The PT where I work disagrees with me as to where ATP-PC is stored and when and how it is used. I suspect it is in the type II fibers because they are the fibers that predominantly use it. He says it is in all the fibers and can be used in type I fibers if the type I fibers become exhausted. I feel that ATP-PC is only used in explosive or high intensity situations which require more involvement from the type II fibers. I understand that some (little) type I involvement will occur. Neither of us can find anything written explaining this. Can you give some guidance on this or give a reference source that can explain it?
dennis…Timbo likes the post, my man. Good stuff, this exercise phys jargon. Here goes what I know…
The basics: ATP-PCr is an energy system that provides energy (ATP) through the splitting of a phosphocreatine (PCr) molecule into creatine, an inorganic phosphate (Pi) and energy (E) by creatine kinase; the Pi and E join with ADP to create the high-energy ATP. The system is predominantly used in short-duration, maximal effort exercise (sprints, shot put, etc) and can only supply energy for around 3 to 15 sec; PCr stores become rapidly depleted and exhaustion soon follows. As you mention, type II fibers (particularly IIb) are the big daddies in this type of exercise…however, I believe that both IIa and I fibers would be called into play as well, as the force production is quite high and demands maximal involvement. I’m not sure about your question exactly though…are you asking if ATP is stored in all fibers or if PCr is stored in all fibers? ATP-PCr is not a single compound by itself, as I mapped out above. Whatever the case, it is to my understanding that both are stored in all fiber types. Type II fibers (a and b) have much faster myosin ATPase than I fibers and can produce ATP at a much quicker rate (more force production, quicker).
First understand that all three major energy systems are functioning at the same time with constant changes in dominance. I think you need to look more toward duration of activity. ATP-PC system is for short duration activity because it is right there in the cells ready for action. Not necessarily for high intensity activity. If you stand up and sit back down what energy system are you using? ATP-PC dominant. Turn your head to the left. What energy system? The movement is most likely requiring an immediate increase muscle activity from resting which would require readily accessible energy. Again ATP-PC offers a little energy which is replaced by aerobic and anaerobic sources. These are by no means high intensity effort activities but require immediate changes available energy.
To make a long story short ATP-PC is used in all fiber types.
Both timbo and Bill are right. This fuel source is stored in all fibers. Like bill said it isn’t just the intensity but the TIME as well that stimulates the PCr system. Glycoloysis doesn’t “turn on” until about 20-30 secs so it is the creatine phoshpate shuttle that gets us moving in the first place. Whether that is getting up, like timbo said, which does NOT use type IIb fibers, or a quick sprint which will type IIb fibers. So both fibers store creatine phosphate. Muscle fibers are “fired” due to demand and the motor unit is what fires it. Intensity and time is what makes the different fibers and energy systems go.
Bill and siscokid…thanks for clearing the fact that ATP-PCr system is used in all activities…as I re-read my post I saw where it was somewhat misleading. The bottom line that I was trying to get across, as were you, is that ATP and PCr are available in all muscle cells and fiber types, just like glycogen.
just consult a basic human biology textbook and it would lay this information out. ATP and PC is present in all cells, just in greater concentrations in fast twitch fibers. The basic processor for making a muscle cell contract is to break apart an ATP into ADP and P which creates energy used to move the myosin chains. Then this ADP and P are rejoined into ATP either by using PC, or one of the other aerobic or anerobic pathways.