Effects of High-intensity Training on Performance and Physiology of Endurance Athletes
Carl D Paton, Will G Hopkins
Sportscience 8, 25-40, 2004
Endurance in relation to athletic performance has been defined in various ways. In this article we have reviewed effects of high-intensity training not only on athletic endurance performance but also on underlying changes in the aerobic energy system. Endurance for our purposes therefore refers to sustained high-intensity events powered mainly by aerobic metabolism. Such events last ~30 s or more (Greenhaff and Timmons, 1998).
Training for endurance athletes generally emphasizes participation in long-duration low- or moderate-intensity exercise during the base or preparation phase of the season, with the inclusion of shorter-duration high-intensity efforts as the competitive phase approaches. The effects of low- to moderate-intensity endurance training on aerobic fitness are well documented (see Jones and Carter, 2000 for review), but reviews of high-intensity training on endurance performance have focused only on describing the effects of resistance training (Tanaka and Swensen, 1998), the effects of resistance training with runners (Jung, 2003), and the different types of interval training used by athletes (Billat, 2001a) and studied by researchers (Billat, 2001b). Furthermore, previous reviews have included the effects of high-intensity training on untrained or recreationally active subjects, so findings may not be applicable to competitive athletes. The purpose of this review was therefore to describe the effects of high-intensity training on performance and relevant physiological characteristics of endurance athletes.
We identified most relevant publications through previous reviews and our own reference collections. We found 22 original-research peer-reviewed articles that identified competitive endurance athletes as the subjects in a study of effects of high-intensity training on performance or related physiology. We excluded studies of recreationally active subjects or of subjects whose characteristics were not consistent with those of competitive athletes, including Daniels et al. (1978), Hickson et al. (1988), Tabata et al. (1996), Franch et al. (1998), and Norris and Petersen (1998). We did not perform a systematic search of SportDiscus or Medline databases for theses or for non-English articles, and we did not include data from chapters in books.
We assigned the training to two categories:
Resistance training: sets of explosive sport-specific movements against added resistance, usual or traditional weight training (slow repeated movements of weights), explosive weight training, or plyometrics and other explosive movements resisted only by body mass (Table 1).
Interval training: single or repeated intervals of sport-specific exercise with no additional resistance (Table 2).
Classification of some resistance-training studies was difficult, owing to the mix of exercises or lack of detail. In particular, all the studies we classified under explosive sport-specific resisted movements probably included some non-explosive resisted movements and some plyometrics.
We classified the duration and intensity of intervals in Table 2 as follows: supramaximal (<2 min), maximal (2-10 min) and submaximal (>10 min), where "maximal" refers to the intensity corresponding to maximum oxygen consumption (VO2max). The supramaximal intervals will have been performed at or near all-out effort; the maximal intervals will have started at less than maximum effort, but effort will have approached maximum by the end of each interval; the submaximal intervals can be considered as being close to anaerobic threshold pace (a pace that can be sustained for ~45 min), and effort will have risen to near maximum by the end of each interval.
A major concern with all but one of the studies we reviewed is that the high-intensity training interventions were performed in the non-competitive phases of the athletes’ season, when there was otherwise little or no intense training. Authors who have monitored endurance athletes throughout a season have reported substantial improvements in performance and changes in related physiological measures as athletes progress from the base training to competitive phases (Barbeau et al., 1993; Lucia et al., 2000; Galy et al., 2003). Indeed, our own unpublished observations show that well-trained cyclists ordinarily make improvements in power output of ~8% in laboratory time trials as they progress from base through competitive phases of their season. The large improvement in performance as the competitive phase approaches occurs because athletes normally include higher intensity endurance training as part of a periodized program. It therefore seems unlikely that the large improvements reported in studies performed during a non-competitive phase would be of the same magnitude if the studies were performed in the competitive phase, when the athletes ordinarily include higher intensity training in their program. Indeed, in the only training study we could find performed during the competitive phase of a season, Toussaint and Vervoorn (1990) found that 10 weeks of sport-specific resistance training improved race performance time in national level competitive swimmers by ~1%. Though such improvements appear small, they are important for elite swimmers (Pyne et al., 2004), and the estimated change in power of ~3% is certainly greater than the ~0.5% that is considered important in other high-level sports (Hopkins et al., 1999).
Analysis of Performance
Measures of performance in real or staged competitions are best for evaluating the effects of training interventions on competitive athletes (Hopkins et al., 1999). Toussaint et al. (1990) were the only researchers to use competitive performance in a study of high-intensity training. The others have opted instead for laboratory-based ergometer tests or solo field tests, which may not reproduce the motivating effect of competition. Appendix 1 summarizes the effects from sport-specific time trials and constant-power tests, sorted into the same three intensity/duration categories as the interval training. Appendix 2 summarizes the effects on maximum power in incremental tests. To permit comparison of effects, we have converted outcomes in the various performance tests into percent changes in mean or maximum power, using the methods of Hopkins et al. (2001). Footnotes in the appendices indicate which measures needed conversion.
The remaining tables show the effects of high-intensity training on physiological measures related to endurance performance: maximum oxygen consumption (VO2max, Appendix 3), anaerobic threshold, exercise economy (Appendix 4), and body mass (Appendix 5). Most endurance events are performed at a nearly constant pace, and for those performed at an intensity below VO2max mean performance power or speed is the product of VO2max, the fraction of VO2max sustained, and aerobic energy economy (di Prampero, 1986). Provided they can be measured with sufficient precision, percent changes in each of these components are therefore worth documenting, because they translate directly into percent changes in endurance power. Of course, training is likely to change more than one of these components, so researchers serious about identifying the mechanism of a change in performance should assess all three.
Most authors of the studies we reviewed measured VO2max, usually in an incremental test. Some also measured economy (work done per liter of oxygen consumed) from VO2 measurement either in middle stages of the incremental test or at a fixed work rate in a separate test. Where necessary, we re-expressed percent changes in VO2max and economy for VO2 measured in units of L.min‑1, to avoid difficulties in interpretation arising from changes in mass when VO2 is expressed as ml.min‑1.kg‑1.
No authors measured the fraction of VO2max sustained in the endurance test itself (requiring measurement of VO2 throughout the test), but some measured the anaerobic threshold, usually from an analysis of blood lactate concentration during an incremental test. Depending in its method of measurement, the anaerobic threshold occurs at ~85% of VO2max, an intensity that an athlete can sustain for ~30-60 min (Jones and Carter, 2000). One can therefore assume that percent changes in the anaerobic threshold will translate directly into percent changes in fractional utilization of VO2max in a sub-VO2maximal event. Authors in two studies provided the anaerobic threshold as a power rather than a percent of VO2max; in this form the measure is effectively already a nett measure of submaximal endurance performance, with contributions from VO2max, fractional utilization of VO2max, and economy. We therefore included these measures in Appendix 1 in the subgroup of submaximal tests.
The relevance of changes in anaerobic threshold to changes in endurance performance at maximal and supramaximal intensities is unclear, but for such events (lasting up to ~10 min) anaerobic capacity makes a substantial contribution to performance (Greenhaff and Timmons, 1998). None of the studies we reviewed included critical-power or other modeling of performance to estimate the contribution of changes in anaerobic capacity resulting from high-intensity training. However, a practical and much more reliable measure of anaerobic capacity is performance in sprints lasting ~30 s, which we have included as supramaximal tests in Appendix 1.
mass is an important determinant of performance in running
The outcomes from individual studies are shown in Appendices 1-5, at the end of this article. Table 3 represents a summary derived from the appendices and justified in the following sections.
Appendix 1 shows that maximal and supramaximal intervals produced equally impressive gains (3.0-8.3%) on performance at submaximal intensities. The magnitude of the largest improvement (Westgarth-Taylor et al., 1997) is likely to be due to either sampling variation or a computational error, because it is not consistent with the smaller gains (4.6 and 8.3%) in two similar studies by the same group (Lindsay et al., 1996; Weston et al., 1997). Explosive resistance training was less effective (0.3 and 1.0%) over the same time frame as the interval training studies (~4 wk), and even after 9 wk the gains were still not as great (2.9 and 4.0%) as with interval training. In the only study of the effect of usual weight training on submaximal endurance, there were opposing effects on anaerobic threshold power (2.6%) and time-trial power (‑1.8%) in the same subjects after 12 wk. The authors suggested that the non-specific movement and speed of the weight training accounted for its failure to enhance time-trial performance (Bishop et al., 1999).
Explosive sport-specific movements produced the greatest gains in maximal endurance tests (1.9-5.2%) after 8-9 wk (Appendix 1). Maximum intervals were less effective (2.8%), although the duration of training was only 4 wk. Plyometric jumps were less beneficial (1.2%).
Not surprisingly, the highest-intensity training produced the greatest enhancements in the supramaximal tests (Appendix 1). The very large gain with explosive weights (11%) was more than twice that with supramaximal intervals and explosive sport-specific resistance (3.0-4.6%). Maximal intervals had little effect (0.4%).
There was only one study of the effects of submaximal intervals (Sjodin et al., 1982), and it did not include measures of performance power. The effects on VO2max, anaerobic threshold, and economy in that study, if they were additive, would be consistent with ~6% enhancement of submaximal endurance and possibly 2-4% on supramaximal and maximal endurance respectively.
Maximum-intensity intervals appear to be the most effective form of high-intensity training for improving maximum incremental power (by 2.5-7.0%; Appendix 2). Gains appear to be smaller with explosive sport-specific resistance training (2.3% and 6.0%) and supramaximal intervals (1.0-4.7%), and possibly smaller still with explosive weights (2.0%). Remarkably, a gain of 4.7% was achieved in only four sessions of supramaximal intervals (Laursen et al., 2002a).
These improvements will transfer to time-trial performance to some extent, because maximum power achieved in an incremental test correlates well with time-trial performance (Noakes et al., 1990; Hawley and Noakes, 1992; Bourdin et al., 2004). Exactly how they will transfer might depend on the duration of the time trial. Most of an incremental test is performed at submaximal intensities, but the last minute or two is maximal and supramaximal. Performance in the test will therefore be determined by a mix of VO2max, anaerobic threshold, economy, and anaerobic capacity. If the mix does not reproduce that of the time trial, enhancements of one or more components of the mix will produce changes in maximum incremental power that differ from those in time-trial performance.
It is evident from Appendix 3 that the largest improvements in VO2max occurred with maximal-intensity interval training (gains of 2.3-7.1%). Supramaximal intervals were probably less effective (impairment of 0.6% in one study, enhancements of 2.2% and 3.5% in two others). The changes can occur rapidly: Laursen et al. (2002a) recorded an increase of 3.5% after a total of only four supramaximal sessions in two weeks. Explosive weight training can produce smaller gains (up to 2.0%), but the various forms of resistance training had a predominantly negative effect on VO2max. Improvements in other physiological measures can offset this effect and result in nett improvements in endurance performance following resistance training.
One cannot draw a firm conclusion about the effect of explosive resistance training on the anaerobic threshold in Appendix 4, given that there were major enhancements in three studies (5.0-7.1%) and substantial impairments in two others (2.0 and 2.1%). In the only study of presumably maximal intervals, the gain was ~5.0%, whereas the gain was less (1.5%) in the only study of submaximal intervals.
Although the claim of 39% increase in economy from explosive sport-specific resistance training in Appendix 4 is almost certainly erroneous, it is clear from the other studies in the table that explosive resistance training in general produced spectacular beneficial effects (3.5-18%) on this endurance parameter. Plyometrics may be only a little less effective (3.1-8.6%). The effects of interval training were least for submaximal (2.8%) and greater for a mixture of submaximal and maximal (6.5%).
It is reasonably clear from Appendix 5 that explosive resistance training increased body mass by ~1%, presumably via an increase in muscle mass. Any direct harmful effects of this increase in mass on performance were inconsequential, given the large enhancements that this form of training produced in power output of all durations. Usual weight training may produce increases in body mass that are greater (2.8% in one study) and therefore more likely to impair performance in some sports.
High-intensity interval and resistance training in an endurance athlete’s non-competitive phase can substantially improve performance and related physiological measures. Interval training at intensities around VO2max (intervals lasting 2-10 min) improves mainly submaximal endurance performance (by ~6%) through improvements of all three components of the aerobic system (VO2max, anaerobic threshold, economy). Effects of longer intervals at lower intensity have unclear but possibly similar effects on performance, judging by their effects on the components of the aerobic system. Higher intensities of interval training (intervals of <2 min) probably have similar benefit for submaximal endurance and possibly less benefit (~4%) for shorter durations of endurance performance, but the contribution of aerobic components is unclear. Explosive resistance training produces some benefit (~2%) for submaximal endurance, but probably more benefit (4-8%) for maximal and supramaximal endurance. The effects of explosive resistance training are mediated at least partly by major increases in economy, possibly by increases in anaerobic threshold, but probably not by increases in VO2max. Increases in body mass with this kind of resistance training are not an issue.
Many high-level endurance athletes will already include high-intensity intervals in their training leading up to and including the competitive phase. For these athletes adding more intervals is not necessarily a good strategy, but altering the mix to reduce the volume of lower intensity intervals and increase the volume of higher intensity intervals may be beneficial. Athletes who do not currently include sport-specific explosive resistance training are almost certain to experience substantial gains in performance by adding this form of training to their programs.
A partially selective effect of the different kinds of training on physiological measures raises the possibility of prescribing training to correct weaknesses in these measures. On the basis of the existing research one can tentatively recommend adding or increasing explosive resistance training for an athlete with a poor economy and/or poor anaerobic capacity, and adding or increasing maximal intervals for an athlete with a poor VO2max.
We need more research aimed at filling voids in the matrix of different kinds of training vs effects on performance and physiology. In particular:
We need to know more about the effects of non-specific resistance training (especially plyometrics and usual weights) on performance and some aspects of physiology.
The effects of supramaximal intervals on anaerobic threshold and economy need more research.
The one study on physiological effects of submaximal intervals needs augmenting with studies that include performance measures.
High-intensity sport-specific resistance training of the non-explosive variety has not been investigated other than in the one study that was performed in the competitive phase.
This new research will give us a more complete understanding of how each type of high-intensity training in isolation affects endurance performance. More importantly, it will give us a better indication of the possibility of prescribing training to correct deficits in an athlete's physiological profile. Well-designed studies of individualized training prescription will further address this issue.
From the perspective of the athlete and coach, the most important question is how best to combine the various kinds of high-intensity training before and during the competitive phase of the season. There is currently only one study of high-intensity training of athletes in the competitive phase. We need more, and we need studies of periodization of high-intensity training in the phases leading to competition.
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