Does Lactate-Guided Threshold Interval Training within a High-Volume Low-Intensity Approach Represent the “Next Step” in the Evolution of Distance Running Training?

of the present study was to describe a novel training model based on lactate- guided threshold interval training (LGTIT) within a high-volume, low-intensity approach, which characterizes the training pattern in some world-class middle- and long-distance runners and to review the potential physiological mechanisms explaining its effectiveness. This training model consists of performing three to four LGTIT sessions and one VO 2maxintensity session weekly. In addition, low intensity running is performed up to an overall volume of 150–180 km/week. During LGTIT sessions, the training pace is dictated by a blood lactate concentration target (i.e., internal rather than external training load), typically ranging from 2 to 4.5 mmol L 1 , measured every one to three repetitions. That intensity may allow for a more rapid recovery through a lower central and peripheral fatigue between high-intensity sessions compared with that of greater intensities and, therefore, a greater weekly volume of these speci c workouts. The interval character of LGTIT

external training load), typically ranging from 2 to 4.5 mmol L 1 , measured every one to three repetitions. That intensity may allow for a more rapid recovery through a lower central and peripheral fatigue between high-intensity sessions compared with that of greater intensities and, therefore, a greater weekly volume of these speci c workouts. The interval character of LGTIT allows for the achievement of high absolute training speeds and, thus, maximizing the number of motor units recruited, despite a relatively low metabolic intensity (i.e., threshold zone). This model may increase the mitochondrial proliferation through the optimization of both calcium and adenosine monophosphate activated protein kinase (AMPK) signaling pathways. Keywords: running; performance; physiological adaptations; endurance sports; lactate; training monitoring 1. Introduction On 7 August 2021, 20-year-old Norwegian middle-distance runner Jakob Ingebrigtsen won the 1500 m Olympic title in Tokyo while breaking the Olympic and European records with a time of 3:28.32 (min:s). He also has won the World 5000 m and European 1500 m, 3000 m, 5000 m, and cross-country titles and owns the current indoor 1500 m world record (3:30.60 (min:s)). Further, his brothers Henrik and Filip, also Olympians, won the European 1500 m championships in 2012 and 2016, respectively. Their training pattern was described in a recent article [1] and is considered critical for their development as athletes. While it does not differ greatly from usual training modes in world-class runners [2,3], there is one speci c characteristic which makes it unique and innovative: they were typically measuring their blood lactate concentration ([BLa]) during most of their high-intensity training sessions with the intent of matching a speci c physiological intensity [1]. The main physiological performance determinants which account for success in dis- tance running events are: maximal oxygen uptake (VO2max) [4–6]; running economy (RE), de ned as steady-state VO2at a given submaximal speed or as the VO2per unit of distance [5,7–9]; the ability to sustain a high percentage of VO2maxduring competition (% VO2max) [10–12]; the lactate threshold (LT), de ned either as the velocity at which a non-linear increase in blood lactate occurs, the maximal lactate steady-state (MLSS),

uptake (VO2max) [4–6]; running economy (RE), de ned as steady-state VO2at a given submaximal speed or as the VO2per unit of distance [5,7–9]; the ability to sustain a high percentage of VO2maxduring competition (% VO2max) [10–12]; the lactate threshold (LT), de ned either as the velocity at which a non-linear increase in blood lactate occurs, the maximal lactate steady-state (MLSS), Int. J. Environ. Res. Public Health2023,20, 3782.

Int. J. Environ. Res. Public Health2023,20, 3782 2 of 15 or the velocity corresponding to a blood lactate concentration of 4 mmol L 1 [13]; ve- locity at LT (vLT)/MLSS [14,15]; and the minimum velocity needed to achieve VO2max (vVO2max) [6,16,17]. To improve distance running performance, the training stimulus must enhance one or more of these factors [18]. The training stimulus represents the interaction among training volume (km per week), training frequency, and training intensity designed to enhance the aforementioned performance physiological determinants and performance in distance runners (19). The ideal relationship among these three training variables has, through several decades, been a topic of discussion in both the scienti c [19–25] and coaching [26–30] literature. However, it remains unclear whether selecting the absolute training intensity com- posing the training stimulus through the control of an internal training load marker (i.e., blood lactate concentration) to match speci c metabolic (relative) intensities may represent a training pattern optimizing the improvement of performance and its physiological deter- minants in distance runners. Accordingly, the present article aims to describe this training model and its similarities with those considered optimal according to the current scienti c literature and examine the potential physiological mechanisms which may support its effectiveness. It would encourage the conduction of further intervention studies testing its in uence on performance and its physiological determinants. If this model represents a more ef cient training approach than those currently accepted, it may be useful for coaches and athletes, thereby optimizing performance in the latter. 2. Historical Trends in Distance Runners' Training Principles During the last 100 years, the training principles used by middle- and long-distance runners have been inspired by training theories that provided success for contemporary outstanding runners. To a lesser extent, principles derived from physiological research have contributed to our understanding of how to train runners. In the 1920s and 1930s, international distance running was dominated by Finnish runners. The Finnish sports professor Lauri Pikhala inspired Pavo Nurmi (nine-time Olympic champion from 1920–1928 in events ranging from 1500 m to 10,000 m and cross country) and other Finnish runners with training

principles derived from physiological research have contributed to our understanding of how to train runners. In the 1920s and 1930s, international distance running was dominated by Finnish runners. The Finnish sports professor Lauri Pikhala inspired Pavo Nurmi (nine-time Olympic champion from 1920–1928 in events ranging from 1500 m to 10,000 m and cross country) and other Finnish runners with training principles he brought home from the United States. Their training system during the spring and summer seasons was a precursor to interval training [27]. Nurmi could, for instance, incorporate 6 400 m in 60 s into a slow run of 10 to 20 km in the forest [31]. The term “interval training” was introduced in the 1930s by the German coach Woldmar Gerschler and physician Herbert Reindel [27]. Their interval training represented a way to quantify the training load on the basis of repetitive runs to a heart rate of 180 beats/min, with a recovery interval to a heart rate of 120 beats/min. An interval training session consisted of repetitions of shorter (100 m to 400 m) runs. Gerschler was the coach of elite German middle-distance runners, such as Rudolf Harbig, who broke the 800 m world record in 1939 with a time of 1:46.6 (min:s). Importantly, in the 1930s, many years before the advent of portable heart rate monitors, accurately measuring a heart rate of 180 was nearly impossible, and the rationale for choosing “run to 180, recover to 120” is lost to history. Gösta Holm²r was the coach of the Swedish runners Gunder Hägg and Arne Anderson who set numerous world records (WR) over distances from 1500 m to 5000 m in the 1940s. Holm²r developed “fartlek”, which consisted of intensive efforts of varying distance and duration, interspersed with slower running [32]. It was very similar to Gerschler's interval training but less formally organized and often conducted “by feel” in the forests rather than on a track. Czech runner Emil Zatopek, multiple-time Olympic champion in events from 5000 m to marathon from 1948 to 1952, typically performed an interval training regime consisting of a very high number of

with slower running [32]. It was very similar to Gerschler's interval training but less formally organized and often conducted “by feel” in the forests rather than on a track. Czech runner Emil Zatopek, multiple-time Olympic champion in events from 5000 m to marathon from 1948 to 1952, typically performed an interval training regime consisting of a very high number of repetitions over 400 m (i.e., 60 400 m or 40 400 m with a recovery period between repetitions typically of a 200 m jog). The pace used and effort made during these repetitions were submaximal [27].

Int. J. Environ. Res. Public Health2023,20, 3782 3 of 15 An interval training regime was also used by Mih¡ly Igloi, who coached Hungarian Sandor Iharos. Iharos broke WRs in the events ranging from 1500 m to 10,000 m during the 1950s. Training intensity during their intervals was higher than that used by Gerschler [27]. In the 1960s, the New Zealand coach Arthur Lydiard criticized the hard interval training regimes, primarily on the grounds that predicting when peak performance would occur was dif cult. Lydiard proposed that effective distance running training should be founded on the basis of high volume of continuous low- to moderate-intensity running. He coached his countrymen Peter Snell (three-time Olympic gold medallist in 800 m and 1500 m between 1960 and 1964) and Murray Halberg (5000 m gold in 1960). His training philosophy involved a periodized training pattern. Three main training periods were completed: a 10–12 week preparation period which consisted mainly of high mileage of easy continuous running targeted at reaching 100 miles (160 km) per week, a 6–8 week period characterized by a high volume of hill running, and a 10–12 week competitive period consisting mainly of track interval training at or near race pace leading up to the main competition of the year [26]. In particular, the net effort during the competition period was fairly low, based on Lydiard's saying, “you can't train hard and race hard at the same time”. In the same general timeframe, German coach and physician Ernst Van Aaken proposed the Pure Endurance Training Method, which was based on very similar principles as those proposed by Lydiard, but without xing a speci c training volume (i.e., 100 miles per week), using hill repetitions and developing a periodized pattern. Van Aaken coached German runner Harald Norpoth, who achieved a silver medal at the 1964 Olympic Games in the 5000 m event [29]. In the 1970s and 1980s, many athletes who competed at an international level in distance running used a training regime based on Lydiard's high volume of continuous training principle, but in contrast to Lydiard, they also incorporated sessions of

coached German runner Harald Norpoth, who achieved a silver medal at the 1964 Olympic Games in the 5000 m event [29]. In the 1970s and 1980s, many athletes who competed at an international level in distance running used a training regime based on Lydiard's high volume of continuous training principle, but in contrast to Lydiard, they also incorporated sessions of interval training during the preparation period [33–35]. The “hard day–easy day” approach to training system is usually attributed to University of Oregon coaches Bill Bowerman and Bill Dellinger (bronze medallist in 5000 m in the 1964 Olympic Games), in which two to three high intensity interval sessions per week were separated by easier days (some with a training volume of <5 km/day) with continuous running [30,36]. From the 1970s and 1980s to the present day, most athletes have used a training regime consisting of two to ve weekly sessions of interval training and/or longer tempo runs combined with a relatively high volume of easy and moderate intensity continuous run- ning [33,34,37,38]. A variety of sources have reported that successful distance runners have typically run between 120 and 250 km per week distributed across 11 to18 sessions [37–41] . Most of these training characteristics have been determined through a `trial and error' approach rather than by the outcomes of intervention studies. Furthermore, apart from Gerschler, internal physiological intensity control during high intensity interval sessions has rarely been proposed as a training strategy to improve performance. 3. External and Internal Training Load in Distance Running The training load, which refers to the interaction between training intensity and training volume, can be understood as either external (i.e., measurable aspects of training occurring externally to the athlete such as volume or intensity (i.e., running speed)) or internal (actual psychophysiological response that the body initiates to cope with the requirements elicited by the external load) [42]. Therefore, external load refers to the actual distance covered and speed achieved during a given training session. In turn, internal load can be measured through the monitoring of heart rate or [BLa]. While external training load represents an important

speed)) or internal (actual psychophysiological response that the body initiates to cope with the requirements elicited by the external load) [42]. Therefore, external load refers to the actual distance covered and speed achieved during a given training session. In turn, internal load can be measured through the monitoring of heart rate or [BLa]. While external training load represents an important reference to understand the performance evolution during the training process [3], it is generally believed that internal load may be the most accurate indicator of the effort for distance runners [22] as well as for other sports [42]. Accordingly, measuring internal training load (i.e., [BLa]) during training and using that information to control the absolute training intensity (i.e., speed or duration of repetitions) in order to

Int. J. Environ. Res. Public Health2023,20, 3782 4 of 15 achieve the most optimal stimulus represents a conceptually attractive training protocol which agrees with current recommendations [42]. 4. Training Volume and Intensity Distribution Analysis in Runners Based on Their Internal Response to Exercise The aerobic–anaerobic transition as a framework for predicting performance in en- durance events was introduced in 1979 by Kindermann et al. [43]. During the last ve decades, this framework has been espoused and updated by several scientists using either gas exchange or [BLa] markers [14,40,44,45]. During the last 50–60 years, several de nitions related to the LT parameter have been presented [14]. Today it is common to refer to two breakpoints from a plot of the [BLa] during an incremental exercise test in a laboratory. The rst threshold (LT1) was named aerobic threshold by Skinner and McLellan [44] and refers to the upper limit of aerobic metabolism. Intensities up to this point could last for hours. The second threshold or second lactate threshold (LT2) that has also been associated with the MLSS is known as the highest constant workload during continuous dynamic work, where there is an equilibrium between lactate production and lactate elimination [14,41,46,47]. At a slightly higher intensity than MLSS, the critical power (CP) concept, which is related to the hyperbolic relationship between speed or power output and the duration for which that speed or power output can be sustained, is an alternative approach to de ning the maximal metabolic steady state [48]. According to these concepts, three training intensity zones (see Table) for endurance athletes are commonly used [22,49]. Zone 1 represents speeds below rst ventilatory threshold or 2 mmol L 1 [BLa]. Zone 2 is represented by speeds between the two ventilatory thresholds or 2 and 4.5 mmol L 1 [BLa] (vLT1 and vLT2, respectively). Zone 3 represents speeds above vLT2 [50]. However, this classi cation does not differentiate between low- and high-intensity Zone 2 training, nor does it demarcate the different intensity zones that are in Zone 3, such as lactate tolerance and sprint training, being both above the VO2maxintensity. Table 1.Intensity

or 2 and 4.5 mmol L 1 [BLa] (vLT1 and vLT2, respectively). Zone 3 represents speeds above vLT2 [50]. However, this classi cation does not differentiate between low- and high-intensity Zone 2 training, nor does it demarcate the different intensity zones that are in Zone 3, such as lactate tolerance and sprint training, being both above the VO2maxintensity. Table 1.Intensity scale for distance runners. Scale [BLa] HR VO 2max RPE Training Methods 6-Zone 3-Zone mmol L 1 % Max % 6–20 SST (6) 3 n/a n/a n/a n/a Sprint VHIT (5) 3 8–18 >97 94–140 18–20 Lactate tolerance (i.e., 800 m and 1500 m pace) HIT (4) 3 4.5–8 92–97 88–94 16–18 Intensive aerobic interval (i.e., 5000 m pace) MIT (3) 2 3.5–4.5 87–92 84–88 14–16 Threshold training: interval running (10,000 m pace) MIT (2) 2 2–3.5 82–87 80–84 12–14 Threshold training: continuous/interval running (marathon pace) LIT (1) 1 0.7–2 62–82 55–80 9–12 Easy and moderate continuous running [BLa]: Blood lactate concentration; HR: heart rate; VO2max: maximal oxygen uptake; RPE: rate of perceived exertion according to original Borg scale; SST: short sprint training; VHIT; very-high-intensity training, HIT: high-intensity training; MIT: moderate-intensity training; LIT: low-intensity training; n/a: not applicable; numbers in parentheses in the rst column refer to each zone of the 6-zone scale and numbers in the second column refer to each zone of the 3-zone scale. Furthermore, the transition between the different intensity zones does not follow clearly de ned limits and are not anchored on exactly de ned physiological markers [22]. The relationship between HR and [BLa] will also vary among different runners and in the same athlete across different training periods or seasons [51]. Table training performed, typical [BLa], typical % of HRmax, and % VO2maxin the various zones for well-trained distance runners. Table models) that will be referred to in this article and is elaborated upon according to previous suggestions [1,52,53]. Further mentions of training zones in the present article are referred to by the six-zone scale as z1, z2, . . . , and z6.

various zones for well-trained distance runners. Table models) that will be referred to in this article and is elaborated upon according to previous suggestions [1,52,53]. Further mentions of training zones in the present article are referred to by the six-zone scale as z1, z2, . . . , and z6.

This article reviews a training model for distance runners focusing on lactate-guided training methods.