How does high-intensity intermittent training affect recreational endurance runners?Acute and chronic adaptations:A systematic review

Felipe Grcí-Pinillos,Víctor M.Soto-Hermoso,Pedro A.Ltorre-Román

aDepartment of Corporal Expression,University of Jaen,Jaen 23071,Spain

bDepartment of Sports Sciences,University of Granada,Granada 18071,Spain

Review

How does high-intensity intermittent training affect recreational endurance runners?Acute and chronic adaptations:A systematic review

Felipe García-Pinillosa,*,Víctor M.Soto-Hermosob,Pedro A.Latorre-Romána

aDepartment of Corporal Expression,University of Jaen,Jaen 23071,Spain

bDepartment of Sports Sciences,University of Granada,Granada 18071,Spain

Objective:This systematic review aimed to critically analyze the literature to determine how high-intensity intermittent training(HIIT)affects recreational endurance runners in the short-and long-term.

Methods:Electronic databases were searched for literature dating from January 2000 to October 2015.The search was conducted using the key words“high-intensity intermittent training”or“high-intensity interval exercise”or“interval running”or“sprint interval training”and“endurance runners”or“l(fā)ong distance runners”.A systematic approach was used to evaluate the 783 articles identifie for initial review.Studies were included if they investigated HIIT in recreational endurance runners.The methodological quality of the studies was evaluated using the Physiotherapy Evidence Database(PEDro)scale(for intervention studies)and the modifie Downs and Black Quality Index(for cross-sectional studies).

Results:Twenty-three studies met the inclusionary criteria for review.The results are presented in 2 parts:cross-sectional(n=15)and intervention studies(n=8).In the 15 cross-sectional studies selected,endurance runners performed at least 1 HIIT protocol,and the acute impact on physiological,neuromuscular,metabolic and/or biomechanical variables was assessed.Intervention studies lasted a minimum of 4 weeks,with 10 weeks being the longest intervention period,and included 2 to 4 HIIT sessions per week.Most of these studies combined HIIT sessions with continuous run(CR)sessions;2 studies’subjects performed HIIT exclusively.

Conclusion:HIIT-based running plans(2 to 3 HIIT sessions per week,combining HIIT and CR runs)show athletic performance improvements in endurance runners by improving maximal oxygen uptake and running economy along with muscular and metabolic adaptations.To maximize the adaptations to training,both HIIT and CR must be part of training programs for endurance runners.

?2017 Production and hosting by Elsevier B.V.on behalf of Shanghai University of Sport.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Endurance;High-intensity training;Intermittent exercises;Interval running;Long-distance runners;Running

1.Introduction

Running has no age or gender restrictions,and it does not require expensive equipment or workout facilities.These are just some of the reasons that make running an increasingly popular sport.As the popularity of races such as the New York, London,and Madrid marathons shows,there is a growing number of recreational runners becoming involved in competition.Runners essentially pursue 2 objectives:(1)to improve their athletic performances and(2)to be healthy enough to keep training and achieving their aims.

With respect to the firs objective,recent reviews have highlighted the potential of varying quantities of both high-intensity intermittent training(HIIT)and continuous high-volume,lowintensity training on performance in athletes.1–3Although there is no doubt that both types of training can effectively improve cardiac and skeletal muscle metabolic function4,5and that a dose of both types of training is an important constituent of an athlete’s training program,endurance runners often think“more is better”and so accumulate great volumes of running kilometers,6spending most of their time training at low or moderate intensities.6

A growing body of literature highlights the role of mean training intensity over a season in optimizing athletic performance.1,4,5,7,8A clear example for endurance runners was reported by Billat et al.,9who showed that male Kenyan runnerstraining at higher speeds had a significanty better 10 km performance than Kenyan athletes training at lower speeds,despite the elite status of both groups.In this context,as a type of training that results in athletes running faster,previous studies have remarked on the importance of HIIT1–5,7,8,10–16for maximizing performance in endurance athletes.Although there is no universal definition HIIT generally refers to repeated short to long bouts of high-intensity exercise—performed at close to 100%maximal oxygen uptake(VO2max)—interspersed with recovery periods,and it is considered one of the most effective forms of exercise for improving the physical performance of athletes.1,4,5,7,8

Compared with lower-intensity cyclic workloads,intensive running requires activation of a larger motor unit,with increased recruitment of fast oxidative and glycolytic muscle fiber and an increase in the intensity of chemical processes in the muscle.17,18Additionally,increases in running speed lead to greater levels of neuromuscular engagement(mainly in the hamstring muscles).19Likewise,some differences have been observed between HIIT protocols(with different durations of work and relief intervals)in both physiological and neuromuscular impact.20–26Coaches must themselves decide how to manage HIIT inclusion in running plans for endurance athletes, so knowledge about the acute changes induced by HIIT protocols and the long-term adaptations induced by HIIT-based interventions in endurance runners plays a key role in the training prescription.

Because there is strong evidence that a greater training distance per week is a risk factor for lower extremity running injuries,27HIIT also seems to be an“interesting option”for avoiding injuries(regarding the second aim mentioned at the beginning of this section,“to be healthy enough to keep training and achieving their aims”).The incidence of running-related injuries on an annual basis is high,occurring in 40%to 50%of runners.28Even though it is widely accepted that injuries in endurance runners are multifactorial,it is also well known that running-related injuries are often attributable to training errors.29Because of a lack of studies evaluating injury occurrence,the effects of more strenuous runs on markers related to risk of injury are still unknown,so this review mainly focuses on the effects of HIIT on endurance performance.

To the best knowledge of the authors,14 reviews have so far been written about HIIT,1–5,7,8,10–16of which only 212,13were systematically performed by and included information about literature search strategies.Five of these 14 studies focused on sprint interval training(SIT),with work periods at maximal intensities,10–14whereas the other 9 considered different HIIT regimens at submaximal intensities.As for the type of population,3 studies focused on active healthy people,12,13,15whereas the other 11 related to trained athletes.Among them,only Billat7,8focused on endurance runners,with 2 works published in 2001.Therefore,a systematic review that summarizes find ings and new evidence about how HIIT affects recreational endurance runners from a multidisciplinary perspective(physiological,neuromuscular,and biomechanical)in the short and long term is needed,and this is the main purpose of the current work.

2.Methods

2.1.Search strateg y

Electronic databases,including PubMed,ScienceDirect, Web of Science,and SPORTDiscuss,were searched for literature dating from January 2000 to October 2015.The keywords used were“high-intensity intermittent training”or“highintensity interval exercise”or“interval running”or“sprint interval training”and“endurance runners”or“l(fā)ong distance runners”.The search was limited based on text availability (full-text available),publication date(from January 2000 to October 2015),species(humans),language(English),and age (≥18 years).Duplicates between searches were removed. Results of the search procedures are summarized in Fig.1.

2.2.Selection criteria

Fig.1.Flowchart illustrating the different phases of the search and selection of the studies.

Studies were included in the review if they met the following criteria:(1)published in peer-reviewed journals;(2)included participants 18 years or older;(3)involved recreational endurance runners;and(4)used run-based testing sessions and,in the case of intervention studies,run-based training programs. Studies were excluded if they(1)did not meet the minimum requirements of an experimental study design(e.g.,case reports),(2)did not meet the minimum requirements regardingtraining design(e.g.,lack of information on volume,frequency, and/or intensity of training),(3)were not written in English,or (4)involved untrained subjects,team sport athletes,or nonendurance runners.Additionally,review articles were not included in this systematic review.Based on the inclusion and exclusion criteria,2 independent reviewers(FGP and PALR) screened the citations of potentially relevant publications.If the citation showed any potential relevance,it was screened at the abstract level.When abstracts indicated potential inclusion, full-text articles were reviewed.A third-party consensus meeting was held with a third author(VMSH)if the 2 reviewers were not able to reach agreement on inclusion of an article.

2.3.Quality assessment

For cross-sectional studies(those focused on examining the acute effects of HIIT protocols on physiological,metabolic, neuromuscular,and biomechanics measurements),quality was assessed using the modifie version of the Quality Index developed by Downs and Black.30The original scale was reported to have good test–retest(r=0.88)and inter-rater(r=0.75)reliability and high internal consistency (Kuder–Richardson Formula 20(KR-20)=0.89).The modifie version of the Downs and Black Quality Index is scored from 1 to 14,with higher scores indicating higher-quality studies.

For intervention studies(those focused on the impact of HIIT-based running programs on physiological,metabolic,neuromuscular,and biomechanics measurements),methodological quality was assessed using the Physiotherapy Evidence Database(PEDro)scale,31an 11-item scale that rates randomized controlled trials from 0 to 10,with 6 representing the cutoff score for high-quality studies.One question was used to establish external validity and was not included in the score.Only studies with PEDro scores of 6 or higher were considered for the systematic review.31Maher et al.31demonstrated fair-togood inter-rater reliability with an intraclass correlation coefficien of 0.68 when using consensus ratings generated by 2 or 3 raters.Eight studies met the inclusion criteria.32–39Consensus was achieved on scores given to the 8 articles.

For both cross-sectional and intervention studies,2 independentreviewers(FGPand PALR)performed quality assessmentsof the included studies,and disagreements were resolved through a consensus meeting or a rating by a third assessor(VMSH).

3.Results

The results for cross-sectional and longitudinal studies are presented separately.Table 1(cross-sectional studies,n=15) and Table 2(intervention studies,n=8)summarize the essential parameters of the selected studies.

3.1.Cross-sectional studies

Results from the Downs and Black scale are shown in Table 3.Scores for the Downs and Black scale ranged from 9 to 12 of a possible 14.Of particular note was that no study included a sample size representative of the entire population (Item 12)or considered confounding factors(Item 25).

As for the cross-sectional studies selected in Table 1 (n=15),some focused on describing the response to a specifi HIIT running protocol,40–42whereas others made a comparison between the responses to HIIT and a continuous run(CR),20,43,44or between different HIIT running protocols.21–26,45–47In the study by García-Pinillos et al.,47participants performed typical running workouts,varying in intensity(in terms of average running pace),duration of work,and relief intervals but with similar density and total distance(4 km).Similarly,Kaikkonen et al.21utilized running protocols with the same volume(3 km) but different intensity(85%–105%velocity associated to VO2max(vVO2max))and different durations of work and rest periods.On the other hand,Seiler and Hetlelid22and Collins et al.45focused on the manipulation of resting time but maintained work intervals,with workouts performed at a selfselected pace,whereas Millet et al.,46Wallner et al.,25and Billat et al.24maintained constant work and rest intervals but modifie the intensity(30–30 s during Tlim,10–20 s during 30 min,and 15–15 s up to exhaustion,respectively).Finally,Vuorimaa et al.23and Seiler and Sjursen26compared HIIT protocols withidentical volume and work–rest ratios but differing work and rest intervals(at avVO2maxand self-selected pace,respectively).

Table 1 Studies(n=15)examining the acute effects of HIIT on physiological,metabolic,neuromuscular,and biomechanics measurements in recreationally trained endurance runners(cross-sectional studies).

Table 1(continued)

Table 1(continued)

Table 2 Studies(n=8)examining the impact of HIIT-based running programs on physiological,metabolic,neuromuscular,and biomechanics measurements in recreationally trained endurance runners(intervention studies).

Table 2(continued)

Table 3 Modifie Downs and Black scale.39

Most studies used heart rate(HR)and blood lactate accumulation(BLa)to control the exhaustion level reached and to monitor the physiological and metabolic response to HIITs, whereas some of them also included hormone response,40energy expenditure,43lipids response,43gas exchange analysis,20–26,43–46and running economy(RE).45Biomechanical variables were controlled in some of the aforementioned works,23,45whereas the impact of HIIT protocols at a neuromuscular level was assessed in 4 studies.23,41,42,47

3.2.Intervention studies

PEDro scores for the 8 selected articles ranged from 6 to 7 out of a maximum of 11(Table 4).One article48was excluded because of the score obtained.Concealment of allocation is not entirely relevant in studies of this nature;given the nature of endurance training and the sample selection methods used,it is difficul for researchers to keep themselves and participants unaware of the treatment and groups involved.Blinding of subjects and therapists(i.e.,trainers)was also not applicable in this case.

From the 8 articles included in Table 2,7 used a highvolume,low-moderate intensity continuous training program for the control group.32–35,37–39Likewise,3 studies included 2 HIIT-based intervention groups,36–38with one of them36not including a control group.All these studies lasted a minimum of 4 weeks,with 10 weeks being the longest intervention period,37and included up to 2,35–383,32,34,39or 433HIIT sessions per week.Most of these studies combined HIIT sessionswith CR sessions for intervention groups,with only 2 studies exclusively performing HIIT32,34(In both studies this included 3 sessions per week).To check the effectiveness of the training programs,all these studies included gas exchange analysis during an incremental running test.Likewise,HR and BLa were used to control possible changes in the acute response to running protocols.Moreover,among the outcome measures,1 study included some indexes of oxidative stress,32whereas others included muscle proteins and enzymes33or parameters related to muscle morphology.35Blood analysis,in addition to BLa,was performed in 4 of these studies,33–35,39whereas all studies assessed the athletic performance of participants.

Table 4 Physiotherapy evidence database scale(PEDro).31

4.Discussion

The purpose of this systematic review was to critically analyze the literature to determine how HIIT affects recreational endurance runners from a multidisciplinary perspective (physiological,neuromuscular,and biomechanical)in the short and long term.The main finding from the cross-sectional studies included in this review are(1)at a neuromuscular level, trained endurance runners are able to maintain an adequate muscular performance after a HIIT workout,whereas highintensity CR impairs muscular performance;(2)at a physiological level,the main difference between CR and HIIT is the energetic metabolic pathway that is activated(there is a greater activation of anaerobic lactic metabolism during HIIT);and(3) at a biomechanical level,HIIT sessions including runs for 1–2 min and performed at intensity close to VO2maxdo not consistently perturb the running kinematics of trained male runners.On the other hand,the major outcomes from intervention studies included in this review are as follows:(1)HIIT-based training programs are effective in improving athletic performance in recreational endurance runners;(2)exercise bouts at an intensity close to or above the intensity corresponding to VO2maxappear to be more effective in improving performance and VO2maxcompared with moderate-intensity exercise training;(3)HIIT-based running plans appear to be effective in improving RE in trained endurance runners;and(4)HIIT causes an increased oxidative capacity of a greater number of muscle fiber and a reduced plasma K+concentration,which contributes to the maintenance of muscle function during intense exercise and delays the appearance of fatigue.However, caution should be exercised when interpreting these findings owing to the heterogeneity that exists among study protocols.In the next section,acute responses to HIIT(including crosssectional studies)and long-term adaptations to HIIT interventions(including HIIT-based training programs)are discussed separately.

4.1.Acute responses to HIIT-based running protocols

Many variables,at least 9,can be manipulated to prescribe different HIIT sessions,and,among them,the intensity and duration of work and relief intervals are the key influencin factors.3–5Then the number of intervals and the number of series and between-series recovery durations and intensities determine the total work performed.From the analysis of cross-sectional studies included in this review,the authors state that the manipulation of each variable in isolation likely has a direct impact on metabolic,cardiopulmonary,and/or neuromuscular responses.When more than 1 variable is manipulated simultaneously,responses are more difficul to predict because the factors are inter-related,making it unclear which combination of work-interval duration and intensity,if any,is most effective in allowing an individual to spend prolonged time atvVO2maxwhile “controlling”for the level of anaerobic engagement7and/or neuromuscular load.8

4.1.1.Acute neuromuscular changes after HIIT-based running exercises

The available evidence about neuromuscular engagement after run-based HIIT is limited.In the current review,4 of the revised manuscripts23,41,42,47examined the neuromuscular response to a HIIT workout in recreational endurance runners, and all of them did this through indirect measures related to muscular performance(i.e.,jumping,balancing,and gripstrength testing).Whereas Latorre-Román et al.41and García-Pinillos et al.42examined the impact induced by a single HIIT protocol,García-Pinillos et al.47and Vuorimaa et al.23compared the changes induced by different HIIT workouts,but none of these studies made a comparison with a CR protocol.Despite differences in the running protocols,all were performed at a velocity close tovVO2max,accumulated longer work periods than 10 min at the aforementioned velocity,and,consequently, led to high levels of exhaustion in terms of BLa,rate of perceived exertion,and mean and peak HR.

In general,all these studies agree on the lack of impairment in muscular performance parameters for trained endurance runners performing a HIIT workout.Some of these studies41,42,47even discussed the presence of the postactivation potentiation phenomenon,whereby there is a significan improvement in vertical jump performance after running.It is known that endurance training causes,on the one hand,a greater amount of phosphorylation of regulatory myosin light chains in slow fiber and,on the other hand,a greater resistance to fatigue,which allows for the prevalence of potentiation and may explain the postactivation potentiation presence in endurance athletes.49Therefore,the ability to sustain adequate muscular performance and to tolerate fatigue during HIIT seems to be typical for endurance runners.

Nevertheless,none of these studies included a comparison with CR,so the effects of CR at a neuromuscular level remain unknown.Contradictory results can be found in recent literature;although some previous works have reported 8%–16%reductions in jumping test performance(drop jump and repeated jump tests)after a marathon50and a negative influ ence of intensive aerobic running(6 km at velocity related to lactate threshold)on some muscle contractile characteristics (i.e.,an impaired excitation–contraction coupling),a previous work by Vuorimaa et al.51investigated acute changes in muscle activation and muscular power performance after 40 min of CR at an intensity of 80%vVO2maxin elite longdistance runners and showed an enhanced jumping performance postexercise.

4.1.2.Acute effect of HIIT-based running protocols on physiological parameters

Compared with CR,there is no doubt that differences in the impact of HIIT-based runs exist at a physiological level.20,40,43,44A different hormone response,in terms of salivary cortisol and testosterone concentrations postexercise,was found after CR (30 min at lactate threshold intensity)and HIIT(6×3.5 min at 90%VO2max),with increased concentrations after HIIT compared with CR.40O’Brien et al.20found that despite total work durations of CR and HIIT protocols being similar(～20 min), HIITs with intervals decreasing from 100%to 50%vVO2maxresulted in greater mean average VO2than CR,with CR runners spending 1–7 min above 90%VO2maxand no participant exceeding 90%VO2max.Similar results were found during runs up to exhaustion44(both CR and HIIT,with work periods performed ～90%–95%vVO2max),with higher VO2maxvalues reached during HIIT and longer times above 90%VO2max,so that HIIT was more effective than CR in stimulating aerobic metabolism,with a longer time to exhaustion,a longer time atvVO2max,and higher VO2maxwith lower BLa.Besides supporting these find ings,Hernández-Torres et al.43added that energy expenditure was higher during HIIT(based on higher VO2values)and reported different effects on the lipids response;both HIIT and CR increased total cholesterol,where high-density lipoprotein cholesterol increased with HIIT and low-density lipoprotein cholesterol increased with CR.Taken together,the intermittent profil of HIIT workouts allows a high stimulation of aerobic metabolism(even greater than in CR)as well as a greater activation of anaerobic lactic metabolism.Thus,the main difference between CR and HIIT is the energetic metabolic pathway that is activated.

Different physiological responses to CR and HIIT might be expected,but what about between different HIIT protocols? Whereas some studies found differences in the physiological response to the compared protocols,20–26others did not.45,47Differences in protocols make comparisons difficult It seems clear that during short HIIT-based protocols with fi ed durations of work and relief intervals(30–30 s;4615–15 s;2410–20 s25),an increased intensity in terms of running pace elicits greater VO2max,BLa,peak HR,and rate of perceived exertion and a longer time above 90%vVO2max.But what happens during longer intervals?Some studies have compared the response to different HIIT protocols.By using short HIITs(100 m runs at ～130%vVO2max),a study by García-Pinillos et al.47reported a physiological impact similar in terms of BLa and HR response to that of a longer HIIT(based on 400 m runs at～105%vVO2max).However,when doubling the duration of work and relief intervals(from 1–2 min)but maintaining running intensity(atvVO2max)and the work–rest ratio(1:1),the physiological demands changed significanty with increased aerobic energy release,BLa,and VO2max.20,23In a similar study performed at a self-selected pace,Seiler and a Sjursen26concluded that a higher number of shorter runs increases VO2maxduring recovery and decreases it during exercise,but protocols with intervals lasting 2–6 min showed a similar VO2kinetic.Additionally, duration and intensity of relief intervals during HIIT workouts are influencin factors.In Seiler and Hetlelid’s study,22longer work bouts(lasting 4 min)were undertaken,and changes in recovery periods(1,2,or 4 min)induced a 2%increase in average work intensity with no differences in VO2.Likewise, RE impairment with changes in substrate utilization—an increased dependency on fat oxidation—has been reported after HIIT(based on 400 m runs)and independent of recovery time (1,2,or 30 min45).Taken together,HIIT protocols involving short work periods(<1 min)and work–rest ratios approximately 1:1–1:2 and that are performed close to maximum intensities(with indications such as“complete the protocol as fast as you can”)enable athletes,compared with longer HIIT or CR protocols,to train at an increased running pace(widely abovevVO2max)and to elicit similar,or even greater,mean average VO2.

4.1.3.Acute fatigue-induced changes in biomechanics of running during HIIT

The effect of exertion on running kinematics has been extensively studied.45,52–59However,most of these studies were performed under laboratory conditions and with athletes performing prolonged treadmill runs53,54,60or running-induced fatigue protocol on treadmills.55,56,61The generalization of results from studies that analyze running on a treadmill may become controversial if treadmill and overground running biomechanics are eventually not proven to be equivalent.62,63

The evidence about changes induced by HIIT running protocols is quite limited.From all the studies included in this review,only 2 studies23,45assessed the HIIT-induced changes to the biomechanics of running.Both agreed that HIIT sessions including runs for 1–2 min performed at intensity close to VO2maxdid not consistently perturb the running kinematics of trained male runners.

In turn,after CR protocols,some studies found fatigueinduced changes during running at kinematic level,for example,increased trunk inclination peak angle,59decreased knee fl xion angle at foot strike,54increased step length with a corresponding decrease in cadence,53and changes in foot strike pattern.57,58Thus,based on the biomechanical response to CR and HIIT protocols,and being especially cautious because of the wide variety of running protocols used,the authors suggest that CR causes greater impairments to running kinematics than HIIT protocols,including runs for 1–2 min and performed at intensity close to VO2max.

Cross-sectional studies have limitations because the outcomes from correlative analyses do not allow the identificatio of a cause-and-effect relationship.Accordingly,intervention studies have to be conducted to detect cause-and-effect relationships.The subsequent section will discuss intervention studies that examined the effects of HIIT-based running programs on parameters related to endurance performance(neuromuscular,physiological,and biomechanical parameters).

4.2.Long-term adaptations to HIIT-based running programmes

In addition to the elevated number of variables that can influenc the acute effect of every single HIIT session(seeearlier),determining the effectiveness of an intervention requires parameters such as duration(weeks or months),frequency(sessions per week),methodology(type of workouts), and periodization(progress of the training load)to be taken into consideration.Additionally,as we mentioned earlier,when coaches prescribe training programs,they essentially pursue 2 objectives:(1)to improve athletic performance and(2)to avoid injuries,so these elements will be covered in this section.

4.2.1.Changes in athletic performance after HIIT interventions

Despite differences in training programs conducted by these studies,32–39all agree that athletic performance improved after HIIT intervention.Esfarjani and Laursen37underwent the longest intervention included in this review(10 weeks)by combining CR with HIIT(at 100%vVO2max,G1)or SIT(30 s runs at 130%vVO2max,G2)in 4 sessions per week;performance in a 3 km time trial increased by 7.3%and 3.4%(G1and G2,respectively).After 9 weeks of combining low-moderate intensity CR with HIIT and SIT sessions in a total of 6 sessions per week, Bangsbo et al.33found that performance in 3 km and 10 km time trials increased(3.3%and 3.1%,respectively).Even during shorter interventions,36,38the combination of habitual CR sessions with 2 HIIT sessions per week over 4 weeks has shown improvements in 1.5 km(2.0%),3 km(1.1%–2.7%), and 5 km time trials(1.5%–2.3%).All these studies implemented traditional endurance training sessions with HIIT-based workouts,but other authors went further and prescribed running plans exclusively using HIITs.Gunnarsson and Bangsbo34replaced the regular endurance-training program(high-volume and low-intensity)with a HIIT-based intervention(10–20–30 training concept)3 times per week and reported 6%and 4% improvements in 1.5 km and 5 km time trials,respectively,after 7 weeks of intervention.Based on these results,the presence of at least 2 sessions of HIIT workouts in a running plan allows trained endurance runners to improve their athletic performance.It is also important to examine the duration of work intervals during HIIT.Some of these studies included SIT(all-out efforts lasting 20–30 s with long resting periods of 3–5 min),33,37others aerobic HIIT with long work intervals(2–4 min at intensity of≤100%vVO2max),33,36–38and others32,34,35,39HIIT with short work intervals(lasting 20–60 s) at intensities>vVO2max.Based on these findings the authors suggest that HIIT and SIT must be a part of running plans for endurance athletes,but training periodization should take the progressive overload principle into consideration.For example, during a traditional periodization(increasing intensities and decreasing volumes),HIIT should move from long runs to shorter and faster runs,whereas SIT should be progressively included from short sprints to 25–30 s all-out efforts.

Despite the suggested association between increased running speed and running injury,52,64none of these studies has directly measured or monitored injury risk factors during HIIT intervention.Only Smith et al.38monitored subjective ratings of sleep,fatigue,stress,and muscle soreness,with no changes reported during the 4-week intervention.Therefore,it seems that consensus exists about the benefit of HIIT interventions for endurance performance,even though more longitudinal studies covering the effects of HIIT-based training programs on injury risk factors for endurance runners are needed.Moreover, why does endurance performance improve when running intensities during workouts are increased?To answer this question, long-term neuromuscular and physiological adaptations to HIIT interventions are examined next.

4.2.2.Changes in gas exchange measurements after HIIT-based running plans

Related to VO2maxis the concept of RE,the energetic cost of running at a given speed.65Most of these studies33,35,36,38considered RE to be an influencin factor in endurance performance and hypothesized that including repeated bouts of faster runs(HIIT)in their running plans would lead to improvements in RE for endurance-trained runners.However,the results reported by these studies are equivocal.Whereas Gliemann et al.35found no change in RE after 8 weeks of combining HIIT (10–20–30 training concept,2 sessions per week)and CR(1 session per week),other studies reported RE improvements after 436,38and 933weeks of training programs that included HIIT sessions.When one looks at the training programs performed in those studies,the equivocal results obtained may depend on 2 factors:the weekly running distance and the intensity of the HIIT.As suggested by Denadai et al.,36improvements in RE with HIIT may result from improved muscle oxidative capacity and associated changes in motor unit recruitment patterns.Ensuring a minimum weekly mileage is important in improving muscle oxidative capacity,and Gliemann et al.35reduced it to approximately 15 km/week,although the studies reporting RE improvement reached greater weekly mileage.As for the intensity of the HIIT,the importance of neuromuscular characteristics(motor unit recruitment and contractile properties)in determining RE and performance has recently been pointed out by Nummela et al.,66whereas Gliemann et al.35based their running plan on a 10–20–30 training concept performed on average at 85%HRmaxand under 100%vVO2max.34The HIIT workouts included in the studies reported that RE improvements were seen atvVO2maxor above.The finding of Denadai et al.36support this rationale,with RE improving after the training program that included HIIT at 100%vVO2maxbut not after HIIT at 95%vVO2max.These data suggest that to improve RE in trained endurance runners,coaches should pay special attention to weekly mileage(combining HIIT and CR may be a good way to ensure a minimum mileage)and intensity of HIITs(close to or above 100%vVO2max).

4.2.3.Muscular adaptations to HIIT-based running plans

Improved global oxygen consumption and delivery also correspond with changes in muscle fibe,in which Type I fiber have greater oxidative capacity than Types IIA and IIX fibers Interval training,by affecting glycolytic capacity,may also lead to increased mitochondrial activity in Type II fiber and thus show characteristics similar to those of Type I fibers67Training at maximal and near-maximal exercise intensities also seems to be effective in creating muscular adaptations such as increasesin the activity of oxidative enzymes and expression of Na+-K+pump subunits and lactate and H+transporters.32–35Moreover, HIIT causes repeated VO2fluctuation related to changes in exercise intensity as opposed to CR,where VO2is nearly constant during the exercise.Because of this,a higher exerciseinduced oxidative stress could be expected;however,HIIT-and CR-based training programs induced similar beneficia effects in endurance runners,reducing the resting levels of oxidative stress biomarkers in plasma and urine.32Therefore,because all these studies reported athletic performance improvements after a HIIT intervention longer than 7 weeks but did not all fin VO2maxor RE improvements,muscular adaptations to a HIIT period may play a critical role in the performance improvement of endurance runners.

On the other hand,no changes in muscle morphology occurred after 7–8 weeks of run-based endurance training34,35in either CR-or HIIT-based training programs.Likewise, capillary-to-fibe ratio and capillary density were unaltered after 7–8 weeks of HIIT-based running protocols(10–20–30 training concept).34,35These data suggest that HIIT running protocols are less effective in improving capillarization than prolonged running and that 10–20–30 training evokes weaker angiogenic stimuli than moderate-intensity exercise training. Because muscle capillarization is important for the delivery of oxygen and nutrients to the exercising muscle(a higher capillary density can increase muscle-to-blood exchange surface, decrease oxygen diffusion distance,and increase red blood cell mean transit time),these finding lead the authors to support the idea that both HIIT and CR must be part of training programs for endurance runners to maximize the physiological adaptations to training.

4.2.4.Changes induced in blood variables—at rest and after exhausting runs

Most of the intervention studies included in this review collected BLa at the end of an exhaustive running protocol,33,34,38,39with some of these studies reporting no adaptations after HIIT intervention,34,39so it seems that improved short-term performance can occur without changes in some of the key H+transport proteins.Bangsbo et al.33found changes in BLa clearance(but not in peak BLa)in athletes who had completed the HIIT intervention,whereas the CR group remained unchanged.Because maximal muscle oxidative capacity is related to BLa removal ability,the authors suggest that the differences in BLa clearance might be due to an oxidative capacity improvement during the HIIT period.From this thinking,either the lack of changes in BLa together with the performance improvement(similar BLa despite a greater athletic performance)reported by some studies34,39or the reduction of BLa after a running protocol performed at the same relative intensity38is an indication of improved buffer capacity and H+clearance in working muscle.Hence,training at high intensity can delay the accumulation of lactate in the blood,which may be due to an increased oxidative capacity of a greater number of muscle fiber and/or a reduced plasma K+concentration (Plasma K+contributes to the maintenance of muscle function during intense exercise33).The training protocols used by these studies are different;although results must be interpreted with caution,increased intensity in a running plan seems to be effective in improving oxidative capacity when compared with a CR-based plan.

Regarding resting blood variables,2 studies34,35examined changes in blood hemoglobin and plasma iron,glucose,myoglobin,creatine kinase,cortisol,insulin,and triglycerides induced by intervention training programs.Although intense aerobic training is generally associated with improved blood lipid profil and insulin sensitivity,the results reported by these studies are equivocal,probably because the athletes were already trained at the beginning of the intervention.

5.Conclusion

Because HIIT running sessions lead,in the short term,to increased cortisol and testosterone concentrations,greater mean VO2,longer times above 90%VO2max,higher energy expenditure,and different effects on lipids response—and there being no reported“extra”neuromuscular strain or consistent perturbations in running kinematics(when compared with moderate-intensity CR)—they are an efficien option for endurance runners in response to demands of higher average intensities and lower weekly running distances(for injury prevention and performance improvements,respectively).Because of this, some studies have checked the effectiveness of HIIT-based running plans(minimum of 4-week program,at least 2 HIIT sessions per week,and mostly combining HIIT and CR workouts)and have shown athletic performance improvements in trained endurance runners by improving VO2maxand RE along with muscular and metabolic adaptations(increased oxidative capacity of a greater number of muscle fiber and reduced plasma K+concentration).

From a practical point of view,the authors support the idea that both HIIT and CR must be part of training programs for endurance runners to maximize adaptations to training.Additionally,the authors suggest that the inclusion of 2 to 3 HIIT sessions in a running plan,accumulating work periods longer than 10 min and working at close to or abovevVO2maxper session,lets recreational endurance runners improve their athletic performance.But what type of HIIT?In general,a good practice for endurance runners would include HIIT protocols involving short work periods(<1 min)with work–rest ratios of approximately 1:1 to 1:2,performed at close to all-out intensities,which enable athletes to elicit similar or greater mean average VO2and to train at an increased running pace(above 100%vVO2max)when compared with longer HIIT or CR protocols.Nevertheless,the authors highlight that although HIIT and SIT,together with CR,must be a part of running plans for endurance athletes,the HIIT-based workload will vary according to training periodization,which must be based on the progressive overload principle.

Acknowledgment

Authors would like to thank the University of Jaén for its support to the present study.

Authors’contributions

FGP revised literature and drafted the manuscript;VMSH critically revised the manuscript;PALR revised literature and helped to draft the manuscript.All authors have read and approved the fina version of the manuscript,and agree with the order of presentation of the authors.

Competing interests

None of the authors declare competing financia interests.

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Received 5 December 2015;revised 9 May 2016;accepted 6 June 2016 Available online 31 August 2016

Peer review under responsibility of Shanghai University of Sport.

*Corresponding author.

E-mail address:fegarpi@gmail.com(F.García-Pinillos).

http://dx.doi.org/10.1016/j.jshs.2016.08.010

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