Does holding your breath increase running performance? I've noticed in the slow-motion replay of the Olympics it looks like many athletes are not breathing. -N
Most athletes in sprint-length events do hold their breath, for part or even all of the race.
The 50m freestyle swim is routinely done with one mid-race breath, as is the first part of the 100m run, where runners are straining to pushing off the blocks and accelerate. Weightlifters also breath hold and bear down, or Valsalva, during their power lifts. If the sport requires maximal effort over a short period, many athletes breath hold. The common theme with all these sports is that they're anaerobic.
The body already has all the oxygen it needs for 20 seconds or so of hard effort, bound to the blood's hemoglobin. The negative after-effect will be a lactic acid build-up, which in a distance athlete would be crippling, but for a sprinter doesn't matter.
I was able to find one scientific study of Norwegian national level swimmers that broke it down to simple numbers. Every breath cost the swimmers 0.03 secs. Breathing reduces the amount of time spent in the optimal power-producing position. Interestingly, the top two swimmers breathed the least: every fourth-stroke cycle over the course of a 100m swim, or just once in the 50m race.
Is there a down side to breath-holding? Another study looked at power lifters, and found their blood pressure rose as high as 370/360 (normal is 110/70) while breath-holding during a power lift.
Breathing during effort dropped their blood pressure in half, but also lowered the maximum weight they could lift. There are of course, problems associated with breath-holding during severe exertion: ruptured arteries, retinal damage, and even strokes, making it a really bad idea for recreational athletes.