Workout Plans: How To See If Muscles, Heart or Lungs Are the Weak Link

Workout Plans: How To See If Muscles or Lungs Are the Weak Link

Where should I focus workout plans for my clients to make them better?  Why are they getting tired during workouts? What is the weak link where training should focus?  Is it the muscles? Is the the lungs and cardiovascular system.  These are questions frequently asked by clients and coaches.  Cardio-metabolic testing can provide the answers, and in this video and post we will show how it is done.

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What Is My Weakest Link To Improve?

Why am I getting tired now during my workout? What is the weak link in my system, or the part of my body that will fail first? Where should I focus my training in order to become better? These are questions frequently asked by all recreational and professional athletes. 

These questions express the goals behind every training plan: Go farther, go faster, advance skills, win more, become better. 

The Systems Our Bodies Use: Heart, Lungs, Muscles

Before answering these questions and spotting the limiting factor in a person’s body we need to first understand the different systems that can fail. 

What are the different systems in a person’s body that can fail?

To answer let’s first look at how our body operates during physical activity. We’ll review the three systems that work together to put our body in motion: heart, lungs and muscles. 


The energy needed for movement is generated in our muscles and specifically in the mitochondria of our cells. For muscles to get the energy to move, oxygen is needed to burn nutrients stored in our body such as fats and carbohydrates. When these nutrients are burned and used by the cells, they release the energy which is used to create movement. 

Cardio-respiratory system: Heart & Lungs

The oxygen needed for this chemical reaction is delivered through our cardio-respiratory system: our heart and lungs. When we inhale oxygen rich air, the oxygen is absorbed by our lungs and enters the bloodstream. The heart then pumps the oxygen rich blood into our arteries and in to the working muscles. 

The working muscles then use the oxygen to release energy from fats and carbohydrates and power the movement of the muscle. 

All 3 major systems work together to create movement: Our lungs, heart and muscles. 

How To Tell Which System Fails First

With this in mind we can now go back to the original question about the system most likely to fail. It is either our heart, lungs or muscles. 

The next question is:“How can we tell which system is likely to fail first?”

The scientific method that can reliably and accurately pinpoint the weakest link is cardio-metabolic testing and muscle oxygenation measurement. 

Cardio-metabolic Testing & Muscle Oxygenation Measurement

With cardio-metabolic testing we measure how much oxygen our body consumes and how much carbon dioxide it is producing. 

With muscle oxygenation measurement we can measure how much oxygen is being delivered to the working muscles.

 By measuring both we can find the weakest link.  

Is it the cardio-respiratory system, which is the combination of our heart and lungs that is failing to deliver enough oxygen in the bloodstream?  Or is it the muscle that cannot absorb the oxygen available to produce more energy? 

In both cases not enough oxygen will be delivered into the working muscle and the muscle will go from an aerobic to anaerobic energy generation mode.  

Aerobic and Anaerobic Energy 

Now let’s explain aerobic and anaerobic modes.  When our muscles generate energy aerobically it means that they are using oxygen to burn nutrients like fats and carbohydrates. This is a process that doesn’t generate fatigue and can be sustained for long workouts and periods of physical exercise. The downside is that the aerobic mode can’t generate high amounts of energy over a short period of time.  

On the contrary, when our muscles start to generate energy anaerobically it means the demand for energy increases and the muscles don’t use oxygen in the burning of nutrients. Burning nutrients without using oxygen creates fatigue can’t be sustained for long periods of time by the human body.  Anaerobic energy burning will quickly lead to failure. The advantage of this process is that it can create high amounts of energy over a short period of time. 

Determining the weak link and what system fails first is the most important piece of information needed to determine where to focus our training. 

Let’s see now how we can use the information collected from a cardio-metabolic analyzer and a muscle oxygenation monitoring system to understand the weakest system in our body. 

What To Measure To Determine The Weakest Link

Before we examine the data collected from the two devices let’s explain the measurements:

  • VO@ is the volume of oxygen consumed by a person’s body in every breath cycle
  • TV is the total amount of air a person’s lungs can exchange per breathing cycle
  • Breathing frequency is the number of breaths a person takes every minute. 
  • SmO2 is the muscle oxygen saturation
  • FeO2 is the fraction of exhaled O2, in other words the average concentration of O2 when we exhale a breath
  • FEV1 is the maximum amount of air that can be forcefully exhaled in 1 second. 

Weak Link: Lungs

In the first chart we see the cardio-metabolic profile during a step test where the person remains at specific exercise intensity for 4-5 minutes. 

The lines plotted here are the following:

  • The yellow line is TV/ FEV1.  This shows us the percentage of total volume at which his lungs are operating 
  • The blue line is VO2/bf.  This shows us how much oxygen the person is consuming per breathing cycle
  • The purple line is VO2/hr. This shows us how much oxygen the person is consuming per heart beat. 

By looking at the chart we can easily spot the following trends:

  • VO2/hr remains steady throughout the test. This shows that the person’s heart is not the weak link for workout performance.
  • On the other hand, we see that the amount of oxygen the person consumes in a breathing cycle drops rapidly after a certain exercise intensity.  The lung capacity used also drops. This clearly shows that the lungs are the weak link and this person needs to train his respiratory system to improve performance. 

Weak Link: Muscles

Now let’s see another example where the muscles are the weak link. In this chart we see a combination of 2 factors:

  1. First, the High saturation of O2 on a muscle level is shown by SmO2 being at ~70% at high intensity and 
  2. Second, FeO2, the fraction of exhaled O2 is high throughout the test. 

Combined, these measurements show the following:

High oxygen being exhaled means that oxygen is being returned to the lungs without being used.  This means the muscles are not using the oxygen. When the blood brings oxygen back to the lungs then the hemoglobin, the transporter of oxygen in the blood stream is already full and can’t absorb more oxygen in the next breath. This increases the average oxygen concentration that is exhaled because the oxygen coming in to the lung is not absorbed and is exhaled back to the environment. 

If you are interested in learning more about how PNOE can help you develop training recommendations based on the limiting factor detected by cardio-metabolic and muscle oxygen saturation analysis, contact us now at

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