If you start running at a full sprint, you will experience a high level of effort immediately, before you get tired. That’s because your brain has to drive your muscles very hard when you run fast. On the other hand, when you run slowly, your sense of effort is low initially, but then it increases gradually as you keep going. That’s because your brain has to work harder and harder to drive the muscles as they become fatigued. What’s more, as we saw in the preceding chapter, the brain itself becomes fatigued during prolonged low-intensity exercise so that actual stimulation of the muscles by
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