Last week we discussed the basic principles of resistance training, including the pros and cons of practising weightlifting with free weights, resistance machines and body weight exercises. We can practice any one or a combination of these modalities using certain exercise protocols.
Though volume based training, which focuses on the number of repetitions (reps) of a particular exercise (set); and number of sets has traditionally been in vogue; intensity based plans, where intensity of workout instead of its volume has now become a rage.
High Intensity Interval Training (HIIT) is one such intensity based plan which we'll discuss through this article.
Resistance training works on the basic premise that the workout causes a bearable stress upon the exercised muscle groups, resulting in muscle fatigue, utilisation of muscle glycogen, and micro-trauma to the muscle fibers.
When followed by good diet and rest, the body repairs the tissues making them stronger and better able to lift a potentially heavier weight next time.
The debate has always been around whether it is better:
To lift moderately heavy weights to bring muscle fatigue in one set?;
To lift heavy weights but stop short of total muscle fatigue?; or
To perform multiple sets and reps of an exercise in achieving this goal?
HIIT vs. Low Intensity High Volume
To examine these issues, Prof Martin Gibala and his colleagues at McMaster University conducted a series of studies beginning in 2006.
They compared the efficacy:
Low Volume Sprint Interval Training (or "HIIT" group)
Low Intensity High Volume (i.e. conventional High-volume Endurance Training or "ET" group)
in improving effort tolerance and inducing positive changes in skeletal muscles.
The HIIT group performed 30-second bursts of intense cycling on a stationary bike (at 250% of their maximum rate of oxygen consumption or "VO2 max") which was repeated 3-5 times with 4 minutes rest between the bursts, while the ET group cycled at 65% of VO2 max steadily for 90-120 minutes.
Both groups performed the same workout on 3 non-consecutive days per week for 2 weeks. So the HIIT group exercised for a total of 12-18 minutes of actual training in 2 weeks (total duration 90-120 minutes including rest periods), whereas the ET group spent 9-12 hours training per week. Both groups completed an 18.6-mile cycling test both before and after the two-week training.
Surprisingly, both groups had equal improvements in exercise performance and adaptations in skeletal muscle biopsies, despite a much smaller exercise time in the HIIT group.
Given the excess time spent in running long distances slowly and the risk of musculoskeletal injury noted in a previous article, HIIT appears to be a much more effective and safer method of training.
As the heart and lungs don’t differentiate between lower or upper body exercise, nor between the stress caused by intense cycling, sprinting or lifting heavy weights, the adaptations occurring in the skeletal muscles and the benefits accrued to the cardiovascular system (heart, blood vessels and lungs) are similar.
The Physiology of HIIT
To understand how this happens, we have to briefly discuss human physiology.
When a muscle needs energy, glucose gets broken down in the presence of oxygen by a process called the Krebs cycle - producing a large amount of energy (measured as 36 ATPs, with ATP being a unit of energy). This process is called Aerobic metabolism.
The Krebs cycle can produce greater energy, but works relatively slowly.
However, Glycolysis, which is an Anaerobic (i.e. without oxygen) process of converting glucose into energy, generates only only 2 ATPs.
Interestingly, when intense effort is put in, its Glycolysis, rather than Krebs cycle, which accelerates to produce molecules called pyruvate, along with energy.
Due to limits on processing capacity of a cell, the pyruvate accumulates in the muscle, and gets converted to lactic acid, which causes the burn noted in muscles after a short period (30-60 seconds or so) of intense effort.
At this stage, the exercise is usually stopped and the lactic acid is converted back to pyruvate, which can then gradually undergo Aerobic metabolism. This is what is called the EPOC (Excess Post-exercise Oxygen Consumption) or the oxygen debt, alluded to in a previous article.
Thus recovery from a high intensity exercise stimulates the aerobic metabolism, as much or even greater than a steady-state aerobic exercise.
But there is more! If one can manage to continue exercise despite the lactic acid burn (as happens with regular training), the lactate in the muscles enters the circulation, reaches the liver, where it gets converted to pyruvate and then glucose, by a process called gluconeogenesis.
This glucose is again available for the muscles, for immediate use (if exercise is continued) or for deposition as muscle glycogen, if exercise has been stopped. This process, called the “Cori Cycle”, is responsible for the so-called “second wind” that an athlete gets if one manages to push forward despite a lactic acid burn.
Further, as one continues a strenuous activity, or during crisis situations, the body releases the emergency hormones - adrenaline and glucagon, which in turn activate a process of fat burn (by stimulating an enzyme called hormone sensitive lipase). This should put to rest the misconception that only steady-state aerobic exercise can burn fat, and not HIIT.
HIIT - A Savior For Diabetics?
Other benefits from intense exercise include the complete breakdown of muscle glycogen in the exercised muscles, which improves insulin sensitivity (as glucose has to re-enter to get converted to glycogen) and thus blood glucose levels as well.
HIIT is thus one of the most effective techniques for decreasing insulin resistance that is so common in overweight individuals and is the root cause of most cases of diabetes. Muscle strength and bulk also increase more with intense effort, due to the greater micro-trauma caused to muscle, and the greater release of anabolic hormones like testosterone and growth hormone.
Why Low Intensity Exercise May Not Be Enough?
On the other hand, low intensity, steady state exercise or moderate weight lifting (not to complete failure) for multiple sets are useful but not optimal. They do not deplete the muscles completely of glycogen and while helpful in lowering blood glucose, are not as effective in reversing insulin resistance as HIIT.
If low intensity exercise (which is tiring, nevertheless) is followed by a high calorie meal or diet, the liver and muscles rapidly get saturated with glycogen, and the excess glucose can have no metabolic fate, except be converted to fat.
This fatty acid generation leads to a rise in the potentially harmful VLDL and LDL cholesterol fractions. The inflammation generated by the high circulating levels of glucose and insulin in liver and blood vessels is patched up by LDL cholesterol, which may ultimately predispose to heart attacks.
A possible disadvantage of prolonged periods of slow jogging (i.e., Low Intensity High Volume) which are long enough to completely deplete muscle glycogen stores, is the potential breakdown of muscle needed for energy generation, a phenomenon that doesn’t occur with HIIT due its shorter duration and the release of emergency hormones like adrenaline that cause fat breakdown instead.
You might also have wondered why overweight and obese people often fail to lose weight despite caloric restriction and aerobic exercise. In insulin resistant individuals, the baseline high insulin levels prevent fat breakdown despite a calorie deficit. Thus, they face fatigue and food cravings after exercise (for carbohydrates, in particular) and fail to lose as much weight as they want to.
Further, studies show that the metabolic adaptations to aerobic exercise have peripheral effects i.e. only in the particular muscles being exercised, i.e., the training effect of regular jogging does not increase the effort tolerance for cycling equally.
Anaerobic routines such as HIIT, in contrast have a much central effect, resulting in overall tolerance and strength.
Aerobic exercise is useful in many ways for promoting health and fitness, but the benefits are predominantly owing to a training effect of the muscle groups in use.
On the other hand, anaerobic exercise like HIIT provides muscle strengthening and hypertrophy along with all the advantages of aerobic training and more: greater EPOC, greater fat loss and better restoration of insulin sensitivity, all despite spending much lesser time exercising, though with greater effort.
Next week, we shall discuss how HIIT can be incorporated into our workouts.
This is a recurring column published every Sunday under the title: A Guide to Exercise.
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