All right so let's talk about single bout sprinting for now. The word sprinting is typically used to describe bursts of maximal effort lasting less than 60 seconds. The way in which the athlete uses speed differs according to the sport. In track and field, speed is used to perform a single, or out effort. Training for that single effort, however, consists of performing multiple bouts of maximum speed to induce the necessary structural adaptations for just that one single bout of speed during competition. In team sports, sprinting only occurs in multiple bouts. In between each bout the athlete may stand still, they may walk, or they may jog. So we are going to examine how the energy systems are used when the athlete sprints just one time versus sprinting multiple times. We begin by examining a single bout of sprinting. And the 100 meters and the 400 meter races are examples of single bout sprinting. There's always quite a significant recovery period between the performance of these races during a track and field competition. During a 100-meter sprint most of the cells' phosphocreatine is used during the first 20 meters of the race. And here is the 100 meter speed curve, showing the athlete's speed throughout the race. And here is the curve showing the amount of phosphocreatine left in the cell. phosphocreatine, remember is the fuel supply for the PCr energy system. During the first 20 meters, the athlete rapidly accelerates. And this is the yellow line. And the phosphocreatine energy system is the critical source of ATP during this acceleration phase because of its ability to produce very, very high force and power. Speed begins to decrease between 40 to 60 meters, when the PCr stores have declined. And at this point, glycolysis is producing a significant proportion of the ATP. And this is the reason the athlete can keep going. During the 100 meter sprint, glycolysis provides around 65 to 70% of the necessary ATP. And this is the reason acidosis increases throughout the 100 meter race. The fastest sprinters produce the highest acidosis levels, indicating the importance of the glycolytic energy system to ATP production in the 100 meter race. However, acidosis does not contribute to fatigue during the 100 meter race. Fatigue in the 100 meter race, causing the athlete to slow down, is due to the phosphocreatine levels in the muscle. They are very low. Fatigue begins around 60 meters, as indicated by the slowing speed on the speed curve. And even though acidosis is increasing at this point in the race, the race is over before the acidosis can interfere with the muscle activity. Okay, now the situation is very different in the 400 meter race. The phosphocreatine energy system certainly plays a role. However, the acidity associated with glycolysis is a critical fatiguing factor. So let's take a look at this. The concentration of phosphocreatine falls 89% during the race, and the athlete's speed is also slowing, as you can see here. Speed peaks at 100 meters, and then slowly declines after that. The rise in the lactate indicates the activity of glycolysis, and the amount of acidity in the blood. And the rate of ATP production from glycolysis reaches its maximum between the 200 and 300 meter point, when the phosphocreatine is relatively low, or almost depleted. Over the last 100 meters, when the phosphocreatine supply is really low and acidity is very high. And this is why the athlete begins to tie up and feel considerable pain. The 400 meter race is brutal over the last 100 meters. There is a dramatic decrease in running speed over the final 100 meters, because of this acidosis. It's interfering with the ability of the muscle to work properly. And at the end of the 400 meter race, the high rate of acidosis is thought to inhibit the cross-bridge cycling of the muscle that we've looked at this animation quite frequently. But as well, the aerobic energy system is becoming the dominant supplier of ATP. And over the last 100 meters, the decrease in speed is due to a much higher reliance on the aerobic energy system to support the struggling glycolytic energy system due to high acidosis. And also to the interference of the acidosis with the muscle's activity.
