Why is lactic acid produced at submaximal oxygen consumption.

because the human body is complex, and this cannot be represented precisely with simple equations? Not that I would know. )

You mean Lactate, not Lactic acid.

The answer is because some Anaerobic fueling is always present during Aerobic exercise.

Lactate is always used even at rest because it is part of the conversion of glycogen and glucose to ATP. Along with Lactate production, 3 ATP are produced with every 2 Lactate molecules when muscle Glycogen is the source and 2 ATP when blood glucose is the source. These 3 or 2 extra ATPs are the Anaerobic contribution to Aerobic exercise.

So at 90% VO2max you are also getting about 9% Anaerobic fueling including a small amount of energy from Creatine Phosphate.

I should add that Lactate is finally converted to ATP Aerobically in the Mitochondria.

thanks

You're welcome. It's a very good question actually that has puzzled Biochemists for over 100 years.

Wow! Thanks, and my apologies to the OP for my snarky comment.

Hill and Meyerhof won the Nobel prize for Chemistry in 1920. Meyerhof worked out the Chemical pathways which are incredibly complex equations, so you were correct.

Hill calculated the calorific content of "Lactic acid (as Lactate)" a few years later.

He seems to have been implying that Lactate is the Aerobic fuel, and he did this almost 100 years ago. It took another 50 years for this idea to begin to gain acceptance and several more years to be general acceptance. Not the the complete picture is clear though. But we are a long way forward from what you read in many text books, websites and blogs old and new.

Preface: Below I offer info. related to the question posed by the OP. Note that although I studied biochemistry as part of my Ph.D. program, I am not an expert.

Pyruvate from substrate phosphorylation (non-oxidative -anaerobic - glycolysis), which serves to increase the ratio of NADH to NAD+, enters the mitochondria via a transport protein called pyruvate translocase. Tgen, pyruvate is decarboxylated, which produces acetyl CoA - an irreversible reaction - which thereafter enter the Krebs (TCA) cycle where electron carriers are produced. The process of oxidizing Acetyl Co-A generates chemical energy sources in the forms of ATP, NADH, and FADH2. Also, the Krebs cycle is produces citrate and gluconeogenisis. Following the Krebs Cycle is the Electron Transport Chain (on the inner mitochondrial membrane) that shuttles electrons from NADH and FADH2 to molecular oxygen. Eventually, protons are shuttled to the intermembrane space, and oxygen is reduced to water. There are more details to the process, but essentially 32-34 ATP are produced from ADP and Inorganic Phosphate, which can be used to do work (exercise) or repair and rebuild the cell.

Hi Tom. Do you mean the total number of ATP including ADP + Pi added to the yield from mitochondrial respiration = 32 - 34 ATP?

I'm using 30 ATP as the mitochondrial yield.

In other words Anaerobic yield is 3 ATP from muscle glycogen 2 ATP from blood glucose and 30 ATP from aerobic metabolism. I think I've seen estimates from 29 - 31 in different biochemistry papers.

Indeed, a net of 2 ATP (glucose) or 3 ATP (glycogen) is the consequence of substrate glycolysis (the 10-steps of anaerobic metabolism). The Krebs (TCA) cycle yields only 2 ATP. However, the Electron Transport Chain (ETC) yields 30 ATP from NADH and 4 ATP from FADH2. Yet, some transport costs use part of the NADH and ATP, so the final tally is 32-34, perhaps 35. Long ago there was no understanding that some ATP was used during oxidative phosphorylation; only that it generated ATP.

The process of accounting for ATP is a bit messy because there is variance in how many ATP are used in oxidative phosporylation. Also, other sugars beside glucose can be used in ATP formation, and some ATP can be used for protein metabolism outside of oxidative phosphorylation. It seems to me we should use a wider range, such as 29-35 ATP is the final tally.

very interesting stuff guys, thanks for your replies.

Cheers Tom.

ILLEGITIMI NON CARBORVNDVM

The biochem stuff goes way over my head.

Is the whole point of training to become better at recycling or tolerating lactate?

Or is it to become better at burning fat?

Or both?

Non of those things. They happen anyway as normal metabolic functions.

The term 'Lactate Tolerance' is somewhat misleading because the acidity we feel is due to glycogen depletion which can happen quickly in a high intensity effort and is happening only briefly in the most powerful muscle fibers. Or it can happen slowly as a general glycogen depletion in a long run. The obvious solution is good pace judgement, so that is an important part of the training.The point of training is to improve your efficiency and endurance.

You're training your nervous system to use energy more efficiently for longer by improving motor skills. As with any skill or task you get better with practice. Pain is a good general guide. In my opinion most training should be mostly pain free because you can't fight increasing acidity in the muscles without making it worse. But you can delay it with better pace judgement.

Things like adapting to different weather conditions are short terms gains. But improvements in motor skills involve both short and long term gains.

Tinman's intermediate muscle fiber adaption hypothesis is interesting. These fibers can adapt in Myosin protein expression to give more power or more endurance. These are adaptions relative to the specificity of the training.

Biological systems (that includes you and I) don't work in a way that one energy system is on, suddenly shuts off 100%, and another one suddenly turns on 100%, like a light switch. As the primary power system (aerobic) reaches it's max, it gradually stops increasing in power. As that happens, the other backup system (anaerobic), gradually starts ramping up. Once the primary (aerobic) system's capacity has been exceeded, the backup system (anaerobic) increases towards it's peak.

It's a gradual transfer of power, from one system, to the next. The various "thresholds" that you hear about, are where one system meets the other.

Great stuff ATP, forest and the trees.