout our journey you’ve seen how age, BMI, and sex each shape your physiological response to graded exercise. Younger runners spike their heart rate highest, 19–30-year-olds hold the widest healthy “gap” between thresholds, underweight folks breathe fastest, and heavier groups produce more CO₂. While males take in more oxygen on average, both sexes cross their second threshold at almost the same point.

These insights remind us that there’s no one-size-fits-all in training, knowing your own demographics and performance bracket can help you tailor workouts for safer, more effective progress.

Conclusion

On this site, we want you to understand that treadmill running, and fitness training more broadly, is highly individualized. There’s no one-size-fits-all approach. From the very start, we saw that ventilation thresholds vary significantly from person to person, and you learned how to spot those thresholds on physiological graphs.

Our demographic visuals showed that while factors like BMI, age, and sex do exert some influence on threshold times, their impact is relatively small. Training itself is the key driver of performance and of delaying the point at which you hit your ventilation threshold. By combining personalized, per-runner plots with aggregated, demographic views, we hope you’ll take away this core message: effort and tailored training matter far more than innate traits.