In a previous article, titled The Paradigm Shift In Bioenergetics, I outlined a contemporary model of bioenergetics. The key takeaways from that article are as follows:
The support of muscle contraction requires rapid non-oxidative ATP production on the millisecond time scale and within 0–15 milliseconds of muscle contraction phosphocreatine is broken down to restore ATP.
In order to sustain muscle contractions we need a non-oxidative energy supply. However, we run into an issue given that glycolytic intermediates like glucose within a muscle are limited.
Biochemical evidence shows that glycogen phosphorylase can rapidly increase its activity, and as a result glycogen can be broken down to restore the phosphocreatine needed to sustain contractions.
Between contractions the ATP needed to re-synthesize glycogen, phosphocreatine, and re-establish ion gradients comes from the oxidation of lactate. However, only a fraction of the lactate produced needs to be oxidized to restore these energy pools. So, lactate accumulates in muscle cells which is not due to it being a fatigue by-product, but rather it’s due to an inefficiency in this process.
In effect, this means that oxygen always part of energy production, whether direct or indirect. As a result, all training is aerobic. Additionally, lactate is always present. Therefore, all training is lactic. Contrary to popular belief, lactate is not a fatigue product, rather it’s a fuel source. Bruce Gladen said it best in his 2004 paper titled Lactate metabolism: a new paradigm for the third millennium where he stated “Lactate can no longer be considered the usual suspect for metabolic ‘crimes’, but is instead a central player in cellular, regional and whole-body metabolism.”
If you accept the above statements to be true, then it opens up a whole dialogue about how we classify different work capacity based sports. If all training is aerobic, then it’s nonsensical to call a sprinter an anaerobic-power athlete, which is an idea I introduced in an article called The Bioenergetics of Sprinting. Below you’ll find a reproduced image from that article which shows a muscle oxygen saturation trend for an individual performing a 60-second sprint on an air bike. Notice that the second they begin their sprint, oxygen is utilized instantaneously in the working muscle. In fact, oxygen is consumed at a much greater rate than it is supplied to the active muscle, which is why muscle oxygen saturation is declining. As soon as the athlete stops pedaling oxygen supply supersedes oxygen utilization, and oxygen saturation rises rapidly. Among other things, this tells us that Oxygen is utilized immediately upon loading. That is to say, that the energy contribution from oxidative processes are quite high at the start of a sprint, and they decrease over the course of the sprint as muscle oxygen supplies are depleted. Additionally, when oxygen is depleted we can infer that glycolysis is increasingly being relied upon to power activities.
If sprinting is aerobic and lactic, then it seems nonsensical to try and classify athletes based on energetic demands. However, it does raise an interesting question which is how we can differentiate athletes that compete in seemingly opposite work capacity events (ie, sprinters vs marathon runners) if both of them are ‘aerobic-lactic’ athletes. This is where the idea of rates of change becomes useful.
Most NIRS users are familiar the muscle oxygen saturation (SmO2) measurements (if you’re unfamiliar with NIRS, i’d recommend reading a previous article titled Applied Bioenergetics before continuing). Muscle oxygen saturation reflects the balance of oxygen supply and demand. If muscle oxygen saturation is going up, then it means oxygen supply is greater than oxygen utilization and vice versa. However, we can gain additional information by taking the first derivative of muscle oxygen saturation, which is the rate of change of muscle oxygen saturation (termed ΔSmO2). Because ΔSmO2 reflects the rate of change of SmO2, higher positive values reflect greater rates of oxygenation whereas lower negative values reflect greater rates of oxygen utilization. This is important so far as it adds a temporal component to oxygen utilization, which is typically though of only in terms of sheer magnitude.
Great sprinters, and power athletes in general, are able to utilize oxygen at an incredibly fast rate. For example, an elite 100m sprinter starting who starts their race with ~70% SmO2 may finish the event with their 5-10% SmO2. That would mean that their maximal rate of oxygen utilization is ~7% per second. On the flip side, an elite 10k runner may start their race with ~80% SmO2 and after clipping off multiple miles at a 4:30 pace they may finish the event with ~20-25% SmO2. That means they are utilizing oxygen at a rate of ~0.4 % per second. Both the sprinter and the 10k runner are aerobic athletes, but they differ substantially in their rate of oxygen utilization. One athlete trains to empty the tank as fast as possible, while the other athlete trains to extend their fuel supply for as long as they can. Crossfit athletes fall somewhere in the middle.
Rather than thinking in terms of what energy systems need to be trained based on an individuals event, we can instead think in terms of rate limiting factors for energy production. For example, is an individual limited by their pulmonary systems ability to uptake oxygen (and get rid of carbon dioxide), the cardiovascular systems ability to transport oxygen, or the skeletal muscle’s systems ability to utilize oxygen? If we know the event demands and individuals physiological and sport specific limitations, then the training becomes simple.
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