So, we just talked about a single bout of sprinting and what happens to the fuel supplies. Now, let's take a look at what happens during multiple bouts of sprinting. Athletes train for speed by performing multiple bouts of high intensity sprinting separated by short periods of rest or very low recovery. And here you see an animation of what the activity to intensity during one training session might look like. There are three maximum speed bouts in this particular training session, but there can be more if you're training a track sprinter. Team sport coaches will try to mimic the speed variation typical during a game. And here is an example of the workload for a sprinter who performs one single burst of maximum speed. And here is an endurance runner, running a 5K training run. And the speed is very consistent and below their VO2max. And here is a basketball player and here is a hockey player. Now, it's clear there is quite a bit of variability in both of these team sport athletes. Sometimes they're below their maximum VO2 and sometimes they're above. Accompanying this speed variability are fluctuations in muscle levels of phosphocreatine. During the short bouts of intense sprinting, the phosphocreatine concentrations can be as low as 40% of the resting levels. After 15 seconds of rest, the phosphocreatine can increase to about 70% of resting levels. And here you see the fluctuating phosphocreatine concentration in a muscle cell over six minutes, and as you can see it fluctuates quite a bit. The level of phosphocreatine in the muscle when the athlete needs to sprint determines their power. And during a team game the phosphocreatine energy system is supported by glycolysis, as you can imagine that becomes a very important energy source. But as a result, the lactate production during team sport training sessions and competitions can be very high, indicating relatively high levels of acidosis. The more speed bouts the athlete does, the higher their level of acidosis will be. And throughout a one hour training session, the player's glycogen supply will gradually decline. Now part of the role played by the aerobic energy system is to provide the ATP needed to rebuild the phosphocreatine. However, glycogen stores will only be replenished between gains through the diet. If the athlete runs out of glycogen, they will have to rely on their aerobic energy system, and their performance will decline because they cannot run as fast. They cannot produce any significant levels of power if they're low on glycogen. Now over the course of one hour, a team sport athlete uses a lot of glycogen. During a typical team competition the glycogen stored within the exercising muscle is used. Now some resynthesis of glycogen occurs from the lactate during periods of rest and low intensity activity, but it's not a really significant amount. Some fat is used to support multiple bursts of speed by the aerobic energy system. However, it's really important, for you team sport coaches out there to realize, that glycogen is the most important substrate for team sport players, and it must be replenished between the training sessions by way of a good diet, and that includes carbohydrates.