Join Date: Oct 2006
The first thing to keep in mind is that there is never a moment when the body is operating solely on one metabolic pathway; all are working all the time, but each contributes a greater portion of the energy supply depending on intensity and duration of activity.
When muscles are producing ATP lthrough fast glycolosis, lactic acid is a by-product. That lactic acid can be buffered into lactate through oxidation in the local muscle or transported for oxidation in another muscle - OR, it can be transported to the liver, where the Cori Cycle converts lactate to glucose, which can then be returned to the working muscles as a substrate for continued gylcolysis.
So, the metabolic pathway depends first on activity intensity and second on duration. The phosphagen pathway functions at the start of ALL activity, and is the primary source of ATP for very brief, very intense activity such as a single snatch. The phosphagen pathway uses existing stored ATP in the muscle, breaking it into ADP + 1 phosphate and releasing energy. ADP can then react with creatine phosphate to create ATP + creatine. Finally, 2 ADP can be broken down into 1 ATP and 1 AMP.
Glycolysis is the breakdown of blood glucose or muscle glycogen to create ATP. If the intensity of activity is low enough to allow the slower oxidation to supply energy, then pyruvate enters the Krebs Cycle and lactic acid is not produced. If, however, the demand for energy is too great for this, like in the case of a CF workout, pyruvate is converted to lactic acid, which allows a faster supply of ATP.
Now, lactic acid can disrupt muscle activation--it's not clear how exactly, but something like interfering with actin/myosin coupling or calcium ion binding, etc. The presence of lactic acid also lowers the pH of the muscle, which decreases metabolic enzyme activity. Result = fatigue and reduced force production capacity.
BUT, if that lactic acid is buffered into lactate, those problems are avoided. Again, lactate can be used to generate more energy.
So if you have a well-developed lactic acid buffering capacity, you'll be able to avoid the build up of acid in the working muscles, avoiding its fatiguing characteristics, and add the resulting lactate to your list of available energy substrates. If, however, your clearance/buffering of lactic acid is poor, you'll end up fatigued and weak. So all this means you need to train in a way that improves your lactic acid buffering - and that's CrossFit and/or threshold training.
Yes, CF workouts are still producing lactic acid/lactate after 3 minutes if the intensity is high enough--although now Brooks et al is suggesting that lactate is being produced and utilized at all times regardless of activity intensity. But of course the quantities involved in a CF workout vs me typing right now are clearly very different.
As far as muscle fiber types, like the metabolic pathway situation, you're going to have a mix of fiber types working at any given time. The greater the intensity, the more fibers, so for example if you're running a marathon, you'll be running primarily on slow-twitch (type I) fibers. But if you're performing a 1RM deadlift, you'll be using a lot of all fiber types. The actual contribution of type I fibers to a max deadlift is probably very small, but they're still working. All that said, all muscle fiber types are able to use all 3 metabolic pathways, it's just that each is suited better for certain ones. Type I of course have greater mitochondrial density and oxidative enzyme activity, so they function better with oxidative metabolism. Type II a and b function better with phosphagen and glycolytic metabolism. So in your scenario of a largely glycolytic CF workout, all fiber types will be contributing, all using a mix of metabolic pathways, but primarily glycolytic, and the Type IIs will be better at this.
This new research has suggested that lactate can be used directly by the muscle as an energy substrate--it does not need to be transported first to the liver to be converted to glucose. What Brooks et al found is that lactate can be oxidized directly by the mitochondria of the working muscle, and that lactate oxidation is occuring at all times (much like all 3 metabolic pathways are contributing to varying extents at all time). So to answer your question about by which muscle fiber types lactate is used, it would be more by Type-I because of their greater oxidative capacities, but no doubt by Type-IIs also to a lesser extent.
Hopefully that answered your question. Maybe.