Review

Training for intense exercise performance: high-intensity orhigh-volume training?

P. B. Laursen1,2,3

1New Zealand Academy of Sport, Auckland, New Zealand, 2Sport Performance Research Institute New Zealand (SPRINZ), Schoolof Sport and Recreation, Auckland University of Technology, Auckland, New Zealand, 3School of Exercise, Biomedical and HealthSciences, Edith Cowan University, Joondalup, Western Australia, AustraliaCorresponding author: Paul B. Laursen, New Zealand Academy of Sport North Island, PO Box 18444, Glen Innes, Auckland1743, New Zealand. Tel: 164 9 477 5427, Fax: 164 9 479 1486, E-mail: paull@nzasni.org.nz

Accepted for publication 4 March 2010

Performance in intense exercise events, such as Olympicrowing, swimming, kayak, track running and track cyclingevents, involves energy contribution from aerobic and anae-robic sources. As aerobic energy supply dominates the totalenergy requirements after � 75 s of near maximal effort,and has the greatest potential for improvement with train-ing, the majority of training for these events is generallyaimed at increasing aerobic metabolic capacity. A short-term period (six to eight sessions over 2–4 weeks) of high-intensity interval training (consisting of repeated exercisebouts performed close to or well above the maximal oxygenuptake intensity, interspersed with low-intensity exercise orcomplete rest) can elicit increases in intense exercise per-

formance of 2–4% in well-trained athletes. The influence ofhigh-volume training is less discussed, but its importanceshould not be downplayed, as high-volume training alsoinduces important metabolic adaptations. While the meta-bolic adaptations that occur with high-volume training andhigh-intensity training show considerable overlap, the mo-lecular events that signal for these adaptations may bedifferent. A polarized approach to training, whereby� 75% of total training volume is performed at lowintensities, and 10–15% is performed at very high intensi-ties, has been suggested as an optimal training intensitydistribution for elite athletes who perform intense exerciseevents.

Both high-intensity (short-duration) training andlow-intensity (high-volume) training are importantcomponents of training programs for athletes whocompete successfully in intense exercise events. In thecontext of this review, an intense exercise event isconsidered to be one lasting between 1 and 8min,where there is a mix of adenosine triphosphate(ATP)-derived energy from both aerobic and anae-robic energy systems. Examples of such intenseexercise events include individual sports such asOlympic rowing, kayak and canoe events, mostswimming races, running events up to 3000m andtrack cycling events.Exercise training, in a variety of forms, is known to

improve the energy status of working muscle, subse-quently resulting in the ability to maintain highermuscle force outputs for longer periods of time.While both high-volume training and high-intensitytraining are important components of an athlete’straining program, it is still unclear how to bestmanipulate these components in order to achieveoptimal intense exercise performance in well-trainedathletes. While a short-term period of high-intensitytraining is known to improve performance in theseathletes (Laursen & Jenkins, 2002), a high training

volume may also be important (Fiskerstrand &Seiler, 2004). More recent work by exercise scientistsis revealing how the combination of these distinctlydifferent forms of training may work to optimize thedevelopment of the aerobic muscle phenotype andenhance intense exercise performance.The purpose of this discourse is to: (i) review the

energy system contribution to intense exercise per-formance, (ii) examine the effect of high-intensitytraining and high-volume training on performanceand physiological factors, (iii) assess some of themolecular events that have been implicated in signal-ing for these important metabolic adaptations and(iv) make recommendations, based on this informa-tion, for the structuring of training programs toimprove intense exercise performance.

Energy system contribution to intense exerciseperformance – what is it we are trying to enhance?

Intense exercise events involve a near maximal en-ergy delivery for a sustained period of time. Thesenear maximal efforts require a mix of anaerobic andaerobic energy provision. To illustrate this, Duffield

Scand J Med Sci Sports 2010: 20 (Suppl. 2): 1–10 & 2010 John Wiley & Sons A/S

doi: 10.1111/j.1600-0838.2010.01184.x

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et al. (2004, 2005a, b) examined the aerobic andanaerobic energy system contributions to 100, 200,400, 800, 1500 and 3000m track running in well-trained runners. The data from the male runners inthese studies are plotted in Fig. 1, revealing that theenergy contribution to an intense exercise eventarises from a mix of aerobic and anaerobic sources.The crossover point, where aerobic and anaerobicenergy contributes equally, occurs approximately at600m of near maximal running. This compares wellwith an earlier crossover estimate made by Gastin(2001) of about 75 s of near maximal exercise. Thus,for an intense exercise event that lasts beyond 75 s,total energy output is mostly aerobically driven. Thisis a convenient situation for the exercise conditioner,because the aerobic energy system appears to be amore malleable system to adjust. Indeed, both high-intensity training and high-volume training can elicitimprovements in aerobic power and capacity.

Effect of training on physiological variables andintense exercise performance

The purpose of exercise training is to alter physiolo-gical systems in such a way that physical workcapacity is enhanced through an improved capacityto deviate from resting homeostasis during subse-quent exercise sessions (Hawley et al., 1997). Manip-ulation of the intensity and duration of work and restintervals changes the relative demands on particularmetabolic pathways within muscle cells, as well asoxygen delivery to muscle (Holloszy & Coyle, 1984).

In response, changes occur in both central andperipheral systems, including improved cardiovascu-lar dynamics (Buchheit et al., 2009), neural recruit-ment patterns (Enoka & Duchateau, 2008), musclebioenergetics (Hawley, 2002), as well as enhancedmorphological (Zierath & Hawley, 2004), metabolicsubstrate (Hawley, 2002) and skeletal muscle acid–base status (Hawley & Stepto, 2001). The rate atwhich these adaptations occur is variable (Vollaardet al., 2009) and appears to depend on the volume,intensity and frequency of the training. Importantly,development of the physiological capacities wit-nessed in elite athletes does not occur quickly, andmay take many years of high training loads beforepeak levels are reached.Training can be structured in an infinite number of

ways, but in general, coaches tend to prescribeperiods of prolonged submaximal exercise, moderateperiods of training at ‘‘threshold’’ or shorter high-intensity exercise sessions (Hawley et al., 1997). Inthe context of this review, low-intensity traininggenerally refers to exercise performed below the firstventilatory threshold, ‘‘threshold’’ intensity refers toexercise performed between the first and secondventilatory thresholds and high-intensity trainingrefers to exercise performed above the second venti-latory threshold (Seiler & Kjerland, 2006). Submax-imal low-intensity endurance training performed forlong durations involves predominantly slow twitchmotor unit recruitment, while higher intensity train-ing (usually completed as high intensity intervaltraining) will recruit additional fast twitch motorunits for relatively short durations (Enoka & Duch-

Fig. 1. Percent aerobic and anaerobic energy system contributions to near maximal running over distances ranging from 100to 3000m. Figure derived based on the male data obtained from the studies of Duffield et al. (2004, 2005a, b).

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ateau, 2008). Both forms of training are importantfor enhancing the aforementioned physiological sys-tems and intense exercise performance, but the de-gree and rate at which these variables change in theshort term appear to be affected more acutely byhigh-intensity training (Londeree, 1997).

Performance and physiological effects of increasedtraining intensity

The marked influence of high-intensity training onperformance and physiological factors is well known(Laursen & Jenkins, 2002), but an athlete’s ability toperform this type of training is limited (Billat et al.,1999). One successful method of performing highervolumes of high-intensity training is termed high-intensity interval training. High-intensity intervaltraining, also called transition training, is defined asrepeated bouts of high-intensity exercise (i.e. frommaximal lactate steady state or second ventilatorythreshold to ‘‘all-out’’ supramaximal exercise intensi-ties), interspersed with recovery periods of low-inten-sity exercise or complete rest (Hawley et al., 1997).In already well-trained athletes, the effect of sup-

plementing high-intensity training on top of analready high training volume appears to be extremelyeffective. In well-trained cyclists, for instance, high-intensity interval training (six to eight sessions),completed at a variety of intensities (i.e. 80–150%VO2max power output) for 2–4 weeks, has beenshown to have a significant influence (i.e. 12–4%)on measures of intense exercise performance (i.e.time-to-fatigue at 150% of peak power output;60.5 � 9.3 vs 72.5 � 7.6 s; Po0.01), peak power out-put and 40 km time trial performance (Lindsay et al.,1996; Westgarth-Taylor et al., 1997; Weston et al.,1997; Stepto et al., 1998). In well-trained middledistance runners, Smith et al. (1999, 2003) foundimprovements in 3000m running performance whenrunners performed high-intensity interval training(8 � � 2–3min at VO2max running speed, 2:1work-to-rest ratio) twice a week for 4 weeks. In aretrospective study performed on elite swimmers,Mujika et al. (1995) found that mean training in-tensity over a season was the key factor explainingperformance improvements (r5 0.69, Po0.01), butnot training volume or frequency. Clearly, a short-term period of high-intensity interval training sup-plemented into the already high training volumes ofwell-trained athletes can elicit improvements in bothintense and prolonged exercise performance (Laur-sen & Jenkins, 2002).While the potent influence that a short-term dose

of high-intensity interval training has on intense andprolonged endurance performance is well known,the mechanisms responsible for these performancechanges with well-trained individuals are not clear.

For example, Weston et al. (1997) had six highlytrained cyclists perform six high-intensity intervaltraining sessions (8 � 5min at 80% peak poweroutput, 60-s recovery) over 3 weeks, and showedsignificant improvements in intense exercise perfor-mance (time-to-fatigue at 150% peak power output)and 40 km time trial performance, without changes inskeletal muscle glycolytic or oxidative enzyme activ-ities. Thus, despite the likely high rates of carbohy-drate oxidation (340mmol/kg/min) required by theseefforts (Stepto et al., 2001), this acute perturbation inenergy status of working muscle did not appear toincrease metabolic enzyme function in the skeletalmuscle of these six cyclists (Weston et al., 1997), aswould be predicted based on findings made in less-trained subjects (Gibala & McGee, 2008). Instead, anincrease in skeletal muscle buffering capacity wasreported (Weston et al., 1997). Other physiologicalfactors that have been shown to increase in parallelwith improvements in performance following theaddition of high-intensity interval training to thealready high training volume of the well-trainedathlete include improvements in the ventilatory (Ace-vedo & Goldfarb, 1989; Hoogeveen, 2000) and lactatethresholds (Edge et al., 2005; Esfarjani & Laursen,2007; Driller et al., 2009), an increased abilityto engage a greater volume of muscle mass (Luciaet al., 2000; Creer et al., 2004) and an increased abilityto oxidize fat relative to carbohydrate (Westgarth-Taylor et al., 1997; Yeo et al., 2008).In a recent study, Iaia et al. (2008, 2009) asked

runners who were training 45 km/week to lower theirtraining volume to only 15 km/week for 4 weeks, andinstead perform speed endurance training (8–12 � 30 s sprints; three to five times per week). Afterthis distinct change in training, runners in the speedendurance training groups had maintained their10 km run performance, VO2max, skeletal muscleoxidative enzyme activities and capillarization com-pared with the 45 km/week control group (Iaia et al.,2009). However, 30-s sprint (17%), Yo-Yo inter-mittent recovery test (119%) and supramaximalrunning (119–27%) performances had increased inthe speed endurance training group (Iaia et al., 2008).This study indicates that 4 weeks of low-volumehigh-intensity interval training can maintain an ath-lete’s endurance performance and muscle oxidativepotential (Iaia et al., 2009), and additionally increaseintense exercise performance (Iaia et al., 2008).In summary, it is clear that when a period of high-

intensity interval training is supplemented into thealready high training volumes of well-trained endur-ance athletes, further enhancements in both intenseand prolonged endurance performance are possible.As well, lower volume high-intensity interval train-ing can maintain endurance performance ability inalready well-trained endurance athletes. While high-

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intensity training can have the aforementioned pro-found effects acutely on various aspects of intenseexercise performance, the importance of a high-training-volume background should not be over-looked (Fiskerstrand & Seiler, 2004; Esteve-Lanaoet al., 2005, 2007; Seiler & Kjerland, 2006).

Performance and physiological effects of additionaltraining volume

When individuals are untrained, and commence aperiod of training characterized by long duration,low-intensity sessions, profound adaptations to ske-letal muscle and supporting systems are witnessed,including increases in the mitochondrial content andrespiratory capacity of muscle fibers (Holloszy &Coyle, 1984). As a consequence of the increase inmitochondria, exercise of the same intensity results ina disturbance in homeostasis that is smaller intrained than in untrained muscles, leaving someauthors to suggest that the influence of prolongedtraining in already trained muscle may be limited(Laursen & Jenkins, 2002). Costill and colleagues(1991) do well to summarize an outsider’s viewpointon the issue, stating that, ‘‘it is difficult to understandhow training at speeds that are markedly slower thancompetitive pace for 3–4 h/day will prepare (anathlete) for the supramaximal efforts of competi-tion.’’ Nevertheless, athletes commonly perform alarge number of long-duration, low-intensity trainingsessions per week, which combine to form an ath-lete’s high weekly training volume. Indeed, it hasbeen estimated that well-trained (including world-elite) athletes perform � 75% of their training atintensities below the first ventilatory threshold, de-spite competing at much higher intensities (Seiler &Kjerland, 2006). This type of training likely contri-butes to their high skeletal muscle energy status (Yeoet al., 2008), their ability to sustain high muscularpower outputs for long durations (Coyle et al., 1988)and their ability to recover from high-intensity ex-ercise (Seiler et al., 2007).Relative to the number of studies showing en-

hancements in intense exercise performance withhigh-intensity interval training (Laursen & Jenkins,2002), there are relatively few studies documentingimprovements in performance with step increases intraining volume (Costill et al., 1991). This may bedue to the fact that the time course for performanceimprovement with increases in training volume maynot occur as rapidly (Costill et al., 1991) comparedwith acute increases in high-intensity training (Wes-ton et al., 1997; Laursen et al., 2002), makinginvestigation into its influence more challenging forresearchers. In one study that managed to achievethis, Costill et al. (1991) divided collegiate swimmers

into groups that trained either once or twice a day for6 weeks. As a result, one group performed twice thevolume of training of the other (4950 vs 9435m/day)at similar high-training intensities (estimated by theauthors to be 95 vs 93.5% VO2max per interval).Despite higher levels of citrate synthase activity fromthe deltoid muscle shown in the group that doubledtheir training volume, performance times following ataper over distances ranging from 43.2 to 2743m werenot different between the groups. While this studydemonstrates that a relatively acute period of high-volume training (with similar high training intensities)does not appear to enhance performance, there maybe subtle positive effects of low-intensity high trainingvolumes (Fiskerstrand & Seiler, 2004; Esteve-Lanaoet al., 2005, 2007; Seiler & Kjerland, 2006).The efficacy of low-intensity long-duration train-

ing sessions has been shown in at least three studies.In one of the first studies to show the importance oflow-intensity high-volume training, Fiskerstrand andSeiler (2004) retrospectively investigated changes intraining volume, intensity and performance in 21international medal-winning Norwegian rowersfrom 1970 to 2001. From the 1970s to the 1990s,VO2max increased by 12% (5.8–6.5 L/min) while 6-min rowing ergometer performance increased by10%. Coinciding with these performance changeswas an increase in low-intensity training (i.e. bloodlactate o2mM; 30–50 h/month), or high trainingvolume, coupled with reductions in race pace andsupramaximal intensity training (blood lactate 8–14mM; 23–7 h/month). As a result, training volumeincreased by 20% over this period of time (924–1128 h/year), as did intense exercise performanceresults (Fiskerstrand & Seiler, 2004). In anotherlongitudinal study conducted over a 6-month period,Esteve-Lanao et al. (2005) compared the influence ofdifferent amounts of intensity and volume trainingon running performance in eight sub-elite endu-rance runners (VO2max 5 70.0 � 7.3mL/kg/min).The authors found strong relationships betweentime spent training at intensities below the firstventilatory threshold and both 4 km (r5� 0.79;P5 0.06) and 10 km (r5� 0.97; P5 0.008) run per-formance (Esteve-Lanao et al., 2005). In an anotherrecent study, Ingham et al. (2008) divided elite Britishrowers into groups that performed either 12 weeks oflow-intensity (100% of training performed below thelactate threshold) or mixed-intensity training (30%above, 70% below lactate threshold). While bothgroups improved similarly in terms of their perfor-mance, the low-intensity training group improvedtheir speed at lactate threshold to a greater extentthan the mixed training group (Ingham et al., 2008).Clearly, important adaptations appear to occur withlow-intensity continuous training that are not ob-served with mixed or high-intensity training.

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While the immediate effect of low-intensity high-volume training on intense exercise performance canbe difficult to assess, it would appear that the inser-tion of these low-intensity training sessions has apositive impact on performance, despite being per-formed at an intensity that is markedly less than thatwhich is specifically performed at during intenseexercise competition. It is often purported that theseperiods of relatively low-intensity, high trainingvolumes may provide the aerobic platform neededto facilitate the specific adaptations that occur inresponse to the high-intensity or specific workouts.Others feel that a high training volume may beimportant for achieving optimal body compositionand engraining the neuromuscular blueprint neededto optimize performance. While the author is una-ware of any research to back these claims, it wouldappear that intense exercise athletes do tend toorganize their training into periods of both high-intensity and prolonged low-intensity phases (Fisker-strand & Seiler, 2004; Esteve-Lanao et al., 2005,2007; Seiler & Kjerland, 2006; Ingham et al., 2008).The disconnect between the scientific literature andthe practices of elite intense exercise athletes high-lights the urgent need for scientific research into theeffects of high-volume training in high-level athletes,including the time course of adaptations as well asthe long-term effects (i.e. detraining effects).

The important interplay between high-intensity trainingand high-volume training

The review of studies that manipulate training in-tensity and volume over a short-term period revealsthat successful training programs may benefit fromboth forms of training at particular periods within anathlete’s training program. When training does nothave an appropriate blend of both high-intensitytraining and high-volume training inserted into theprogram, performance ability can stagnate. For ex-ample, Iaia et al. (2008, 2009) examined the influenceof marked changes in intensity and volume trainingon performance and metabolic enzyme activity inendurance-trained runners. In this study, runnerstraining 45 km/week lowered their training volumeto only 15 km/week for 4 weeks, but instead per-formed speed endurance training (8–12 � 30 ssprints; 3–5 times/week). While markers of sprintperformance were improved, 10 km run performancewas only maintained, and not enhanced (Iaia et al.,2008, 2009). In a study on competitive swimmers,Faude et al. (2008) used a randomized cross-overdesign where swimmers performed two different 4-week training periods, each followed by an identicaltaper week. One training period was characterized bya high training volume, while the other involvedhigh-intensity training; neither program involved

aspects of both. The authors found no differencebetween the training periods for 100 and 400m swimperformance times, or individual anaerobic thresh-olds (Faude et al., 2008). Clearly, a mix of both high-intensity training and high-volume training is im-portant, but predominance of one form of training orthe other does not appear to be as beneficial. In astudy demonstrating the importance of having equalamounts of distinctly different training, Esteve-Lanao et al. (2007) divided 12 sub-elite runners intotwo separate groups that performed equal amountsof high-intensity training (� 8.4% of training aboverespiratory compensation point). The difference be-tween the groups in terms of their training, however,was the amount of low- vs moderate-intensity train-ing they performed. In one group, more low-intensitytraining (below the first ventilatory threshold; 81 vs12%) was performed. In the other group, moremoderate-intensity training (above first ventilatorythreshold but below the second ventilatory threshold;67 vs 25%) was performed. While intense exerciseperformance was not assessed, it is interesting to notethat the magnitude of the improvement in 10.4 kmrunning performance 5 months following the inter-vention was significantly greater (P5 0.03) inthe group that performed more low-intensity training(� 157 � 13 vs � 122 � 7 s). Admittedly, the 10.4 kmtest used to assess running performance falls outsideof the intense exercise spectrum, but does suggestthat the aerobic power and capacity of these runnerswere enhanced by this training scheme; a capacityidentified previously in this article as critical tointense exercise performance success beyond � 75 sof all-out near maximal activity.The synthesis of these studies reveals the impor-

tance of combining periods of both high and low-intensity training into the training programs of theintense exercise athlete. Seiler and Kjerland (2006)refer to this training distribution as a polarizedmodel, where approximately 75% of sessions areperformed below the first ventilatory threshold,with 15% above the second ventilatory thresholdand o10% performed between the first and secondventilatory thresholds. For the exercise scientist,these observations beg the question: why might themixing of distinct high and low-intensity trainingsessions be so effective at increasing the energy statusof working muscle and subsequent exercise perfor-mance? Seiler et al. (2007) offer a plausible hypoth-esis for why successful elite intense exercise athletesbenefit from periods of both high training intensitiesand volumes. In this study, the authors monitoredacute disturbances in autonomic balance using heartrate variability following different types of exercise inhighly trained Norwegian orienteers (Seiler et al.,2007). On separate occasions, athletes ran for 60 and120min below their first ventilatory threshold, for

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30min between their first and second ventilatorythreshold and intermittently for 60min above theirsecond ventilatory threshold. Irrespective of runningfor either 1 or 2 h below the first ventilatory thresh-old, markers of autonomic balance disturbance werenot altered to the same degree as compared witheither a 30-min session performed between the firstand second ventilatory threshold, or a 60-min high-intensity interval training session performed abovethe second ventilatory threshold. The authors pro-posed that the first ventilatory threshold may be animportant demarcation point identifying the trainingintensity above which autonomic balance may bealtered. However, two recently published manu-scripts (Meyer et al., 2004; Faude et al., 2009) suggestthat prolonged (i.e. 3 h) low-intensity training (belowventilatory threshold; v-slope method) may not beadequate to induce recovery or regeneration in the4-day period following a 13-day intensive trainingphase. Nevertheless, the period of overreaching ap-plied to the athletes in this study could be consideredabrupt, where more well-trained, experienced ath-letes might build gradually into the same relativetraining load. When phases of high training loads arerepeated without adequate recovery, and autonomicbalance is continually disturbed, this results in over-training (Billat et al., 1999). Notwithstanding thelimitations of monitoring staleness in athletes usingheart rate variability (Bosquet et al., 2008), and thatother factors such as hormonal disturbances, psy-chological stressors, muscle damage, injury and ill-ness may be responsible for prolonged fatigue inathletes (Bosquet et al., 2008), the documentedtraining organization of elite athletes (Seiler & Kjer-land, 2006) may serve to maximize performance byoptimizing mitochondrial protein synthesis signalingwithout significantly compromising autonomic bal-ance (Seiler et al., 2007).

How does it happen? Beginning to understandmolecular signaling

While the picture is far from complete, scientistshave begun to make impressive inroads towardunderstanding how skeletal muscle adapts to varyingexercise stimuli, and for an excellent review on thetopic, the reader may refer to the work of Coffey andHawley (2007). As assessed by these authors (Coffey& Hawley, 2007), there appear to be at least fourprimary signals (along with a number of secondarymessengers, redundancy and cross-talk) that can leadto an increase in mitochondrial mass and glucosetransport capacity in skeletal muscle following sev-eral forms of exercise training. These include (i)mechanical stretch or muscle tension, (ii) an increasein reactive oxygen species that occurs when oxygen is

processed through the respiratory pathways, (iii) anincrease in muscle calcium concentration as requiredfor excitation–contraction coupling and (iv) the al-tered energy status (i.e. lower ATP concentrations) inmuscle. These mechanisms and pathways are com-plex, with many beyond the scope of this review. Forthe purpose of this discourse, however, the focus willbe on the last two of these primary signals, whichhave received increased attention in recent studies.The first of these mentioned molecular signals is

the prolonged rise in intramuscular calcium, such asthat which occurs during prolonged endurance ex-ercise or high exercise training volumes. These highcalcium concentrations activate a mitochondrial bio-genesis messenger called the calcium–calmodulinkinases (Fig. 2). Second, the altered energy statusin muscle associated with small reductions in ATPconcentrations, such as that present during high-intensity exercise, elicits a relatively large concomi-tant rise in adenosine monophosphate (AMP), whichactivates the AMP-activated protein kinase (AMPK).With these two secondary phenotypic adaptationsignals identified, it becomes apparent how differenttypes of endurance training modes might elicit simi-lar adaptive responses (Burgomaster et al., 2008). Insupport of these distinct pathways, Gibala et al.(2009) showed significant increases in AMPK imme-diately following four repeated 30-s ‘‘all-out’’ sprints.This was associated 3 h later with a twofold increasein peroxisome proliferator-activated receptor-g coac-tivator-1a (PGC-1a) mRNA, a transcriptional coac-tivator that has been described by some as the‘‘master switch’’ for mitochondrial biogenesis (Ad-hihetty et al., 2003) (Fig. 2). Of note, however, is thatthis occurred without an increase in the calcium–calmodulin kinases (Gibala et al., 2009), which areknown to be stimulated during prolonged repeatedcontractions (Rose et al., 2007).With these results in mind, it becomes clear what

has been known by coaches for decades; that is, withrespect to prescribing training that improves perfor-mance, ‘‘there’s more than one way to skin a cat.’’The high mitochondrial oxidative capacity, improvedfat oxidation potential, and increased glucose trans-port capacity in the skeletal muscle of enduranceathletes may be achieved through either high volumesof endurance training, high intensities of endurancetraining or various combinations of both. Highervolumes of exercise training are likely to signal forthese adaptations through the calcium–calmodulinkinases (Rose et al., 2007), while higher intensities ofendurance training, which lowers ATP concentra-tions and raises AMP levels, appear more likely tosignal for mitochondrial biogenesis through theAMP-activated protein kinase pathway (Gibalaet al., 2009). As shown in Fig. 2, these differentsignaling molecules have similar downstream targets

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(Baar, 2006). The result is an increased capacity togenerate ATP aerobically. Thus, at the molecularlevel, it may be the blend of signals induced fromcombined high-volume training and high-intensitytraining that elicits either a stronger or more frequentpromotion of the aerobic muscle phenotype throughPGC-1a mRNA transcription (Fig. 2). As well, thelower intensity higher volume training sessions arelikely to promote the development of the aerobicphenotype without disturbing autonomic balancethat could lead to overtraining (Seiler et al., 2007).These speculative comments highlight an importantarea for future research.

How do we optimally structure training programs forhigh-performing endurance athletes?

While this manuscript offers a unique discoursedescribing a binary model by which training isorganized into periods characterized by either hightraining intensities, or high training volumes, thereality of the matter is that athletes often performsessions where there are mixed amounts of both (e.g.a 6-h group bike training session over hilly terrain).Thus, characterizing all training sessions as beingeither a prolonged low-intensity, moderate-intensityor high-intensity session can be problematic. Never-theless, the synthesis of this information reveals apattern highlighting the importance of applyingperiods of both high-intensity training and high-volume training at the appropriate time in a training

program, in order to elicit an optimal intense exerciseperformance. Experts in training program designrefer to this as the art of periodization (Issurin,2008). While the high-intensity training stimulusover the lead up period to intense exercise perfor-mance appears critical (Londeree, 1997), the sub-maximal or prolonged training durations (volume ofrepeated muscular contractions) cannot be down-played (Fiskerstrand & Seiler, 2004). These high-volume training periods may elicit the molecularsignals needed to stimulate mitochondrial proteinsynthesis without creating undue autonomic distur-bance that could lead to overtraining (Seiler et al.,2007). Over time, the progressive result is likely to bean improved efficiency of skeletal muscle and adevelopment of the fatigue-resistant aerobic musclephenotype. Indeed, development of the successfulintense exercise athlete tends to require a numberof years exposure to high training volumes andintensities (Schumacher et al., 2006). The art ofsuccessful intense exercise coaching, therefore, ap-pears to involve the manipulation of training sessionsthat combine long duration low-intensity periodswith phases of very high-intensity work, appropriaterecovery and tapering (Mujika et al., 2000; Issurin,2008; Pyne et al., 2009).The paper will finish with two practical examples

that demonstrate the effectiveness of this model. Thefirst example is New Zealand’s Olympic 800-m run-ning legend, Sir Peter Snell. Snell was a protege of thelate New Zealand athletics coach Arthur Lydiard,who was renowned for organizing the training of

Fig. 2. Simplified model of the adenosine monophosphate kinase (AMPK) and calcium–calmodulin kinase (CaMK) signalingpathways, as well as their similar downstream target, the peroxisome proliferator-activated receptor-g coactivator-1a (PGC-1a). This ‘‘master switch’’ is thought to be involved in promoting the development of the aerobic muscle phenotype. High-intensity training appears more likely to signal via the AMPK pathway, while high-volume training appears more likely tooperate through the CaMK pathway. ATP, adenosine triphosphate; AMP, adenosine monophosphate; GLUT4, glucosetransporter 4; [Ca21], intramuscular calcium concentration.

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middle- and long-distance runners into early phasesof high training volumes (up to 160 km/week), before‘‘strength’’ phases consisting of hill running, fol-lowed by high-intensity track sessions in the acuteperiod leading up to a major event. This was inno-vative at the time because 160 km training weekswere the sort of training only completed by mostmarathon runners (Snell P, personal communica-tion). The result for Snell was an 800m world recordin 1962 (1:44.3), and winning double Gold in the 800and 1500m events at the 1964 Tokyo OlympicSummer Games.Another report of a high-volume training plan that

elicited a winning intense exercise performance wasthat of the German 4000-m team pursuit cyclingworld record achieved at the Sydney 2000 OlympicGames (Schumacher & Mueller, 2002). In this paper,Schumacher and Mueller (2002) provide a detailedaccount of the training performed by the cyclists overthe 7-month lead-up to the critical event. In general,training involved extremely high volumes (29 000–35 000 km/year) that included long periods of low-intensity road training ( � 50% VO2max) interspersedwith stage racing (grand tour) events. While the roadracing component of the cyclists’ training programwould have entailed numerous periods of both high-volume and high-intensity stimuli, it was not until thefinal 8 days before the Sydney Olympics that aspecific high-intensity training taper period on thetrack was prescribed. Nevertheless, this training de-sign yielded outstanding results, and the model hassince been replicated by both the Australian andBritish cycling teams to break this record repeatedlyover the last two Olympic Games (Quod M, CyclingAustralia, Australian Institute of Sport, personalcommunication).

Summary

Our understanding of how best to manipulate thetraining programs of athletes competing in intense

exercise events so that performance is optimized isfar from complete. It would appear that a polarizedapproach to training may be optimal, where periodsof both high and low-intensity training but high-volume training are performed. The supplementationof high-intensity training to the high-volume pro-gram of the already highly trained athlete can elicitfurther enhancements in endurance performance,which appears to be largely due to an improvedability of the engaged skeletal muscle to generateATP aerobically. Prolonged durations of low-inten-sity or high-volume training are likely to facilitateadaptation by signaling for the aerobic phenotype,yet the intensity may be low enough to promoteautonomic balance, recovery and athlete health.Some of the important molecular signals arisingfrom various forms of exercise training include theAMPK and calcium–calmodulin kinases, likely to beactivated in response to intense and prolonged ex-ercise, respectively. Both of these signals have similardownstream targets in the skeletal muscle that pro-mote the development of the aerobic muscle pheno-type. A further understanding of how best toorganize and manipulate the training programs forfuture intense exercise athletes will require the con-tinued cooperation of sport scientists, coaches andathletes alike.

Key words: interval training, aerobic capacity, energysystem, molecular signaling, mitochondrial biogenesis,AMPK, CaMK.

Acknowledgements

Special thanks to Inigo Mujika, Chris Abbiss, Marc Quod,Alison Hall and two anonymous reviewers for their helpfulcomments and editorial assistance during the preparation ofthis manuscript.

Conflicts of interest: The author has no potential conflicts ofinterest.

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