We have all been told to devote some time in the gym to “cardio” exercise (that is, exercise that is supposed to make your heart stronger). Your body needs a strong heart to overcome circumstances that challenge the heart’s maximal capacity. For example, when a heart attack occurs it is a result of cardiac demand challenging the maximal capacity of the heart. This “attack” has nothing to do with the endurance of the heart, but its ability to overcome this type of challenge dependant on its strength.
When you perform cardio exercise for long durations and at low intensities, you can improve the endurance of your heart but are actually training it to get weaker by downsizing its output. Long-duration, low-intensity cardio downsizes the heart because it signals the cardiovascular system to do as little work as possible in order to go further and last longer. Conversely, short-duration, high-intensity cardio activities strengthen the heart because they challenge and signal it to immediately generate enough force to overcome the demand placed on it. In the case of a heart attack, bigger, faster, and immediate cardiac output is what you need. Additionally, short, intense bouts of cardio exercise are more effective at lowering fat and triglyceride levels in the blood than long, light bouts.
The health of your heart and how much you can depend on it to keep you alive depends on how efficiently you train it. So instead of walking on that treadmill for 45 minutes or stepping on that StairMaster for 30 minutes, try mixing it up. Do some interval training with your cardio; for example, you might want to run for a minute and then walk for a minute for 10 minutes on the treadmill. Then, go as hard as you can for 4-5 minute intervals on the StairMaster. The combined 30 minutes of interval training that you spend on the treadmill and StairMaster is more efficient at improving heart health than the 75 minutes you would otherwise spend trying to pace yourself.
If you want to make the most of your cardio exercise, put your heart into it!