Your body produces energy anaerobically at the start of any race.
During exercise of any kind, your body requires fuel to produce the energy needed to keep you going. In speed training, this energy comes in the form of oxygen and glycogen stores in your muscles. You utilize both the aerobic and anaerobic system in speed training, but finding the correct balance between the two will improve your body's efficiency during intense exercise.
The primary fuel source for your body during any exercise is oxygen. Oxygen uptake, which is also known as VO2 uptake, increases proportionally to the intensity of your workout. The faster you move, the more oxygen your muscles need to keep them going. When you run fast enough, the oxygen demand of your muscles will exceed your ability to supply it. Once you've exceeded your aerobic capacity, you'll begin to create an oxygen deficit.
In most forms of speed training, including sprinting on the track or bike, you'll produce energy anaerobically, meaning without oxygen. Your heart rate increases rapidly during sprint training and you exceed your aerobic capacity, which forces your body to convert glycogen into instant energy. Because of the intensity of the practice, most people can only keep up anaerobic exercise for up to three minutes at a time. For speed training, increasing your anaerobic endurance is critical to improving your explosive power.
Through consistent speed training, you can increase your lactate threshold. Increasing your lactate threshold will allow you to extend the duration of your speed bursts. While aerobic training will allow you to increase your cardiovascular endurance, it won't do much for your speed. To increase speed endurance, you need to get used to processing and tolerating lactic acid buildups through interval training.
According to a 2007 study conducted by the Norwegian University of Science and Technology, high-intensity interval training actually improves VO2 max more than steady-state, moderate cardio training. With this in mind, and considering that steady-state cardio will do little to improve speed, interval training is the way to go for improving your sprints. By alternating between periods of high and low intensity, you get the benefits of anaerobic training and allow your body to recover aerobically between sets.