There is the intensive care aspect, ventilatory support, anaesthetics, specialist nursing staff, nutritional support, and intensive rehabilitation.

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But beyond the ventilatory threshold, the release of carbon dioxide begins to exceed the body's intake of oxygen.

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For most individuals, the ventilatory threshold is around 50% to 60% of the way to their maximum capacity, though there is tremendous individual variation.

WSJ: Hard-Wired to Hate Exercise?

By using tricks such as listening to music, people can continue to feel good even slightly past their ventilatory threshold, Dr. Ekkekakis and his colleagues have found.

WSJ: Hard-Wired to Hate Exercise?

And while both ventilatory threshold and maximum capacity can be slowly increased, people have to have enough positive experiences to stick with exercise over time so they actually can boost these limits.

WSJ: Hard-Wired to Hate Exercise?

In continuing studies of obese, sedentary but otherwise healthy middle-aged women, Dr. Ekkekakis found that some individuals reach their ventilatory threshold after just one minute at a slow pace on a treadmill.

WSJ: Hard-Wired to Hate Exercise?