J Appl Physiol 119: 961–967, 2015.
First published September 10, 2015; doi:10.1152/japplphysiol.00498.2015.
Effects of ischemic preconditioning on maximal constant-load cycling
performance
Rogério Santos de Oliveira Cruz, Rafael Alves de Aguiar, Tiago Turnes, Kayo Leonardo Pereira,
and Fabrizio Caputo
Human Performance Research Group, College of Health and Sport Science, Santa Catarina State University, Florianopolis,
Santa Catarina, Brazil
Submitted 15 June 2015; accepted in final form 3 September 2015
endurance exercise; aerobic metabolism; surface electromyography; ratings of perceived exertion; pulmonary oxygen uptake
kinetics
the aim of protecting cardiac muscle fibers (51), local acute ischemic preconditioning (IPC)
consists of a potent endogenous mechanism that has been
shown to protect various tissues against ischemia-reperfusion
injury (27). Particularly, the benefits of IPC on noncontracting
skeletal muscles during prolonged periods of blood flow occlusion have been described in detail (1, 31, 55, 60). Hence, it
has not taken long for the procedure to be translated to improve
exercise performance in humans (16, 17, 32).
In the last few years, IPC has been shown to enhance
endurance performance during cycling (16, 17, 37), running
(6), rowing (39), and more recently in a handgrip exercise (7).
ORIGINALLY DEVELOPED WITH
Address for reprint requests and other correspondence: R. Santos de Oliveira
Cruz, Laboratório de Pesquisas em Desempenho Humano - CEFID/UDESC,
Rua Pascoal Simone, 358, Coqueiros, Florianópolis - SC, CEP 88080-350,
Brazil.
http://www.jappl.org
However, there are also studies reporting unchanged performance after the procedure (14, 23, 61). Of note, while the
benefits of IPC on endurance are currently being investigated,
the mechanisms underlying these improvements are still unclear. Research has identified a greater ability for aerobic
energy transduction after IPC (7, 17, 37). For example, IPC
accelerated muscle deoxygenation kinetics in a moderate-intensity cycling exercise (37) and increased oxygen uptake
(V̇O2) peak during a maximal ramp exercise test (17). Conversely, other studies reported that IPC could increase endurance performance without affecting the V̇O2 responses (5, 6,
14, 16). On the other hand, an increase in neural drive to the
active muscles has also been speculated (16). In the only
relevant study, surface electromyographic (EMG) potentials
and maximal isometric force were increased in skeletal muscle
of animals following IPC (57), suggesting increased motor unit
recruitment. However, the effects of IPC on EMG amplitude
has not yet been investigated in exercising humans. Finally, it
has also been suggested that IPC lowers the sensitivity of the
body to fatigue signals (16). If so, this should result in lower
ratings of conscious perception of effort [rating of perceived
exertion (RPE)] during a constant-load exercise, as it is derived
from sensory input arriving from many different biological
systems, including the cardiovascular, respiratory, and musculoskeletal systems (15).
While time trials are the preferred choice for studies simulating the actual nature of a sporting event, they are not optimal
for tracking eventual changes induced by IPC on V̇O2 kinetics
or the rates of increase in RPE and EMG amplitude. On the
other hand, time-to-exhaustion (Tlim) tests have a similar
sensitivity to that of time trials to changes in endurance (3),
with an advantage in that they do not permit fluctuations of
exercise intensity that can add “noise” to the measurement of
physiological markers (45). Therefore, we have examined the
hypothesis that IPC could improve the exercise tolerance
during a constant-load cycling exercise performed at the peak
power output attained during an incremental test (PPO). The
choice of the test could be relevant to athlete performance, as
it closely resembles the requirements of a track 4-km pursuit
event. Additionally, we investigated the hypotheses that the
ergogenic effects of IPC on endurance performance could be
related to 1) an improved aerobic metabolism, 2) an increase in
neural drive to the active muscles, and/or 3) a reduction in the
perception of effort. Therefore, the primary aim of this study
was to evaluate the effect of IPC of the lower limbs on
endurance performance. Our secondary aim was to investigate
the IPC effects on V̇O2 kinetics, neuromuscular function, and
perceptual responses.
8750-7587/15 Copyright © 2015 the American Physiological Society
961
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Cruz RSO, de Aguiar RA, Turnes T, Pereira KL, Caputo F.
Effects of ischemic preconditioning on maximal constant-load cycling
performance. J Appl Physiol 119: 961–967, 2015. First published September 10, 2015; doi:10.1152/japplphysiol.00498.2015.—This study investigated the effects of ischemic preconditioning (IPC) on the ratings
of perceived exertion (RPE), surface electromyography, and pulmonary oxygen uptake (V̇O2) onset kinetics during cycling until exhaustion at the peak power output attained during an incremental test. A
group of 12 recreationally trained cyclists volunteered for this study.
After determination of peak power output during an incremental test,
they were randomly subjected on different days to a performance
protocol preceded by intermittent bilateral cuff pressure inflation to
220 mmHg (IPC) or 20 mmHg (control). To increase data reliability,
the performance visits were replicated, also in a random manner.
There was an 8.0% improvement in performance after IPC (control:
303 s, IPC 327 s, factor SDs of ⫻/⫼1.13, P ⫽ 0.01). This change was
followed by a 2.9% increase in peak V̇O2 (control: 3.95 l/min, IPC:
4.06 l/min, factor SDs of ⫻/⫼1.15, P ⫽ 0.04), owing to a higher
amplitude of the slow component of the V̇O2 kinetics (control: 0.45
l/min, IPC: 0.63 l/min, factor SDs of ⫻/⫼2.21, P ⫽ 0.05). There was
also an attenuation in the rate of increase in RPE (P ⫽ 0.01) and a
progressive increase in the myoelectrical activity of the vastus lateralis muscle (P ⫽ 0.04). Furthermore, the changes in peak V̇O2 (r ⫽
0.73, P ⫽ 0.007) and the amplitude of the slow component (r ⫽ 0.79,
P ⫽ 0.002) largely correlated with performance improvement. These
findings provide a link between improved aerobic metabolism and
enhanced severe-intensity cycling performance after IPC. Furthermore, the delayed exhaustion after IPC under lower RPE and higher
skeletal muscle activation suggest they have a role on the ergogenic
effects of IPC on endurance performance.
962
IPC and Cycling Performance
METHODS
Cruz RSO et al.
time between the end of the procedure and the start of Tlim test was
90 min (6).
Incremental exercise test. In the first visit, subjects underwent an
incremental test to determine the maximum V̇O2 (V̇O2max) and PPO.
The test consisted of 3 min of unloaded baseline pedaling (16 ⫾ 4 W,
equivalent to the lowest workload provided by the cycle ergometer),
followed by a step increase in power output of 0.5 W/kg body mass
every 3 min. PPO was defined as the power output attained at
exhaustion if the test was terminated at the end of a 3-min stage.
If the test was terminated before the last stage had been completed,
PPO was calculated as the power of the previous stage plus the
power increment times the duration (s) of exercise at the final stage
divided by 180 s (43).
Tlim tests. Before the Tlim protocol, subjects were submitted to a
standard warm-up protocol, followed by 5 min of passive recovery on
the bike. The performance test consisted of 3 min of unloaded baseline
pedaling, followed by a sudden increase to the required power output
(i.e., 100% of PPO), which should be kept until volitional exhaustion,
or participants could no longer maintain the chosen cadence minus 5
rpm for ⬎5 s, despite strong verbal encouragement.
Gas exchange measurements. Throughout each testing protocol,
cyclists wore a facemask, and respiratory gas exchange was measured
breath by breath using an automated open-circuit gas analysis system
(Quark CPET, Cosmed, Rome, Italy). Gas analyzers were always
previously calibrated using ambient air, and gases contained 16%
oxygen and 5% carbon dioxide. The turbine flow meter used for the
determination of minute ventilation (V̇E) was calibrated with a 3-liter
calibration syringe. During the incremental test, the pulmonary gas
exchange was measured with the aid of a 7-liter mixing chamber (52).
The pulmonary V̇O2 data from each test were initially examined to
exclude occasional errant breaths caused by coughing, swallowing,
sighing, etc., which were considered not to be reflective of the
underlying kinetics; i.e., only values greater than 3 SDs from the local
mean were omitted. During the incremental exercise, the pulmonary
V̇O2max attained was taken as the highest 15-breath rolling average
value attained. An identical procedure was employed on the Tlim tests
for the determination of peak V̇O2 and its associated heart rate (HR)
and V̇E. To avoid being influenced by the amount of data used in the
comparison between the control and IPC conditions, the V̇O2 onset
kinetics were analyzed by fixing the time window to the shortest Tlim
recorded for each subject (i.e., isotime). The breath-by-breath data
were first linearly interpolated to provide second-by-second values,
and, for each individual, identical repetitions were time-aligned to the
start of exercise, and the ensemble was averaged. A biexponential
model was then applied to characterize the V̇O2 kinetics in its fast and
slow components:
V̇o2共t兲 ⫽ V̇o2b ⫹ A1共1 ⫺ e⫺t⫺TD1⁄␶1兲 ⫹ A2共1 ⫺ e⫺t⫺TD2⁄␶2兲
where V̇O2(t) represents the absolute V̇O2 at a given time t; V̇O2b
represents the mean V̇O2 in the final 30 s of the baseline period; A1,
TD1, and ␶1 represent the amplitude, time delay, and time constant,
respectively, describing the fundamental increase in V̇O2 above baseline; and A2, TD2, and ␶2 represent the amplitude, time delay before
the onset of, and time constant describing the development of, the V̇O2
slow component (V̇O2 SC), respectively. The initial phase was not
modeled, and, therefore, the first 20 s on-transient were omitted to
obviate any distorting influence of phase I on the subsequent kinetics.
Blood lactate measurements. At the end of each stage during the
incremental test, capillary blood samples (25 ␮l) were taken from the
earlobe and analyzed for blood lactate concentration. For the performance trials, samples were collected at the end of the 3 min of
unloaded baseline pedaling and at exhaustion. All blood samples were
stored in Eppendorf tubes containing 50 ␮l of 1% NaF in a ⫺20°C
environment. Later, samples were analyzed by enzyme electrode
technology (YSI 1500 SPORT, Yellow Springs, OH). Calibration was
made using a standard solution of 5 mmol/l.
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Ethical approval. This study was carried out in accordance with the
guidelines contained in the Declaration of Helsinki and was approved
by the Ethics Committee in Human Research of the Santa Catarina
State University. The subjects were fully informed of any risks and
discomforts associated with the experiments before giving their written, informed consent to participate.
Subjects. A group of 12 recreationally trained cyclists (18), ranging
in age from 20 to 36 yr, with an average height of 177 cm (range:
171–188 cm) and an average body mass of 79 kg (range: 68 –96 kg),
volunteered for this study. None was receiving any pharmacological
or specific dietetic treatment. All participants attended properly fed
and hydrated and were instructed not to perform strenuous exercise
and to abstain from alcohol on the day before each session. They were
also asked to maintain the same dietary pattern throughout the experiment and to refrain from consuming caffeine for at least 2 h before
each trial.
Study design. Subjects were required to report to the laboratory on
five occasions over a 15-day period (⫾4 days), and all tests were
interspersed with ⬃48 h of recovery. After an incremental test to
determine PPO, subjects were then randomly submitted in sessions 2
and 3 to a performance protocol preceded by either intermittent
bilateral cuff inflation to 220 mmHg (IPC) or to 20 mmHg (control).
To increase data reliability, visits 2 and 3 were replicated in visits 4
and 5, also in a random manner. The inclusion of an additional round
of exercise in each condition reduces the typical error by a factor of
0.7 (i.e., 1/兹n, where n is the number of transitions), and thus
improved the signal-to-noise ratio by a factor of 1.4 (29). Each subject
was always tested at the same time of day (⫾2 h) to minimize the
effects of diurnal biological variation in a temperature-controlled
laboratory (21 ⫾ 1°C). All cycle tests were performed on an electrodynamically braked cycle ergometer (Lode Excalibur Sport, Groningen, The Netherlands). The ergometer seat and handlebar were
adjusted for comfort, and the settings were replicated for subsequent
tests. During the tests, subjects were instructed to maintain their
preferred cadence for as long as possible until volitional exhaustion.
They were verbally encouraged to give their best effort and were
blinded to the time elapsed during exercise. Breath-by-breath V̇O2 and
EMG responses were continuously registered, and capillary blood
samples were taken at specific time points throughout the tests. During
performance trials, the RPE were measured every minute by using
Borg’s 6 –20 scale (8), but subjects were uninformed about this
frequency of collection.
Ischemic preconditioning. IPC was performed in the supine position using bilateral arterial occlusion. The occlusion cuffs were placed
proximally around the upper thighs and inflated to 220 mmHg to
restrict arterial inflow for 5 min. The ischemic procedure was repeated
four times, each separated by 5 min of reperfusion. This protocol (i.e.,
four 5-min cycles of limb ischemia interspaced with 5 min of reperfusion) was chosen for two main reasons: 1) it has been successfully
applied in previous studies investigating the ergogenic effects of IPC
(23, 32, 39, 56); and 2) clinical studies have shown that at least three
ischemia-reperfusion cycles are necessary to protect against skeletal
muscle necrosis (i.e., infarction or tissue death) and endothelial
dysfunction after prolonged periods of imposed ischemia (47, 55).
Limb IPC comprising three to five cycles of 5-min inflations was
found to be safe and well tolerated in both patients and healthy
volunteers (25, 40, 46). Of interest, no objective signs of neurovascular injury were observed, with no subjects experiencing skin breakdown, prolonged discoloration, or temperature or pulse disparities in
the treated limb. There were no cases of deep vein thrombosis or
injury to the preconditioned limb, and no protocol was prematurely
terminated due to subject discomfort. Furthermore, 5 min of blood
flow restriction is far from depleting the muscle energy charge
potential (55). On control condition, participants followed an identical
protocol, but, instead, the cuff was inflated only to 20 mmHg. The
•
963
Cruz RSO et al.
subject and treatment (i.e., individual responses to the IPC procedure)
had been taken into account. The Pearson product moment correlation
coefficient (r) was used to examine the relationship between residuals.
The uncertainties in the effects were expressed as 95% confidence
limits, and all tests were analyzed at an ␣-level of 0.05.
*
Control
19
•
IPC
16
RESULTS
13
Moment: P< 0.001
Treatment: P= 0.01
Interaction: P= 0.07
10
min 1
min 2
min 3
min 4
Last
Fig. 1. Subjective ratings of perceived exertion during the initial 4 min of
constant workload performance and in the last minute measured for each
subject. IPC, ischemic preconditioning. Values are means ⫾ SD. *P ⱕ 0.05.
400
Control
380
IPC
360
583
340
320
Moment: P< 0.001
Treatment: P= 0.04
Interaction: P= 0.55
300
280
563
min 1
min 2
min 3
min 4
Exhaustion
Fig. 2. Myoelectrical activity of the vastus lateralis during the first 4 min of
constant workload performance and in the minute preceding exhaustion. EMG,
electromyography. Values are means ⫾ SD.
J Appl Physiol • doi:10.1152/japplphysiol.00498.2015 • www.jappl.org
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EMG measurements. In visits 5 and 6, quadriceps EMG were
recorded from the right vastus lateralis using bipolar 10-mm-diameter
Ag-AgCl surface electrodes with full-surface solid adhesive hydrogel
(Tyco Healthcare Group). Before the electrodes were placed, the skin
over the muscle was shaved, sandpapered, and cleaned with ethanol.
The placement and location of the electrodes are according to the
standard recommendations (28). Therefore, two electrodes were carefully taped to the belly of each muscle, in the approximate direction
of the muscle fibers with an interelectrode distance of 20 mm. The
reference electrode was placed over an electrically neutral site (processus spinosus of C7 vertebra). To minimize movement artifacts,
electrode cables were fastened to the subject’s quadriceps using
adhesive tape. The positioning of the electrodes was marked with
indelible ink to ensure that they were placed in the same location at
subsequent visits. EMG was recorded with a common mode rejection
ratio of 110 dB and an input impedance of 10 G⍀ with a four-channel
EMG system (Miotool 400 USB, Miotec, Porto Alegre, Brazil). The
EMG signals were amplified (⫻400), analog-to-digital converted (14
bits resolution) at a sampling rate of 2 kHz, and stored for later
analysis. The raw digital EMG signal was processed using MATLAB
7.12 (MathWorks, Natick, MA). The signal was initially filtered using
a 10- to 500-Hz band-pass, fifth-order Butterworth filter. The data
were then rectified and smoothed using root-mean-squared analysis,
calculated for a 25-ms window. A computer algorithm identified the
onset of activity where the signal deviated by more than 4 SDs above
the baselines. Each EMG burst was inspected to verify the timing
identified by the computer. The average root-mean-squared analysis
during each minute was then calculated.
Statistical analysis. Comparisons were performed with the mixed
linear modeling procedure of the IBM SPSS statistics (version 19.0,
IBM), inserting the subject term as a random effect. Treatment was a
fixed effect, and order of treatment was included as an additional fixed
effect to account for continuing familiarization or other order effects
(except for V̇O2 kinetics analysis, which requires at least two transitions to be satisfactorily modeled). For neuromuscular and perceptual
responses, moment was also included as a fixed factor, and analysis
was performed controlling for the corresponding responses during the
first minute of exercise in each condition. Except for RPE, data were
log transformed for the analysis to reduce nonuniformity of error.
Means and between-subject SD for subject characteristics and RPE
were derived from the raw values of the measures; for all other
measures, they were derived by back-transformation of the logtransformed values and the dispersion are shown as a ⫻/⫼ factor SD
(30). These factors come from the assumption that the log-transformed variable rather than the variable itself is normally distributed,
and thus the dispersion becomes slightly asymmetrical after backtransformation. Typical errors of the measurement were derived by
back-transformation of the residual term after any interaction between
Incremental exercise test. Subjects attained a PPO of 297 W
(⫻/⫼1.11), and the V̇O2max was 4.05 l/min (51 ml·kg⫺1·min⫺1), with
a factor SD of ⫻/⫼1.15.
Tlim tests. The typical variation of Tlim measured at PPO
was 9.9% (factor limits of ⫻/⫼1.3). After the application of
IPC, performance was improved by 8.0% (confidence limits
of ⫾ 5.7%, P ⫽ 0.01). The back-transformed means (i.e., the
mean of the log-transformed variable converted back into raw
units) for control and IPC conditions were 303 and 327 s,
respectively, with a factor SD of ⫻/⫼1.13.
RPE. There was an attenuation in the rate of increase in RPE
during performance (Fig. 1), with the overall RPE being lower
after IPC (P ⫽ 0.01). At the end of the 4th minute of exercise,
the average RPE was 0.8 units lower in the IPC condition
(confidence limits of ⫾0.5 units, P ⫽ 0.003). No changes were
observed between conditions in RPE in the last minute before
exhaustion (P ⫽ 0.55).
EMG. The typical error of controlled EMG measurements
was 3.8% (factor limits of ⫻/⫼1.2). The neuromuscular activity increased from the 1st minute of exercise to voluntary
exhaustion in both the control and IPC conditions (P ⬍ 0.001,
Fig. 2), being uniformly higher during the IPC trial (overall
effect of 2.3%, confidence limits of ⫾2.1%; P ⫽ 0.04).
Physiological and metabolic responses. The effects of IPC
on cardiorespiratory and metabolic responses are presented in
Table 1. At the end of 3 min of unloaded baseline pedaling, the
blood lactate concentration and the pulmonary V̇O2 were identical in both conditions. During performance, the peak V̇O2
attained was significantly higher in the IPC condition, despite
similar HR and V̇E. As there were no important changes in
phase II of V̇O2 onset kinetics, this increase in peak V̇O2
resulted in a higher amplitude of the V̇o2 SC, as seen in Fig. 3.
Correlations. The residuals of peak V̇O2 largely correlated
with 1) the residuals of the accumulated V̇O2 at isotime (r ⫽
0.65, P ⫽ 0.02); and 2) those of performance improvement
(r ⫽ 0.73, P ⫽ 0.007). The performance improvement was also
Surface EMG (mV)
Ratings of Perceived Exertion
(units)
IPC and Cycling Performance
964
IPC and Cycling Performance
•
Cruz RSO et al.
Table 1. Effects of ischemic preconditioning on performance and metabolic responses during the constant workload exercise
performed at PPO
Baseline [La], mmol/l
Baseline V̇O2, l/min
Phase II ␶, s
Phase II amplitude, l/min
Phase II time delay, s
V̇O2 SC time delay, s
V̇O2 SC amplitude, l/min
V̇O2 SC ␶, s
V̇O2 at isotime, l/min
Accumulated V̇O2 at isotime, liters
Peak V̇O2, l/min
HR at peak V̇O2, beats/min
V̇E at peak V̇O2, l/min
End [La], mmol/l
Control
IPC
1.9 ⫻/⫼ 1.44
1.02 ⫻/⫼ 1.10
11.0 ⫻/⫼ 1.94
2.47 ⫻/⫼ 1.12
10 ⫻/⫼ 1.76
128 ⫻/⫼ 1.33
0.45 ⫻/⫼ 2.21
122 ⫻/⫼ 1.88
3.81 ⫻/⫼ 1.15
15.63 ⫻/⫼ 1.18
3.95 ⫻/⫼ 1.15
177 ⫻/⫼ 1.04
179 ⫻/⫼ 1.18
7.6 ⫻/⫼ 1.15
1.9 ⫻/⫼ 1.44
1.02 ⫻/⫼ 1.10
8.9 ⫻/⫼ 1.94
2.49 ⫻/⫼ 1.12
9 ⫻/⫼ 1.68
126 ⫻/⫼ 1.33
0.63 ⫻/⫼ 2.21
157 ⫻/⫼ 1.88
3.88 ⫻/⫼ 1.15
15.92 ⫻/⫼ 1.18
4.06 ⫻/⫼ 1.15
178 ⫻/⫼ 1.04
179 ⫻/⫼ 1.18
8.2 ⫻/⫼ 1.15
Typical Error‡
28
11
3.8
2.7
4.6
1.6
6.0
12
%Change ⫾ 95% CL
P Value
0.1 ⫾ 16
0.4 ⫾ 6.2
⫺19 ⫾ 45
0.9 ⫾ 1.9
⫺8.6 ⫾ 58
⫺1.7 ⫾ 11
41 ⫾ 41
29 ⫾ 46
1.8 ⫾ 2.2
1.8 ⫾ 1.6
2.9 ⫾ 2.7
0.5 ⫾ 1.2
0.1 ⫾ 3.5
7.5 ⫾ 8.2
0.99
0.90
0.23
0.30
0.69
0.72
0.05*
0.17
0.10†
0.03*
0.04*
0.37
0.94
0.07†
related to the changes in the slow component phenomenon (r ⫽
0.79, P ⫽ 0.002).
DISCUSSION
The major original findings of this investigation include the
following. 1) Preconditioning of the lower limbs increased
A
Control
VO2 (L·min -1)
3.5
2.5
Ap
1.5
0.5
-0.5
B
Time (s)
IPC
VO2 (L·min -1)
3.5
2.5
Ap
1.5
0.5
-0.5
Time (s)
Fig. 3. Group mean pulmonary O2 uptake (V̇O2) during cycling performance in
the control (A) and IPC (B) conditions. Data were matched at the shortest
time-to-exhaustion recorded, and error bars were omitted for clarity. Fitting
curves and the residuals of those fits were also showed. The vertical dashed
lines represent the transition from baseline pedaling to exercise performance.
The top and bottom horizontal dashed lines represent the baseline V̇O2 and the
phase II absolute amplitude of V̇O2 onset-kinetics (Ap), respectively. The
hatched areas above Ap represent the slow-component phenomenon.
Tlim by 8% during maximal constant-load performance in
recreational cyclists, which was closely related to a significantly higher V̇o2 SC (and hence peak V̇O2), rather than an
acceleration in phase II kinetics. Such increased aerobic energy
provision might have possibly reduced the rate of utilization of
the finite anaerobic energy stores, delaying exhaustion (19, 34,
62). 2) These changes on endurance performance and aerobic
metabolism were accompanied by an attenuation in the rate
of increase in RPE and a progressive increase in the myoelectrical activity of the vastus lateralis throughout exercise.
Together, these results suggest that a lower sensitivity of the
body to fatigue signals and increases in central motor
drive/output may possibly play a part in the beneficial
effects of IPC on endurance performance, as previously
speculated by Crisafulli et al. (16).
The benefit of IPC on aerobic metabolism has been suggested to be mediated by peripheral mechanisms (59), although
possible effects on central hemodynamics cannot yet be fully
ruled out. The active skeletal muscle is the predominant site of
the V̇o2 SC phenomenon (58), which is consistent with the
progressive increase in the EMG signal observed herein. Furthermore, the similar HR and V̇E at peak V̇O2 suggests no
excessive energetic requirements of cardiorespiratory work
contributing to the differences (20, 38). The present results are
thus in accordance with the recent observations of Barbosa et
al. (7). The authors investigated the effects of remote IPC on
forearm hemodynamics and deoxygenation during a constantload rhythmic handgrip protocol by means of Doppler ultrasound and near-infrared spectroscopy. At the time of task
failure, brachial artery blood flow was unaffected, but the
deoxygenated hemoglobin and myoglobin concentration in the
forearm was higher, indicating higher fractional O2 extraction.
Although speculative, this lower blood flow/V̇O2 could be due
to the recruitment of additional fibers, i.e., a baseline effect as
seen for fast- vs. slow-twitch muscles (22, 49). Alternatively,
an increase in maximal mitochondrial O2 flux capacity after
IPC remains to be investigated, despite the fact that, during
high-intensity cycling, in vivo maximal mitochondrial function seems to be limited by convective O2 delivery rather
than an intrinsic mitochondrial limitation (10, 11).
J Appl Physiol • doi:10.1152/japplphysiol.00498.2015 • www.jappl.org
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Values are means ⫻/⫼ factor SD. ‡Multiply by 0.7 to correct by the number of trials in each condition. IPC, ischemic preconditioning; CL, confidence limits;
[La], lactate concentration; V̇O2, pulmonary oxygen uptake; ␶, time constant; V̇O2 SC, V̇O2 slow component; HR, heart rate; V̇E, minute ventilation. *P ⱕ 0.05.
†P ⱕ 0.10.
IPC and Cycling Performance
Cruz RSO et al.
965
the sciatic nerve (i.e., despite similar stimulation frequencies),
along with lower decreases in maximal isometric force during
prolonged ischemic periods (57). Together with the lower rate
of increase in RPE throughout endurance performance, these
findings could be suggesting that IPC inhibits central fatigue
(24), which deserves further attention. If true, this response
could possibly be associated with lower discharges from opioid-mediated muscle afferents, which are known to modulate
endurance performance (2). Indeed, activation of opioid receptors in the striated muscle tissue by circulating endogenous
opioid peptide(s) has been associated with the early phase of
protection of either local acute or remote IPC (1, 21). Although
further studies are necessary, the inhibition of spontaneous
discharges from these nerve fibers to the central nervous
system could be resulting in an overshoot in central motor
drive (4), thereby allowing the utilization of a higher fraction
of the skeletal muscle recruitment (functional) reserve (4, 16,
53). The fact that the V̇o2 SC have been greater after IPC (rather
than delayed) would support this notion, as additional fiber
recruitment is the only putative mediator of the V̇o2 SC that
could improve performance (26, 33). This possibility has
already been suggested in previous studies (16, 59). Thus,
instead of representing a loss of muscle efficiency commonly
demonstrated during severe-intensity cycling exercises (26),
the higher V̇o2 SC phenomenon observed after IPC may have
possibly spared an important amount of the finite anaerobic
energy stores. This scenario is evidenced by the higher accumulated V̇O2 at isotime and the correlations found in the
present study. After isotime, the anaerobic energy spared could
then be used to prolong exercise tolerance (19, 34, 62).
Perhaps the main methodological constraint of IPC studies is
the inability to completely “blind” the subjects (17, 23, 44),
because the sensations induced by the IPC and the lowpressure protocol are clearly distinct (44). In the present study,
subjects were all deprived from any information until the end
of the data collection, and they were always verbally encouraged to realize their maximum. However, a placebo effect
cannot be discarded, though it is unlikely that the subjects had
managed their own metabolism. Second, as we employed a
time window of ⬃48 h between performance visits, the extent
at which the less intense second window of protection lasting
48 –72 h could have affected the results is unknown (48). In an
attempt to mitigate this limitation, the order of trials was
included as an additional fixed effect in the mixed model.
Finally, while the present results might suggest an increased
V̇O2max after IPC, it can be noted that the peak V̇O2 attained in
the IPC condition was not really different from the V̇O2max
measured during the incremental test. Therefore, even though
we have used a different apparatus during the incremental
exercise (i.e., a mixing chamber system coupled to the gas
analyzer), we cannot comment on the potential effects of IPC
on V̇O2max (14, 16, 17), which remains an open question.
In conclusion, the present findings provide a link between
improved aerobic energy metabolism and enhanced severeintensity cycling performance after IPC. Furthermore, the delayed exhaustion after IPC was accomplished under lower RPE
and higher skeletal muscle activation, suggesting they have a
role on the ergogenic effects of IPC on endurance performance.
ACKNOWLEDGMENTS
We thank all of the colleagues who contributed.
J Appl Physiol • doi:10.1152/japplphysiol.00498.2015 • www.jappl.org
Downloaded from http://jap.physiology.org/ by 10.220.33.5 on June 18, 2017
It has been speculated that activation of ATP-sensitive
potassium channels and/or elevated adenosine levels may play
a role in the effects of IPC on aerobic metabolism and endurance performance (17). These suggestions find wide support in
animal studies investigating the IPC-induced protection against
ischemia-reperfusion injury on noncontracting muscles (31, 54,
60). An important consequence of both mechanisms would be
an enhanced vasodilatory response following limb occlusion
(50). This reactive hyperemia could possibly be improving the
distribution of the regional skeletal muscle blood flow to the
most active fibers, matching O2 demand with O2 delivery, or
increasing mean capillary transit time, allowing a higher O2
extraction per unit of blood flowing through the fully activated
muscle fibers. Indeed, there are studies showing that regular
IPC episodes can cause improvements in endothelial function
in healthy individuals (35, 36). However, to the best of our
knowledge, there is no evidence indicating improvements in
skeletal muscle blood flow or vascular conductance after acute
IPC under exercise conditions (7). Meanwhile, in the present
study, as well as in a recent report (37), the phase II of V̇O2
kinetics during severe-intensity cycling was unchanged, which
could have been sensitive to an improved matching of local
muscle O2 availability to O2 utilization. Furthermore, it may
not be possible to increase peak leg V̇O2 by inducing vasodilation during maximal cycling exercises, likely due to an
alteration in the perfusion/V̇O2 relationship (12).
Traditionally, the V̇o2 SC is at least partly associated with
additional motor unit recruitment (26, 33, 41, 42), as exercise
proceeds and the initially recruited fibers become fatigued. On
the other hand, it has also been postulated that a progressive
recruitment of muscle fibers is not obligatory for the development of the V̇o2 SC, and a reduction in the efficiency of
contractions of fatigued fibers has been implicated (64). This
interpretation, originating from an experiment involving isolated dog gastrocnemius muscle in situ, has recently received a
significant support from studies involving humans (13, 63).
The fact that endurance performance was improved after IPC
suggest that, at least in this particular situation, the increase in
V̇o2 SC (i.e., relative to control, not the entire V̇o2 SC) may have
been driven by the former, which has been considered the
unique solution to maintain the same power output, even if
there is a decrease in fiber efficiency (9). If IPC would make
the muscle fibers less efficient owing to a lower level of
“metabolic stability” (e.g., decreases in the Gibbs free energy
of ATP hydrolysis and [phosphocreatine], increases in [lactate], [H⫹], [ADP], [Pi], [IMP], [NH3], etc., where brackets
denote concentration), the relationship between power output
and V̇O2 at submaximal intensities should have also been
affected. This was proved not to be the case in a range of
submaximal intensities reported by previously published literature (14, 17) and would be somewhat inconsistent with the
ATP-sparing effect observed after IPC in noncontracting muscles during prolonged periods of ischemia (1, 55), although it
is unknown whether the latter would stand during normal
muscular activity with normal blood supply.
A higher voluntary neural drive/output from the spinal
motoneuron pool is suggested by the increase in the response
of the quadriceps on EMG throughout exercise following IPC
(2). While it could be due to increases in motoneuronal firing
rates, this higher EMG potential after IPC was already observed in feline hindlimbs submitted to electrical stimulation of
•
966
IPC and Cycling Performance
GRANTS
We thank Conselho Nacional de Desenvolvimento Científico e Tecnológico
and Fundação de Amparo à Pesquisa e Inovação do Estado de Santa Catarina
for financial support.
17.
18.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
19.
AUTHOR CONTRIBUTIONS
Author contributions: R.S.O.C. and F.C. conception and design of research;
R.S.O.C., R.A.A., T.T., and K.L.P. performed experiments; R.S.O.C., R.A.A.,
T.T., and K.L.P. analyzed data; R.S.O.C., R.A.A., T.T., K.L.P., and F.C.
interpreted results of experiments; R.S.O.C. prepared figures; R.S.O.C.,
R.A.A., T.T., K.L.P., and F.C. drafted manuscript; R.S.O.C. and F.C. edited
and revised manuscript; R.S.O.C., R.A.A., T.T., K.L.P., and F.C. approved
final version of manuscript.
20.
21.
22.
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