Unfortunately most of us interested in exercise took an American approach to exercise when Dr. Cooper first popularized exercise in the late 1960s. We took the "more is better" approach and started racking up the miles or hours in the gym or aerobics classes and competing in marathons or triathlons. Turns out that this excessive cardio was likely not much better at improving longevity than being sedentary. Most exercise programs today are built based upon a very incomplete picture of the physiology of your body. Getting cardiovascular benefits requires working all your muscle fibers and their associated energy systems. Unfortunately for the long slow cardio folks out there, this cannot be achieved with traditional slow cardio. Unfortunately, anywhere from 90 to 98 percent of people who exercise are NOT doing high intensity exercises. By focusing on slow endurance-type exercises, such as running on a treadmill, you actually forgo many of the most profound benefits of exercise. Your heart has two different metabolic processes: * The Aerobic, which require oxygen for fuel, and * The Anaerobic, which do not require oxygen.
Let's simplify this. Studies show that we have three different types of muscle fibers; slow twitch (type I), and fast twitch (fast twitch IIa & IIb). If you don't actively engage and strengthen all three muscle fiber types and energy systems, then you're not going to work both processes of your heart muscle. Many mistakenly believe that cardio works out your heart muscle, but what you're really working is your slow twitch muscle fibers. You're not effectively engaging the anaerobic process of your heart.
I want to paint the picture of what high intensity is, what the benefits are, and why ANYONE can partake in this type of exercise. High intensity exercises sequentially recruit all the different types of muscle fibers in your body, starting with the smaller motor units made up of slow-twitch fibers which are primarily aerobic (have a lot of endurance and recover quickly) -- to the intermediate fibers -- followed by the fast twitch fibers. If you stopped and gave it some thought, it's actually easy to see that your body was designed for high intensity, short interval exercise. In nature you will never see an animal jogging -- hint hint. Instead of being sedentary for much of the day and then running for an hour on a treadmill, our ancient ancestors combined lots of walking with regular lifting and short bursts of high-intensity activities. It is safe to say that science backs up the notion that they did not have near the amount of cardiovascular diseases and metabolic issues that we face today. So why did we fix what wasn't broken? Steady state aka long slow exercise trains the plasticity out of your physiologic system. This is the ability to handle widely varying levels of exertion within a short span of time. Yes, you can make yourself less adaptable to physical stress in general.
Your fast twitch fibers are largely glycolytic and store a lot of glucose (carbs). When these muscles are recruited, it creates the stimulus needed to grow muscle. We have to remember that muscle is very metabolically expensive. If you become sedentary and send your body the signal that this tissue is not being used, then that tissue is metabolically expensive. The adaptation the body takes is to deconstruct that tissue (Use It Or Lose It). At the same time, it enlarges the glucose storage reservoir in the muscle, which in turn enhances your insulin sensitivity. Normalizing your insulin is one of the primary health benefits of exercise, and this is particularly true in the case of high intensity exercise. Slow long distance cardio recruits the slow twitch fibers only. If you remember, earlier I said those fibers recover quickly. So rather than moving to the next set of motor units, you're just recruiting that one group over and over again. As a result, your intermediate and fast twitch fibers actually begin to atrophy aka SHRINK AND DIE! Aside from losing muscle mass, you'll also experience earlier onset of loss of insulin sensitivity, leaving yourself open to a full scale of health ramifications, such as metabolic syndrome (Diabetes).
High intensity short interval training can be as simple as getting on a recumbent bike and pedaling as hard as you can for 30 seconds, then resting for 30 seconds and repeating that process for 10 minutes. But it also relates to the individual who can perform a set of squats for 6+ repetitions, move on to a set of step ups for 6+ repetitions on each leg and starting over until 2+ rounds are completed. There is no one size fits all. Everyone has to start from somewhere.