Ischemic preconditioning (IPC; brief cycles of ischemia followed by reperfusion) is well known to induce protection against ischemia–reperfusion injury (9,26,36). Adenosine triphosphate (ATP) sparing (36) and increase of electron mitochondrial flux (9) are some of the mechanisms responsible for the IPC protection. In addition, IPC abolishes the endothelial function impairment induced by ischemia–reperfusion (26), which is mediated by the autonomic nervous system (26). The beneficial effects of IPC on mitochondria and blood vessels, if translated to exercising muscles, could, supposedly, enhance efficiency of ATP use and resynthesis by the aerobic metabolism, as well as could increase skeletal muscle blood flow, leading to higher oxygen delivery and metabolites removal. The hypothetical effects of IPC on mitochondrial and vascular function have, consequently, driven studies that investigated the ergogenic effect of IPC on endurance performance (19). A meta-analysis recently reviewed 8 studies in this area (38), and concluded the IPC induces a small beneficial effect on endurance performance. Of note, however, no study has sufficiently controlled placebo and nocebo effects (37).
Previous studies have compared the IPC to a sham intervention where cuffs have been inflated at low pressure and subjects have not been informed about the possible effect of IPC on endurance performance (2,3,12–14,37). Such approach can possibly bias the interpretation about the IPC effect on endurance performance, because IPC induces pain during circulatory arrest and relief during reperfusion (25), which do not occur during the low cuff inflation. Thus, the net effect on subjects’ expectancy about the interventions is unpredictable, such that subjects may think the IPC is either beneficial (i.e., placebo) or harmful (i.e., nocebo). Placebo and nocebo effects, in turn, could per se modify exercise performance, according to randomized, controlled, and double-blinded studies (4,6,11). One of these studies, for example, showed a placebo supplement ingested during a cycling time trial enhanced performance by 4.3% (11). On the other hand, another study showed negative information provided about ingestion of a capsule reduced sprint performance by 1.6% (4). Therefore, the isolated effect of IPC on endurance performance remains unclear, due to possible biases from placebo and nocebo effects.
The effect of IPC on aerobic metabolism parameters that determine endurance performance has been equivocal (2,12,14,17,18,24,38), perhaps, in part, because subjects have not been told about the potential ergogenic effect of IPC. The reason for that is that uncertainty about an intervention increases individual variability in endurance performance (11), which may reduce precision of performance outcomes, and so, could possibly mask a small effect of IPC on both aerobic metabolism parameters and endurance performance. The varied effect of IPC on the maximal oxygen uptake (V˙O2max) (2,12–14,24), particularly, may still be attributed to the lack of strict criteria to confirm its measurement. Previous studies have used traditional criteria based on attainment of V˙O2 plateau and a given level of RER, HR, capillary blood lactate concentration, and perceived effort (2,12,14). However, V˙O2 plateau is observed in only approximately 50% of well-trained subjects during a continuous incremental test (30), and other criteria may be insufficient to confirm the V˙O2max attainment (34). Thus, a verification protocol has been developed to confirm the V˙O2max identification (30), and its use would be helpful to elucidate the effect of IPC on the V˙O2max.
The aim of this study was to investigate the effect of IPC on aerobic metabolism parameters and endurance performance in well-trained runners circumventing limitations from previous studies. To dissect the placebo effect, we compared IPC with a sham intervention (SHAM), and resting control (CT). One of the researchers told the subjects that IPC and SHAM would similarly improve performance compared with CT (i.e., similar placebo induction), and IPC would be harmless despite circulatory occlusion sensations (i.e., nocebo avoidance). In addition, we used a verification protocol to confirm the V˙O2max (30). We hypothesized that 1) IPC would improve aerobic metabolism parameters compared to SHAM and CT, and 2) both IPC and SHAM would improve endurance performance compared with CT, but the IPC improvement would surpass the SHAM effect (i.e., placebo effect).
Twenty subjects volunteered to participate. However, during the study, a man was excluded due to a muscle injury, and a woman was excluded due to symptoms related to menstruation. Among the 18 subjects that completed all the study’s visits, 14 were men (mean ± SEM; age = 22.3 ± 0.9 yr; V˙O2max = 66.4 ± 1.2 mL·kg−1·min−1; body fat = 9.1% ± 0.8%) and four women (age = 24.0 ± 2.5 yr; V˙O2max = 56.7 ± 1.8 mL·kg−1·min−1; body fat = 23.6% ± 0.7%). All subjects were nonsmokers and were not taking medications, supplements, treating orthopedic injuries, or presented history of chronic diseases. They had been training 6 times per week, for 5.0 ± 0.5 yr, with supervision of a coach graduated in physical education. Moreover, they had been competing in official middle and long distance races. The ethics committee of the Federal University of São Paulo (process 610.367) approved the study, and all subjects signed an informed consent form before participating in the study.
The study was randomized, placebo-controlled and nocebo-controlled, and crossed-over. Subjects visited the laboratory four times. Visit 1 was used for familiarization with the interventions and execution of a continuous incremental test. In addition, in this visit, one of the researchers told subjects that IPC and SHAM would similarly improve performance compared to CT, and IPC would be harmless despite circulatory occlusion sensations. Then, on visits 2, 3, and 4, subjects were exposed to IPC, SHAM, or CT, in random order, and next submitted to a discontinuous incremental test and a supramaximal test. Visits occurred at the same time of day for a given subject. Interval between visits was 7 d. Subjects were asked to arrive fully hydrated, ingest a light meal 2 h before the tests, and not to ingest caffeine or alcoholic beverages for 24 h. Training intensity was reduced on the day before the testing sessions to allow adequate recovery from training, and subjects were assessed only if they reported score of at least 15 (i.e., good recovery) in the total quality recovery scale (23). Only one subject’s visit had to be postponed 1 d to allow adequate recovery.
Ischemic preconditioning, sham, and control
IPC was performed with subjects in supine position, using customized cuffs (16). One cuff was placed in each thigh, as close to the groin as possible. Cuffs had two independent chambers installed in series. Each chamber was 36 cm long and 17.5 cm wide. Together, they covered at least 80% of the thigh’s circumference. Each cuff had a Velcro strap that, in most cases, surrounded the tight at least two times. Cuffs were inflated to 220 mm Hg for 5 min and deflated to 0 mm Hg for 5 min (2). When one cuff was inflated, the other was deflated (2). Four pressure cycles were performed on each leg (2), thus the IPC procedure lasted 40 min. Circulatory occlusion was confirmed with a vascular Doppler (Doppler vascular 610B, MEDMEGA, Brazil) at the posterior tibial artery. No pulse was detected in any subject while cuffs were inflated.
The SHAM procedure consisted of simulating the administration of therapeutic ultrasound. Physical therapists commonly use therapeutic ultrasound in an attempt to accelerate the healing process of musculoskeletal injuries, supposedly, in part, because it increases deep tissues temperature (15). All subjects had been treated with ultrasound sometime before the study, and all had a positive belief that it could aid the treatment of musculoskeletal injuries. Nevertheless, before the study, subjects had little or none information about the mechanisms involved on the ultrasound therapeutic effects. In our study, one of the researchers (J.L.S.) informed the subjects that the beneficial effects of ultrasound application could not only heal musculoskeletal injuries but also enhance exercise performance. If subjects had doubts or were interested to know more details about the possible mechanisms involved, the ultrasound heating effect (15) was used to support the researcher’s explanation.
The ultrasound device was plugged in a power socket, turned on, and its front lights were on. However, the start button to deliver the ultrasound was not pushed, and subjects did not notice that. The subjects lie down before and during the ultrasound application. The ultrasound device was placed close to the subjects’ leg and its panel was facing the researcher that conducted the procedure (J.L.S.). Before applying the ultrasound, this researcher pushed buttons on the ultrasound to simulate the adjustment of settings and, at last, always pushed the timer button, which emitted a sound. Ultrasound gel was applied on the skin, and the ultrasound probe was rubbed over the muscles of the anterior thigh, posterior thigh, anterior leg, and posterior leg. The sham ultrasound application was done for 5 min in each region, alternating the application between limbs, which totaled 40 min. Noteworthy, active administration of therapeutic ultrasound does not provoke any relevant sensation, than the sensations provoked by gel application and probe movement on the skin. Thus, it was impossible for the subjects to notice that the device was not delivering ultrasound.
The CT procedure consisted of lying supine for 40 min. Subjects were not allowed to sleep during this period. After the interventions administration, subjects stood on a treadmill to be instrumented for the exercise tests. At the end of the instrumentation, on visits 3 and 4, or only on visit 4, one of the researchers (J.L.S.) asked the following question: “Do you think your performance today will be equal, better, or worse than the performance on the previous visit(s)?” Subjects were instructed to compare their performance versus the CT day. In detail, if the CT was used on visit 2, subjects compared the IPC and SHAM versus CT (e.g., IPC better than CT). If the CT was used on visit 4, subjects were asked on this visit to compare their expected performance versus the previous visits, and later responses were inverted for statistical analyses (e.g., the response CT worse than IPC was analyzed as IPC better than CT). These questions were used to assess the placebo induction and nocebo avoidance. The same researcher conducted all exposures to IPC, SHAM, and CT (J.L.S.). This researcher was also responsible to inform the subjects that IPC and SHAM would similarly improve performance compared with CT, and IPC would be harmless despite circulatory occlusion sensations. Moreover, this researcher asked the question about performance expectancy. Just one researcher executed these methods, according with a systematic ritual, to standardize the placebo induction and nocebo avoidance.
Exercise tests were performed on a treadmill (Super ATL, Inbrasport, Brazil), with grade set at 1% to approximate the energy expenditure of outdoor running (39). The continuous incremental test, conducted on visit 1, provided data to define the discontinuous incremental test, used on visits 2, 3, and 4. The continuous incremental test consisted of baseline at 8 km·h−1 for 3 min, 1 km·h−1 increase in velocity per minute until voluntary exhaustion, and recovery at 5 km·h−1 for 5 min.
The discontinuous incremental test started with 6 min of baseline at velocity 1 km·h−1 lower than the velocity where the ventilatory threshold was identified on the continuous incremental test, which aimed to obtain steady state V˙O2 data for the oxygen cost of running interpretation (39). After that, the discontinuous incremental test consisted of 3 min at velocity 2 km·h−1 higher than the baseline velocity, followed by 1 km·h−1 increase in velocity per stage until voluntary exhaustion (Fig. 1). Each stage lasted 3 min and was followed by a 30-s break for blood sampling. At the end of each stage, the treadmill velocity was reduced to zero, and, as soon as subjects could, they stepped off the treadmill belt. They stood for blood sampling and then velocity was increased to the next stage. When subjects reached exhaustion, treadmill velocity was reduced to zero and subjects recovered standing on the treadmill for 3 min. Next, they recovery walking for 7 min at 5 km·h−1.
After the recovery period from the discontinuous incremental test, a supramaximal exercise test was started (Fig. 1). The treadmill velocity was firstly set for 2 min at 60% of the velocity of last completed stage on the previous discontinuous incremental test [i.e., peak velocity (Vpeak)]. Next, velocity was set at 0.5 km·h−1 higher than Vpeak until volitional exhaustion (30). Subjects were blinded about the treadmill velocity and all other data collected during the study. The researcher that gave verbal encouragement during the test was always the same for a given subject and was blinded about the test results and the intervention that was administered. All tests were conducted at temperature between 21°C and 23°C, and relative humidity between 45% and 65%.
Ventilation and pulmonary gas exchange were recorded breath by breath throughout the exercise tests using a metabolic analyzer (Quark CPET, Cosmed, Italy). O2 and CO2 analyzers were calibrated according to the manufacturer’s specifications, using ambient air and gases with known concentration (16% O2 and 4% CO2). The flow meter was calibrated using a 3-L syringe. Electrocardiogram was continuously recorded (Powerlab, AD Instruments, Australia) to quantify HR (LabChart, AD Instruments, Australia). Twenty-five microliters of blood were draw from the earlobe during the breaks of the discontinuous incremental test. A vasodilator ointment was used to arterialize blood samples (Finalgon, Boehringer Mannheim, Germany). Blood was collected using heparinized and calibrated capillaries, and then stored in Eppendorfs containing 50 μL of 1% NaF (i.e., anticoagulant), until analysis of lactate concentration (YSI 1500 SPORT, Yellow Springs Instruments, USA). Rating of perceived exertion was assessed during the continuous and discontinuous incremental tests using 0–10 Borg scale. Time to exhaustion was used to assess endurance performance. Time was recorded using a digital stopwatch. The chronometer was started when subjects achieved the target supramaximal velocity, and stopped when subjects asked to interrupt the test (i.e., volitional exhaustion), despite strong verbal encouragement. Time to exhaustion was recorded from all subjects, but only data obtained at the same supramaximal velocity, for a given subject, were used for statistical analyses. As a result, for this variable, the sample size was 15. Due to technical problems, the sample size for peak ventilation, peak pulmonary gas exchange, and peak HR were reduced to 16. In addition, blood samples were only obtained from the first 14 subjects that participated in the study.
Pulmonary ventilation and gas exchange were filtered to exclude aberrant breaths (two standard deviations from the mean of a 15-breath window). Oxygen cost of running was calculated as the average V˙O2 of the last 2 min of baseline of the discontinuous incremental test, divided by the corresponding velocity in kilometers per minute (39). Two experienced investigators independently identified the ventilatory threshold using 20-s mean data. When there was no agreement on the independent identification, investigators reviewed the identification together. The following criteria were used (1): 1) break point in CO2 output (V˙CO2) plotted versus V˙O2; and 2) increase in ventilatory equivalent of O2 (V˙E/V˙O2) plotted versus time, without increase in ventilatory equivalent of CO2 (V˙E/V˙CO2) plotted versus time. Lactate threshold was determined using a mathematical model that identified the point of intersection between two linear regressions that yielded the least sum of squared differences between the observed lactate values and the fitted values (31). Peak lactate concentration in the discontinuous test was obtained from the blood sample collected after the last completed stage. Peak lactate concentration in the supramaximal test was the highest value obtained from blood samples collected at exhaustion and the third minute of recovery.
The highest 20-s mean V˙O2 during the exercise tests was considered the peak oxygen uptake (V˙O2peak). If V˙O2peak of the discontinuous and supramaximal tests was within the tolerance of measurement error (i.e., 2%), the V˙O2max was assumed to be achieved (30), and the highest value was used for statistical analyzes. In our study, the V˙O2max was confirmed in all cases. Beat by beat HR was reduced to 20-s means, and peak HR was the highest value during each test. V˙O2, V˙CO2, lactate concentration, and perceived effort during the discontinuous test were linearly interpolated to compare data throughout the test among the interventions (Origin, Microcal, USA). V˙O2 and V˙CO2 interpolation was done using 20-s mean data. The highest common velocity among the interventions was considered 100% and, based on that, interpolated data corresponding to 70%, 75%, 80%, 85%, 90%, and 95% were identified and used for statistical analyses.
Sample size was calculated considering the endurance performance as the main endpoint in one-way repeated measures ANOVA (G*Power 3.1 Dusseldorf University, Germany), as well as considering a mean difference of 3% in endurance performance among interventions (2,3,14), P value at 0.05 and power at 0.80. As a result, the sample size calculation indicated that 15 subjects would be necessary to conduct the study. All variables showed normal distribution in the Shapiro–Wilk test. One-way repeated measures ANOVA was used to compare IPC, SHAM, and CT. The Greenhouse–Geisser correction was used to adjust ANOVA results, whenever sphericity was violated in the Mauchly test. The Bonferroni post hoc was used when significant F values were found. Results are presented as mean ± SEM. All analyses were two-tailed, and statistical significance was accepted for P < 0.05. Statistical analyses were done using the software STATISTICA (Statsoft, USA).
The subject’s best time is reported and compared with the corresponding world record in Table 1. On average, men and women’s best time were, respectively, 123% ± 3% and 130% ± 3% of the corresponding world record (i.e., ~23% and ~30% above the world record). Score on the total quality recovery scale was similar among interventions (IPC: 16.5 ± 0.3 a.u. vs SHAM: 16.3 ± 0.3 a.u. vs CT: 16.3 ± 0.39 a.u., P = 0.52). Occlusion pressure during IPC was recorded in the first four subjects that participated in the study, and it varied between 140 and 150 mm Hg. The majority of the subjects expected performance would better after IPC than CT (better, 83%; equal, 17%; worse, 0%) and after SHAM than CT (better, 78%; equal, 22%; worse, 0%). The percentage of subjects who responded performance would be better versus CT was statistically similar between IPC and SHAM (83% vs 78%; P = 0.41).
Peak data were similar among interventions in the discontinuous and supramaximal tests (Table 2). Ventilation, V˙O2, V˙CO2, lactate concentration, and perceived effort were similar among IPC, SHAM, and CT throughout the discontinuous test (P > 0.05; Fig. 2). Oxygen cost of running, lactate threshold, and V˙O2max were also similar among interventions (P > 0.05; Fig. 3). Noteworthy, V˙O2 was similar between the fourth and sixth minute of baseline within all interventions (IPC: fourth minute, 39.70 mL·kg−1·min−1 vs sixth minute, 39.66 mL·kg−1·min−1, P = 0.98; SHAM: fourth minute, 39.74 mL·kg−1·min−1 vs sixth minute, 39.85 mL·kg−1·min−1, P = 0.95; CT: fourth minute, 39.78 mL·kg−1·min−1 vs sixth minute, 39.93 mL·kg−1·min−1, P = 0.93). These results indicate that V˙O2 was at steady state. Time to exhaustion was longer after IPC (mean ± SEM, 165.34 ± 12.34 s; Fig. 4) and SHAM (164.38 ± 11.71 s) than CT (143.98 ± 12.09 s; P = 0.02 and 0.03, respectively), but similar between IPC and SHAM (P = 1.00).
We circumvented limitations from previous studies to test the effect of IPC on aerobic metabolism parameters and endurance performance in well-trained runners. The data showed no effect of IPC on aerobic metabolism parameters. Time to exhaustion, on the other hand, was longer in the IPC and SHAM versus CT, but similar between IPC and SHAM. Therefore, the results indicate that 1) the improvement in performance after IPC did not surpass the effect of a placebo intervention, which refuted the ergogenic effect specifically mediated by the IPC; and 2) the improvement in performance after IPC and SHAM was probably mediated by neural mechanisms related to placebo induction, instead of change in metabolic responses.
The present study sought to induce similar placebo effect on the IPC and SHAM interventions, and to avoid a possible nocebo effect related to circulatory occlusion sensations. Only one of the researchers interacted with the subjects to induce placebo and avoid nocebo, according with a systematic ritual, as the interlocutor interaction with the subjects can modulate the placebo effect in clinical studies (7). Most subjects, in fact, expected IPC and SHAM would improve performance compared with CT, which indicates that, in general, the placebo induction and nocebo avoidance was successful. However, the positive expectancy about the interventions effect was not unanimous. The lack of unanimity may be explained by the question used in the study, because subjects were asked to compare their expected performance with the performance on previous visits. The response to this question may rely not only on the researcher’s explanation about the possible effect of interventions but also on any factor that may modulate the subject well-being and motivation on the experiment day. In addition, the effect of placebo induction on endurance performance also depends on the subjects’ personality (6), which may explain part of the between-subject variability on expectancy about the interventions effect.
A recent meta-analysis reported a small beneficial effect of IPC on endurance performance (effect size, 0.51; 90% confidence interval, 0.35–0.67), based on the analysis of eight studies (38). Some of these studies specifically investigated the effect of IPC on time to exhaustion during constant workload tasks (3,13,24), which is similar to the endurance assessment used in the present study. IPC prolonged time to exhaustion during rhythmic handgrip exercise (3), cycling at heavy intensity (24), and cycling at maximal intensity (13). However, these and other studies about the effect of IPC on endurance performance did not sufficiently control placebo and nocebo effects, which complicate the interpretation about the isolated effect of IPC (37). In this sense, Marocolo et al. (28) attempted to exclude the placebo effect to test the ergogenic effect of IPC on a 100-m time trial and a bout of lower limb resistance exercise (29). Authors told subjects’ IPC and low-pressure cuff inflation would improve performance versus resting CT. They found both interventions improved performance compared with CT, but there was no difference between IPC and low-pressure cuff inflation, which does not support the ergogenic effect specifically mediated by the IPC.
A recent study from our group, on the other hand, compared the effect of IPC, a sham intervention, and low cuff inflation on swimming performance in a repeated sprint swimming task (16). The sham intervention consisted of short-time circulatory occlusion, which approximated signs and symptoms of IPC, without attaining enough duration that could possibly induce ergogenic effect. Similarly to the present study, a researcher told subjects that IPC and a sham intervention would improve performance compared with the CT condition, and that IPC and sham intervention would cause absolutely no harm. Most subjects in fact believed IPC and sham intervention would improve performance. At the end, the IPC enhanced swimming performance, whereas the sham intervention did not, which supported the ergogenic effect specifically mediated by the IPC.
In the present study, data did not support that IPC per se mediated ergogenic effect on endurance performance. The findings herein presented are therefore in contrast to those from the recent study of our group (16). Of note, however, in the present study, we applied IPC once, 10 min before the performance assessment. In the former study, we applied IPC on three occasions (48 h, 24 h, and 30 min) before the performance assessment, which may have summed early and late effects of IPC and amplified the IPC effect (26,40). In addition, the exercise mode was different (running vs swimming), as well as the testing protocol (constant velocity at supramaximal intensity vs repeated sprints). Consequently, these characteristics may be related to the distinct effect of IPC between these studies, which should be further investigated to enable the use of IPC to enhance exercise performance.
Multiple mechanisms are known to regulate endurance performance, including determinants of oxygen delivery and utilization (20), and mechanisms that modulate perceived effort (27). Nevertheless, three variables summarize most of the contribution of aerobic mechanisms to endurance performance (20,22), that is oxygen cost of running, lactate threshold, and V˙O2max (20,22). In our study, oxygen cost of running was unaffected by IPC and, as far as we know, no study has reported reduced oxygen cost at submaximal exercise intensities (2,14,18,24,32). Altogether, the present and previous findings suggest that IPC does not change the efficiency of the aerobic metabolism during whole body exercise performed at submaximal intensity. The lack of IPC effect on the aerobic metabolism during exercise is somewhat surprising, because IPC induces ATP sparing (36) and augment electron mitochondrial flux during prolonged ischemia (9). Therefore, the mechanisms that mediate ergogenic effects of IPC during whole body aerobic exercise, if any, may not be the same to the protective effects mediated by IPC during prolonged ischemia and subsequent reperfusion.
Bailey et al. (2) reported that IPC decreased blood lactate concentration during an incremental treadmill test, which trended to dislocate the onset of blood lactate accumulation to higher running velocity. In contrast, Crisafulli et al. (12), de Groot et al. (14), Jean-St-Michel et al. (21), and Patterson et al. (33) found no effect of IPC on blood lactate concentration during varied exercise tests. In the present study, after controlling placebo and nocebo effects, there was no effect of IPC on the blood lactate concentration throughout exercise intensities, as well as on the lactate threshold. Thus, the present results supported that IPC did not change blood lactate levels during an incremental treadmill test in well-trained runners.
The effect of IPC on the V˙O2max has also been equivocal. For example, de Groot et al. (14) and Cruz et al. (13) reported IPC increased V˙O2max during cycling tests, but others did not corroborate this finding (2,12). Nevertheless, all previous IPC studies did not use strict criteria to confirm the V˙O2max. In the present study, therefore, we used a verification protocol. The protocol successfully confirmed the V˙O2max in all subjects, although peak RER was relatively low and peak perceived effort was not always at score 10. These findings have been previously reported in discontinuous incremental tests (30), and rest–exercise transitions (10). Of note, the low peak RER has been attributed to increase in endogenous CO2 stores during rest–exercise transitions (10). However, without the use of the verification protocol, doubts about the V˙O2max attainment could have been raised. Consequently, the verification protocol strengthens the interpretation that IPC did not change the V˙O2max in the present study.
Given that IPC and SHAM similarly enhanced endurance performance, without any change in metabolic responses, neural mechanisms related to placebo induction could be responsible for the performance enhancement. In fact, strong evidences support that neural factors can per se determine endurance exercise performance, independent from changes in metabolic responses (27,32), but the mechanisms specifically involved in placebo-induced exercise performance enhancement are unknown (5). It is possible, however, that some mechanisms already described in clinical settings may be involved. For instance, Benedetti et al. (8) manipulated the meaning of a painful experience from negative to positive through verbal suggestions. The manipulation resulted in increased pain endurance, which was mediated by coactivation of opioid and cannabinoid pathways (8).
One of the study’s limitations was the assessment of endurance performance using an open-ended laboratory test (i.e., the end is not fixed) performed at constant velocity, instead of a close-ended task (i.e., the end is fixed) performed at subject’s preferred pace. The latter is nearer to a real sport event and is more physiologically complex. Thus, our findings may not be extrapolated to other situations. Another limitation was the measurement of V˙O2 at the lungs and lactate concentration at ear capillaries. These measurements may not exactly reflect the V˙O2 and lactate concentration at contracting skeletal muscles nor differentiate metabolic responses between subtypes of skeletal fibers. IPC was applied just once, 10 min before the performance assessment, which may have not been enough to induce ergogenic effects in well-trained subjects. Or, alternatively, the effect of IPC in well-trained subjects is blunted compared with less fit subjects, which, for example, has been reported for the ergogenic effect of nitrate supplementation (35). Furthermore, IPC may not change metabolic responses, but instead, modifies neuromuscular mechanisms, such as muscle activation (13) and/or rate of muscle contraction and relaxation (3), which deserves further investigation. At last, we assessed men and women, but our sample size was not enough to dissect whether gender modulates the effect of IPC.
In conclusion, IPC did not change aerobic metabolism parameters, whereas improved endurance performance in well-trained runners. The IPC improvement, however, did not surpass the effect of a placebo intervention. Therefore, the results refute that the ergogenic effect specifically mediated by the IPC and suggest that the improvement in performance after IPC and SHAM was probably mediated by neural mechanisms related to placebo induction, instead of change in metabolic responses.
J. L. S. and T. O. received scholarship from the São Paulo Research Foundation (FAPESP; grants: 2014/15877-8 and 2015/03572-0, respectively). B. M. S. received funding from FAPESP (grant: 2014/25683-6 and 2014/24294-6) and the National Counsel of Technological and Scientific Development (CNPq; grant: 461516/2014-4) to conduct the study. We are grateful to Luis Gustavo Andrade Candido, who was the trainer of most of the athletes that participated in the study, for collaboration with subjects’ enrolment and scheduling of experimental visits.
Authors have no conflict of interest to disclose. The results of the present study do not constitute endorsement by ACSM. Also, the authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
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