Boutcher 2010 published a review article of the previous two decades’ research on interval training in relation to fat loss. The dogma within the exercise world has been steady-state ever since Dr. Cooper’s 1970s research if not earlier with the advent of the modern marathon in the early days of the Olympics.
Since the early to mid-1990s, sport science has been researching the use of sprints, intervals and general high intensity activity and its effects. The most well-known study in this group was the study by Dr. Tabata whose research we discuss here.
Dr. Boutcher outlines most high intensity intermittent exercise (HIIE) programs reviewed as those 3-4 minutes of exercise, divided into 8-60secs all out sprints with recovery time between each interval three times a week for several weeks.
Acute responses to HIIE include changes in heart rate, venous blood glucose, lactate levels and metabolic reactivity. Heart rate at rest has been measured between 140bpm and 170bpm depending on the protocol used. Considering that most recreational runners will not break 145bpm as a general rule, this interval running clearly happens at a different level and using a different metabolic pathway. Catecholamine’s have been demonstrated to become elevated by this form of exercise. While epinephrine and norepinephrine rise ~10% in steady state running, they have been shown to increase 630-1450% from HIIE! This is likely a major reason why fat loss is increased with HIIE, as epinephrine helps to drive lipolysis. Growth hormone and cortisol have also shown dramatic rises from HIIE. . It has been hypothesized though not measured that excess post-exercise oxygen consumption plays a major role in driving the hormonal changes that occur.
Lactate levels have ranged from 6 to 13 mmol/L depending on the protocol used, interestingly using the 8s on-12s off protocol for 20 min generated merely 4 to 5 mmol/L. Boutcher cites Bussau et al’s study comparing one 10s sprint to 20 min of aerobics for type 1 diabetics and its effects on blood sugar. It was found that from a hormone levels perspective, 1 10s sprint was more effective than 20 min of steady state aerobics. The next time you feel you can’t make it to the gym, consider what a 10 second sprint will get you.
Immediately post-exercise, as one would expect, blood ATP-PC and glucose are low as they have been used to fuel the anaerobic activity. Aerobic and anaerobic metabolic changes and muscular changes have been documented. Chronic changes include adaptations in various proteins and enzymes which regulate cellular and mitochondrial metabolism such as citrate synthase and cytochrome oxidase. VO2max, an aerobic measure of fitness, has shown 7-13% improvement with 4-6 Wingate tests with 4 min recovery, 3 times a week after 2 weeks of training. One study using the Wingate protocol for 24 weeks showed a 46% increase in VO2max in untrained male athletes. The 8s/12s protocol over 15-20 min has demonstrated 18-24% increases. Very few programs can generate this level of aerobic improvement, never mind programs which are primarily anaerobic in nature. Anaerobic response also shows dramatic improvement from 5%-28%.
Blood insulin sensitivity has also shown dramatic improvement, 19%-58% depending on the protocol and the length of the study. For pre-diabetics in a medically supervised program, this may be an easy way to get their insulin levels under control and in short order begin regulating their blood sugar and lose fat. In fact, a study of HIIE on type 2 diabetics (stage 1) showed a 46% improvement in glucose tolerance after 2 weeks of training 3 a week.
Clearly, this is a powerful tool in a training arsenal. It can be overused, particularly in untrained and unadapted individuals but none the less, should be carefully implemented into an athlete’s regimen. If one starts incorporating some HIIE during the off-season, then doing one hard session in-season to maintain the benefits should not be too taxing on the system. As always, use your common sense.