First up, I think the fault probably lies with the writer, not the scientist, for misrepresenting valid ideas and drawing spurious conclusions based on them. It states a reasonable hypothesis, which is that lactic acid may not be directly or solely responsible for muscle fatigue and post-exercise muscle pain. There may not be a need for people to exercise below a "lactic threshold" (the first time I've heard that term).
However, the article contains many statements that I think are misleading:
Lactic acid is actually a fuel, not a caustic waste product. Muscles make it deliberately, producing it from glucose, and they burn it to obtain energy. The reason trained athletes can perform so hard and so long is because their intense training causes their muscles to adapt so they more readily and efficiently absorb lactic acid.
Lactic acid can be a fuel, but it is originally produced from pyruvic acid during anaerobic respiration because the body is having difficulty providing enough oxygen or utilizing that oxygen to carry out aerobic respiration. The conversion from pyruvic acid to lactic acid is needed to regenerate NAD+ for glycolysis. In order to make use of that lactic acid as fuel, the body needs to pay back its "oxygen debt" and convert lactic acid back to pyruvic acid. Training would help reduce the overall accumulation and production of lactic acid probably by two important means: people are more cardiovascularly fit, so they can oxygenate their tissue better; and a valid idea mentioned in the article, which is muscles having more mitochondria to use that oxygen and produce more energy via aerobic respiration.
"I gave rats radioactive lactic acid, and I found that they burned it faster than anything else I could give them," Dr. Brooks said.
It looked as if lactic acid was there for a reason. It was a source of energy.
Well yes, the body prefers not to keep lactic acid around since it is, in an anaerobic state, more or less just a relatively safe waste product of glycolysis. In an properly oxygenated being, they should readily metabolize excess lactic acid for energy by converting it back to pyruvic acid.
The understanding now is that muscle cells convert glucose or glycogen to lactic acid. The lactic acid is taken up and used as a fuel by mitochondria, the energy factories in muscle cells.
It's been well-known for a long time now that any cell can produce lactic acid as a product of anaerobic respiration--red blood cells produce all of their energy by this method. However, the fact remains that again, in order for lactic to be used for energy, it needs to be converted back to pyruvic acid, a mandatory intermediate product in aerobic respiration. Lactic acid is not a mandatory intermediate product, and therefore it is misleading to say that muscles necessarily convert glucose to lactic acid in order to make energy.
Through trial and error, coaches learned that athletic performance improved when athletes worked on endurance, running longer and longer distances, for example. That, it turns out, increased the mass of their muscle mitochondria, letting them burn more lactic acid and allowing the muscles to work harder and longer.
Again, I'd argue that the muscles are able to be better oxygenated and make better use of that oxygen, negating the need to produce as much lactic acid during exertion.
Muscles make it deliberately, producing it from glucose, and they burn it to obtain energy.
is horrendously misleading. Muscles make it from glucose because there isn't enough oxygen to completely oxidise the glucose, so they make lactic acid which is still a thermodynamically favourable route, but releases much less energy. Lactic acid is, to speak, "half burnt" glucose.
Muscles do not burn lactic acid to "obtain energy", they do it in post-exertion respiration when oxygen is plentiful and they either squander the energy in just heat or use it in respiration. Mostly squander it.
The rat result is simple - Lactic acid should only be there after exertion and since it impedes muscle performance, it's an evolutionary advantage to get rid of it by any means necessary ASAP. It does burn for energy, but rarely useful or used energy.