I play hockey (field hockey) at a slightly-below-elite level. I train (but don't play) with players who play in the UK National Prem.

I, and other players I play with, and coaches I've trained under, have always talked about us and our brains being "oxygen-starved", when you're playing beyond your fitness level. (Especially common during pre-season when everyone's unusually un-fit :D )

What we mean by this in terms of external observable actions is that, when we get very tired we stop making good decisions, and our fine motor control drops - we chose bad passes, we take people on when we wouldn't normally do so when not tired, our hand-eye cooridnation drops so that we can no longer execute skills as well, etc. etc. etc.

Certainly the effect exist - those actions are clearly observable.

But is there truly a physiological effect behind it, and if so, is that effect truly about oxygen levels in the brain?

If one were able to install an O2 sensor in one of major arteries leading to the brain, would you be able to see actual drops in O2 levels, when someone is reaching a point of combined aerobic and anaerobic exhaustion (hockey is a heavy mix of both forms of activity, like many team sports)

  • 1
    Short answer yes. I have more details when not on mobile.
    – JohnP
    Commented Aug 19, 2019 at 0:51
  • Will be interested in a detailed answer. For actually measuring this, Moxy provides such apparatus. Commented Aug 19, 2019 at 5:45
  • @FenryrMKIII - DC Rainmaker (An independent reviewer I respect immensely) has a review of the moxy on his site.
    – JohnP
    Commented Aug 19, 2019 at 15:06
  • Yup I know about DCRainmaker :) Some studies have also been made by Evan Peikon on crossfit athletes Commented Aug 20, 2019 at 9:12

1 Answer 1


Oxygen deficit and oxygen debt are real things in the world of exercise, and the base reason is a short term systemic energy cycle known as the ATP-ADP cycle (Adenosine Tri Phosphate- Adenosine Di Phosphate), and long term, the Krebs cycle.

The base reason (i.e. tl; dr;) is that the breakdown of ATP to provide energy to drive the cell is an oxidative process, and can only occur in the presence of oxygen. No oxygen to the muscle, no oxidation, no energy.

There are a lot of physiological adaptations that occur with exercise, the heart enlarges, the lungs become more efficient at oxygen transfer, the body will grow new blood vessels into the muscles, more mitochondria are created in the cells, etc. Most of these are related to the need to get oxygen to the muscles.

Now for the long stuff - The basic chemical driver of cell function is the breaking of the phosphate bond in ATP, turning it into ADP. This energy is used to drive the cell function, whether it is muscular or whatever. In extreme exertion, the cell can also break ADP down into AMP (Adenosine Mono Phosphate). This is done using oxygen.

Now to turn the ADP/AMP back into ATP, the body has a few different methods.

First, there is available creatine phosphate that can be immediately used to recreate the phosphate bond. This is short term, until the available creatine phosphate has been used up. (On a side note, this is why creatine supplementation works. It fills cells with extra CP for use in this process).

Next up is the glycogen-lactic acid system. This is slower, but uses glycogen and lactic acid (Yes, lactic acid is a fuel in addition to its other implications in exercise). This system actually works in the absence of oxygen, but will only supply energy for up to around 80-100 seconds.

Finally, aerobic respiration kicks in (Krebs cycle) and starts supplying ATP replenishment through glycogen breakdown from liver stores, circulating glucose in the blood, fat oxidation (Note the word oxidation again), and in extreme cases, breakdown of proteins. The last can be dangerous, as that is the beginning stages of rhabdomyolysis.

All of these are driven by oxygen, as is the recovery. While lactic acid is a fuel, too much buildup in the muscles can lead to temporary muscle failure, it's suspected in muscle cramps (But has not been proven), and it is flushed from the system also using oxygen. Panting/heavy breathing helps drive this process post exercise. Any time you see something that says it's an oxidative process, oxygen is a driving force. The oxygen debt is what causes you to have to stop, to get enough oxygen to replenish energy, take out the trash, etc.

So basically, oxygen is the spark that drives almost every single energy process in the body. When you are out of shape, your body is not very efficient as delivering oxygen to where it is needed to make all these cycles work. As a result, you breathe heavier than you might when you are fit, your energy systems don't deliver as fast, and you tire and have to stop sooner. As you get fitter (the physiological adaptations noted above), your body delivers more oxygen to where it is needed, and you last longer before you are done.

On a final side note, this also affects motor control. You start losing fine motor control at around 115 beats per minute (BPM) heart rate, and you start losing gross motor control at around 145 BPM. (You also start losing some visual acuity, and other effects, but those aren't totally relevant). So if you are not fit, your heart is trying to supply blood as fast as possible, and your heart rate skyrockets. This starts affecting your play, you stumble, aren't as agile, don't see things as well, etc. Long time training can remediate this somewhat, but the bottom line is that the fitter you are, the better you will be at your sport.

  • This is a great, and detailed answer! But I was specifically thinking about cognitive effects (It was confusing of me to also mention the muscle effects in passing). Does the same oxygen depletion occur in the brain, or does the body prioritise keeping brain 02 levels constant?
    – Brondahl
    Commented Aug 20, 2019 at 6:35
  • Note that from recent readings, it seems that lack of oxygen is not the reason the body uses other pathway than the oxidative pathway. See here for example : ncbi.nlm.nih.gov/pmc/articles/PMC5016084 Commented Aug 20, 2019 at 9:16
  • @FenryrMKIII - Thank you for that link, I had not seen that article. I have it bookmarked to read when I have time to work through it. Admittedly, the systems are not "stand alone", they all work to some extent all the time. The base breakdown is valid. It's like "fat burning" zones, you're using fat all the time to some extent or another.
    – JohnP
    Commented Aug 20, 2019 at 14:09
  • @Brondahl - See ncbi.nlm.nih.gov/pubmed/22316722 - Yes, there are some cognitive effects associated with mild hypoxia.
    – JohnP
    Commented Aug 20, 2019 at 14:12

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