Muscular acidosis is due to the hydrolysis of ATP to ATP and other substrates being used for energy in the muscles.
Basically what happens is this:
ATP + H2O → ADP(hydrated) + Pi(hydrated) + H+(hydrated) ΔG˚ = -30.54 kJ/mol (−7.3 kcal/mol)
Lactic acid is directly through the following equation (FYI pyruvate is the end result of glycolysis):
Pyruvate + NADH <-> Lac + NAD+
The reason why this doesn't actually produce a H+ ion is because pyruvate itself is already a carboxylic acid Ch3-CO-COOH which is deprotonated already at body pH. Therefore, it's already pyruvic acid (pyr- + H+ + NADH <-> Lac- + H+ + NAD+) where the H+ cancel on each side of the equation. This stoichiometric mistake was why people THOUGHT lactic acid was the cause of acidosis; however, as you can see this is not true.
Lac has a pKa of ~3.85 which means that at equilibrium in the body some of the lac will be pronated but most will have no H on the carboxylic acid. However, as the pH drops, a greater proportion of the carboxylic acid becomes pronated thus effectively buffering acidosis as described above.
Alright, here's the fun part which I was actually just discussing today with someone regarding the "lactic acid threshold." What is actually happening here is a couple of things.
Basically, what happens when intensity becomes high enough (thereby increasing demand in energy) is that the body needs to produce energy. The oxidative pathway becomes bottlenecked at pyruvate/acetyl-CoA because it cannot handle the load of NADH that is being produced. Thus, NADH accumulates.
The reason why NADH accumulating is bad is because glycolysis uses the non-hydrogenated carrying form of NADH (namely NAD+) to produce NADH. Thus, lactic acid is produced for the body to change the extra NADH back to NAD+ so that glycolysis can keep running.
Here's the brilliant illustration I thought up today. Think of the oxidative pathway as a pipe. This actual "pipe" consists of the citric acid cycle (or TCA/kreb's cycle whatever you want to call it) which consists of 10 steps
, transport of electron carrying coenzymes such in the form of NADH and FADH2 (from glycolysis and CAC) from the cytoplasm into the mitochondria along with ADP, dumping of electrons into the proteins of the mitochondria wall (the electron transport chain) whereby these proteins pump H+ across the membrane to create a H+ gradient. This gradient is then harnessed by ATP synthase to produce ATP. NAD+ and ATP must be shipped out of the mitochondria into the muscles. This takes some time obviously to get "running."
Note: this also why people with large aerobic engines are also buffered well against acidosis because their mitochondria pumping H+ across the membrane provides a greater buffering effect.
In any case, glycolysis is a simple pathway which consists of about 10 steps all of which occur in the cytoplasm. This occurs very quickly because ADP is readily available because the myosin-actin is close by and need very little transport activity.
Thus, let's take into account the scenario from the original/cross post:
10mph for essentially forever
10.5 mph for an hour but it works the hell out of them
11 mph and they fatigue in a few minutes
At 10 mph the effect of what is happening is that the glycolysis funnel (we'll just call it a funnel) dumps NADH and pyruvate into the oxidative "pipe." It can handle the capacity. Thus, it can basically run forever without getting overloaded.
What happens at 10.5 MPH when it is slightly overloaded is that there is some spill over out of the glycolytic "funnel" that the oxidative "pipe" cannot handle the total volume. Thus, what occurs is that the pyruvate is converted to lactic acid to use up the NADH converting it back to NAD+. This allows the glycolytic cycle to continue to provide energy to the muscles because there is a limited amount NAD/NADH in the body. However, since there is "little" spill over, the body can handle the pace and keep continuing.
Now we arrive at 11 MPH. What is occurring here is that there is a large spill over out of the glycolytic pipe as glycolysis consumes a lot of the muscles glycogen stores very quickly. Since there is a large spill over, the body can only convert some of the pyruvate to lac to generate NAD+. Thus, it can only keep producing a limited amount of energy as the lac keep accumulating. Eventually you will reach a point where there is too much ADP (and hence pH drops enough) that glyolysis + oxidative pathway cannot keep up to reproduce ATP. This leads to muscular failure.
Here's a nice picture I made in paint to illustrate the above (click to enlarge):
In any case, what is also interesting is the adaptations that occur because of the former scenarios.
With the 10 MPH that is sustainable, it clearly strongly work the oxidative "pipe" at max capacity for a long time. This produces a strong response on the oxidative pathway increasing mitochondria, type I fibers, and other aerobic changes which is why you NEED to do long distance work to build up a sufficient aerobic base if you ever plan on competing in elite long distance racing.
What happens here is that since there is an overflow of the glycolytic funnel, the glycolytic pathway is stressed as the glycolyticenzymes need to keep up with the rate as NAD+ is resynthesized quickly. The oxidative pathway is stressed because it is working at full capacity, but the duration is shorter than the former. What you end up with is some increases in anaerobic and aerobic qualities in the muscles.
This is the interesting concept because it shows clear "pushing" against the "lactate" threshold. So basically since there is a large overflow from the glycolytic funnel we get an extremely strong stress on the glycolytic energy pathway. This induces strong anaerobic changes in the muscles. However, since the duration is extremely short, the oxidative pathway receives little to moderate stress resulting in only poor-decent gains in aerobic fitness UNLESS this is continually pressed through for a while. For example, a HIIT session that continues after you get gassed or a metcon that keeps going a couple minutes longer after max intensity drops will help produce aerobic changes.
This is why HIIT/tabata/metcon all have the tendency to produce anywhere from mediocre to strong aerobic changes as well as strong anaerobic changes in energy pathway metabolism.
Basically, for longer duration rowers/runners/etc. this is COUNTERPRODUCTIVE because they do not need improvements in the anaerobic engines which will induce changes that will counteract their aerobic engines. This is easily observable and agrees with the OP & anecdotal evidence.
I think that's about it. Hope you learned something.