As a personal trainer, I have to make sure that my clients do each and every exercise correctly. I watch them like a hawk, looking for misalignment and any signs of extreme struggle. Both of which can negate any potential benefits. I also have to make sure that my clients breathe. Sounds silly as breathing is somewhat natural, but when you struggle with a weight or any other type of exercise activity—the natural tendency is to stop breathing as you power through the move. And that, I do believe, is the one thing I'm constantly pointing out to my clients. "BREATHE," I shout (nicely) almost regularly.
And so, in a universal message to my readers and my clients, I say this—don't forget to breathe when you train! It keeps the oxygen flowing, without which your muscles wouldn't function. And you could even faint. Which isn't fun for you. Or your trainer. In addition, a held breath can elevate your heart rate. And yes, you want to raise your heart rate during exercise to achieve maximum calorie burn—but the goal is to raise it naturally via increased activity. Not because you're doing something unnatural like preventing your lungs from taking in oxygen. So remember to inhale when the effort is easy and exhale when the going gets tough. For example, inhale when your back is flat and exhale as you enter the crunch. Inhale as you squat, exhale as you return to standing. So on, and so forth. Now go forth, enjoy your workout. And don't forget to breathe!