 At the professional level , sprinters begin the race by assuming a <unk> 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 <unk> dictates that a runner 's near @-@ top speed cannot be maintained for more than thirty seconds or so because <unk> acid builds up once leg muscles begin to suffer oxygen <unk> . Top speed can only be maintained for up to 20 metres . 
