400 meters is characterized by a combination of speed and strength. The functional feature is a mixed function with glycolysis functional system as the main function.
You must exercise your lactic acid function. You can use interval training to run for 30 seconds at near maximum speed, then jog for 30 seconds to one minute, and then accelerate for 30 seconds. Repeated several times, the speed did not decrease significantly.
You should also pay attention to retrogression and abdominal strength exercises.
You should pay attention to your posture, running backwards helps to lift your legs.