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This study was designed to quantify the daily distribution of training intensity in a group of well-trained junior cross-country skiers and compare the results of three different methods of training intensity quantification. Eleven male athletes performed treadmill tests to exhaustion to determine heart rate and VO2 corresponding to ventilatory thresholds (VT1, VT2), maximal oxygen consumption (VO2max), and maximal heart rate. VT1 and VT2 were used to delineate three intensity zones. During the same time period, all training sessions (N=384, 37 strength training, 347 endurance) performed over 32 consecutive days were quantified using continuous heart rate registration and session Rating of Perceived Exertion (RPE). In addition, a subset of 60 consecutive training sessions was quantified using blood lactate measurements. Intensity distribution across endurance training sessions (n=318) was similar when based on heart rate analysis (75±3%, zone 1; 8±3%, zone 2; 17±4%, zone 3) or session RPE (76±4%, zone 1; 6±5%, zone 2; 18±7%, zone 3). Similarly, from measurements of 60 consecutive sessions, 71% were performed with ≤2.0 mM blood lactate, 7% between 2 and 4 mM, and 22% with over 4 mM (mean=9.5±2.8 mM). In this group of nationally competitive junior skiers, training was organized after a polarized pattern, with most sessions performed clearly below (about 75%) or with substantial periods above (15–20%) the lactate accommodation zone, which is bounded by VT1 and VT2. The pattern quantified here is similar to that reported in observational studies of elite endurance athletes across several sports. It appears that elite endurance athletes train surprisingly little at the lactate threshold intensity.
There is general agreement regarding the physiological factors limiting endurance performance (Pate & Kiska, 1984; Coyle, 1995; Hawley & Stepto, 2001). However, debate continues about how the daily training process should be organized to best develop these components and improve performance. Of the essential training variables, exercise intensity and its distribution is probably the most critical and most heavily debated.
We propose that two basic patterns of training intensity distribution emerge from the research literature (Fig. 1). The threshold-training model emerges from a number of studies demonstrating significant improvements among untrained subjects training at their lactate threshold intensity (Kindermann et al., 1979; Denis et al., 1984; Londeree, 1997; Gaskill et al., 2001). In this pattern of training organization, training at intensities at or very near the lactate threshold is emphasized. In contrast, a polarized-training model emerges from a limited number of published observations of international class rowers (Steinacker, 1993; Steinacker et al., 1998), gold medal winning time-trial cyclists (Schumacker & Mueller, 2002), and internationally elite marathoners (Billat et al., 2001). These studies suggest that at high-performance levels, athletes generally train below the lactate threshold intensity (perhaps 75% of the sessions or training distance), or clearly above the threshold intensity (15–20% of the time), but surprisingly little at their lactate threshold intensity. In essence, the training intensity distribution is polarized away from the moderately hard intensity range represented by the lactate threshold. This appears to be true even for marathoners of international class, who compete at an intensity approximating their lactate threshold (Billat et al., 2001).
Figure 1. Conceptual training intensity distributions associated with (a) the threshold training model – emphasizing training between the first and second lactate/ventilatory thresholds and (b) the polarized training model – emphasizing a large volume of training below the first lactate or ventilatory threshold combined with significant doses of training with loads eliciting 90–100% of VO2max.
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Organizing the training intensity continuum into specific zones is common, with the zones often defined in terms of heart rate or blood lactate concentration ranges. Training zones have been recommended in coaching literature (Gaskill, 1998; Noakes, 2001) and national and international sports governing bodies have implemented standardized intensity-zone scales consisting of up to five different aerobic intensity zones. However, these numerous intensity zones suggest a degree of physiological specificity that is not really present, as the intensity zone boundaries are not clearly anchored in underlying physiological events. Kindermann (Kindermann et al., 1979) first described the “aerobic–anaerobic transition” beginning with the aerobic threshold, marking the first increase in blood lactate, and ending with the anaerobic threshold, corresponding to the maximal lactate steady state. Studies using breath-by-breath gas exchange measurements (Lucía et al., 1998, 1999) have identified two specific ventilatory changes that correspond to the aerobic (LT1) and anaerobic (LT2) thresholds introduced by Kindermann and colleagues. These reproducible ventilatory changes are associated with simultaneous changes in blood lactate, EMG amplitude, and catecholamine concentration (Chwalbinska-Moneta et al., 1998). While questions remain regarding the cause–effect relationships among ventilatory, lactate, EMG, and sympathetic hormone changes, VT1 and VT2 appear to provide useful laboratory markers for the identification of three training intensity zones that are distinguished by meaningful differences in sympathetic stress load, motor unit involvement, and duration to fatigue. The ventilatory threshold approach to defining three intensity zones has been used by several groups in recent years (e.g. Boulay et al., 1997; Lucía et al., 1999; Fernandez et al., 2000), often for the purpose of describing intensity distribution in long endurance competitions. We have named these three intensity zones in terms of blood lactate characteristics: a low lactate zone, a lactate accommodation zone (where blood lactate concentration is elevated but production and removal rates re-establish equilibrium), and a lactate accumulation zone, where blood lactate production exceeds maximum clearance rates, and muscle fatigue is imminent.
In this study, we used the three intensity-zone model to quantify the training intensity distribution of a group of well-trained, male Norwegian cross-country skiers during a critical period of training preceding their competitive season. We hypothesized that these athletes would train according to a polarized model of training, where relatively little training was performed at lactate threshold intensity. We also compared training intensity distribution using three independent measurements: heart rate, blood lactate, and session perceived exertion.
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The key finding of this study is that well-trained junior cross-country skiers, training in a manner consistent with the intensity distribution recommended for highly successful international cross-country skiers, adopt a polarized model of intensity distribution. About 75% of their training sessions are performed with essentially the entire session below the first ventilatory threshold (≤2.0 mM blood lactate). In 5–10% of training sessions, major portions of the training are performed between VT1 and VT2. The remaining 15–20% of training sessions are performed as interval bouts, with substantial periods of work above VT2.
The quantification of daily training performed here appears to be one of the most rigorous available in the literature, with nearly 400 individual training sessions quantified over 32 consecutive days during an important period of competition preparation. However, this was not an experimental study. We did not compare the impact of two different training intensity distributions on performance enhancement. Experimental studies are extremely difficult to perform on high level athletes because neither the athletes nor their coaches wish to suddenly alter methods they have developed over perhaps years of coaching. The yearly training process at this level is indeed an experimental setting involving years of iterative adjustments to training in response to performance results. The training intensity distribution observed in these junior males skiers is very similar to the distribution of training observed in elite rowers (Steinacker, 1993; Steinacker et al., 1998), gold medal winning track cyclists (Schumacker & Mueller, 2002), and international class marathoners (Billat et al., 2001). Because this “75-5-20” distribution of training intensity across the three intensity zones demarcated by VT1 and VT2 emerges from several different studies of highly successful performers in different sports from different countries, we hypothesize that it approximates an optimal intensity distribution for training of high-performance endurance athletes. However, experimental studies comparing this intensity distribution with, for example, greater emphasis on training within the lactate accommodation zone, need to be performed. In addition, studies examining training intensity distribution during different phases of the training cycle are needed. The present study focused only on the pre-competition preparation period. One goal of future investigations should be to quantify to what extent training intensity distribution changes from the preparation period to the competitive period.
A potential shortcoming of the three intensity-zone quantification model used here is that training at very high intensities where heart rate is irrelevant is not incorporated into the quantification structure. For example, Paavolainen et al. (1999) have demonstrated that sprint and explosive plyometric type training can improve endurance performance, pointing to the role of neuromuscular factors in endurance performance. This type of training constituted only 3.5% of all endurance sessions quantified in the present study. It is unclear how these volumes of very high intensity, short duration training impact the total stress load on the endurance athlete.
Ventilatory thresholds were used as a surrogate measure for lactate thresholds when establishing exercise intensity zones. Ventilatory measurements provide two clearly defined physiological events that are practical to identify in a laboratory setting with modern breath-by-breath gas exchange measurement equipment. Corresponding lactate thresholds LT1 and LT2 (sometimes called the aerobic and anaerobic thresholds) are more difficult to define given that blood lactate concentration is a continuous function of increasing exercise intensity. The second threshold (LT2) is sometimes fixed to a specific concentration such as 4.0 mM. However, this approach is imprecise given substantial individual and exercise mode (Beneke & Von Duvillard, 1996) variation in the lactate concentration corresponding to the maximum lactate steady state (MLSS). Lucia and colleagues have successfully used the ventilatory threshold approach to distinguish the physiological characteristics of professional and elite amateur cyclists (Lucía et al., 1998).
That VT1 and VT2 correspond to LT1 and LT2 is supported by studies of Lucía et al. (1999). They demonstrated that in 28 professional or elite amateur cyclists undergoing a continuous ramp test to exhaustion, there were no significant differences in the power output corresponding to VT1, LT1 or the root mean squared EMG amplitude threshold identified as EMGT1. Further, there were no significant differences in power output corresponding to LT2, VT2, and EMGT2. Chwalbinska-Moneta et al. (1998) have demonstrated a close correlation between lactate, EMG, and catecholamine thresholds. Together, these studies support LT1/VT1 and LT2/VT2 as defensible physiological anchor points for the establishment of three training intensity zones. In the present study, the field measurements of blood lactate during training were highly consistent with the ventilatory threshold determinations made in the laboratory, further supporting the validity of the approach. We chose blood lactate concentrations of 2.0 and 4.0 mM as estimates for LT1 and LT2, based on published studies (Lucía et al., 1999) and present results. These values appear to be reasonable for running, cross-country skiing, and rowing. However, for some individuals, and perhaps in general for activities like cycling and speedskating, the LT2 (MLSS) may be substantially higher than 4 mM (Beneke & Von Duvillard, 1996).
The practical link between laboratory testing measurements and training quantification is often heart rate. Downloadable heart rate monitors make it possible to quantify the total time spent during a workout within any specific heart rate range. This function, combined with the identification of threshold heart rates, makes the determination of average heart rate for an exercise bout or “total time in zone” a practical and popular approach to evaluating training intensity. However, we found that this method agreed poorly with both session RPE and blood lactate measurements. Averaging heart rate over an entire session may underestimate the energetic and sympathetic stress of repeated high-intensity bouts such as interval training. In addition, well-trained athletes tend to spend more time warming up and cooling down at lower intensities, which will inflate the time spent in the lowest intensity zone. The session–goal method of heart rate analysis employed here resulted in a distribution of training intensity that was in close agreement with both the athletes' perception and lactate measurements taken during the various types of training performed.
The third intensity quantification approach used in this study was the session RPE method developed by Foster and colleagues. The session RPE method attempts to quantify the athlete's global perception of the stress of an entire training bout, based on an evaluation performed 30 min after training cessation. In studies of cyclists and speedskaters, Foster's group found session RPE multiplied by training duration to be a valid measure of training load, when compared with heart rate quantification. In a retrospective analysis, over 80% of upper respiratory tract infections incurred during the observation period were explained by preceding (within 10 days) elevations in training load quantified using session RPE (Foster, 1998). In our group of well-trained junior athletes, session RPE appeared to be a practical method of monitoring daily training stress that corresponded closely with heart rate and blood lactate measures. Intensity zone determinations based on session RPE and the session–goal heart rate method were in agreement for 92% of all sessions. In the remaining sessions, the session RPE method identified lower intensity zone than heart rate, perhaps because of heart rate drift over the course of a longer workout. Session RPE may be particularly useful in capturing elevations in exercise stress that are not because of acute intensity alone, but also because of the duration of an individual bout, and the background training load and accumulated fatigue experienced by the athlete.
Accepting that we have accurately quantified the day-to-day intensity of training in this group of successful junior athletes, the question remains “Why do successful endurance athletes train above and below their lactate threshold, but surprisingly little at their lactate threshold intensity?” From a biological perspective, the goal of training is to stimulate appropriate changes in gene expression and protein synthesis. This regular, cellular level stimulation must be achieved while preserving the autonomic balance of the organism so that over training is avoided and the capacity for maximal sympathetic mobilization is retained. This balance is severely challenged by elite endurance athletes. The practical manifestation of this interplay between training as cellular adaptive signal and training as negative stressor is the day-to-day manipulation of the intensity, frequency, and duration of exercise.
In untrained subjects, training for 2–3 months, 4–5 days per week at an intensity within the lactate accommodation zone has been shown to stimulate significant improvements in VO2max, lactate or ventilatory thresholds, and endurance performance (e.g. Kindermann et al., 1979; Denis et al. 1984; Londeree, 1997; Gaskill et al., 2001). The intensity region at or near LT2 represents the highest work rate that can be maintained for an extended period, making it an attractive intensity for daily training. However, in well-trained athletes training once or twice daily through most of the year, repeated training bouts at the lactate threshold might generate excessive sympathetic stress (Chwalbinska-Moneta et al., 1998) while still providing a sub-optimal stimulus for eliciting further gains in capacity (Londeree, 1997). Rather than train monotonically within the lactate accommodation zone, elite endurance athletes with unrestricted training time appear to select a training pattern involving the accumulation of large volumes of work at lower intensities combined with 1–3 weekly bouts where significant time is spent at intensities ≥90% of VO2max. High-performance endurance athletes have also gravitated toward multiple daily training sessions instead of one longer daily session. While experimental data is lacking, we assume that there are long-term advantages to this training approach since it has become so common. For example, multiple daily sessions, often at comparatively low intensity, may ensure a high degree of induction of genes for the synthesis of mitochondrial (and other relevant) proteins while ensuring better energy availability and less autonomic stress on the organism. In the present study, 75% of the training sessions were spent training at an intensity of ∼65% VO2max. For well-trained athletes with a high VO2max (70–80 mL kg−1 min−1 in this group), this “low” intensity still generates a high-oxidative flux in the working muscle. Assuming similar active muscle mass, the athletes here training at 65% of their maximal oxygen consumption would have about the same muscular oxidative flux as an untrained person performing at or near VO2max. This magnitude of cellular energy turnover coupled with the relatively long duration the workloads are sustained appears sufficient to provide an effective stimulus for the induction of the various genes involved in mitochondrial biogenesis (Hood et al., 2000). This approach may be preferable in the long-term training of high-performance athletes since combining frequent bouts and moderately hard intensities on most days seems to increase the risk of over training (Bruin et al., 1994).
The hard training sessions quantified here were clearly quite demanding, consisting of repeated 4–8 min work bouts performed at ≥90% of VO2max. These sessions were also reasonably long, 70–100 min in total duration. While these high-intensity sessions are believed to be critical to achieving maximal performances, they cannot be performed optimally if intervening basic endurance sessions are performed at too high an intensity (Bruin et al., 1994). However, high-intensity training sessions appear to be well tolerated when variation in intensity of training is ensured (Lehmann et al., 1991, 1992). Less experienced athletes may tend to train harder than prescribed during low-intensity sessions and not hard enough during prescribed high-intensity sessions (Foster et al., 2001). The junior athletes in the present study, training under the close supervision of an experienced coach of elite athletes, appeared to manage their training intensity quite rigidly. Although they trained substantially fewer hours than top senior-age skiers (who train up to 25–30 h per week), their organization of training intensity was essentially the same as that recommended for the senior national team (personal communication).