What's the healthiest way for runners to train?
Which is better for fitness development and health: aerobic endurance training or high-intensity interval training? This question has been the subject of debate among both health professionals and sports physiologists alike for decades. Each has unique metabolic benefits and can help you improve yourself in different ways. So which is better? Spoiler alert: both are good, and training like a miler gives you the best of both worlds!
A quick overview: Aerobic endurance training is what we all think of as going out for a steady jog: a continuous session at a low-to-moderate effort. This is usually thought of as 30+ minutes at 50-70% of your maximal heart rate, or a "conversational" effort. Going out for your 10 km afternoon jog or your weekend long run would be exhibits A and B for this type of session.
High-intensity interval training is the opposite: very segmented sessions at a high effort. The intense intervals might be anywhere from 30 seconds to a few minutes repeated several (or many!) times close to or at your maximal heart rate with very easy recovery following each interval. Classic examples might be going down to the track to do repeat 200s or 400s or hitting the roads and doing a fartlek (alternating hard and easy running), or finding a nice steep hill to tortuously run up and down.
Both of these training methods confer unique adaptations in your body all the way from the cellular level up to the whole body that benefit your performance and your day-to-day well-being. So what's the benefit of aerobic training? We all know that "running" is good for your health, and it's logical that if you want to improve performance, running more is probably a good idea. This is true, and the benefits of "long slow distance" running are numerous: when you start building up the distance of your runs, you get some noticeable changes in your cardiovascular system. Namely, your blood pressure and resting heart rate drop, while your plasma volume and vessel wall compliance increase. Long runs increase the capillarization of your muscles, meaning they have a better blood supply and more efficient access to oxygen and nutrients. These continuous aerobic sessions also do a doozie on your mitochondria. What are mitochondria, you ask? They're the little machines inside your muscles that are responsible for your athletic endurance performance! They convert oxygen and fuel (carbohydrates or fat) into the molecules that your muscles use to contract and propel you around the track. Steady, aerobic runs increase both the size and number of these guys in your cells, meaning you develop a greater efficiency at breathing in air and burning it in your muscles. This is seen as an increase in what physiologists call your VO2max, which is the maximum amount of oxygen you can take in at once. Steady running also enhances your ability to use lipids as fuel, or put another way, helps increase your ability to burn fat. Aerobic work primarily relies on what are called "Type I" muscle fibers. Quick physiology lesson: you have two muscle fiber types, these Type I slow-twitch fibers (resistant to fatigue and good for endurance) and Type II fast-twitch fibers (powerful and good for sprinting), and you have roughly a 50/50 mix of each (we'll get to Type II later!). These Type I muscle fibers need oxygen and prefer to burn fat to generate power. As you develop the ability to run further and further at submaximal intensities, you begin waking up these Type I fibers to help with the task, and your body becomes better at using fats to do the work. This is certainly good news from the standpoints of health, body composition, and performance capacity.
Aside from the physiological adaptations to steady running, there are some great structural and psychological benefits. The long, receptive loading on your bones and joints will actually provide a stimulus to make them more robust to degeneration. If you increase the volume of these aerobic sessions cautiously and patiently, these continuous sessions will signal your skeletal system to adapt, and you'll slowly build stronger, denser bones and more resilient tendons, ligaments, and joints (though too much too soon can put you on the other side of that curve!). Further, aerobic sessions help balance and control many neurological and endocrine pathways that keep your mind and your hormones ticking along.
But what about the intense interval training? These sessions contrast with the above sessions in that they use your anaerobic system (without oxygen) to help power you through. They are likely to put you in a state of relative "hypoxia" (too little oxygen), and this state triggers all sorts of great (albeit extremely uncomfortable) changes in your physiology. Unlike the aerobic steady sessions, these sessions depend heavily on the second type of muscle fiber, the Type II fibers. These fibers use carbohydrates (sugars and starches) to generate a lot of energy very quickly. They work both with and without oxygen (the Type I fibers we talked about earlier need oxygen to contract), so when you go to run or sprint at intense paces with your heart races nearing its maximum and your lungs begging for more air, these muscle fibers step up to do the work. By incorporating interval training, you develop your ability to use these fibers for longer periods and further train the capacity of the anaerobic system. When you're running at these paces and intensities and oxygen is in short supply, your muscles start generating lactate, and the acidity of your blood starts to increase. Interval training helps you tolerate this acidity better, and makes your liver more efficient at converting that lactate back into usable fuel! Furthermore, interval training prompts some of those Type II fibers to increase their mitochondrial content and use oxygen. This means that the parts of your muscles that normally drive the intense sprinting and power start to develop the ability to burn oxygen (which means lactate isn't generated) and keep functioning for long periods of time. By churning out those shorter, more intense intervals on the track or charging up hills, you're pushing the limits of your system and forcing it to adapt. Another unique benefit of intervals is that the coupling of intensity and recovery allows some of the beneficial stresses of the intense phases to be carried over to the recovery phases. This means that by using intervals, you can keep the metabolic engine "revved" for the duration of the session. If you do intervals at the pace of your maximal oxygen consumption (about the pace you'd race a mile!), your body continues consuming and burning oxygen at the high rate required by the intervals during the rest periods, but your muscles stop generating lactate. The end result is that you can spend more time stressing your metabolic capacity at that high level without all the uncomfortable, limiting waste products that would accumulate if you were running steadily the whole time.
Like the aerobic training, the faster, higher-intensity interval training is also highly beneficial for your mechanics. By running at paces much faster than you'd jog at, you drive your system to behave with more coordination. You call upon both muscle fiber types to work together, and the end result is a smoother running form. The power required to go faster will help your tendons and ligaments further adapt to the new stress, and your system as a whole will become more resilient.
To excel at the mile, you need both the aerobic and anaerobic energy systems working together in harmony at full capacity. A miler's training program gives you a healthy mix of the continuous, steady running and the short, fast intervals. The human body is incredibly responsive to new stresses, and it plateaus in its response to repetitive or redundant stresses. If you just go out and run steadily every day of the week, or just do some intense sprint intervals a few times a week, you'll make some improvements initially, but you'll certainly plateau pretty quickly. By mixing it up with steady runs, intervals down at the track, fartleks, hills, and long runs, you tap into all components of your metabolic capabilities. Those afternoon jogs and weekend long runs challenge the aerobic metabolism and wake up your Type I fibers. This will help you burn more fat to fuel the run and make your cardiovascular system more capable of delivering oxygen to the muscles. Those repeat 200s or 400s will call upon your Type II fibers and your anaerobic metabolism to keep your muscles churning when the oxygen is in short supply. This will help you burn carbohydrates more efficiently, give you a higher threshold for discomfort, and help some of those powerful Type II fibers develop the ability to burn oxygen (so you can run faster longer!).
These two types of training, steady continuous running and intense intervals, are both ways to enhance your physiology and develop as a runner. You can think of your training like building a house: the steady aerobic runs increase the size of the foundation, and the fast intervals increase the number of floors! By using both types of training, you maximize the house's capacity. In training like a miler, you tap into the all the sources of energy in the human body. You become more metabolically flexible and develop your fitness more holistically. This will not only yield performance improvements in the near term, but also set the system up for more robust long-term development. Runners of all sorts, from casual joggers to seasoned marathoners, will see improvements by incorporating both of these types of sessions into their training methods, as the surest way to build the biggest house is to use all available resources! The variety in a miler's training provides continual novel stimuli to your body, which promotes continual steady improvement in your racing, training, and general health. By exploiting both training paradigms, a miler gets to have his or her metabolic cake, and eat it too!