Time to exhaustion at intermittent maximal lactate steady state is longer than continuous cycling exercise

Abstract

The maximal lactate steady state (MLSS) represents a submaximal intensity that may be important in prescribing both continuous and interval endurance training. This study compared time to exhaustion (TTE) at MLSS in continuous and intermittent (i.e., with pauses) exercise, investigating whether physiological variables differ between these exercise modes. Fourteen trained male cyclists volunteered for this investigation and performed an incremental test, several 30-min tests to determine two MLSS intensities (continuous and discontinuous protocol), and two randomized tests until exhaustion at MLSS intensities on a cycle ergometer. The intermittent or discontinuous protocol was performed using 5 min of cycling, with an interval of 1 min of passive rest. TTE at intermittent MLSS was 24% longer than TTE at continuous exercise (67.8 ± 14.3 min vs. 54.7 ± 10.9 min; p < 0.05; effect sizes = 1.04), even though the absolute power output of intermittent MLSS was higher than continuous (268 ± 29 W vs. 251 ± 29 W; p < 0.05). Additionally, the total mechanical work done was significantly lower at continuous exercise than at intermittent exercise. Likewise, regarding cardiorespiratory and metabolic variables, we observed greater responses during intermittent exercise than during continuous exercise at MLSS. Thus, for endurance training prescription, this is an important finding to apply in extensive interval sessions at MLSS. This result suggests that interval sessions at discontinuous MLSS should be used instead of continuous MLSS, as discontinuous MLSS allows for a larger amount of total work during the exhaustion trial.

Full text

Time to exhaustion at intermittent maximal lactate
steady state is longer than continuous cycling
exercise
Talita Grossl, Ricardo Dantas de Lucas, Kristopher Mendes de Souza, and
Luiz Guilherme Antonacci Guglielmo
Abstract: The maximal lactate steady state (MLSS) represents a submaximal intensity that may be important in prescribing
both continuous and interval endurance training. This study compared time to exhaustion (TTE) at MLSS in continuous and
intermittent (i.e., with pauses) exercise, investigating whether physiological variables differ between these exercise modes.
Fourteen trained male cyclists volunteered for this investigation and performed an incremental test, several 30-min tests to
determine two MLSS intensities (continuous and discontinuous protocol), and two randomized tests until exhaustion at
MLSS intensities on a cycle ergometer. The intermittent or discontinuous protocol was performed using 5 min of cycling,
with an interval of 1 min of passive rest. TTE at intermittent MLSS was 24% longer than TTE at continuous exercise
(67.8 ± 14.3 min vs. 54.7 ± 10.9 min; p< 0.05; effect sizes = 1.04), even though the absolute power output of intermittent
MLSS was higher than continuous (268 ± 29 W vs. 251 ± 29 W; p< 0.05). Additionally, the total mechanical work done
was significantly lower at continuous exercise than at intermittent exercise. Likewise, regarding cardiorespiratory and meta-
bolic variables, we observed greater responses during intermittent exercise than during continuous exercise at MLSS. Thus,
for endurance training prescription, this is an important finding to apply in extensive interval sessions at MLSS. This result
suggests that interval sessions at discontinuous MLSS should be used instead of continuous MLSS, as discontinuous MLSS
allows for a larger amount of total work during the exhaustion trial.
Key words: physiological responses, endurance capacity, submaximal performance, time to exhaustion, intermittent exercise.
Résumé : La concentration maximale de lactate en régime stable (MLSS) correspond à une intensité sous-maximale d’effort
à prescrire pour des programmes d’entraînement à l’endurance, continu ou par intervalle. Cette étude compare les temps
d’épuisement (TTE) à la MLSS d’un exercice continu et par intervalle (avec pauses) et analyse les différences des variables
physiologiques entre ces modes d’exercice. Quatorze cyclistes masculins entraînés participent à cette étude et effectuent un
test d’effort progressif et plusieurs tests d’une durée de 30 min pour déterminer deux intensités d’effort (continu et discon-
tinu) correspondant à la MLSS; de plus, ils participent à deux tests d’effort jusqu’à épuisement sur une bicyclette ergomé-
trique, et ce, de façon aléatoire. Le protocole discontinu ou par intervalle est constitué de périodes de 5 min d’exercice sur
vélo intercalées de périodes de repos passif d’une durée de 60 s. Le TTE à la MLSS du protocole discontinu est de 24 %
plus long que le TTE du protocole continu (67,8 ± 14,3 min vs. 54,7 ± 10,9 min; p< 0,05; amplitude de l’effet = 1,04)
même si la production absolue de puissance au cours du protocole intermittent à la MLSS est plus grande que dans le proto-
cole continu (268 ± 29 W vs. 251 ± 29 W; p< 0,05). De plus, la quantité totale de travail mécanique réalisé au cours de
l’exercice continu est significativement plus faible qu’au cours de l’exercice intermittent. Au sujet des variables cardiorespi-
ratoires et métaboliques, on observe à la MLSS des réponses plus grandes durant l’exercice intermittent comparativement à
l’exercice continu. En matière de prescription d’exercices dans un programme d’entraînement à l’endurance, il est important
d’appliquer cette observation dans les séances exhaustives d’exercices par intervalles à la MLSS. D’après cette observation,
on devrait privilégier les séances d’effort discontinu à la MLSS comparativement aux séances d’effort continu, car les pre-
mières permettent de réaliser une plus grande quantité totale de travail au cours des tests d’effort jusqu’à épuisement.
Mots‐clés : réponses physiologiques, capacité d’endurance, performance sous-maximale, temps jusqu’à épuisement, exercice
intermittent.
[Traduit par la Rédaction]
Introduction
Through the measurement of blood lactate concentration
([La]) during incremental exercise testing, it is possible to ac-
cept the existence of physiological domains separated by two
typical breakpoints: the intensity at which [La] begins to rise
above baseline levels (i.e., lactate threshold, LT) and the in-
Received 12 September 2011. Accepted 24 May 2012. Published at www.nrcresearchpress.com/apnm on xx August 2012.
T. Grossl, R.D. de Lucas, K.M. de Souza, and L.G.A. Guglielmo. Sports Center, Federal University of Santa Catarina, Centro de
Desportos –Laboratório de Esforço Físico, Campus Universitário –Trindade, CEP: 88040-900 Florianópolis (SC), Brazil.
Corresponding author: Talita Grossl (e-mail: talitagrossl@gmail.com).
1047
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tensity corresponding to the onset of blood lactate accumula-
tion (OBLA). OBLA is an indirect way to determine the
maximal lactate steady state (MLSS) (Bentley et al. 2007;
Faude et al. 2009).
The MLSS can be defined as the highest running velocity
or cycling power output at which [La] remains stable during
prolonged submaximal constant-workload exercise (Beneke
2003) and has been considered the upper limit of the heavy
intensity domain (Pringle and Jones 2002; Beneke et al.
2011).
The physiological importance of the MLSS is that it de-
fines the exercise intensity above which there is a net contri-
bution of energy associated with lactate accumulation due to
an increased rate of glycolysis that exceeds the rate of mito-
chondrial pyruvate utilization (Heck et al. 1985).
It is important to highlight that MLSS is usually deter-
mined by continuous (MLSScon) and long protocols. Never-
theless, the prescription of aerobic training in many sports is
conducted by interval sessions, in which case, it is necessary
to make adjustments on the intensity. Both continuous and
interval training sessions designed to enhance endurance ca-
pacity have been conducted at heavy or severe intensity do-
mains and used the MLSS (measured directly or indirectly)
to define the exercise intensity (Billat et al. 2004; Philp et al.
2008; Striegel et al. 2008).
It has been demonstrated that the inclusion of rest periods
during MLSS determination modifies the level of physiologi-
cal exertion during the tests and may modulate the MLSS
(Beneke et al. 2003). Hence, Beneke et al. (2003) found that
the MLSS power output determined in discontinuous or inter-
mittent protocol (MLSSint) is approximately 10% higher than
the MLSScon. This could be explained, at least in part, by the
partial resynthesis of creatine phosphate stores and blood lac-
tate removal during recovery periods (Dupont et al. 2004).
Furthermore, the recovery periods during intermittent exer-
cises reduce the glycolytic flux of the previously active
muscles (Beneke 2003), which could generate a delay in the
[La] kinetics before the steady-state condition is attained
(Barbosa et al. 2011). Thus, using training loads higher than
MLSScon during interval training may also determine stability
of [La]. Therefore, the protocol used to determine MLSScon
may have limited applications for extensive interval training
prescriptions for endurance athletes.
Usually, the volume of the training session at MLSS
ranges between 30 and 60 min and has been applied using
both continuous and interval exercise modes (Billat et al.
2004; Philp et al. 2008). The relationship between the train-
ing volume at this intensity and its time to exhaustion (TTE)
could improve the individualization of training load. To date,
few studies have focused on the time to exhaustion at
MLSScon (TTEcon), and these studies have demonstrated that
the exercise can be sustained for ~60 min (Billat et al. 2004;
Baron et al. 2008; Legaz-Arrese et al. 2011). To our knowl-
edge, no study has attempted to measure the TTE at discon-
tinuous MLSS (i.e., MLSSint).
Thus, there is a lack of information about endurance time
at MLSSint (TTEint). Therefore, based on the aforementioned
references (Beneke 2003; Barbosa et al. 2011), it can be hy-
pothesized that although power output at MLSSint is higher
than at MLSScon, TTE is similar for both intensities, that is,
the rest intervals (from MLSSint) could compensate for the
expected higher power output. Hence, the aim of the present
study was to determine and to compare the TTE and physio-
logical responses at continuous and intermittent MLSS in
trained cyclists.
Materials and methods
Participants
Fourteen trained male cyclists (n= 14, 29.7 ± 5.3 years,
76.5 ± 7.0 kg, 176.9 ± 5.5 cm, 12.6% ± 4.6% body fat) vol-
unteered for this study. All participants had at least three
years of experience with training and competitions. In the pe-
riod preceding this study, participants trained 5–6 days·week–1,
with a weekly training volume of 320–360 km. After being
fully informed of the risks and stresses associated with the
study, subjects gave their written, informed consent to par-
ticipate in the study. The study was performed according to
the Declaration of Helsinki, and the protocol was approved
by the Ethics Committee of the Federal University of Santa
Catarina, Florianopolis, Brazil.
Overview of experimental design and the equipment used
The participants were instructed to arrive at the laboratory
in a rested and fully hydrated state at least 3 h postprandial
and to avoid strenuous exercise in the 48 h preceding a test
session. Each subject was tested at the same time of day
(±2 h) to minimize the effects of biological variation (Carter
et al. 2002). Participants visited the laboratory for physiolog-
ical testing on eight to 10 separate occasions within a 3- to 4-
week period and performed only one test on any given day.
The tests were separated by at least 48 h.
Initially, all participants were assessed for body mass (kg),
height (cm), and seven skinfolds (chest, midaxillary, suprail-
iac, abdomen, triceps, subscapular, and thigh) to estimate
body fat (Jackson and Pollock 1978). After that, they per-
formed an incremental maximal test on an electromagnetic
braked cycle ergometer (Ergofit 167 Cycle, Pirmasens, Ger-
many) to determine the maximal oxygen uptake ( _
VO2max),
peak power output (PPO), maximal heart rate (HRmax), max-
imal ventilation ( _
VEmax), and OBLA.
After the determination of OBLA, several constant load
tests were performed with the continuous and intermittent
protocols to determine the MLSS power outputs. Next, each
participant performed two TTE tests in a randomized order
(continuous vs. intermittent). The preferred cadence (±5 rev-
olutions·min–1) of each participant was adopted in all tests
and remained constant throughout the experiment. All tests
for determination of MLSScon, MLSSint, and TTEcon and
TTEint, respectively, started with a 5-min warm-up phase at
50% of PPO.
Oxygen uptake ( _
VO2) and ventilation ( _
VE) were measured
breath by breath using a gas analyser (Quark PFT Ergo,
Cosmed, Rome, Italy). The gas analyzers were calibrated im-
mediately before each test using ambient air (assumed to con-
tain 20.94% oxygen and 0.03% carbon dioxide) and certified
alpha standard gases containing 16.0% oxygen and 5.0% car-
bon dioxide (White Martins Ltda, Osasco, Brazil). The tur-
bine flowmeter used for the determination of minute
ventilation has a resistance of <0.7 cm H2O·L–1·s–1at a flow
rate of 12 L·s–1and an accuracy of ±2% and was calibrated
with a 3 L syringe (Quark PFT Ergo, Cosmed, Rome, Italy).
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Heart rate (HR) was recorded continuously during the test by
a HR monitor incorporated into the gas analyser. Blood sam-
ples (25 µL) were collected from the ear lobe into microcen-
trifuge tubes containing 50 µL NaF (1%), and the [La] was
determined by an electrochemical method (YSI 2700 STAT,
YSI Incorporated, Yellow Springs, Ohio, USA).
Incremental maximal test
The incremental test started at 105 W, and the workload
was increased by 35 W every 3 min until to exhaustion (De-
nadai et al. 2004). Each participant was verbally encouraged
to undertake maximum effort. _
VO2and _
VE data were re-
duced to 15 s mean values. _
VO2max and _
VEmax were consid-
ered as the highest value obtained in a 15 s interval. The
attainment of _
VO2max was defined using the criteria proposed
by Lacour et al. (1991). Blood samples were collected during
the final 15 s of every 3 min. HRmax and peak [La] ([La]peak)
were the highest HR and [La] obtained during the test, re-
spectively. PPO was determined according to Kuipers et al.
(1985).
Determination of MLSScon and MLSSint
For the determination of MLSScon, each constant power
output test lasted 30 min. The power output of the first test
corresponded to a [La] of 3.5 mmol·L–1(OBLA) measured
during the incremental maximal test. A fixed blood [La] of
3.5 mmol·L–1, instead of 4.0 mmol·L–1, has been used in in-
cremental tests with 3-min stages (Heck et al. 1985; Smith
and Jones 2001). Blood samples were collected on the 10th,
20th, and 30th min of the constant power output tests.
The initial intensity for determination of MLSSint was 5%
above the MLSScon. The identification of MLSSint was simi-
lar to the continuous protocol, but with a total duration of
35 min due to 1-min rest intervals (passive recovery) for
each 5 min of cycling, characterizing an exercise–rest ratio
of 5:1. Blood samples for measurement of [La] were col-
lected on the 10th, 20th, and 30th min of exercise (i.e., at
the end of 2nd, 4th, and 6th exercise intervals).
If a steady state or a decrease in [La] was observed during
the first constant power output test of both protocols, further
subsequent 30-min constant power output tests with a 5%
higher power output were performed on separate days until
no [La] steady state could be maintained. If the first constant
power output test resulted in a clearly identifiable increase in
the [La] and (or) could not be completed due to exhaustion,
further constant power output tests were conducted with sub-
sequent reduced (5%) power output. The MLSS in both pro-
tocols was determined for each subject as the highest power
output that could be maintained with [La] increase lower than
1 mmol·L–1during the last final 20 min of appropriate tests
(Beneke 1995; Beneke et al. 2001).
Determination of TTEcon and TTEint
All participants were asked to perform a test until exhaus-
tion at the intensity corresponding to MLSScon and at previ-
ously determined MLSSint._
VO2, HR, and _
VE were
continuously measured according to the procedures used in
the incremental maximal test. At TTEcon, blood samples
were collected on the 10th, 20th, and 30th min and at ex-
haustion. Furthermore, from the 30th min, the participants
were given ~100 mL of water every 10 min to avoid dehy-
dration. At TTEint, blood samples were collected on the
10th, 20th, and 30th min of exercise (i.e., at the end of 2nd,
4th, and 6th exercise intervals) and at exhaustion. In the in-
termittent protocol, the participants were given ~100 mL of
water from the 35th min and every 10 min until the end of
exercise.
The interruptions in the intermittent protocol were not
counted when determining the duration of the test (i.e.,
TTEint). The criterion of exhaustion was the incapacity to
maintain the preferred cadence (±5 revolutions·min–1) for the
second time or the participant stopped cycling voluntarily
(Fontana et al. 2009). The total amount of mechanical work
(kJ) completed in each TTE trial was calculated as a product
of power outputs (W) and time (s).
Nevertheless, because of the different TTEcon and TTEint of
the 14 participants, _
VO2,_
VE, and HR were expressed and
analysed as percentages of TTEcon and TTEint between 20%
and 100% of TTE (t20%,t40%,t60%,t80%, and t100%). The mean
values of the last 2 min of all percentages were used. More-
over, blood lactate was analysed on the 10th, 20th, and 30th
min and at exercise termination on TTE (t10,t20,t30, and tend).
These values were used to calculate the average [La] at TTE
([La]TTE).
Statistical analysis
Data are presented as mean ± standard deviation (SD).
Normality was assessed by the Shapiro–Wilk test. Student’s
ttest for paired data was used to compare variables (TTE,
MLSS intensity, total work done, and [La]TTE) between con-
tinuous and intermittent exercise.
Two-way ANOVA with repeated measures was used to an-
alyze differences in physiological variables between the exer-
cise conditions (continuous versus intermittent) and within
conditions at different times of TTE. Multiple comparisons
were made with the Bonferroni post hoc test when necessary.
Cohen’sdeffect sizes (ES) were calculated as the differ-
ence between the means divided by the mean SD to charac-
terize the practical (clinical) significance rather than the
statistical significance. The following criteria for effect sizes
were used (Cohen 1988): <0.1, trivial; 0.1–0.3, trivial to
small; 0.3–0.5, small; 0.5–0.7, small to moderate; 0.7–1.1,
moderate; 1.1–1.3, moderate to large; 1.3–1.9, large; 1.9–2.1,
large to very large; and >2.1, very large.
The relationship between HR, _
VO2, and _
VE at TTEcon and
TTEint was examined using the Pearson product moment cor-
relation coefficients. Analyses were carried out using the
GraphPad Prism software package for Windows (version 5.0;
GraphPad Prism Software Inc., San Diego, California, USA).
The level of significance was p< 0.05 for all analyses.
Results
Incremental maximal test
The values of PPO, HRmax,_
VO2max,_
VEmax, and [La]max
determined during the incremental test are reported in Ta-
ble 1.
Time to exhaustion
MLSSint (268 ± 29 W) was 6.5% higher than MLSScon
(251 ± 29 W), and even so, TTEint was 24% longer than
TTEcon (67.8 ± 14.3 min and 54.7 ± 10.9 min, respectively;
Grossl et al. 1049
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p< 0.05). The effect size showed a moderate effect (i.e.,
1.04) when compared with the TTE in both protocols. Dur-
ing the TTEint trial, subjects were able to complete an aver-
age of 13 intervals of 5-min cycling. Furthermore, [La]TTE
was lower during continuous cycling than during intermittent
cycling (p< 0.05; Table 2). Additionally, the total mechani-
cal work done at MLSScon was significantly lower than that
done at MLSSint (p< 0.05; Table 2) during TTE trials and
presented a moderate to large effect size (i.e., 1.24).
The participants maintained a higher percentage of PPO at
MLSSint (79.3% ± 3.2%) than at MLSScon (74.3% ± 3.0%;
p< 0.05). Table 3 presents the values of HR, _
VO2, and _
VE
relative to time percentages of TTEcon and TTEint. It can be
observed that only _
VO2values were kept stable during both
TTE. HR, _
VO2, and _
VE values at MLSScon and MLSSint are
depicted in Fig. 1. The _
VO2at TTEint was significantly
higher (p< 0.05) than the _
VO2at TTEcon in all percentages
of TTE. Conversely, the HR showed no significant difference
between continuous and intermittent exercise from t60% until
exhaustion (Fig. 1). Conversely, the HR showed no signifi-
cant difference between continuous and intermittent exercise
from t60% until exhaustion (Fig. 1). However, with the excep-
tion of t60%, the _
VE at TTEint was significantly higher (p<
0.05) than the _
VE at TTEcon in remaining percentages of
TTE. Furthermore, there were high correlations between
TTEcon and TTEint for the physiological variables HR, _
VO2,
and _
VE(r= 0.83, 0.88, and 0.87, respectively).
Figure 2 shows the blood lactate parameters during the
TTEcon and TTEint. With the exception of tend, all [La] values
at t10,t20, and t30 were significantly lower (p< 0.05) during
continuous exercise (3.5 ± 0.8, 3.8 ± 0.8, and 4.1 ±
0.9 mmol·L–1, respectively) compared with intermittent exer-
cise (4.3 ± 1.0, 4.8 ± 1.1, and 4.9 ± 1.2 mmol·L–1, respec-
tively). Furthermore, in both protocols, [La] at tend (4.9 ± 1.2
and 5.5 ± 1.8 mmol·L–1at TTEcon and TTEint, respectively)
was significantly higher than in the first 30 min (Fig. 2).
Discussion
The major finding of this study was that the time to ex-
haustion at MLSSint was about 24% higher than at MLSScon,
even though the power output at MLSSint was higher than at
MLSScon (268 ± 29 vs. 251 ± 29 W; p< 0.05). In fact, this
higher (6.5%) power output at MLSSint found in the present
investigation is in agreement with the previous study (Beneke
et al. 2003). Therefore, these results demonstrate that the in-
clusion of systematical rest periods to determine MLSS leads
to an overestimation of the power output of traditional MLSS
determination. On the other hand, if an athlete uses the
MLSScon during an interval training session, the physiologi-
cal exertion will be underestimated; hence, this might lead to
an inappropriate prescription of MLSS training intensity. This
fact was reported by Beneke et al. (2003), who showed dif-
ferent blood lactate responses during exercises performed in
intermittent and continuous modes at MLSS (i.e., with and
without pauses, respectively). The change in [La] during the
final 20 min was higher in continuous cycling (1.2 mmol·L–1)
than in intermittent cycling (0.2 mmol·L–1for 30 s pauses
and –0.3 mmol·L–1for 90 s pauses), both performed at the
same power output (MLSScon).
Based on the aforementioned evidence. we hypothesized
that the endurance time could be similar between two MLSS
modes, because the rest intervals (from MLSSint) could com-
pensate for the higher power output compared with MLSScon.
Surprisingly, the length of time spent at MLSSint found in the
present study did not support this hypothesis. Unfortunately,
we were unable to explain the exact mechanism involved in
the longer TTE during discontinuous trial, as the physiologi-
cal measures obtained in this study did not help to under-
stand the differences in TTE.
One possible reason for stopping prolonged exercise could
be related to substrate availability (i.e., muscle and liver gly-
cogen depletion). Considering the direct relationship between
exercise intensity and glycolytic energy flux (Vøllestad and
Blom 1985), this could not support the longer TTE score ob-
served during MLSSint, as the power output at this intensity
was higher than during the continuous protocol. Likewise,
the cardiorespiratory and metabolic responses were higher
during intermittent exercise.
The duration limits of exercising at MLSS have been in-
vestigated over the last decade, and many doubts remain
about the mechanisms involved in the intolerance at this in-
tensity (Baron et al. 2008). To our knowledge, the first study
that determined the endurance time at MLSScon was pub-
lished by Billat et al. (2004). These authors found an average
TTE of 44 min before and 63 min after a training period in
runners. It is important to note that before the training period,
the mean velocity at MLSS was 13.8 km·h–1, whereas after
training, this velocity was increased to 15.2 km·h–1, although
the relative intensities were the same (i.e., ~85% of velocity
at _
VO2max). A subsequent study conducted by Baron et al.
(2008) reported a TTE of 55 ± 8 min in well-trained cyclists
during MLSScon exercise, while Legaz-Arrese et al. (2011)
reported 64 ± 15 min in endurance-trained runners. These re-
sults are highly consistent with 54.7 ± 10.9 min found in
present study for MLSScon and 67.8 ± 14.3 min for MLSSint.
The results of the present study could be interesting for the
endurance training area, as we demonstrated that during
MLSSint, the athletes were able to perform a higher total me-
chanical work level at the same relative intensity (i.e., maxi-
mal equilibrium between [La] appearance and
disappearance). Thus, for endurance training prescription,
this is an important finding to apply in extensive interval ses-
sions at MLSS.
During an endurance exercise at the same power output,
greater physiological stress during continuous versus inter-
mittent effort may be expected (Beneke et al. 2003) because
of two factors associated with the latter: (i) the partial recovery
Table 1. Mean ± SD for physiological variables at-
tained during the incremental maximal test (n= 14).
Variables Values
PPO (W) 337.4±32.4
HRmax (bpm) 195±5
_
VO2max (mL·kg–1·min–1)59.9±9.6
_
VO2max (L·min–1)4.6±0.6
_
VEmax (L·min–1)163.4±29.5
[La]peak (mmol·L–1) 11.4±2.0
Note: PPO, peak power output; HRmax, maximal heart rate;
_
VO2max, maximal oxygen uptake; _
VEmax, maximal ventilation;
[La]peak, peak blood lactate concentration.
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of muscle phosphagens and O2stores, and (ii) increased
blood lactate removal. However, it is important to highlight
that in the current study, the exercise intensity between con-
tinuous and intermittent cycling was normalized by [La] bal-
ance, which resulted in different power outputs. During
intermittent exercise, [La] are affected by the rate at which
lactate is being produced, the duration of time during which
high levels of lactate production are necessary (i.e., exercise
duration), and the period of time during which the rate of
lactate production is low (i.e., rest periods). During TTE tri-
als, the [La] response showed higher values for MLSSint
compared with MLSScon during the first 30 min. At the ex-
haustion point, the [La] showed no difference between the
conditions. Analysing the mean values of [La] during TTE,
the present study disagrees with the study published by Ben-
eke et al. (2003), as they found similar values during 30 min
of exercise, instead of TTE.
Moreover, we found [La] accumulation after the first
30 min in both protocols (Fig. 2), as the [La] was higher at
the completion than in the 30th min. Baron et al. (2008)
found an opposite behaviour, with significantly lower [La] at
the end compared with minutes 20 and 30. More studies de-
signed with intermittent exercises over 30 min at MLSS are
necessary to better understand the kinetics of [La] and its re-
lationship with energy metabolism.
A supposed mechanism that could be highlighted to eluci-
date our finding is the greater recruitment of type II fibres
after 30 min of exercise, which may explain the accumulation
of [La]. It has been shown that during prolonged moderate to
heavy exercise, there is an increased recruitment of type II fi-
bres due to the depletion of oxidative fibres (Krustrup et al.
2004). There is evidence that the increase in recruitment of
type II fibers is related to a slow component of O2(Saunders
et al. 2000; Pringle et al. 2003); however, this was not ob-
served in the present study. Nevertheless, to confirm those
assumptions, a biopsy-based study needs to be conducted to
directly measure the glycogen depletion of different types of
fibres during a TTE test at MLSS.
Regarding cardiorespiratory variables, we observed greater
responses during intermittent exercise than during continuous
exercise at MLSS. Moreover, Baron et al. (2008) also re-
ported that _
VO2did not rise significantly between the begin-
ning and the end of TTE at MLSScon, in accordance with our
findings. Therefore, the raising of _
VO2values at MLSS over
time does not seem to occur or explain the earlier exhaustion
found at MLSScon.
On the other hand, _
VE increased over time, a fact also ob-
served for HR (Table 3). The study published by Lajoie et al.
(2000) observed changes in physiological parameters during
60 min of exercise at MLSScon in well-trained cyclists. They
also found that HR and _
VE increased over time, whereas
[La] remained stable. In our study, HR increased during the
time completed in both protocols, reaching 93% of HRmax
values. The difference between the beginning (t20%) and the
end of TTE were in order of 17 bpm for MLSScon and
13 bpm for MLSSint. Baron et al. (2008) also found that HR
increased significantly between t20% and t100% (~11 bpm).
These values should be taken into account when this index
is used to control the training intensity during MLSS exer-
cise.
In addition, Baron et al. (2008) noticed that the HR on
TTE was significantly lower than the HRmax during the max-
imal incremental test and could not have caused the termina-
tion of exercise during the MLSS test (Baron et al. 2008).
The findings of the current study are in agreement with this,
given that we also found that the final HR at TTEcon (179 ±
9 bpm) and TTEint (179 ± 7 bpm) were significantly lower
than HRmax. Interestingly, however, the final HR was the
same for both modes of exercise, which did not occur with
the respiratory variables investigated (Fig. 1).
Finally, these findings showed that the continuous cycling
at MLSS underestimates the exercise intensity that an athlete
could realize under intermittent condition. This aspect may
have important implications for prescribing endurance inter-
val training.
Table 2. Mean ± SD for physiological variables in continuous and intermittent MLSS exer-
cise (n= 14).
Variables Continuous Intermittent pvalue ES
Power output (W) 251±29* 268±29 <0.0001 0.58
[La]TTE (mmol·L–1) 4.1±0.9* 4.9±1.2 0.009 0.76
TTE (min) 54.7±10.9* 67.8±14.3 0.0025 1.04
Total work (kJ) 49.4±3.1* 65.4±3.8 <0.0001 1.24
Note: TTE, time to exhaustion; [La]TTE, blood lactate concentration at time to exhaustion protocol; ES,
effect sizes. An asterisk (*) indicates p< 0.05 compared with intermittent exercise.
Table 3. Mean ± SD for HR, V
_O2, and V
_E during percentages of
TTEcon and TTEint between 20% and 100% of TTE (t20%,t40%,t60%,
t80%, and t100%) at MLSS.
Time (%) HR (bpm) _
VO2(mL·kg–1·min–1)_
VE (L·min–1)
TTEcon
t20% 162±8* 47.3±6.9 85.1±11.1‡
t40% 168±9* 47.9±6.8 91.1±13.2§
t60% 172±9†48.3±7.5 95.5±16.1†
t80% 175±9†49.0±7.8 99.3±16.4†
t100% 179±9 48.3±7.4 103.7±19.8
TTEint
t20% 166±8* 50.1±7.3 89.6±12.6‡
t40% 171±8* 50.5±7.3 96.5±12.1§
t60% 174±8* 51.0±7.2 100.0±15.3†
t80% 176±7* 51.0±7.8 106.2±20.0†
t100% 179±7* 50.6±7.7 113.3±23.4
Note: TTEcon, time to exhaustion continuous; TTEint, time to exhaustion
intermittent; HR, heart rate; _
VO2, oxygen uptake; _
VE, ventilation.
*p< 0.05 compared with all percentages of TTE within the same proto-
col.
†p< 0.05 compared with t100% within the same protocol.
‡p< 0.05 compared with t60%,t80%, and t100% within the same protocol.
§p< 0.05 compared with t80% and t100% within the same protocol.
Grossl et al. 1051
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Endurance time at MLSSint was analysed for the first time
in the present study, and the possible mechanisms that ex-
plain the differences between two modes are unclear. Accord-
ingly, one could suppose that the model used in current study
(i.e., a 1-min rest period after each 5 min of exercise) im-
proves the endurance capacity at this particular metabolic in-
tensity. Moreover, the training volume of an interval session
designed at MLSS should take into consideration this higher
power output and also TTE when compared with continuous
exercise. Consequently, the number of intervals completed at
TTE trial could also be useful to prescribe submaximal inter-
val training.
More studies are necessary to clarify the fatigue mecha-
nisms involved at MLSS in continuous and discontinuous
protocols, as well as the influence of different exercise and
recovery durations.
Acknowledgment
We thank the 14 dedicated subjects who gave such an in-
credible effort for the tests. We also thank the reviewers for
the invaluable review, which improved the quality of manu-
script.
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