The Difference Between Muscle Fatigue and Oxygen Debt

We all know the feeling. Every muscle is tense, every pore is dripping sweat, even your lungs feel like they are on fire, and then you hit the wall. No matter how hard you try to push them, your muscles just do not want to move. And then comes the hurt. Deep soreness fills your limbs and it seems to last for hours or sometimes even a couple of days.

These two reactions to vigorous exercise, first muscle fatigue, and then muscle pain, are actually brought on by two completely different causes. Complex chemical reactions in your body take place both during and after a workout affecting every part of your body. It is important to understand the difference between training inhibitions caused by muscle fatigue and those caused by oxygen debt in muscles so that you can improve your workout, and your overall health and capacity.

Chances are you’ve experienced the occasionally embarrassing effects of muscle fatigue. Ten minutes ago you were doing more push-ups than you have ever done in your life, and now you can't even lift your arm to turn on the shower. To understand why this happens, it helps to know how muscles work.

Muscles contract when their cells pick up signals from the brain to release calcium; when the functions of either the nerves or the metabolism are interrupted, the muscle is no longer able to generate the same level of force.

Nervous fatigue happens when the nerves are simply no longer able to sustain high-frequency signals. After prolonged periods of maximum-level contraction, nervous signals drop off rapidly until the nervous system has recovered. Strength training is all about increasing the duration of time that your nerves can sustain those signals.

Muscle fatigue, on the other hand, can be caused by a few different factors depending on what chemicals are present in the body. Excess amounts or build-ups of potassium, chlorides, and metabolites like magnesium can all inhibit muscle cells from releasing calcium. Despite the conventional wisdom, lactic acid is not actually responsible for muscle fatigue. But it does play a big role in oxygen debt in muscles.

Excess post-exercise oxygen consumption is the wordy sports science way of saying, "Why you tend to hyperventilate for a while after working out;" oxygen debt in muscles is the reason. As your body starts to recover from exercise, it uses oxygen to return itself to stasis through a number of different functions. One of the most important of these is to oxidize excess lactic acid so that it can be recycled throughout the body.

Huge amounts of lactic acid are produced during strenuous activity, and it needs to be converted back to pyruvic acid in the liver, kidneys, and heart. These organs use oxygen to make the conversion, and the more oxygen is available in the blood stream, the faster they can do so.

The duration of this effect is greatest just after a workout, and decreases exponentially thereafter. However, some studies have shown that oxygen debt can last up to 38 hours.

Oxygen measurement is an important tool in developing your training, and being able to reference your muscle oxygen levels can give you an edge when it comes to maximizing your workout.

By monitoring the oxygen level of specific muscles, you can know when to slow down before oxygen debt becomes too great, lessening both recovery time and post-workout soreness as a result. Recording muscle oxygen levels over time can also be a great way to chart the progress of your training regimen, allowing you to track as your body enhances its ability to take in oxygen.