High-intensity interval training (HIIT) is a popular and time-efficient training strategy to induce cardiorespiratory and metabolic adaptations which can lead to improved fitness and endurance performance in athletes (11,12). HIIT protocols typically last no more than 30 minutes in total and involve the performance of multiple periods of activities at near maximal or maximal intensities interspersed by periods of passive rest or low-intensity exercise (11,12). Depending on the format, HIIT protocols can be divided into various subcategories (12). Specifically, subcategories of HIIT include (a) multiple sets of continuous periods of work (≥2–3 minutes) at ≥ 90% of the intensity associated with the minimal velocity eliciting the maximal oxygen uptake (vV̇o2max) (HIIT with long intervals or also known as small-sided games when work specifically include game activities); (b) multiple sets of intermittent periods of work where short exercising intervals (≥15 seconds) at 100–120 %vVO2max are interspersed with recovery periods of equal of lower duration (HIIT with short intervals); (c) multiple sets of near to or all-out short efforts (>4 seconds) interspersed by recovery period longer than 20 seconds (repeated sprint training); and (d) multiple sets of slightly longer near to all-out effort (>20 seconds) interspersed with recovery periods of equal or longer than 2 minutes (sprint interval training) (12).
Each HIIT format can induce high levels of physical stress (11,12), with protocols including higher duration or intensity generally leading to higher glycogen depletion, metabolite accumulation in the muscle, and neuromuscular strain (11,12). As a consequence, previous recommendations have suggested avoiding the prescription of other high-intensity activities within 48 hours of HIIT sessions to allow athletes adequate recovery between HIIT sessions (44). Adopting these prescription guidelines, significant improvements in athletes' endurance parameters (i.e., 2.7 ± 4.6% increase in maximal oxygen uptake) have been commonly reported after a minimum period of 6-week training (48).
In recent years, there has been interest in distributing a higher number of HIIT sessions within a shorter period. Such an approach has been referred as an HIIT shock microcycle (HIITSM) (46,47), and it has been designed to induce faster development of endurance performance capability (40). Although there is an attractive rationale behind the application of an HIITSM (faster improvements in endurance than those possible throughout a traditional approach), to date, an analysis of its sustainability, physiological, and performance outcomes across different HIITSM designs and athletes' categories has not been undertaken yet, leaving the application and optimization of such training intervention more complex for fitness/strength and conditioning coaches. Hence, the purpose of this article is to review original studies and provide a summary characterization of the HIITSM approach to develop athletic endurance capacity by analyzing its sustainability, changes in performance test, endurance parameters and potential mechanisms for physiological adaptations in athletes.
Original research, peer-reviewed articles reporting physiological, perceptual, and performance adaptations or changes after an HIITSM were collated after an extensive search of the English language literature using scholarly search engines (PubMed, Scopus, and Google Scholar). The search terms used were shock microcycle, HIIT shock microcycle, block periodization, HIIT block, and HIIT training. No specific subject characteristics and durations or designs of HIITSM were used to determine inclusion or exclusion criteria. In general for inclusion, all studies were ascertained as having prescribed HIIT protocols by measuring intensity based on percentage of (a) maximal oxygen uptake (V̇o2max); (b) measured heart rate; and (c) maximal aerobic speed or surrogate speeds obtained by specific field test (i.e., 30–15 intermittent fitness test) to ensure for accurate HIIT intensity training prescriptions. Supplementary studies were subsequently located by using the aforementioned search engines to help to contextualize, support, and discuss HIITSM analysis.
This is a brief review and as such does not need to obtain ethics approval. We confirm, that the PhD program of study which this paper forms part was approved by the School of Health Sciences, and ratified by the university's Research Office.
Definition of an HIITSM
Training periodization is a planned manipulation of different training stimuli with the aim of optimizing training adaptation and performance (29). The specific training periodization strategy where training plan is divided into multiple blocks, each one providing a concentrated training and specific stimulus for developing just one or few specific aspects of performance, is better known as block periodization (8,27). An HIITSM is one of these short training blocks where a congested distribution of HIIT sessions is implemented to provide high stimuli for endurance adaptation (27,40,46). More specifically, HIIT shock microcycles reported in the literature have consisted of periods shorter than 28 days and have usually included a day to HIIT ratio equal or lower than 3:2 (i.e., ≤2 HIIT session every 3 days) (10,32,40,41,46,47,50) with studies implementing up to 11 HIIT sessions over 6 days (24). Such congested periods of HIIT have often been followed by 5–7 days of recovery where HIIT sessions are not performed (32,46,47) or significantly reduced to 1 or 2 sessions per week (4,40) to dissipate the fatigue induced by the density of HIIT sessions within a short timeframe. High-intensity interval training sessions occurring during an HIITSM are not different in mode or duration from those applied during other longer traditional HIIT periodization models (11) and can occur up to twice a day (10,46,47). An example of HIIT session distributions over an HIITSM is reported in Table 1.
Sustainability of an HIITSM in Athletes
When implementing an HIITSM into a training program, training load increases significantly, and it is therefore worthwhile to initially consider whether the training required is sustainable for the athletes without inducing a harmful level of stress. Specifically, an abrupt increase in training load can lead the athlete in a stress phase, called overreaching and requiring a few days (functional overreaching) or several days (nonfunctional overreaching) of rest before fatigue can dissipate and training adaptation occurs (22,33) Furthermore, an overreaching syndrome persisting over time along with external life stressors can also develop into the overtraining syndrome, where the cumulative physical and psychological stress seriously harm athlete's performance and health which can take up to several months to restore (22,33).
Levels of physical stress during an HIITSM have been regularly reported in most of the studies by using different wellness and questionnaire scales. For instance, 15 HIIT sessions over 14 days negatively affected the perception in readiness to train physical flexibility and energy as assessed over perceived wellness scales in junior triathletes after (47). Similar HIIT to day ratio (9 HIIT sessions over a week) induced significantly higher rate of perceived exertion during training and significantly lower perception of recovery also in junior cross-country skiers (28); 7 HIIT sessions over 4 days negatively affected the “Short Recovery and Stress Scale” with international junior tennis players (49); 11 sessions over 6 days induced large detrimental change in the “Acute Recovery and Stress Scale” and its abridged version the “Short Recovery and Stress Scale” with a group of well-trained intermittent-sport athletes (24). Further signs of acute effect of fatigue during an HIITSM have been also supported by a significant increase in delayed onset of muscle soreness; a significant increase in creatine kinase (marker of inflammation) levels compared with the values before the intervention (39,49); a decrease in repeated sprints (24), and in countermovement jump (10,49).
Although all these findings support a decrease in wellness, recovery, and performance scores (as sign of high acute fatigue) during the HIITSM period, there is also evidence of a rapid restoration in these psychophysical and performance measures after such a training block (32,50). None of the reviewed studies reported the exclusion of subjects from the postintervention test because of overstress-related causes. Hence, although acute fatigue effect during an HIITSM is expected, and functional overreaching symptoms observed (45), they do not appear to be a limiting factor for its application as they can dissipate after few days of recovery. Nonetheless, coaches and sport scientists should be encouraged to monitor athletes closely to allow proper recovery when fatigue levels exceed the safe threshold as established by the monitoring devices/scales applied. This can be further optimized by educating athletes and implementing specific strategies regarding appropriate recovery processes.
Physiological Adaptations to an HIITSM
There are 3 main physiological parameters affecting endurance performance: (a) maximal oxygen uptake (V̇o2max), (b) lactate threshold, and (c) movement economy. V̇o2max indicates the maximal amount of energy an athlete can obtain through the oxidative process per unit of time during exhaustive activities (9). The lactate threshold (LTan) represents the highest intensity sustainable before a nonlinear (sudden) increase in lactate production, which indicates an imbalance between lactate production and clearance (17). Movement economy represents the oxygen cost required to perform a specific activity (i.e., running, cycling, and skiing) at submaximal intensities and provides information about an athlete's aerobic efficiency to perform such given task (6).
Among these physiological parameters of endurance performance, V̇o2max has been the most frequently assessed after an HIITSM (4,10,23,34,40,41,43). Studies reporting this parameter have been mainly undertaken on endurance athletes (such as cross-country/alpine skiers and cyclists) (4,10,32,40,41) and have reported a significant improvement in V̇o2max when compared with baseline (39) or when compared with both baseline and control group performing regular training (therefore less overall HIIT sessions) (10,34). However, greater improvements in V̇o2max have been also reported when concentrating the same number of HIIT sessions within a shock microcycle, rather than a typical HIIT periodization sequence over multiple weeks (hence the same HIIT sessions differently distributed) (4,40,41,43).
Although most of the studies support V̇o2max improvements after an HIITSM, only 1 study reported no changes after a shock vs. traditional HIIT periodization in junior cross-country skiers (32). However, this particular study was delivered in-season; hence, it could be argued that at this stage, subjects had already reached a ceiling in adaptations (32). Furthermore, this study involved a higher concentration of HIIT sessions (9 HIIT sessions in 1 week) than most of the other effective studies (4,34,40,41,43), and subjects may have required a longer recovery period to dissipate fatigue and accomplish adaptations. In support of this notion, it has been reported that recovery periods are crucial to observe adaptation after an HIITSM with a study examining V̇o2max changes over different time points after an HIITSM revealing that peak improvements in a trained, nonathletic population can be expected after 12 days (23). In addition, the optimization of recovery length after HIITSM should also be evaluated over different athletic populations and in relationship to the number of days and sessions included over the training intervention period. Table 2 summarizes the controlled studies presented in the literature reporting V̇o2max changes as a result of HIITSM inserted into training programs.
In addition to variation in V̇o2max, changes in other physiological parameters have been suggested after an HIITSM. In particular, a consistent body of the literature has reported an increase in power output when working at fixed intensities such as those corresponding to the second ventilatory threshold (10,47), blood concentration equal to 2 (40) and 4 mM·L−1 (41). These changes can be both indicative of an increase in lactate threshold and movement economy (6,20). Nonetheless, lactate threshold is likely to have been the main adaptation and contributor to changes in power output at fixed working intensities. With the exception of Christensen et al. (13), who found movement economy improved after an HIITSM in soccer players, no other studies have observed improvements in movement economy within an intense HIIT period when directly assessing cross-country skiers or cyclist (40,41). These findings, along with evidence that a high volume of training is the main stimulus for improving movement economy (7), might suggest that only specific athletic groups who are not commonly exposed to high volume of continuous activity, such as team sport athletes, might obtain further movement economy improvements after an HIITSM.
Together, the majority of these findings support the theory that a short and intense period of HIIT is likely to induce significant physiological adaptations in parameters of endurance performance, such as V̇o2max and lactate threshold, which are otherwise not occurring with regular training or different HIIT periodization strategies over such a short period. Nonetheless, the limited number of studies warrants further research to strengthen these conclusions and investigate the effect of HIITSM on movement economy as this is still unclear.
Effect of HIITSM on Intermittent Endurance Performance
The transfer of physiological adaptation on performance is usually measured through performance tests, where athletes' score is measured by the ability to complete a competition-related tasks within the shortest period or until exhaustion. When assessing changes in performance, there are 5 studies specifically assessing intermittent running tests, which are more specific for team sport athletes (Table 3). All studies assessing the effect of HIITSM on the intermittent running performance test (i.e., repeated sprint ability [RSA], YO-YO Intermittent Recovery Test Level 2 (YYIR2), and 30–15 Intermittent Fitness Test) (18,19,38,46) have reported significant test improvements, with only 1 exception (13), who found such an HIIT cycle effective for improving RSA but no YYIR2 performance. A possible reason why this study did not find a significant improvement in YYIR2 is that it implemented the HIITSM immediately after the in-season period, with players already exhibiting high YYIR2 scores compared with those in the literature (5), suggesting that these players may have reached their peak performance already. In addition, this study (13) used methodology that included a postintervention test after 36 hours, and as discussed previously, a longer period of recovery may be necessary to accommodate adaptation and improve test scores after such an intense period of training (10 HIIT sessions over 14 days).
Improvements in intermittent endurance performance observed in most of the other studies have been shown to occur by performing ≥5 HIIT sessions per week over periods ranging from 10 to 28 days (13,18,19,46) and also when HIIT activity involved small-sided game format (38). However, it is worth noting that only 1 study included a control group performing regular training and HIIT (38); so, although the literature supports fast improvements in intermittent endurance performance after an HIITSM compared with baseline, further research is required to verify the superiority of such a training strategy approach vs. traditional HIIT periodization on intermittent running performance tests.
Effect of HIITSM on Continuous Endurance Performance
Similarly to the effect of HIITSM on intermittent running performance, most of the studies evaluating the effect of HIITSM on continuous endurance performance (running, cycling, and cross-country skiing) have reported improvements (4,14,32,40,43,47). For example, during cycling time trials (TTs) with trained cyclists, Costa et al. (14) observed a significant increase in power output at the beginning and during the final part of a 20-km event. Similarly, Wahl et al. (47) reported up to 12% improvement in a 20-minute TT on a cycle ergometer with junior triathletes. However, there are also a few other studies that did not report any change over a 600-minute TT (32) or a 20-minute all-out test in elite cross-country skiers (43). Lack of improvements in these latter studies could be attributed to 2 differences in design. First, the period when the HIITSM was undertaken (i.e., end of the cross-country ski season and early during the competitive season, respectively) (32,43) could have influenced subject adaptations to training. As previously discussed, during competitive periods, athletes might have already reached a high level of fitness and to stimulate further improvements is more challenging. Secondly, the greater number of sessions during such HIITSM but a similar postintervention recovery period compared with other effective studies (10,40,41,46) may have hindered full recovery/adaptations of athletes in these studies after training intervention.
In regards to time to exhaustion tests, Bakken (4) found 6% improvement only when a group of cross-country skiing athletes performed an HIITSM. By contrast, Breil et al. (10) did not find any significant change when assessing time limit on a cycle ergometer in alpine skiing athletes. However, it should be acknowledged that time to exhaustion tests have a higher coefficient of variation in contrast to time trials (25–5%, respectively) (15) and might not be the best and most reliable test to reflect actual performance changes. Since continuous endurance races can have different forms and duration (i.e., cycling vs. running; marathon vs. half marathon), future studies are encouraged to expand current knowledge and further support the benefits of HIITSM over different continuous type of races.
Potential Mechanisms for Physiological and Performance Improvements After an HIITSM
An increase in V̇o2max has been regularly reported in the literature after an HIITSM (Table 2). Changes in V̇o2max after traditional endurance training are generally attributed to changes in central components (maximum stroke volume) rather than peripheral changes (arteriovenous O2 difference) (9,35). However, the few studies analyzing stroke volume and cardiac output after an HIITSM have not reported any change associated with V̇o2max (4,34). Another determinant of aerobic capacity, which highly correlates to V̇o2max, is total hemoglobin mass (tHb mass) (42). Interestingly, this factor does not appear to be the main determinant responsible for increase in V̇o2max after an intense HIIT cycle since studies reported no change in tHb mass after an HIITSM regardless of improvements in V̇o2max (4,34). Hence, it can be assumed that other peripheral adaptations might play a more relevant role for improving V̇o2max after an HIITSM.
Among the other peripheral mechanisms affecting V̇o2max are capillary density and mitochondrial content (30). An increase in capillary density might require months to occur in response to training (1,26) and is therefore less likely to justify improvements after a such short training interventions. Nonetheless, there is evidence that an increase in mitochondrial content can occur quickly when increasing the duration and frequency (volume) of HIIT (21). Therefore, it could be plausible to consider that this mechanism may be the main physiological change leading to improvements in maximal aerobic capacity observed after an HIITSM, although no HIITSM study has reported muscle tissue analysis to ascertain this aspect of oxidative mechanisms. Furthermore, an increase in mitochondria content may also be responsible for improvements in other relevant parameters of endurance such as lactate threshold. Specifically, increased mitochondrial enzyme content is associated with an increase in the rate of lipid utilization and in turn a decrease in rate of glycogen depletion (16,25) which might explain the higher power output at the same working intensity after an HIITSM observed in a few studies (10,40,41,47). However, the higher power output observed at the same working intensity could also be attributed to improvement in motor unit recruitment. It has been recently reported that the performance of HIIT over a brief period can induce an increase in discharge rate of high-threshold motor units (31) therefore improving the efficiency of movement.
Nonetheless, although all the aforementioned potential mechanisms for improvements are related to the HIIT training stimulus itself, another separate mechanism for improvements observed after an HIITSM may be attributed to the tapering period allowed before postintervention tests. In fact, although regular training can chronically lower glycogen content, the period of recovery generally allowed after the HIITSM (4–7 days) may lead to taper-mediated muscle glycogen replenishment (36) and justify both improvement in time trial test and V̇o2max (2). Moreover, such a tapering period might also contribute to improved fast-twitch fiber function (36) and in turn increases in power production at the same effort intensity (as observed after the HIITSM).
In conclusion, mitochondria adaptation and improved motor unit recruitment are plausible mechanisms for physiological adaptation and in turn performance improvements observed after an HIITSM. However, the design of the HIITSM, usually allowing longer tapering periods compared with control groups (when having one), does not make it possible to exclude that improvements observed after such training intervention are also caused by pretest tapering effect. Research investigating changes in performance in groups having similar tapering periods regardless of the HIIT periodization approach is recommended.
Advantages of HIITSM in Training Plans
Although the mechanism for HIITSM-induced improvements still needs to be ascertained, the current review suggests HIITSM is an equal or even superior training approach to develop athletes' endurance performance. In addition to its effectiveness, there are also a number of practical advantages which make HIITSM a more appealing HIIT periodization for specific group of athletes than more traditional methods. First, by developing fitness levels quickly, an HIITSM delivered early in the preseason could allow athletes with a short preparation period (i.e., team sport players) to sustain larger part this phase at an optimal fitness. This would be beneficial as athletes with higher levels of fitness can recover faster after sessions and hence can sustain a greater workload during the crucial preseason phase and have more time to train for technical and tactical aspects of the game as getting closer to the competition period. Second, the short duration of an HIITSM intervention enables the development of endurance separately from strength and power, hence avoiding the so-called interference effect when these different forms of training are undertaken concurrently (3). Specifically, the parallel development of strength and endurance (required in most sports) has been often argued to lead to a biological interference effect which reduces the magnitude of physiological adaptations which would be expected by training these physical components separately (3,37).
HIIT Protocol Design During an HIITSM
In the reviewed literature, HIIT protocols applied during an HIITSM have varied from HIIT with long intervals (such as for instance 5 sets of 6 minutes running at intensities corresponding to ∼90% vV̇o2max with 2.5/3 minutes of recovery) (41) to sprint interval training (for instance 10 sets of 3 repeated maximal-effort sprints of 15, 30, and 45 seconds, with an effort to recovery duration ratio of 1:5) (14). In addition, within the same training period, studies have commonly alternated HIIT formats belonging to different subcategories (i.e., HIIT with short intervals and repeated sprints) (13,46) or included HIIT protocols requiring different mode of work, such as running, cycling, swimming, or game-play, according to the athletes specific sport (47,50).
When designing specific HIIT protocols, there are several variables that can be manipulated including the intensity and duration of both working and recovery periods, the intervals and recovery working mode, and set number of intervals and recovery periods (12). There are no specific studies reporting strategies to maximize HIIT protocol design and selection for an HIITSM, with the exception of one, who reported benefits of implementing HIIT protocols with passive recovery rather than with active recovery (47). This is possibly because the passive recovery can slow down lactate clearance; lactate can play a key role as a signaling molecule to stimulate physiological adaptations such as mitochondrial biogenesis (47), which has been previously discussed as the potential main physiological adaptation occurring after an HIITSM. In the absence of further knowledge, traditional guidelines (11,12) have been commonly applied in studies involving HIITSM (4,10,13,18,19,23,34,40,41,43,46) and should be considered to maximize HIIT effect. To improve aerobic capacity, these mainly suggest to apply HIIT protocols that allow athletes to increase exercising time at V̇o2max (T at V̇o2max) (11). Usually, 7-minute T at V̇o2max is a target time for team sport athletes during HIIT training session, whereas long-distance endurance athletes might aim to spend more than 10 minutes at T at V̇o2max (11). Basic guidelines for structuring different formats of HIIT sessions have been provided in Figure 1.
In addition to ensure adequate metabolic and cardiopulmonary stimuli, when planning HIIT protocols over an HIITSM, it is important to consider secondary HIIT-related training stimuli which might affect the neuromuscular/neuromechanical system and increase risk of overstress and injuries during such a period of limited recovery. For instance, protocols involving all-out training intensity might impose a higher muscular stress and increase risk of injuries such as ankle and knee sprains. Furthermore, HIIT protocols involving running activities mode rather than cycling or swimming might be more traumatic for joints, in particular over harder surfaces or when involving also downhill running. Finally, HIIT with long continuous working intervals might expose athletes to higher overuse stress and higher risk of tendon injury than with short intervals (12). Tables 4 and 5 provide practical examples for HIIT protocols design and planning over a HIITSM. For a deeper insight into the various stimuli induced by different HIIT formats, the review articles from Buchheit and Laursen (11,12) provide a good summary. Future research should address how to optimize different HIIT protocols, specifically for an HIITSM, to increase the load of HIIT sustainable by an athlete while providing the highest and most efficient adaptationstimuli.
HIITSM represents an appealing strategy for athletes who need to develop endurance quickly and possibly separately from other different physical qualities such a strength. Most studies presented in the literature report sustainability and effectiveness of such an approach, in particular in developing physiological parameters such as V̇o2max and lactate threshold and improving intermittent and time trial performance. Nonetheless, further research should compare the effect of HIITSM vs. traditional HIIT training approaches on intermittent endurance performance and over multiple endurance specific test. In addition, future studies should clarify the effect of HIITSM on movement economy over various categories of athletes and provide stronger support of mechanisms for improvements after such short training periods. Finally, further research should aim to provide evidence-based guidelines for specifically optimizing HIIT protocol design and selection in relationship to athletes' different background.
HIITSM appears as a viable strategy to quickly improve endurance parameters and performance in athletes. From the current review, to optimize the HIITSM design and effectiveness, strength and conditioning coaches should consider to implement at least 2 HIIT sessions every 3 days for periods ranging from 7 to 21 days. Specifically, strength and conditioning coaches planning HIITSM for team sport athletes should expect better improvement with HIITSM of 12 (±2) days, followed by approximately 6 days of recovery. When training well-trained endurance athletes, who are expected to be able to sustain higher training workload, strength and conditioning coaches might also consider to deliver more intense HIITSM (5 HIIT sessions every 4 days for periods even shorter than 10 days), as similar protocol has been also reported as effective/sustainable for this latter athlete category and improvement could be observed after shorter recovery periods of 3–5. Similarly, volume and intensity of single HIIT protocols over an HIITSM should be better tailored on athlete background and level, with long distance and well-trained endurance athletes likely able to sustain more demanding protocols than team sport or less-trained athletes.
Strength and conditioning coaches should also consider the manipulation of HIIT protocols according to their secondary and complementary neuromuscular and neuromechanical stress (i.e., join overuse, specific muscles damage, etc) to limit excessive stimuli summation and potentially reduce injury risk. Finally, regardless of the athlete background, strength and conditioning coaches should carefully monitor athletes' psychophysical status over this period where training load/stress is substantially increased. Although athletes should expect overreaching symptoms, coaches have to ensure the accumulated fatigue level dissipates during the recovery period of HIITSM to ensure athletes' optimal performance and health.
This research received no external funding. F. Dolci is supported by a Vice-chancellor's International Fee Remission Research Scholarship from the University of Notre Dame, Australia, and supported by a Research Training Program Stipend Scholarship issued by the Australian Government. N. H. Hart is supported by a Cancer Council of Western Australia Research Fellowship. The authors declare they have no competing interests. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association (NSCA).
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