He can't see it, but Olympic athlete Quentin Rew knows it is there - that invisible force-field making his body work even harder when he's pounding out 50km of track.
Now, new Kiwi research has suggested that altitude can affect athletes like the top New Zealand race-walker at levels much lower than previously thought.
An exhaustive analysis by Lincoln University researcher Dr Mike Hamlin, which looked at 132,000 performances by nearly 1900 athletes at close to 800 venues around the world, also found altitude could be a friend to some athletes like long jumpers and sprinters.
But for those in more anaerobic sports like Rew, competing became even more gut-busting when the air was thinner.
He knew a lower amount of oxygen was going to slow down his race time, as his body tried to make do with a smaller intake of air under already intense circumstances.
"The highest race I've been in was at about 1400m above sea level - my race time was a lot slower than I would have been hoping for, even though I had the same physical exertion."
Rew has trained for high altitudes in camps as high up as 2300m above sea level, with the aim of encouraging more oxygen-carrying blood cells to improve performance back on the flat.
In middle and long distance runners, however, altitudes as low as 150m could impair performance, according to Dr Hamlin.
"They start to get substantial decreases in longer distance running performance at altitudes as low as 300m to 600m.
"Most would have thought that this would not happen until much higher altitudes, like 1000m," he said.
On the flip-side, his results indicated that levels of 500m to 600m had a small benefit for men running distances of between 100m and 400m.
Altitudes of between 500m and 1500m carried a slight boost for men competing in hammer throw, yet venues at 1000m or above put men in discus throwing events at a small disadvantage, as the discus lost lift.
Athletes, coaches and administrators needed to be aware of the potential for these performance changes.
Athletics New Zealand's high performance director Scott Goodman said while the findings were interesting, their practical application was less clear and often expensive.
"A possible implication could be for athletes that are very close to selection standards to compete in ideal conditions in order to qualify - however, in some instances this is a very expensive option and we have a limited budget."
The organisation already built altitude training, usually involving stints of 21 days or more, into the schedules of top athletes including Rew, Jake and Zane Robertson, Nick Willis and Nikki Hamblin.
How tough is a game of Super Rugby at 1753 metres above sea level?
That's the question Associate Professor Mike Hamlin, of Lincoln University, is looking to answer by crunching match data stripped from hours of Super Rugby camera footage.
Dr Hamlin and his research partners at Christchurch Polytechnic Institute of Technology are focusing on how players from low-altitude countries like New Zealand fare when playing teams like Johannesburg's Lions or Pretoria's Bulls, whose home turfs sit at 1339m and 1753m above sea level respectively.
The effects of altitude might play a part in how many times a team could gain territory.
"To get across the gain line and increase your chances of scoring, you generally need to first have a number of phases in a row, so with the higher altitude, players from lower altitude countries are likely becoming fatigued as the phases go on and their performance drops away."
While it was difficult to isolate the particular effects altitude had on play, there was enough statistical evidence to suggest higher altitude proved a real home advantage for the South Africans.
Dr Hamlin's research also aimed to provide solutions for combating the altitude factor, which could be difficult for Super Rugby teams whose weekly fixtures denied them time to acclimatise.