Many of the team sports that are popular in North America today involve repeated short duration bursts of high intensity exercise, each of which usually last less than 30 seconds. The intermittent bursts of exercise are usually interspersed with periods of either passive or active rest, during which the athlete is allowed to recover to some extent. The term ‘high intensity intermittent exercise’ (HIIE) has be used to characterize this type of activity.11 Common sports that involve HIIE include American football, ice hockey, basketball, volleyball, and soccer. As these sports often require repeated bouts of maximal, or near maximal exercise, success may be determined by the ability of the athlete to recover as fully as possible during each rest period. It has been suggested that development of the aerobic system my increase recovery by aiding in the replenishment of energy stores, and the removal accumulated metabolites during these rest periods.19,27 Many coaches not only believe that a high level of aerobic fitness is necessary for success in HIIE sports,1 but also stress this type of activity during training and testing. Therefore, the role of aerobic fitness in recovery and performance of HIIE needs to be evaluated in order to: (1) determine if aerobic fitness does indeed play an important role in recovery and performance during HIIE, (2) establish what minimal levels of aerobic fitness are needed for performance in HIIE sports, and (3) establish training guidelines that would allow the athlete to meet the minimum levels of aerobic fitness needed for their particular sport, while at the same time allowing for the maximization of the development of the anaerobic characteristics (ie. strength and power production) that may also be important to success in their sport.

Fatigue and Recovery during Single Bouts of Maximal Exercise

During the initial 10 seconds maximal intensity effort, the majority of the energy produced is derived from the ATP-PC system.7,12 As the duration of the maximal intensity exercise exceeds 10 seconds, and for durations lasting up to approximately 90 to 120 seconds, the energy majority of the energy is then derived from the anaerobic glycolytic system.7,12 During single bouts of short-term (<120s) maximal exercise, there are a number of factors which contribute to fatigue and limit the ability of the muscle to exercise at a given intensity level. These factors include the depletion of stores of ATP, phosphocreatine (PC) and O2-myoglobin, and accumulation of other metabolites such as lactic acid.7,20

During a 30 second sprint, intramuscular ATP stores are only reduced to approximately 70 % of pre-exercise levels, while PC stores are reduced considerably to approximately 20 % of pre-exercise levels.5,20 However, these metabolites stores can be regenerated very quickly, with approximately 70 % of ATP and PC restored within 30 seconds, and full restoration occurring between three to five minutes.18 However, numerous studies have shown that the time to full recovery is increased if blood flow to the muscle is occluded, limiting both blood flow and oxygen delivery to the muscles.12 This indicates the importance of blood flow and oxygen delivery to the working muscles during recovery of the phosphagen system from exercise.

O2-myoglobin is important for two reasons. First, O2-myoglobin acts as a store within the muscle for oxygen. This oxygen is involved functionally in the transfer of oxygen from inside the cellular membrane, to the mitochondria for utilization.12 At the onset of exercise, oxygen stored in the O2-myoglobin complex is readily given up to the mitochondria . Second, O2-myoglobin stores are also restored rapidly, will full restoration occurring in 10 to 80 seconds.9 During recovery , the availability of oxygen in the blood is greatly increased, and the myoglobin stores are recharged. Therefore, these intercellular oxygen store are important during HIIE, as they facilitate the transfer of oxygen from the blood to the mitochondria, and act as an intercellular oxygen store which can be restored very rapidly during the recovery period.

If the exercise period exceeds 10 seconds, the anaerobic glycolytic system will be used to a large extent, with lactic acid production and accumulation occurring. If lactic acid accumulates and is not removed, fatigue will occur as metabolic acidosis is induced due to increased concentrations of H+.12 Metabolic acidosis will contribute to fatigue, as it leads to the impairment of the contractile processes (cross-bridge cycling), impairment of Ca2+ release and uptake from the sarcoplasmic reticulum, and eventually limits the ability of the anaerobic glycolytic system to produce energy because of the decreased activation of glycolytic regulatory enzymes (ie. phosphorylase (PHOS) and phosphofructokinase (PFK)).20 Lactate removal and oxidation takes much longer than the restoration of the other metabolites, with a half-time to recovery of approximately 25 minutes during passive recovery, and full recovery occurring in slightly over an hour.12 During this period, the majority of the lactate is removed through oxidation, much of which occurs in the Type I muscle fibres - especially if the have been conditioned properly.12 Lactate is also oxidized in the muscle fibres of the heart, as well as several other organs.26 The removal and oxidization of lactate during recovery is dependent upon both adequate blood flow for transport of lactate from the sites of production to the sites of removal, and adequate oxygen to supply the mitochondria during the oxidation process.7 Mechanisms of Fatigue During HIIE

In sporting situations involving HIIE, a major factor in success may be the ability of the athlete to repeat a number of short duration high-intensity exercise bouts in succession, with short periods of recovery in between. In order to perform at high intensity during successive exercise bouts, the athlete must be able to restore the energy systems of the body as closely to pre-exercise homeostasis as possible during recovery periods. This will allow the athlete to ensure that subsequent bouts of exercise can be carried out at the highest intensity possible.
During the recovery intervals, if the blood and oxygen supply is poor, or if the sport that the athlete is competing in allows for very short rest periods (ie. < 30 seconds), adequate regeneration of the ATP-PC stores may not occur.30 As the successive exercise bouts continue, the ATP-PC stores will be exhausted more quickly, leading to an increased reliance on the anaerobic glycolytic system.30 The increased reliance on the anaerobic glyoclytic system, and the associated increased levels of lactate production and accumulation could lead to earlier fatigue in successive bouts of exercise.
Lactate accumulation will also be increased during HIIE due to the nature of the exercise itself. High intensity exercise will recruit more of the Type IIb (fast glycolytic) muscle fibres, which produce more lactate.26 The high intensity nature of the exercise will also cause additional adrenally modulated catecholamine production, which acts to accelerate the rate of glycogenolysis and lactate formation.

Lactate accumulation may also increase due to a mismatch between the blood lactate rate of appearance (Ra) versus the rate of disappearance (Rd). This may occur due to available mitochondria being saturated with lactate after several bouts of exercise; because of shunting of blood away from organs that uptake lactate (liver and kidneys), due to the ratio of Type II to Type I muscle fibres in individuals; and due to the capillary density of the exercising muscle (Fleck, 1991).

The Role of The Aerobic System in Recovery and Performance during HIIE

It has been suggested that a well-developed aerobic system will facilitate recovery of the athlete during the rest period of HIIE.19,27 Delivery of oxygen to the muscle tissue is necessary for the replenishment of O2-myoglobin, and the resynthesis of both ATP and PC. This will in turn decrease the reliance on anaerobic glycolysis during successive bouts of exercise. When reliance on the anaerobic glycolytic system does increase, an efficient aerobic system will also help prevent fatigue via removal and buffering of lactate from exercising muscle, and delivery of lactate to sites of aerobic oxidation thereby helping to return the muscle the normal resting pH.25 The closer that these parameters can be returned to pre-exercise levels during the recovery period, the greater the intensity and/or duration of activity that the athlete will be able to perform at in the next exercise bout.1 Adaptations due to aerobic training that may increase recovery and extend the ability of the athlete to perform high intensity work include increased blood flow to the working muscles for nutrient delivery and waste metabolite removal, and increased oxygen extraction at the muscle. Aerobic training has been shown to increase blood flow to the muscles by increasing cardiac output, increasing the capillary density within the muscle, and increasing the ability of the cardiovascular system to vasodilate the vessels that perfuse the active muscle.10, 29 Aerobic training has been shown to increase the oxygen extraction capacity of the muscle by increasing the number, size and density of the mitochondria within the muscle, increase the number and concentration of aerobic enzymes, and increased the concentration of myoglobin within the muscle cell.10,29 Furthermore, aerobic training may increase the proportion of Type I muscle fibres, thereby increasing the total ability of the body to metabolize lactate during exercise, thereby decreasing lactate accumulation.19

However, if the ATP-PC system cannot be regenerated to a sufficient level during the rest period and the reliance on anaerobic glycolotic system increases, the metabolic acidosis cause by the increased [H+] may eventually inhibit glycolysis if the accumulated lactate cannot be sufficiently removed. It has been hypothesized that the aerobic energy system may then play an important role in maintaining power production during subsequent bouts of exercise (from 10-30s) through another mechanism. Bogdanis et al,5 Gaitanos et al 15 and Aziz et al.1 have all suggested that a greater portion of ATP resynthesis during sprint/exercise periods may be directly supplied by the aerobic energy system through oxidation of intramuscular triglycerides. Although this may lead to slower performance times during the work periods, it will extend the total work performed by the athlete, albeit at a lower power level.

Literature Review

There has been a large amount of research that has examined the relationship between cardiovascular fitness and recovery from HIIE. In the majority of the studies, subjects with higher levels of cardiovascular fitness (usually measured by VO2 max) have been shown to significantly perform better, 19, 23, 30 have faster muscle PCr restoration rates,5, 23 have a higher fast component of EPOC,19, 25, 30 have a faster blood lactate clearance rate,19, 28 and faster force or power recovery during HIIE tests.14, 19, 23, 30 Likewise, performing an aerobic interval training program has also shown to increase performance in HIIE tests.14 However, it should be noted that a number of studies have found that the relationship has been very low to moderate.2, 4, 20, 25

One significant finiding that may be relevant to the training of high-level athletes was revealed in a number of studies.1, 2, 4, 25 In these four studies, homogenous groups of highly-trained athletes with similar and high VO2 max were used as the subjects. For all four studies the mean VO2 max for the athletes ranged from a minimum of 53 ml/kg/min to a mximum of 60.4 ml.kg/min. All studies revelaled that there was either a very low to low-moderate correlation between aerobic fitness and performance in HIIE. This may suggest that once an athlete has reached some base level of aerobic fitness, that further increasing aerobic fitness or conditioning may not significantly increase HIIE performance.

Problems with Applying the Findings to HIIE Sports

When applying the findings of these studies to the training and evaluation of athletes, a number of problems should be noted. In many of the studies, the work to rest ratio is not indicative of what is typically seen in the majority of HIIE sports. While the majority of the studies evaluated used fixed ratios, the reality of the competitive situation does not often allow for a fixed work to rest ratio. Furthermore, the number of repetitions performed in these tests is also not indicative of what is seen in the actual competitive situation. The maximal number of high intensity work periods in any of the study protocols was ten. This is far below the number of high intensity work periods that is typically seen in a soccer or American football game. Finally, the length of the work and recovery periods may not adequately reflect the requirements of HIIE sports. While some of the protocols used short recovery periods (30 seconds) that may be indicative of what is seen in HIIE sports, the work periods often involved 30 seconds of maximal intensity work, far above what is typically seen in HIIE sports.

The amount of work performed during the initial repetitions of RSA protocols was also considerably higher for sprint and games athletes when compared to endurance trained athletes when athletes were allowed asked to perform sprints at individual maximal intensity for a set period of time.19, 23 As addressed in individual situations throughout this review, this may have confounded the interpretation of results when evaluating PC recovery rates, and ability to sustain power and force production over subsequent bouts of exercise.
Finally, another problem noted in a number of the studies has to do with the determination if the main aerobic adaptations that increase HIIE performance are centrally or peripherally located.10, 14, 25 Although it has been suggested that the adaptations are mainly peripherally located 10, 25 this may effect the understanding of which aerobic adaptations are most important for maintaining performance and preventing fatigue accumulation during HIIE, and therefore may have implications for how these athletes should be trained.

Considerations for the Training of Athletes Involved in HIIE Sports
Although the evidence seems to suggest that a well developed aerobic system will decrease fatigue and increase performance when performing HIIE activities, two important factors should be mentioned concerning the aerobic conditioning of these athletes. First, high levels of endurance training performed concurrently with strength/speed training has been shown to attenuate strength and anaerobic power development.8, 16, 24 Therefore, although a well-conditioned aerobic system may increase recovery during HIIE, too much aerobic training may decrease anaerobic performance in other aspects of the sport that may be critical to success (ie. Rate of acceleration, maximal sprint speed, jumping ability, etc.)

Second, it has been suggested that once an athlete has reached a certain level of maximal aerobic capacity, further gains may elicit little to no additional benefit to recovery or performance in HIIE. This ‘ceiling effect’ has been demonstrated in basketball players 22, infantry soldiers 21, field hockey and soccer players 1 and ice hockey players.17 The VO2 max. values suggested in these studies ranged between 55-60 ml/kg/min, but would most likely be related to the energy demands of the particular sport. Future research should be directed towards determining if this ceiling effect phenomenon does actually exist for HIIE sports, as well as determining the minimum aerobic capacity that is necessary for optimal recovery during individual HIIE sports. This would greatly aid in setting the parameters for aerobic training, and would allow the coach to maximize gains in anaerobic measures while ensuring that a minimum level of aerobic capacity is developed in these athletes for optimal recovery during HIIE.

Conclusions

A highly trained aerobic system may increase performance in HIIE sports by increasing the rate of blood and oxygen delivery to the exercising muscle, as well as developing the peripheral components that are involved in oxygen uptake and utilization. This would lead to faster restoration of ATP-PC, and blood and muscle oxygen stores during rest periods. In turn, there would be less reliance on the anaerobic glycolytic system, and thereby decrease the amount of lactate produced. Lactate transport and oxidation would also be increased due to peripheral adaptations, leading to less lactate accumulation and fatigue during HIIE.

Therefore, it is important that athletes involved in HIIE sports should achieve some minimal level aerobic fitness. This level of aerobic fitness will related to the characteristics of the sport (work to rest ratio, main energy systems used in the sport, requirements for success in the sport, etc.). However, both coaches and athletes should be educated that excessive performance of aerobic activity will negatively impact gains in strength, power and other anaerobic measures. This will have implications for both the training and physiological testing of athletes. When testing athletes, interpretationVO2 max. values should be used with caution, as they may not accurately predict the ability of the athlete to perform well in HIIE activities, especially if the athletes being tested are a homogenous group of elite athletes with similarly high VO2 max. values. With the negative impact that high volumes of aerobic training may have on anaerobic performance in conjunction with the possibility of a ceiling effect for aerobic contribution to recovery during HIIE, coaches should also use caution when emphasizing aerobic conditioning in their training and testing protocols.

Scott Vass is currently pursuing his Masters in Human Kinetics (Coaching Science – Strength and Conditioning) at the University of British Columbia in Vancouver, Canada. Scott also works in an orthopaedic and sports rehabilitation facility, as well as acting as the asisstant strength and conditioning coordinator for Simon Fraser University.

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