Performance in intense exercise events, such as Olympic rowing, kayak, track running and track cycling events, involves a mix of energy system contributions from aerobic and anaerobic sources. Aerobic energy supply however dominates the total energy requirements of these events after ~75 s of near maximal effort. As the aerobic energy system has the greatest potential for improvement with training, and intense exercise events generally persist for longer than 75 s, training methods for these events are generally aimed at increasing aerobic metabolic capacity. A short-term period (2-4 wk) of high-intensity interval training (HIT; consisting of repeated exercise bouts ranging in intensity from 80-175% of peak power) can elicit increases in intense exercise performance of 2-4% in well trained athletes. While the influence of high volume training (HVT) is less discussed, its importance should not be downplayed, as it may develop the aerobic base needed to support recovery and adaptation from HIT by promoting autonomic balance and athlete health. Indeed, when HIT is performed without a background of HVT, performance can be maintained, but is generally not improved. While the aerobic metabolic adaptations that occur with HVT and HIT are similar, the molecular events that signal for these adaptations may be different. The high levels of intramuscular calcium associated with HVT may signal for metabolic adaptations that improve muscle efficiency through the calcium-calmodulin pathway, while the brief low energy state created with HIT may elicit its effects through the adenosine monophosphate kinase pathway. These distinct molecular signaling pathways, which have similar downstream targets (i.e., mitochondrial biogenesis), may help to explain the potent effect that combined HVT and HIT has on aerobic energy system upregulation and intense exercise performance.
Both high-intensity (short duration) training and low intensity (long duration) high volume training are important components of training programs for athletes who compete successfully in intense exercise events. In the context of this review, an intense exercise event is considered to be one lasting between 1 to 8 min, where there is a mix of adenosine triphosphate (ATP)-derived energy from both aerobic and anaerobic energy systems. Examples of such intense exercise events include individual sports such as Olympic rowing, kayak and canoe events, running events up to 3000 m, and track cycling events.
Exercise training, in a variety of forms, is known to improve the energy status of working muscle, subsequently resulting in the ability to maintain higher muscle force outputs for longer periods of time. While both high volume and high-intensity training are important components of an athlete’s training program, it is still unclear how to best manipulate these components in order to achieve optimal intense exercise performance in well trained athletes. While a short-term period of high-intensity training is known to improve performance in these athletes (Laursen & Jenkins 2002), it is also clear that high training volumes are of equal importance (Fiskerstrand & Seiler 2004). More recent work by exercise scientists is revealing how the combination of these distinctly different forms of training may work to optimize development of the aerobic muscle phenotype and enhance intense exercise performance.
The purpose of this discourse is to i) review the energy system contribution to intense exercise performance, ii) examine the effect of high-intensity training and high volume training on performance and physiological factors, iii) assess some of the molecular events that have been implicated in signaling for these important metabolic adaptations and iv) make recommendations, based on this information, for the structuring of training programs to improve intense exercise performance.
Energy system contribution to intense exercise performance – what is it we are trying to enhance?
Intense exercise events involve a near maximal energy supply for a sustained period of time. These near-maximal efforts require a mix of anaerobic and aerobic energy provision. To illustrate this, Duffield et al. (2004; 2005a; 2005b) examined the aerobic and anaerobic energy system contributions to 100, 200, 400, 800, 1500 and 3000 m track running in well trained runners. The data from the male runners in these studies are plotted in Figure 1, revealing that the energy contribution to an intense exercise event arises from a mix of aerobic and anaerobic sources. The crossover point, where aerobic and anaerobic energy contributes equally, occurs approximately at 600 m of near maximal running. This compares well with 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. This is a convenient situation for the exercise conditioner, because the aerobic energy system appears to be a more malleable system to adjust. Indeed, both high-intensity and high-volume training can elicit improvements in aerobic power and capacity.
Effect of training on physiological variables and intense exercise performance
The purpose of exercise training is to alter physiological systems in such a way that physical work capacity is enhanced through improved homeostasis preservation during subsequent exercise sessions. Manipulation of the intensity and duration of work and rest intervals changes the relative demands on particular metabolic pathways within muscle cells, as well as oxygen delivery to muscle. In response, changes occur in both central and peripheral metabolic systems, including improved cardiovascular dynamics (Buchheit et al. 2009), neural recruitment patterns (Enoka & Duchateau 2008), muscle bioenergetics (Hawley 2002), as well as enhanced morphological (Zierath & Hawley 2004), metabolic-substrate (Hawley 2002) and skeletal muscle acid-base status (Hawley & Stepto 2001). The rate at which these adaptations occur is variable (Vollaard et al. 2009), but appears to depend on the volume, intensity and frequency of the training. Importantly, development of the physiological capacities witnessed in elite athletes do not occur quickly, and may take many years of high training loads before peak levels are reached.
Training can be structured in an infinite number of ways, but in general, coaches tend to prescribe periods of prolonged submaximal or shorter high-intensity exercise sessions. Submaximal endurance training performed for long durations, involves predominantly slow twitch motor unit recruitment, while higher intensity training (usually completed as high-intensity interval training) will recruit additional fast twitch motor units for relatively short durations. Both forms of training are important for enhancing the aforementioned physiological systems and intense exercise performance, but the degree and rate at which these variables change in the short term appears to be affected more acutely by high-intensity training (Londeree 1997).
Performance and physiological effects of additional training intensity
The marked influence of high-intensity training on performance and physiological factors is well known (Laursen & Jenkins 2002), but an athlete’s ability to perform this type of training may be limited. One successful method of performing higher volumes of high-intensity training is termed high-intensity interval training. High-intensity interval training is defined as repeated bouts of high-intensity exercise (i.e., from lactate threshold to ‘all-out’ supramaximal exercise intensities), interspersed with recovery periods of low intensity exercise or complete rest.
In already well trained athletes, the effect of supplementing high-intensity training on top of an already high training volume appears to be extremely effective. In well trained cyclists, high-intensity interval training, completed at a variety of intensities (i.e., 80 – 150% VO2max power output) for two to four weeks, has been shown to have a significant influence (i.e., 2-4%) on measures of intense exercise performance (i.e., time-to-fatigue at 150% of peak power output), peak power output, and 40-km time trial performance (Lindsay et al. 1996; Stepto et al. 1998; Westgarth-Taylor et al. 1997; Weston et al. 1997). In well trained middle distance runners, Smith and colleagues (1999; 2003) found improvements in 3000 m running performance when runners performed high-intensity interval training (8 x ~2-3 min at VO2max running speed, 2:1 work to rest ratio) twice a week for four weeks. In a retrospective study performed on elite swimmers Mujika et al. (1995) found that mean training intensity over a season was the key factor explaining performance improvements (r = 0.69, p < 0.01), but not training volume or frequency. Clearly, a short-term period of high-intensity interval training supplemented into the already high training volumes of well trained athletes can elicit improvements in both intense and prolonged exercise performance (Laursen & Jenkins 2002).
While the potent influence that a short-term dose of high-intensity interval training has on intense and prolonged endurance performance is well known, the mechanisms responsible for these performance changes with well trained individuals are not clear. For example, Weston et al. (1997) had six highly trained cyclists perform six high-intensity interval training sessions (8 x 5 min at 80% peak power output, 60s recovery) over three weeks, and showed significant improvements in intense exercise performance (time to fatigue at 150% peak power output) and 40 km time trial performance, without changes in skeletal muscle glycolytic or oxidative enzyme activities. Thus, despite the likely high rates of carbohydrate oxidation (340 µmol.kg-1.min-1) required by these efforts (Stepto et al. 2001), this acute perturbation in energy status of working muscle did not appear to increase metabolic enzyme function in the skeletal muscle of these six cyclists (Weston et al. 1997), as would be predicted based on findings made in less trained subjects (Gibala & McGee 2008). Instead, an increase in skeletal muscle buffering capacity was reported (Weston et al. 1997). Other physiological factors that have been shown to increase in parallel with improvements in performance following the addition of high-intensity interval training to the already high training volume of the well trained athlete include improvements in the ventilatory threshold (Acevedo & Goldfarb 1989; Hoogeveen 2000), an increased ability to engage a greater volume of muscle mass (Creer et al. 2004; Lucia et al. 2000) and an increased ability to 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 45km/wk to lower their training volume to only 15 km/wk for 4 weeks, and instead perform sprint training (8-12 x 30s sprints; 3-5 times/wk). After this distinct change in training, runners in the sprint training groups had maintained their 10 km run performance, VO2max, skeletal muscle oxidative enzyme activities and capillarisation compared with the 45km/wk control group (Iaia et al. 2009). However, 30-s sprint (+7%), Yo-Yo intermittent recovery test (+19%) and supramaximal running (+19-27%) performances had increased in the sprint training group (Iaia et al. 2008). This study indicates that low volume high-intensity interval training can maintain an athlete’s endurance performance and muscle oxidative potential (Iaia et al. 2009), and additionally increase intense exercise performance (Iaia et al. 2008).
High-intensity interval training has also been shown to be effective at enhancing various aspects of team sport performance. For example, Helgerud et al. (2001) showed that 8 weeks of high-intensity interval training (4 x 4 min @ 95% HRmax, 3 min recovery jog, twice per week) enhanced distance covered during a match (20%), number of sprints (100%), number of ball involvements (24%) and average work intensity during a match. More recently, Bravo et al. (2008) showed that 7 weeks of repeated sprint run training (3 x 6 maximal shuttle sprints of 40 m) was more effective at enhancing Yo-Yo intermittent recovery test performance and repeated sprint ability compared with traditional interval training (4 x 4 min running at 90-95% HRmax).
Finally, when high-intensity training can be woven into training programs using ball-specific drills or small-sided games, markers of intense exercise performance can also be enhanced, with the added benefit of simultaneously receiving skill-based training (Impellizzeri et al., 2006; Buchheit et al., 2009).
In summary, it is clear that when a period of high-intensity interval training is supplemented into the already high training volumes of well trained endurance athletes, further enhancements in both intense and prolonged endurance performance are possible. As well, lower volume high-intensity interval training can maintain endurance performance ability in already well trained endurance athletes. High-intensity interval training inserted into the training programs of team sport athletes is not only effective at enhancing markers of endurance capacity, but can also improve various aspects of game-specific performance. Nevertheless, while high-intensity training can have the aforementioned profound effects on various aspects of intense exercise performance, the importance of a high training volume background should not be overlooked.
Performance and physiological effects of additional training volume
As concluded by Costill and colleagues (1991), “it is difficult to understand how training at speeds that are markedly slower than competitive pace for 3-4 h.d-1 will prepare (an athlete) for the supramaximal efforts of competition”. Nevertheless, it is well known that athletes involved with intense exercise events perform a number of long duration, low intensity training sessions per week, resulting in high weekly training volumes. Indeed, it has been estimated that well-trained (including world-elite) athletes perform ~75% of their training at intensities below the lactate or ventilatory threshold, despite competing at much higher intensities (Seiler and Kjerland, 2006). This type of training likely contributes to the high energy status of their skeletal muscle (Yeo et al. 2008), their ability to sustain high muscular power outputs for long durations, and their ability to recover from high-intensity exercise. Relative to the number of studies showing enhancements in intense exercise performance with high-intensity interval training, there are relatively few studies documenting improvements in performance with increases in training volume. This may be due to the fact that the time-course for performance improvement with high volume training does not occur as rapidly compared with high intensity training, making investigation into its influence difficult for researchers. In one study that managed to achieve this, Costill and co-workers (1991) divided collegiate swimmers into groups that trained either once or twice a day for six weeks. As a result, one group performed twice the volume of training than the other (4,950 m.d-1 vs. 9,435 m.d-1) at similar high training intensities (95 vs. 93.5% VO2max). Despite higher levels of citrate synthase activity from the deltoid muscle shown in the group that doubled their training volume, performance times following a taper over distances ranging from 43.2 to 2743 m were not different between the groups. While this study demonstrates that a relatively acute period of high volume training (with similar high training intensities) does not appear to enhance performance, the subtle effects of low intensity high training volumes over time should be realized.
The importance of low intensity long duration training sessions has been shown in at least two studies. In one longitudinal study conducted over a six-month period, Esteve-Lanao et al. (2005) compared the influence of different amounts of intensity and volume training on running performance in eight sub-elite endurance runners (VO2max = 70.0±7.3 mL.kg.min-1).
The authors found strong relationships between time spent training at intensities below the ventilatory threshold and both 4 km (r=-0.79; P=0.06) and 10 km (r=-0.97; P=0.008) run performance (Esteve-Lanao et al. 2005). In another study, Fiskerstrand and Seiler (2004) retrospectively investigated changes in training volume, intensity and performance in 21 international medal-winning Norwegian rowers over the years from 1970 to 2001. From the 1970s to the 1990s, VO2max increased by 12% (5.8 to 6.5 L.min-1) while 6-min rowing ergometer performance increased by 10%. In parallel with these performance changes was an increase in low intensity training (i.e., blood lactate <2 mM; 30 h.wk-1 to 50 h.wk-1), or high training volume, coupled with reductions in race pace and supra-maximal intensity training (blood lactate 8-14 mM; 23 h.wk-1 to 7 h.wk-1). As a result, training volume increased by 20% over this period of time (924 to 1128 h.yr-1), as did intense exercise performance results (Fiskerstrand & Seiler 2004).
While the immediate effect of high training volumes on intense exercise performance is difficult to assess, it would appear that the insertion of these low-intensity training sessions has a positive impact on performance, despite being performed at an intensity that is markedly less than that which is specifically performed at during intense exercise competition. These relatively low intensity, high training volumes may be a crucial part of competitive training programs and may provide a platform for the specific adaptations that occur in response to the high-intensity or specific workouts.
The important interplay between high-intensity and high-volume training
The review of studies that manipulate training intensity and volume over a short-term period reveals that successful training programs may benefit from both forms of training at particular periods within an athlete’s training program. When training does not have an appropriate blend of both high-intensity and high volume training inserted into the program, performance ability tends to stagnate. For example, Iaia et al. (2008; 2009) examined the influence of marked changes in intensity and volume training on performance and metabolic enzyme activity in endurance-trained runners. In this study, runners training 45km/wk lowered their training volume to only 15 km/wk for 4 weeks, but instead performed sprint interval training (8-12 x 30s sprints; 3-5 times/week). While markers of sprint performance were improved, 10 km run performance was only maintained, and not enhanced (Iaia et al. 2008; 2009). In a study on competitive swimmers, Faude et al. (2008) used a randomised cross-over design where swimmers performed two different 4-wk training periods, each followed by an identical taper week. One training period was characterized by a high training volume, while the other involved high intensity training; neither program involved aspects of both. The authors found no difference between the training periods for 100 m or 400 m swim performance times, or individual anaerobic thresholds (Faude et al. 2008). Clearly, a mix of both high-intensity and high volume training is important, but predominance of one form of training or the other does not seems to be as beneficial. In a study demonstrating the importance of having equal amounts of distinctly different training, Esteve-Lanao et al. (2007) divided 12 sub-elite runners into two separate groups that performed equal amounts of high-intensity training (~8.4% of training above respiratory compensation point). The difference between the groups in terms of their training however was the amount of low vs. moderate-intensity training they performed. In one group, more low-intensity training (below the ventilatory threshold; 81% vs. 12%) was performed. In the other group, more moderate-intensity training (above ventilatory threshold but below respiratory compensation point; 67 vs. 25%) was performed. While intense exercise performance was not assessed, it is interesting to note that the magnitude of the improvement in 10.4 km running performance 5 months following the intervention was significantly greater (p = 0.03) in the group that performed more low-intensity training (-157 ±13 s vs. -122 ±7 s). Admittedly, the 10.4 km test used to assess running performance falls outside of the intense exercise spectrum, but does suggest that the aerobic power and capacity of these runners was enhanced by this training scheme; a capacity identified previously in this article as critical to intense exercise performance success beyond ~75 s of all-out near maximal activity.
The synthesis of these studies reveals the importance of combining periods of both high and low intensity training into the training programs of the intense exercise athlete. Seiler and Kjerland (2006) refer to this training distribution as a polarized model, where approximately 75% of sessions are performed below the first ventilatory threshold, with 15% above the second ventilatory threshold or respiratory compensation point, and <10% performed between the first and second ventilatory thresholds. For the exercise scientist, these observations beg the question: why might the mixing of distinct high- and low-intensity training sessions be so effective at increasing the energy status of working muscle and subsequent exercise performance?
How does it happen? Beginning to understand molecular signaling
While the picture is far from complete, scientists have begun to make impressive inroads towards understanding how skeletal muscle adapts to varying exercise stimuli, and for an excellent review on the topic, the reader may refer to the work of Coffey and Hawley (2007). As assessed by these authors (Coffey & Hawley 2007), there appear to be at least four primary signals (along with a number of secondary messengers, redundancy and cross-talk) that can lead to an increase in mitochondrial mass and glucose transport capacity in skeletal muscle following several forms of exercise training. These include i) mechanical stretch or muscle tension, ii) an increase in reactive oxygen species that occurs when oxygen is processed through the respiratory pathways, iii) the increase in muscle calcium concentration as required for excitation-contraction coupling, and iv) the altered energy status (i.e., lower ATP concentrations) in muscle. These mechanisms and pathways are complex, with many beyond the scope of this review. For the purpose of this discourse however, the focus will be on the last two of these primary signals, which have received increased attention in recent studies.
The first of these mentioned molecular signals is the prolonged rise in intramuscular calcium, such as that which occurs during prolonged endurance exercise or high exercise training volumes. These high calcium concentrations activate a mitochondrial biogenesis messenger called the calcium–calmodulin kinases (Figure 2). Second, the altered energy status in muscle associated with reductions in ATP concentrations, such as that present during high-intensity exercise, elicits a concomitant rise in adenosine monophosphate (AMP), which activates the AMP-activated protein kinase (AMPK). With these two secondary phenotypic adaptation signals identified, it becomes apparent how different types of endurance training modes might elicit similar adaptive responses (Burgomaster et al. 2008). In support of these distinct pathways, Gibala et al. (2009) showed significant increases in AMPK immediately following four repeated 30-s ‘all-out’ sprints. This was associated 3 hours later with a two-fold increase in peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) mRNA, a transcriptional coactivator that has been described by some as the ‘master switch’ for mitochondrial biogenesis (Adhihetty et al. 2003) (Figure 2). Of note, however, is that this occurred without an increase in the calcium–calmodulin kinases (Gibala et al. 2009), which are known to be stimulated during prolonged repeated contractions (Rose et al. 2007).
With these results in mind, it becomes clear what has been known by coaches for decades; that is, with respect to prescribing training that improves performance, “there’s more than one way to skin a cat”. The high mitochondrial oxidative capacity, improved fat oxidation and glucose transport in the skeletal muscle of endurance athletes may be achieved through either high volumes of endurance training, high intensities of endurance training, or various combinations of both. Higher volumes of exercise training are likely to signal for these adaptations through the calcium–calmodulin kinases (Rose et al. 2007), while higher intensities of endurance training, which lowers ATP concentrations and raises AMP levels, appear more likely to signal for mitochondrial biogenesis through the AMP-activated protein kinase pathway (Gibala et al. 2009). As shown in Figure 2, these different signaling molecules have similar downstream targets (Baar 2006). The result is an increased capacity to generate ATP aerobically. Thus, at the molecular level it may be the blend of signals induced from combined high volume and high-intensity training that elicits either a stronger or more frequent promotion of the aerobic muscle phenotype through PGC-1α mRNA transcription (Figure 2). As well, the lower intensity higher volume training sessions are likely to promote autonomic balance by facilitating recovery and muscle remodeling based on the molecular signals received from the high-intensity training sessions.
How do we optimally structure training programs for high performing endurance athletes?
The synthesis of this information reveals a pattern highlighting the importance of applying periods of both high-intensity and high volume training at the appropriate time in a training program, in order to elicit optimal intense exercise performance. Experts in training program design refer to this art as periodisation (Issurin 2008). While the importance of the high-intensity interval training stimulus appears to be critical (Londeree 1997), the submaximal or prolonged training durations (volume of repeated muscular contractions) cannot be downplayed (Fiskerstrand & Seiler 2004). These high volume training periods likely form the aerobic base needed for the rapid recovery between high-intensity training bouts and sessions. Over time, the progressive result is likely to be an improved efficiency of skeletal muscle and a development of the fatigue resistance aerobic muscle phenotype. Indeed, development of the successful intense exercise athlete tends to require a number of years exposure to high training volumes and intensities (Schumacher et al. 2006). The low intensity high training volumes also likely promote the recovery and autonomic balance needed for the facilitation of the mitochondrial protein synthesis signaled for through the high-intensity interval training sessions. The art of successful intense exercise coaching, therefore, appears to involve the manipulation of training sessions that combine long duration low-intensity periods with phases of very high-intensity work, appropriate recovery, and tapering (Mujika et al. 2000; Issurin 2008; Pyne et al. 2009). These same principles are likely to apply for team sport athletes, although other forms of training should likely be incorporated, such as plyometric training (i.e., drop jumps and countermovement jumps), which has recently been shown to improve agility time in semiprofessional football players (Thomas et al. 2009).
The paper will finish with two practical examples that demonstrate the effectiveness of this model. The first example is New Zealand’s Olympic 800 m running legend, Sir Peter Snell. Snell was a protégé of the late New Zealand athletics coach Arthur Lydiard, who was renowned for prescribing very high training volumes to his athletes who performed intense track events. Throughout his years of training, Snell was prescribed training volumes that would replicate those performed by most marathon runners (~160 km per week, interspersed with weekly high-intensity track workouts; P Snell, personal communication). The result was an 800 m world record in 1962 (1:44.3), and winning double Gold in the 800 m and 1500 m events at the 1964 Tokyo Olympic Summer Games.
In another report of a high training volume plan that elicited a winning intense exercise performance was that of the German 4000-m team pursuit cycling world record achieved at the Sydney 2000 Olympic Games (Schumacher & Mueller 2002). In this paper, Schumacher and Mueller (2002) provide a detailed account of the training performed by the cyclists over the 7 month lead-up to the critical event. In general, training involved extremely high training volumes (29,000 – 35,000 km/yr) that included long periods of low-intensity road training (~50% VO2max) interspersed with stage racing (grand tour) events. While the road racing component of the cyclists’ training program would have entailed numerous periods of both high volume and high intensity stimuli, it wasn’t until the final 8 days prior to the Sydney Olympics that a specific high intensity training taper period on the track was prescribed. Nevertheless, this training design yielded outstanding results, and the model has since been replicated by both the Australian and British cycling teams to break this record repeatedly over the last two Olympic Games (M. Quod, Cycling Australia, Australian Institute of Sport, Personal Communication).
Our understanding of how best to manipulate the training programs of athletes competing in intense exercise events so that performance is optimised is far from complete. It would appear that a polarised approach to training is optimal, where periods of both high-intensity and low intensity but high volume training are performed. The supplementation of high-intensity training to the high volume program of the already highly trained athlete can elicit further enhancements in endurance performance, which appears to be largely due to an improved ability of the engaged skeletal muscle to generate ATP aerobically. Prolonged durations of low intensity or high volume training are likely to facilitate recovery and adaptation by promoting autonomic balance and the health of the athlete. Team sport athletes can also benefit from periods of high-intensity interval training, game-based interval training, and plyometrics. Some of the important molecular signals arising from various forms of exercise training include the AMPK and calcium-calmodulin kinases, likely to be activated in response to intense and prolonged exercise, respectively. Both of these signals have similar downstream targets in skeletal muscle that promote development of the aerobic muscle phenotype. Further understanding of how best to manipulate the training programs for future intense exercise athletes and team sportsmen will require the continued cooperation of sport scientists, coaches and athletes alike.
1) Intense exercise events require a blend of anaerobic and aerobic energy, with the aerobic energy system predominating after near-maximal exercise durations of ~75 s.
2) Short term high-intensity training, typically performed as high-intensity interval training, can elicit improvements in intense exercise performance.
3) Low-intensity, long duration work, or high training volumes, are an important component of successful athletes that perform in intense exercise events, and may facilitate recovery and adaptation from high-intensity sessions by promoting autonomic balance and the health of the athlete.
4) From a metabolic perspective, high training volumes may promote development of the aerobic muscle phenotype through the CaMK signalling pathway, while high-intensity training may affect these adaptations through the AMPK signalling pathway.
5) A polarised training approach where relatively small volumes of high training intensities (~10-15%) are manipulated around large volumes of low intensity training (~75%) appears to be an effective means of enhancing intense exercise performance.
Figure 1. Percent aerobic and anaerobic energy system contributions to near-maximal running over distances ranging from 100 m to 3000 m. Figure derived based on the male data obtained from the studies of Duffield et al. (2004; 2005a; 2005b).
Figure 2. Simplified model of the adenosine monophosphate kinase (AMPK) and calcium-calmodulin kinase (CaMK) signalling pathways, as well as their similar downstream target, the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1 α). This ‘master switch’ is thought to be involved in promoting 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 to operate through the CaMK pathway. ATP, Adenosine Triphosphate; AMP, Adenosine Monophosphate; GLUT4, Glucose Transporter 4; [Ca2+], intramuscular calcium concentration.
Special thanks to Iñigo Mujika, Chris Abbiss, Marc Quod and Alison Hall for their helpful comments and editorial assistance during the preparation of this manuscript.
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