Your respiratory system brings air in and pushes it back out with each breath. Lung capacity (the total air your lungs can hold) varies from person to person. You can measure it by blowing up a balloon with a single breath and comparing sizes across different people.

Regular vocal practice may strengthen the organs that bring air in and push it back out. To test this, you recruit 20 participants who are all the same age — half are choir members with at least one year of experience, the other half are non-singers and non-athletes. Each person takes one deep breath with a nose clip on, then blows as much air as possible into a balloon. You measure the balloon diameter with a ruler. Choir members inflate their balloons to a larger size than non-singers, suggesting that sustained breathing tasks like singing build lung capacity over time.

A normal exhale and a forced exhale move very different amounts of air. You recruit 10 males and 10 females of similar age and health. Each person breathes normally into a balloon three times, and you measure the balloon's circumference each time to convert it to a volume. Then each person exhales forcefully into a new balloon, capturing the expiratory reserve volume — the extra air pushed out after a normal breath. Averaging the results for each gender shows whether males or females hold more air in their lungs.

A single breath can generate enough air pressure to push water out of a sealed container. You fill a plastic bottle with water, turn it upside down in a dish pan, and run a tube into the opening. Each volunteer takes a deep breath and blows through the tube into the bottle. The air pushes water out, and you read the markings on the bottle to measure the volume displaced. That volume equals the air the volunteer delivered in one breath. Testing 7 to 10 volunteers with different fitness levels lets you compare results and ask whether people who exercise more have bigger lungs.