Exercise Metabolism

Now I've finally gotten through that post with all the graphs, I can move on with my life! I'm going to write a bit about biochemistry, because surprisingly enough, that's been the easiest subject to write about so far.

This is the last lecture about nutrition, and it probably has more new content in it than all of the other nutrition lectures combined (not like that's an amazing achievement). Enjoy :)

Be familiar with the different fiber types and their metabolic characteristics

There are two main types of muscle fibres: type I and type II. Type II can be further subdivided into type IIa and IIb. Most muscles have a mixture of different types of fibres- it's the proportions that matter.

Type I fibres have a low power output, but they are very fatigue resistant. Muscles that have a high proportion of these fibres include postural muscles and other muscles that work pretty much all the time without you even noticing. They use mostly oxidative metabolism due to their abundant mitochondria, which contributes to their fatigue resistance.

Type IIa fibres have a higher power output. They are not as fatigue resistant as type I fibres, but they still do reasonably well. They generally use a mixture of oxidative and glycolytic metabolism.

Type IIb fibres also have a high power output. They are readily fatigued as they mostly use glycolysis to generate energy. I've mentioned here that glycolysis on its own can result in lactic acid production, resulting in pain and fatigue.

Be able to explain the different types of fuels which can be used during exercise.
Know the advantages and limitations of the different types of metabolic pathways.

As you should know by now, ATP is basically the energy currency of the cell. The main fuels used to produce ATP are glycogen, fatty acids and phosphocreatine (PCr). The first two are probably already pretty familiar to you, so let's talk about phosphocreatine first.

Phosphocreatine (PCr) can be broken down rapidly into creatine and ATP during bursts of heavy activity. It can produce ATP pretty quickly, which is nice, but unfortunately we only have small reserves of PCr to draw on. Fortunately, however, it is fairly easy to restore phosphocreatine levels once you're resting: creatine and ATP can combine back together to form PCr, ADP and H⁺.

Now let's go back into the familiar stuff!

Glycogen stores can be broken down to form glucose and, as mentioned here, glucose can be broken down further via glycolysis. If limited oxygen reserves are available, the process pretty much stops once the glucose becomes pyruvate, at which stage it gets converted into lactic acid. This still produces some energy, and it produces it fairly quickly (in seconds), but this pathway can cause fatigue. If oxygen is available, pyruvate can enter the citric acid cycle to produce more ATP. This process doesn't cause fatigue, but is slower and has a limited capacity to supply ATP (possibly due to limitations on oxygen supply or mitochondria ability).

Fatty acids can also become oxidised to release ATP. Some of the pathways involved, like the citric acid cycle, are similar to those involved in the breakdown of glucose. The limitations are also similar: the process is slow, especially because releasing fatty acids and delivering them to the muscle takes time. It might take around 30min for fatty acid oxidation to become optimal. Also, we only have a limited ability to oxidise fatty acids, so more carbohydrates get used as exercise intensity increases.

Know which metabolic pathways (eg aerobic and anaerobic) are used during different types of exercise
Be able to explain which metabolic pathways are used as exercise intensity increases

All of the pathways pretty much get started as soon as the muscles move, but obviously the faster pathways (the anaerobic ones) will kick in sooner. Hence, short, intense exercise (~5sec) will use mainly PCr at first, which gradually gets replaced by mostly glycolysis at around 30sec (i.e. just the glycolysis to lactate part, not the citric acid cycle part) to eventually mostly oxidative pathways at around 2-3min.

Be able to explain VO₂ max.

This wasn't actually in the lecture outline, but it looks important, so I'm going to include it. VO₂ max is a measure of the maximum capacity to transport and utilise oxygen, as well as an indirect measure of aerobic ATP consumption. It can be measured by exercise tests. Well-trained subjects will usually have a higher VO₂ max as they have a higher density of mitochondria, more enzymes for the citric acid cycle, a higher maximum cardiac output and more muscle and cardiac capillaries.

Be able to explain how the different types of exercise affect protein synthesis in muscles.

The main types of exercise are aerobic ("cardio") and resistance (weight-lifting etc.). Aerobic exercise increases the synthesis of mitochondrial proteins and of type I and IIa fibres. Resistance exercise increases muscle mass and type IIb fibres.