A Sports Training Phenomenon: The Relevance and Versatility of High Intensity Interval Training (HIIT)

 It is well known in the world of sport and exercise that regular participation in endurance training causes improvements to performance in activities that rely mainly on aerobic metabolism. This is largely due to adaptations that allow for greater oxygen transportation and the subsequent utilization of the more efficient energy fuel, fat. On the other hand, high intensity anaerobic training is generally perceived to have less of an impact upon aerobic capacity and oxidative energy metabolism. However, various publications have shown that regular involvement in high intensity interval training (HIIT) for at least 6 weeks can increase VO₂ max and endurance capacity. Much of the recently published evidence also suggests that these biological adaptations associated with aerobic performance enhancement can be obtained more rapidly via HIIT. The current article will present the recent findings regarding speed endurance/high intensity interval training and discuss its relevance for aerobic performance enhancement.

High intensity interval training can be defined as repeated bouts of high intensity exercise (≥90% VO₂ max) lasting between a few seconds and a few minutes, interspersed with relatively longer periods of rest or low intensity active recovery. The length and nature of this recovery period is very much dependant on the athletes training aims.

Several studies have been carried out to investigate the effects of a HIIT regime; one remarkable finding by Burgomaster et al. (2005) was that despite a dramatic reduction in training volume, vast improvements to aerobic performance were seen when eliciting a high intensity protocol. They found subjects could maintain a fixed submaximal work rate for double the length of time (from 26 to 51 minutes, cycling at 80% of pretraining VO₂ max). These results were achieved after just six HIIT workouts, whilst the control group displayed no alteration in performance.
In another study, conducted by Gibala (2006) the experimental subjects performed a generic HIIT protocol whilst the control group carried out six continuous cycling sessions (65% VO₂ max, 90-120 minutes∙d⁻¹). The total training time completed by each group was 2.5 and 10.5 hours respectively, meaning that the HIIT training group saw a 90% reduction in training volume. Although significantly different in almost every sense of the FITT principles(frequency, intensity, time and type), the two protocols attained virtually identical physiological alterations.

Such a markedly improved aerobic performance can be accounted for by the following biological and metabolic changes:

•       Increased resting glycogen content in the skeletal muscle and liver which can be utilized as an energy fuel.
•          A greater total number of muscle glucose transporters, enabling more metabolic fuel to enter the blood and reach active muscles.
•          Increased density of muscle capillarization, providing the network to transport oxygen and nutrients such as liver glycogen to the working muscles. This also allows for the removal of the fatiguing waste product H⁺ ions, associated with lactic acid and a lowered intramuscular pH that leads to a decline in muscle contraction strength.
•          Speed endurance not only increases the presence of glycolytic enzymes involved in anaerobic metabolism, but also causes greater concentrations of beta oxidation enzymes (citrate synthase and cytochrome oxidase). These are responsible for the metabolization  of fat , thus sparing the body's limited glycogen stores and reducing lactate production.
•          There is an increase in the amount of potassium ions pumped back into the cell, limiting its accumulation in the interstitial fluid and therefore delaying progressive membrane depolarisation. This serves to maintain action potential amplitude - consequently delaying the onset of fatigue.
•          Increased  VO₂ max, enabling a greater aerobic endurance performance.
•          Finally, a greater  buffering capacity of lactate has been observed due to HIIT. More H⁺ transporters means that vast amounts of lactic acid's associated H⁺ ions are able to leave the muscle and move into the blood. Here they combine with bicarbonate to give carbonic acid. When carbonic acid dissociates its products of carbon dioxide and water can simply be exhaled, causing no negative effects to athletic performance.

These adaptations have significant implications in enhancing endurance performance. Firstly, running economy has been seen to gain considerable improvement, this means less energy is required to run at the same velocity. Furthering this point, a decreased energy requirement will allow for the already increased glycogen reserves to be maintained even longer. In addition, faster VO₂ kinetics at the onset of physical activity enables oxygen to be utilized faster for aerobic metabolism. As a result there is less break down of the body's anaerobic reserves and a reduction in metabolite (lactate) accumulation.

To conclude, it is the practical applications of HIIT that are of primary concern to elite athletes and recreational exercisers. It is known that highly endurance trained athletes find it increasingly difficult to achieve further biological adaptations through continuous endurance training alone. However, it has been observed that HIIT can improve endurance performance in already trained aerobic performers by increasing  their VO₂ max and running economy. Furthermore, implementation of this efficient training method will allow for more time to be spent on technical/tactical skills training.

On the level of recreational exercise participation and exercise prescription the most common barrier to physical activity is a lack of time, with many adults even failing to meet the minimum exercise guidelines. The innovations of HIIT could significantly contribute to combating this problem as identical adaptations from prolonged endurance training are obtained in a third of the time.

"I do it as a therapy. I do it as something to keep me alive. We all need a little discipline. Exercise is my discipline" - Jack LaLanne