This is a good question and one that a group of researchers at the University of Wisconsin (La Crosse) studied. They measured the heart rate and oxygen use of 16 healthy adults walking or biking while talking at the same time. Oxygen use was the same for both activities when the subjects talked at the same time.

Exercising at a level that allows normal conversation is called the Talk Test. This level of exercise is considered safe and effective. Its use is widely advised for anyone with health concerns.

So long as you can talk while exercising, you won't overstress your heart or lungs. Of course, many people can exercise at a much higher intensity. Humming or talking under your breath increases your breathing more than you think. It's not as reliable a measure as the Talk Test.