Why do runners continue to run past the finish line before they stop?

Runners in the 400m finals appear to slow down in the final stages of the event as a result of moving into the lactic acid system (from the ATP-PC system). This change causes the athletes/runners to experience a decrease in maximal power output and running speed.

The ATP-PC system only lasts for about 8 seconds (before PC stores are depleted) at maximal effort, therefore the lactic acid energy system becomes the dominant supplier of ATP. This is what causes the drop in power and speed. At about 45 seconds, there will be another decline in power output and speed as the body becomes more reliant on the aerobic system for energy.

Kathy Freeman runs 400m in around 49 seconds. This means that she would have used all three energy systems (ATP-PC, lactic acid and aerobic). She would have dropped power and speed at around 8-10 seconds as her ATP-PC stores would have depleted, and moved into the lactic acid system, and at around 45 seconds (towards the end of the race) she would have changed into the aerobic system which also causes a drop in speed and power, just before the end of the race.