At the professional level, sprinters begin the race by assuming a crouching position in the starting blocks before leaning forward and gradually moving into an upright position as the race progresses and momentum is gained. Athletes remain in the same lane on the running track throughout all sprinting events, with the sole exception of the 400 m indoors. Races up to 100 m are largely focused upon acceleration to an athlete's maximum speed. All sprints beyond this distance increasingly incorporate an element of endurance. Human physiology dictates that a runner's near-top speed cannot be maintained for more than thirty seconds or so because lactic acid builds up once leg muscles begin to suffer oxygen deprivation. Top speed can only be maintained for up to 20 metres.