Citation: Casado, A.; Foster, C.;
Bakken, M.; Tjelta, L.I. Does
Lactate-Guided Threshold Interval
Training within a High-Volume
Low-Intensity Approach Represent
the “Next Step” in the Evolution of
Distance Running Training? Int. J.
Environ. Res. Public Health 2023, 20,
3782. https://doi.org/10.3390/
ijerph20053782
Academic Editors: António
Carlos Souse and Paul B.
Tchounwou
Received: 27 December 2022
Revised: 16 February 2023
Accepted: 17 February 2023
Published: 21 February 2023
Copyright:
© 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed
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conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
International  Journal  of
Environmental Research
and Public Health
Review
Does Lactate-Guided Threshold Interval Training within a
High-Volume Low-Intensity Approach Represent the “Next
Step” in the Evolution of Distance Running Training?
Arturo Casado 1,*
, Carl Foster 2, Marius Bakken 3 and Leif Inge Tjelta 4
1
Center for Sport Studies, Rey Juan Carlos University, 28933 Madrid, Spain
2
Department of Exercise and Sport Science, University of Wisconsin-LaCrosse, La Crosse, WI 54601, USA
3
Søm Medical Center, 4637 Kristiansand, Norway
4
Departament of Education and Sports Science, University of Stavanger, 4021 Stavanger, Norway
*
Correspondence: arturo.casado@urjc.es; Tel.: +34-914888401
Abstract: The aim 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 VO2max intensity 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 speciﬁc 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 speciﬁc 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 speciﬁc 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), deﬁned as steady-state VO2 at a given submaximal speed or as the VO2 per unit of
distance [5,7–9]; the ability to sustain a high percentage of VO2max during competition
(% VO2max) [10–12]; the lactate threshold (LT), deﬁned 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 Health 2023, 20, 3782. https://doi.org/10.3390/ijerph20053782
https://www.mdpi.com/journal/ijerph
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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 scientiﬁc [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 speciﬁc 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 scientiﬁc
literature and examine the potential physiological mechanisms which may support its
effectiveness. It would encourage the conduction of further intervention studies testing its
inﬂuence on performance and its physiological determinants. If this model represents a
more efﬁcient 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 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 Holmér 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. Holmér 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 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 Health 2023, 20, 3782
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An interval training regime was also used by Mihály 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 difﬁcult. 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 ﬁxing a speciﬁc 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 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 ﬁve 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 to 18 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 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 Health 2023, 20, 3782
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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 ﬁve
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 deﬁnitions
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
ﬁrst 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 deﬁning the
maximal metabolic steady state [48].
According to these concepts, three training intensity zones (see Table 1) for endurance
athletes are commonly used [22,49]. Zone 1 represents speeds below ﬁrst 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 classiﬁcation 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
VO2max intensity.
Table 1. Intensity scale for distance runners.
Scale
[BLa]
HR
VO2max
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 ﬁrst 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 deﬁned limits and are not anchored on exactly deﬁned 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 1 describes the type of
training performed, typical [BLa], typical % of HRmax, and % VO2max in the various zones
for well-trained distance runners. Table 1 uses the intensity scales (i.e., three- and six-zone
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.
Int. J. Environ. Res. Public Health 2023, 20, 3782
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In order to analyze the effect of particular combinations of training volume and
intensity in each of these zones, different training intensity distribution models (TID) have
been described.
1.
The pyramidal model is characterized by a decreasing training volume from z1 to z2
and z3, respectively. Approximately 70–80% of volume is covered in z1, with the
remaining 20–30% in z2 and z3 [50].
2.
The polarized model is characterized by the completion of approximately 80% of the
volume at z1, with most of the remaining 20% covered at z3 and as little training as
possible in z2 [50].
3.
The threshold model features a greater proportion of overall volume conducted in z2
(i.e., >35%) than other models.
According to recent reviews [3,54], either polarized or pyramidal approaches improved
performance in distance runners to a greater extent than other models, which was also
the main conclusion of a recent debate regarding which of the two models was more
effective [24,25]. However, a more recent review reported that a pyramidal approach was
typically adopted more often in highly trained and elite distance runners, despite the fact
that polarized TID also appears to be effective [55]. Most importantly, a high-volume
low-intensity approach is carried out in both the pyramidal and polarized TID models.
5. Physiological and Performance Development Using Lactate-Guided Threshold
Interval Training (LTIT) within a High-Volume Low-Intensity Approach
5.1. Physiological Mechanisms Underpinning the Effectiveness of the Use of High Training Volume
at Low Intensity
Different hypotheses have been proposed to explain the underpinning mechanisms
regarding the reason why a great proportion (70–80%) of overall training volume conducted
at low intensity yields optimal performance development in endurance athletes who will
race at comparatively high intensities (e.g., low speciﬁcity of training). The improvement
of endurance performance through high volume of low/moderate continuous training
is generated by sustaining increased cardiac output over a prolonged time (therefore
augmenting oxygen delivery to working skeletal muscle) and by increased capacity for
the oxidative metabolism through mitochondrial biogenesis and capillarization in Type I
skeletal muscle ﬁbers [56,57]. Importantly, the mozaic architecture of human skeletal
muscle dictates that increased capillarization in Type I skeletal muscle ﬁbers also serves
to augment O2 delivery in Type II muscle ﬁbers. Two primary signaling pathways for
mitochondrial proliferation (both convergent on PGC1-α expression) exist. One is based
on calcium signaling, which is more likely used with high-volume training [57,58], and
the other is based on signaling derived from adenosine monophosphate (AMP)-activated
protein kinase (AMPK) pathway, which is more likely used with high-intensity training, as
[ATP] and AMP levels are reduced and increased, respectively [59,60]. As recruiting certain
motor units elicited during competitive intensity exercise is needed in order to generate
adaptative responses leading to increase mitochondrial density and aerobic metabolism, it
can be achieved through the completion of at least a modicum of high-intensity training.
The fact that most studies conclude that most of the training volume in distance runners
should be covered at easy intensity to optimize performance development implies that
adaptive potential of calcium signaling pathway is much larger than that of the AMPK
signaling pathway. Accordingly, only relatively small training volume of the latter is
needed to reach saturation in the adaptive response using this pathway [58,61].
Alternatively, evidence suggests that some homeostatic disturbances leading to failure
to adapt to training (i.e., overtraining or non-functional overreaching) may be related
to either inﬂammatory responses [62] or that slow autonomic recovery following high
intensity training [63] may be caused by monotonic loads of high intensity training. These
disturbances could lead to a reduction of the capacity for aerobic ATP generation through
deﬁciencies in the mitochondrial electron transport chain or selective delivery of blood
ﬂow and/or reductions in maximal cardiac output [24]. Despite the mechanisms involved,
Int. J. Environ. Res. Public Health 2023, 20, 3782
6 of 15
quasi-experimental observations [64,65] have suggested the negative effects of an excessive
amount of high intensity training. The optimal combination of low- and high-intensity
training is typically achieved with a hard day–easy day pattern which avoids monotony
during the training process and may act to ensure a sufﬁcient recovery period and to
prevent non-functional overreaching. This may augment adaptive responses, such as gene
expression for mitochondrial proliferation [50,66]. This speciﬁc training pattern is adopted
by well-trained and elite long- and middle-distance runners [52,55,64,67–70]. However,
evidence of the exact balance of different types of training on mitochondrial adaptive
responses is limited. Particularly in already well-trained athletes, the range of options for
achieving additional adaptive responses seems likely to be relatively small. Given the large
volume of low-intensity training already performed by high-level athletes, further adaptive
responses may largely lie in optimizing adaptive responses in Type II muscle ﬁbers.
5.2. Physiological Mechanisms Explaining the Effectiveness of LT2 Intensity Training
It is widely accepted that lactate metabolism serves as a useful index [40,71], although
not likely as a cause [72], of muscular fatigue and that a strong correlation exists between
lactate accumulation and level of performance in endurance events [15,73–76]. The rela-
tionship between running intensity/speed and [BLa] is widely used to predict and identify
performance in distance runners [14,40,74]. A strong correlation between the speed at
vLT2/vMLSS and performance in long-distance running has been consistently observed,
regardless of the method used to determine these physiological variables [73,77–79]. In
this sense, Tjeta et al. [51] demonstrated that VO2max, RE, and %VO2max explained 89% of
the variation in vLT2 among distance runners of national to international level. According
to Billat et al. [41], vLT2/vMLSS is a running speed that a well-trained distance runner
can sustain for approximately one hour (half-marathon pace for elite runners). Similarly,
Roecker et al. [74] found that vLT2/vMLSS was slightly faster than half-marathon pace in
427 competitive runners. This was especially the case for the best runners. As vLT2 during
continuous running is close to half-marathon pace, continuous tempo runs from 8–20 km
are classiﬁed as threshold training in z2 and z3. Tempo runs have been included in the train-
ing regime of distance runners from the 1970s up to now [39,70,80,81]. Casado et al. [82]
found that elite Kenyan distance runners performed more of their total training volume as
tempo runs compared with that in the best Spanish distance runners.
The combination of high training volumes in z1 with moderate volumes in z2 and z3
is a very common pattern in contemporary distance runners. It generated improvements in
performance [64,83] or has been associated with very high performance in highly trained
and elite middle- and long-distance runners [41,67–69]. Furthermore, the use of this ap-
proach was reported to be related to either high levels [67,69,70] or an improvement in
RE [64,83]. Some research also found either improvements in [64,83–85] or were related to
high levels of vVO2max [41,69,70]. A few studies, using high volumes in z1 and moderate
volumes in z2 and z3, were associated with high levels of VO2max [67,70,86]. Studies us-
ing this training pattern also found either improvements in [83,85] or were related with
high levels of vLT2 [41,67,69,70]. In any case, there are comparatively few contempo-
rary elite runners who have a total training volume <100 km/week, and most perform
>160 km/week [53,55]. This approach has, in most cases, one primary characteristic in
common, a high proportion of z2 and z3 training was covered at intensities at or near vLT2
(i.e., high intensity within z2–z3) [41,64,67–69,83,87].
The underpinning mechanisms explaining the relationship between training near/at
vLT2 and the development of performance and its physiological determinants are not clear.
However, it has been hypothesized that the use of this speciﬁc exercise intensity improves
muscle speciﬁc adaptations, including clearing of lactate as opposed to reducing lactate
production [88]. Since only recruited motor units are likely to experience increases in
mitochondrial number and capillary density, with the exception that increases in capillary
density in Type I muscle ﬁbers may beneﬁt O2 delivery to Type II muscle ﬁbers, it may
be speculated that training near vLT2 optimizes the number of motor units recruited
Int. J. Environ. Res. Public Health 2023, 20, 3782
7 of 15
without having to accept the consequences of elevated levels of catecholamines likely to be
experienced with z4 training. It is also important to consider that the speed associated with
a [BLa] of 4 mmol·L−1 is somewhat speciﬁc to the pace of competitions in the 10–20 km
range, which represents a large percentage of available competitions. Additionally, this
velocity can be thought of as «speed work» for marathon runners. Sjodin et al. [76] tried to
elucidate the effects of training at the speed associated with onset of [BLa] (vOBLA or speed
associated with a [BLa] of 4 mmol·L−1) and the mechanisms involved explaining those
effects in eight well-trained middle-distance runners. After the addition of one weekly
training session consisting of 20 min of continuous running at vOBLA to their usual training
regime for 8 weeks, the rate of glycogenolysis during exercise decreased (i.e., reduction of
phosphofructokinase/citrate synthase ratio), while the potential to oxidize pyruvate and/or
lactate increased (i.e., increased relative activity of heart-speciﬁc lactate dehydrogenase).
These enzymatic changes were accompanied by an increase in vOBLA and/or a decrease of
[BLa] at a same absolute speed. This speciﬁc training effect is displayed in Figure 1, which
illustrates the evolution/displacement to the right of the lactate/speed curve yielded from
an incremental intensity test (see Figure 1).
 
vLT2 and the development of performance and its physiological determinants are not 
clear. However, it has been hypothesized that the use of this specific exercise intensity 
improves muscle specific adaptations, including clearing of lactate as opposed to reducing 
lactate production [88]. Since only recruited motor units are likely to experience increases 
in mitochondrial number and capillary density, with the exception that increases in capil-
lary density in Type I muscle fibers may benefit O2 delivery to Type II muscle fibers, it 
may be speculated that training near vLT2 optimizes the number of motor units recruited 
without having to accept the consequences of elevated levels of catecholamines likely to 
be experienced with z4 training. It is also important to consider that the speed associated 
with a [BLa] of 4 mmol·L−1 is somewhat specific to the pace of competitions in the 10–20 
km range, which represents a large percentage of available competitions. Additionally, 
this velocity can be thought of as «speed work» for marathon runners. Sjodin et al. [76] 
tried to elucidate the effects of training at the speed associated with onset of [BLa] (vOBLA 
or speed associated with a [BLa] of 4 mmol·L−1) and the mechanisms involved explaining 
those effects in eight well-trained middle-distance runners. After the addition of one 
weekly training session consisting of 20 min of continuous running at vOBLA to their 
usual training regime for 8 weeks, the rate of glycogenolysis during exercise decreased 
(i.e., reduction of phosphofructokinase/citrate synthase ratio), while the potential to oxi-
dize pyruvate and/or lactate increased (i.e., increased relative activity of heart-specific lac-
tate dehydrogenase). These enzymatic changes were accompanied by an increase in vO-
BLA and/or a decrease of [BLa] at a same absolute speed. This specific training effect is 
displayed in Figure 1, which illustrates the evolution/displacement to the right of the lac-
tate/speed curve yielded from an incremental intensity test (see Figure 1). 
 
Figure 1. Blood lactate concentration changes between two different incremental intensity tests char-
acterized by a displacement of the lactate/speed curve to the right after including a certain amount 
of training at the velocity associated with the second lactate threshold during a training period in a 
hypothetical distance runner. 
In addition, these authors [76] found that the runners who were able to maintain 
[BLa] at 4 mmol·l−1 during the 20 min runs experienced greater performance improvement 
after the training period than runners who allowed [BLa] to “drift”. These data are the 
first to suggest that relatively tight control of [BLa] during training might be advanta-
geous. 
Figure 1. Blood lactate concentration changes between two different incremental intensity tests
characterized by a displacement of the lactate/speed curve to the right after including a certain
amount of training at the velocity associated with the second lactate threshold during a training
period in a hypothetical distance runner.
In addition, these authors [76] found that the runners who were able to maintain [BLa]
at 4 mmol·L−1 during the 20 min runs experienced greater performance improvement after
the training period than runners who allowed [BLa] to “drift”. These data are the ﬁrst to
suggest that relatively tight control of [BLa] during training might be advantageous.
5.3. Potential Beneﬁts of Lactate-Guided Threshold Interval Training
In any case, the association between this physiological intensity (i.e., vLT2 or vOBLA)
and speed is usually assumed when the run is continuous. However, manipulating the
variables composing an interval training session (i.e., repetition velocity, duration, and
inter-repetition recovery time) to match vLT2/vMLSS through [BLa] monitoring during
the session may allow for the adoption of faster speeds (i.e., faster than those derived
from continuous runs) and, thus, optimize the adaptive potential of muscle-ﬁber-type-
speciﬁc adaptations required for race pace achievement (i.e., in middle-distance runners).
In this sense, Kristensen et al. [89] demonstrated that an interval training program using a
higher intensity than that derived from continuous exercise yielded a greater activation of
AMP-activated protein kinase in Type II muscle ﬁbers. In this way, conducting training
Int. J. Environ. Res. Public Health 2023, 20, 3782
8 of 15
in z2 and z3 while recruiting Type II muscle ﬁbers may provide the mechanical and
metabolic advantages both of running close to race pace and at LT2 intensity, respectively.
Furthermore, there is an additional advantage of covering interval training at LT2 intensity
rather than in z4, which is related to fatigue generation. Burnley et al. [90] found that
isometric quadriceps contractions conducted at 10% above the critical torque (i.e., just
above LT2 intensity in z4) generated a rate of global and peripheral fatigue four to ﬁve
times greater than that yielded by the same contractions at 10% below of critical torque
(i.e., just below LT2 intensity in z3). These ﬁndings agree with the existence of a threshold
in fatigue development dependent on whether exercise is carried out at, just below, or
just above LT2 intensity. Accordingly, distance runners may beneﬁt from covering some
of their interval training sessions at z3 but at faster absolute speeds than vLT2 (assessed
through a continuous incremental test) rather than in z4. Nonetheless, this should be
done through short duration repetitions so that [BLa] does not progressively rise, as by
doing so runners would be able to recover faster from ‘high-intensity’ training sessions.
However, the use of intensities within z4–z5 has also been found to be useful in performance
development in distance runners (2, 82). A recent systematic review by Rosenblat et al. [91]
determined that high-intensity interval training at or below intensities of VO2max allows the
improvement in central factors inﬂuencing VO2max, such as plasma volume, left ventricular
mass, maximal stroke volume, and maximal cardiac output. However, peripheral factors
inﬂuencing VO2max, such as skeletal muscle capillary density, maximal citrate synthase
activity, and mitochondrial respiratory capacity in Type II ﬁbers can be developed through
sprint interval training (i.e., 30 s repetitions) [91]. Therefore, given that these physiological
adaptations may not all be achieved through lower intensity training (especially those
derived from sprint interval training), a certain but tolerable [65] amount of high intensity
training within z4–z6 is also needed to improve performance optimally in distance runners.
6. Putting This Training Model into Practice
These theoretical physiological advantages derived from LGTIT within a high-volume
low-intensity model are attributed as beneﬁcial by current Norwegian middle- and long-
distance runners specialized in events ranging from 1500 m to 10,000 m. In the late 1990s,
Marius Bakken (co-author of the present article), a Norwegian elite 5000 m runner, started
to test a new training model on himself, which consisted of accumulating a high volume
of training at an easy pace, a moderate volume of interval training at threshold intensity
while controlling the pace through [BLa] testing during the session and including a low
volume of interval training in z5 [92]. He typically covered 180 km overall, conducted four
interval training sessions (i.e., two double sessions through a hard day–easy day pattern) at
threshold intensity (i.e., at [BLa] ranging from 2 to 4.5 mmol·L−1 depending on the speciﬁc
goal of the session) and one session at z5 per week [92]. Bakken experienced that when
following LGTIT, he could perform a much higher training volume compared with that
when he carried out interval training in z4. On the assumption that a higher total volume
of training is associated with larger adaptive responses, this pattern might be thought of as
beneﬁcial. This assumption also agrees with ﬁndings of Burnley et al. [90] on the reduced
fatigue generation at LT2 intensity when compared with that yielded by z4 training. Bakken
developed this training model through a ‘trial and error’ approach and achieved a personal
best time in 5000 m of 13:06.39 (min:s), which remains as the second all-time best Nordic
best. He transmitted his training knowledge and experience to Gjert Ingebrigtsen, father
and former coach of the three Ingebrigtsen brothers, who developed it for the achievement
of their well-known athletic performances [92]. Bakken’s approach became a model for
contemporary Norwegian runners, and much of the success of Norwegian huge runners
at present is based on Bakken’s training principles. For example, Tokyo 2021 triathlon
Olympic champion, Norwegian Kristian Blummenfelt, also used LGTIT [92]. This model
has been developed within a successful system of endurance training. Norway, with a
population of only 5.5 million, has similar men’s national records in distance running events
to those of the USA: 1:42.58, 3:28.32, 7:27.05, and 12:48.45 (min:s) and 2:05:48 (h:min:s) for
Int. J. Environ. Res. Public Health 2023, 20, 3782
9 of 15
the 800, 1500, 3000, and 5000m and marathon, respectively. For women, Norwegians
have held some of the previous 3000, 5000, and 10,000 m and marathon world records.
They also achieved the top national medal count for the cross-country and biathlon skiing
events at the 2022 Winter Olympic Games, and both the triathlon 2019 and 2021 World
Champion (Gustav Iden) and the aforementioned 2021 Olympic champion (Blummenfelt)
are Norwegians [BLa] measurement and scientiﬁc testing are/were part of their training
processes in most of these athletes.
Interval-training performed with lactate values in z2 and z3 is also classiﬁed as thresh-
old training even though the absolute speed at which they are performed can be faster
than half-marathon pace. This is especially the case for shorter intervals, and the authors
of this article have observed international level distance runners showing 20–25 × 400 m
in 64 s average recovering 30 s between repetitions (13:20 (min:s) pace for 5000 m and
26:40 (min:s) for 10,000 m) and 20 × 400 m in 62 s average recovering 60 s between repeti-
tions (12:55 (min:s) pace for 5000 m and, therefore, much faster than half-marathon pace),
with [BLa] remaining below 4 mmol·L−1. The reason why this can be achieved is that
duration of the running time/distance is too short for [BLa] to rise above LT2, and the rest
period between repetitions is long enough for [BLa] to return to levels near LT1 but not
long enough to decrease under that threshold.
It has been reported that the Ingebrigtsen brothers conducted LGTIT over distances
from 2000 m to 3000 m at close to half-marathon pace as well as over distances from
400 m to 1000 m at paces between 5000 m and 10,000 m race paces. The volume of this
LGTIT sessions ranges between 8 and 12 km, and the recovery time between repetitions
ranges between 20 s and 1.5 min. They often covered two LGTIT sessions in the same day
and a ﬁfth speciﬁc session at a much higher intensity in z4 or z5 (i.e., 20 × 200 m uphill
jogging back in 70 s) (1, 67, 92). Their training intensity has been tightly controlled via
measures of heart rate and [BLa] during all interval sessions (1). While the extensive use
of LGTIT (i.e., up to four sessions per week) represents a novelty in the training of elite
distance runners, several studies have reported the combined use of LT2 and z4/z5 training
during the training week. For example, runners may conduct two (or more) different
interval training sessions per week covered at LT2 and VO2max intensities, respectively
(41, 68–70, 85). On the one hand, the addition of a greater number (i.e., two or three) of
‘high-intensity’ sessions to those typically observed in highly trained and elite runners may
represent an advantage in training adaptation, as assimilating this higher training load
may provide greater performance improvements. On the other hand, it also may represent
an increased risk of injury/overtraining syndrome. Furthermore, the characteristics of
LGTIT are different from those accepted in the current literature in distance runners given
that traditionally LT2 training is conducted as continuous runs at much slower absolute
speeds (31). Furthermore, the use of one sprint training session as well as some strength
training sessions have been suggested as part of this training model [92]. In addition, it
has been reported that this model involved the completion of a high training volume (i.e.,
157–185 km/week) [67,92], which also agrees with the accepted efﬁcacy of high training
volume in elite distance runners [2,55,82]. However, the longest run does not exceed
21 km [92]. Finally, while no mention of the periodization approach adopted by these
runners through this training model exists, the authors’ personal observations suggest that
this training pattern involves the use of a traditional periodization approach, as observed
in other elite distance runners [55]. Furthermore, during the competitive period, the z5
hill interval training session should be partly substituted for track workouts targeting
competition pace at high [BLa] (i.e., from 5 to 10 mmol·L−1), and two LGTIT sessions are
removed from the weekly plan. In this way, the goal during the competitive period is to
achieve the minimum dose of threshold work which can sustain the previously developed
aerobic base allowing for the completion of high volumes of competition pace above z3. This
would be consistent with the current literature regarding optimal training periodization in
highly trained and elite distance runners and shows a trend from a pyramidal TID during
the preparatory period towards a polarized TID during the competitive period [38,55,69,85].
Int. J. Environ. Res. Public Health 2023, 20, 3782
10 of 15
The main goal of the present approach is to improve the speed while keeping [BLa] (and
heart rate) stable during LGTIT sessions across the season. An example of speed and
physiological responses (i.e., [BLa] and heart rate) responses during three similar LGTIT
sessions conducted by Bakken during the 2003–2004 season, leading to his former Nordic
5000 m record of 13:06.39 (min:s) is highlighted in Figure 2 and shows the dramatic ﬁtness
improvement derived from the use of the present training model.
Int. J. Environ. Res. Public Health 2023, 20, x FOR PEER REVIEW 
11 of 17 
 
 
Figure 2. Average speed (A) and heart rate (C) per repetition, and post-repetition blood lactate con-
centration (B) during three lactate-guided threshold interval training sessions conducted by Marius 
Bakken across the 2003–2004 season, leading to his former 5000 m Nordic record of 13:06.39 (min:s). 
Six ×1000, seven × 1000, and five × 1000 m with a recovery time between repetitions of one min were 
completed in December 2003 (mid-preparation period), February 2004 (late-preparation period), 
and June 2004 (competitive period), respectively. 
Rather than a revolutionary training model, it seems much more the result of an evo-
lutionary pattern, as it is based on training practice which has been developed during the 
last 100 years of history of training in distance runners. Gerschler trained his athletes 
within a specific heart rate range; Zatopek covered interval training at submaximal paces 
and effort; Lydiard and Van Aaken established the need for developing a big aerobic base 
Figure 2. Average speed (A) and heart rate (C) per repetition, and post-repetition blood lactate con-
centration (B) during three lactate-guided threshold interval training sessions conducted by Marius
Bakken across the 2003–2004 season, leading to his former 5000 m Nordic record of 13:06.39 (min:s).
Six × 2000, seven × 2000, and ﬁve × 2000 m with a recovery time between repetitions of one min
were completed in December 2003 (mid-preparation period), February 2004 (late-preparation period),
and June 2004 (competitive period), respectively.
Int. J. Environ. Res. Public Health 2023, 20, 3782
11 of 15
Rather than a revolutionary training model, it seems much more the result of an
evolutionary pattern, as it is based on training practice which has been developed during
the last 100 years of history of training in distance runners. Gerschler trained his athletes
within a speciﬁc heart rate range; Zatopek covered interval training at submaximal paces
and effort; Lydiard and Van Aaken established the need for developing a big aerobic base
through high training volumes at an easy pace; and Bowerman demonstrated the usefulness
of a hard day–easy day basis. These characteristics were implemented during the training
process of the Ingebrigtsen brothers. Other coaches and researchers also assisted in the
development of an evidence-based and traditional training pattern, which helped these
Norwegian coaches and scientists to generate this new and effective training model for
distance runners. An example of one training week in which this training model is being
used is described in Table 2.
Table 2. Sample training week. Adapted from Bakken [92].
Morning
Evening
Monday
15 km (z1)
12 km (z1). Sprints (z5) and technique.
Tuesday
5 km (z1). 5 × 6 min at 2.5 mmol·L−1 recovering (r.) 1 min
between repetitions (z2). 2 km (z1)
5 km (z1). 10 × 1000 m at 3.5 mmol·L−1 recovering 1 min
between repetitions (z2). 2 km (z1).
Wednesday
16 km (z1). Strength training.
10 km (z1). Sprints (z5) and technique.
Thursday
5 km (z1). 5 × 2 km at 2.5 mmol·L−1 recovering 1 min between
repetitions (z2). 2 km (z1).
5 km (z1). 25 × 400 m at 3.5 mmol·L−1 recovering 30 s
between repetitions (z2). 2 km (z1).
Friday
15 km (z1).
Rest.
Saturday
5 km (z1). 20 × 200 m uphill at 8 mmol·L−1 recovering 70 s
jogging back (z4). 2 km (z1).
10 km (z1).
Sunday
21 km (z1).
Rest.
Z1–5: Zone 1 to Zone 5 according to the 6-zone scale; mmol·L−1 is a measure of blood lactate concentration.
7. Limitations, Future Studies, and Practical Applications
The present article examined the current training regime of some of the best run-
ners in the world and its derived potential physiological beneﬁts on the basis of only
observational studies and reports. Therefore, the assumptions stated previously should
be taken cautiously since no controlled studies have tested the efﬁcacy of this training
model. Furthermore, whereas [BLa] ranges for training zones were suggested according to
current recommendations [1,52,53] allowing for interindividual variability, speciﬁc values
demarcating zones should be detected for each athlete through physiological tests [14].
Additionally, the training characteristics and its effects on performance and its develop-
ment have been described only in 1500 m and 5000 m runners. Its applicability in other
endurance events, such as the marathon, remains uncertain. However, our article presented
sufﬁcient evidence showing that these training characteristics display agreement with those
reported in the current scientiﬁc literature in highly trained and elite distance runners.
In addition, their differences may, in fact, be considered advantages of this new training
approach from a physiological perspective:
1.
The allowance of a greater number of ‘high-intensity’ sessions compared with adopt-
ing a usual z4 interval training-based approach.
2.
Achieving pre-established goals of internal load during the training session.
3.
The possibility of adjusting and individualizing the speciﬁc training sessions within
the model framework in a periodized approach (i.e., month by month, year by year,
etc.). In this way, it is possible to accurately monitor not only the training adaptations
being achieved without the need of speciﬁc tests but also the response to the different
sessions through [BLa] measurements and make individual adjustments to the training
program on the basis of this information.
4.
Adaptation to altitude training while preventing excessive internal training loads
derived from low air’s O2 partial pressure, given that [BLa] monitoring ensures that
internal load remains at the pre-established levels.
Int. J. Environ. Res. Public Health 2023, 20, 3782
12 of 15
For these reasons, new interventions comparing the physiological and performance
effects of the previously described training characteristics with those of traditional training
methods in highly trained distance runners are particularly encouraged. In this way, this
new training model may represent an evolution of the training characteristics of highly
trained and elite distance runners, and if future studies demonstrate its efﬁcacy and safety,
it may be implemented in other runners. Training characteristics and intensity distribution
characterizing this training model and its derived potential physiological beneﬁts are
illustrated in Figure 3.
Int. J. Environ. Res. Public Health 2023, 20, x FOR PEER REVIEW 
13 of 17 
 
 
For these reasons, new interventions comparing the physiological and performance 
effects of the previously described training characteristics with those of traditional train-
ing methods in highly trained distance runners are particularly encouraged. In this way, 
this new training model may represent an evolution of the training characteristics of 
highly trained and elite distance runners, and if future studies demonstrate its efficacy 
and safety, it may be implemented in other runners. Training characteristics and intensity 
distribution characterizing this training model and its derived potential physiological 
benefits are illustrated in Figure 3. 
 
Figure 3. Training characteristics and intensity distribution characterizing the training methodology 
described in the present article and its derived potential physiological mechanisms leading to per-
formance improvement. LT1: first lactate threshold; LT2: second lactate threshold; vLT2: speed as-
sociated to second lactate threshold; VO2max: maximum oxygen uptake; vVO2max: minimum speed 
needed to achieve maximum oxygen uptake; z1–6: Zone 1 to Zone 6 according to the 6-zone scale; 
AMPK: Adenosine monophosphate activated protein kinase; and PGC1-α: Peroxisome proliferator-
activated receptor-γ coactivator. 
Author Contributions: A.C., L.I.T., M.B. and C.F. contributed to the design of the paper; A.C. and 
L.I.T. prepared the first draft of the manuscript. All authors have read and agreed to the published 
version of the manuscript.  
Funding: No funding was provided for the completion of the present study. 
Institutional Review Board Statement: Not applicable. 
Informed Consent Statement: Not applicable. 
Data Availability Statement: Not applicable. 
Acknowledgments: The present article is dedicated to the memory of one of its coauthors, the re-
cently deceased Leif Inge Tjelta, who significantly contributed to the development of distance run-
ning training research in Norway and worldwide. 
Conflicts of Interest: Arturo Casado, Carl Foster, Marius Bakken, and Leif Inge Tjelta have no con-
ficts of interest relevant to the content of this article. 
References 
Figure 3. Training characteristics and intensity distribution characterizing the training methodol-
ogy described in the present article and its derived potential physiological mechanisms leading
to performance improvement. LT1: ﬁrst lactate threshold; LT2: second lactate threshold; vLT2:
speed associated to second lactate threshold; VO2max: maximum oxygen uptake; vVO2max: mini-
mum speed needed to achieve maximum oxygen uptake; z1–6: Zone 1 to Zone 6 according to the
6-zone scale; AMPK: Adenosine monophosphate activated protein kinase; and PGC1-α: Peroxisome
proliferator-activated receptor-γ coactivator.
Author Contributions: A.C., L.I.T., M.B. and C.F. contributed to the design of the paper; A.C. and
L.I.T. prepared the ﬁrst draft of the manuscript. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The present article is dedicated to the memory of one of its coauthors, the
recently deceased Leif Inge Tjelta, who signiﬁcantly contributed to the development of distance
running training research in Norway and worldwide.
Conﬂicts of Interest: Arturo Casado, Carl Foster, Marius Bakken, and Leif Inge Tjelta have no conﬁcts
of interest relevant to the content of this article.
Int. J. Environ. Res. Public Health 2023, 20, 3782
13 of 15
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Does Lactate-Guided Threshold Interval Training within a High-Volume Low-Intensity Approach Represent the "Next Step" in the Evolution of Distance Running Training?
Casado, Arturo,Foster, Carl,Bakken, Marius,Tjelta, Leif Inge