The two main energy systems used in short, fast bursts of muscle activity are the phosphocreatine and the glycolytic energy systems. And most research of this type is done using a bicycle odometer because it's much easier to collect the data. However, the same results are seen using a treadmill as well. So let's take a look at how research exploring fuel use using short bursts of activity is done. Now here is one of our subjects. And, as you can tell, I've speeded up the video. The subject performs three maximal sprints on a cycle odometer, and each bout lasts 30 seconds. Then there is a 4-minute rest between each of the 30-second bouts. Muscle biopsies are taken at rest and then they're taken after 6, 15, and 30 seconds during the first and third balance of cycling. The muscle biopsies are used to estimate phosphocreatine and glycogen in the muscle. These, remember, are two important fuels. The level of lactate and hydrogen ions indicate the level of acidosis in the muscle, and they both measured as well. Lactate content and hydrogen ions are both byproducts of glycolysis and provide an estimation of how hard the glycolytic energy system is working. So let's examine power output first. During the first 6 seconds, the subject averaged 800 watts between 6 and 15 seconds, the subject averaged 700 watts. And between 15 to 30 seconds, they averaged 500 watts. Now this reduction in power output indicates that fatigue is occurring. The first muscle biopsy at rest provided us with the following results. The phosphocreatine content of the muscle at rest was 88 millimoles per kilogram of dry muscle. And the glycogen content was 480 millimoles per kilogram of dry muscle. Now don't worry about these units, just focus on the number. The lactate content was 5 millimoles per kilogram of dry muscle. Now glycolysis is not very active when the athlete is at rest, and this is the reason for the low lactate content. The hydrogen content is equal to 62 nanomoles. And that is just a quantity of measure of hydrogen ions. Hydrogen ion basically accompanies the lactate content. The subject begins pedaling as fast as possible. Is that power output during the first 6 seconds averaged 800 watts and at 6 seconds, the phosphocreatine content was at 46 millimoles per kilogram of dry muscle. That is the phosphocreatine content and the muscle dropped by almost 50% during the first six seconds. The hydrogen ion content has increased to 110 nanomoles. And this increase in hydrogen ion content is a really good indication that anaerobic glycolysis was supplementing the declining phosphocreatine energy system between 6 and 15 seconds power output drop to an average of 700 watts. And this make sense because the anaerobic energy system cannot produce the power that the PCr energy system can produce. And glycolysis is more dominant between 6 and 15 seconds. At the 15-second point, the muscle biopsy showed that the phosphocreatine content had dropped to 28 millimoles per kilogram of dry muscle. And hydrogen ions had increased to 157 nanomoles. Between the 15 second and 30 second period, average power output dropped to around 500 watts due to rapidly decreasing phosphocreatine stores. And at 30 seconds, the muscle biopsy showed that the PCr content was almost depleted And hydrogen ion content had risen to 183. Now this number means that it's quite acidic in the body. So you can see the PCr content in the muscle has been almost depleted by 30 seconds of high intensity work. The PCr began with 88 units in the muscle cell, and at 30 seconds it is at 8 units. Glycogen content also drops by 80 units, and this means there is still substantial glycogen available. The hydrogen ions have risen dramatically, and this indicates how hard the athlete's glycolytic energy system is working.
