Although ischemic preconditioning (IPC) has demonstrated clinical relevance to limit infarct size in animals (1), the present interest in IPC as a potential ergogenic aid for exercise performance is increasing worldwide (2,3). Originally, IPC involved intermittent peripheral circulatory cycles of occlusion and reperfusion of blood flow to prepare and protect the body’s cells against successive events of similar or greater hypoxic stress (4). In sport, IPC is a noninvasive maneuver that alternates brief periods of occlusion and reperfusion (e.g., 2 to 5 min each cycle) of muscle blood flow, generally applied at rest with an occlusion cuff to the proximal part of lower or upper limbs before exercise (2,5). Partial ischemia, i.e., a lowered blood flow to the tissue, can lead to localized tissue hypoxia. Previous research has typically implemented IPC at rest with total occlusion (usually >200 to 300 mm Hg) with 3 to 4 × 5 min occlusion-reperfusion cycles (5,6).
The ergogenic effects from IPC are still arguable, and its mechanisms are unclear (2,5), although hyperemia during the reperfusion phase appears to play a critical role (7). Libonati et al. (7) reported an improvement in strength performance immediately after IPC (2 min at 200 mm Hg) and attributed it to an increase in muscle blood flow (five- to sixfold from baseline). Increased skeletal muscle [deoxyhemoglobin] (8), higher muscle oxygen extraction (9), and release of nitric oxide (10) are other possible mechanisms in which IPC may improve exercise performance.
The performance enhancement induced by IPC has been reported to range between 1% and 8% in various sport disciplines (2,3,8,11). For instance, IPC has been shown to improve (+2.5%) 5-km running performance (11), and increases in power output (+1.6%) and maximal oxygen uptake (V˙O2max) (+3%) have also been reported (6). These improvements in endurance exercise may also be relevant to exercise at altitude, and a potentially new application of IPC for altitude acclimation has recently emerged (12). It is known that alteration in central convective factors and peripheral oxygen diffusion contributes to the endurance performance decrements at altitude (13). Thus, the IPC-derived hyperemia and the increased oxygen delivery to active muscles could alleviate such detrimental hypoxia-induced effects. Indeed, a recent study suggested a beneficial effect of IPC on endurance time trial (TT) performance in normobaric hypoxia at simulated altitude of 2400 m in trained cyclists (12). The authors attributed the IPC ergogenic effect to a higher blood oxygen saturation, peripheral oxygen use, and lower perception of effort.
In normoxia, IPC seems to lead to beneficial responses during an early (~1 to 2 h) and/or a late (~12 to 72 h) time window (14,15). A recent research showed that the positive effects of IPC on sprint-specific performance at sea level started after ~2 h and lasted for at least 8 h (16). To date, to our knowledge, there is limited evidence of IPC applied within hypoxia, and none regarding a potential delayed effect (i.e., after at least 2 h after IPC application) on performance in hypoxia, which could be relevant for practical application (e.g., competitions with short recovery in between at altitude). Therefore, the present study evaluated whether IPC performed in normobaric hypoxia would generate immediate and/or delayed beneficial effects on two 5-km cycling TT performances separated by ~1 h of rest in hypoxic conditions. We hypothesized that IPC would improve performance in the 5-km TT1 and would mitigate the decline in performance on the 5-km TT2 (1 h later) due to the delayed effect from IPC.
METHODS
Subjects and ethical approval
The sample included 13 healthy male subjects (mean ± SD; age 27.5 ± 3.6 yr, body mass 77.0 ± 10.8 kg, body height 178 ± 7 cm, body mass index 24.3 ± 2.5 kg·m−2) due to a priori sample size calculation based on IPC effect on performance change during a similar TT performed in hypoxia (12). Inclusion criteria were as follows: (a) no smoking history; (b) absence of any cardiovascular or metabolic disease; (c) systemic blood pressure lower than 140/90 mm Hg; (d) no use of creatine supplementation, anabolic steroids, drugs, or medication with potential effects on physical performance; (e) no time spent at altitude >1000 m in the previous 3 months; and (f) no recent musculoskeletal injury. The experimental protocol was approved by the Ethical Commission for Human Research CER-VD 138/15 and conducted according to the Declaration of Helsinki. Participants gave written informed consent after being informed of the procedures and risks involved.
Experimental design
Figure 1 shows the experimental design of this study. Subjects attended the laboratory for three visits (with 3 to 7 d between). On the first visit, there was an initial screening including body mass and height measurements, followed by familiarization with the perceptual scales, equipment, and a 5-km cycling TT at normobaric hypoxia (fraction of inspired oxygen [FiO2] of 16%, ~2400 m). In the second and third visits (~165 min per visit), a randomized crossover assignment of condition (IPC or placebo—SHAM) was adopted. Each session was completed in a normobaric hypoxic chamber (ATS Altitude Training, Sydney, Australia) built in the laboratory. From the start of the IPC/SHAM protocol to the end of the second TT, subjects remained exposed to the same constant hypoxia in the hypoxic chamber. The FiO2 was controlled regularly with an electronic oximeter (GOX 100; Greisinger, Regenstauf, Germany), and the variation remained <0.1%. No information was provided to the subjects regarding the FiO2 level for any trial. However, the subjects were told that the level of hypoxia (if any) would always be the same (i.e., all sessions with the same hypoxia).
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FIGURE 1: Experimental design of the study; n = 13. TT1, first TT; TT2, second TT.
To evaluate a potential delayed effect from IPC on performance (16), the 5-km TT was performed twice per session: 25 min (TT1) and 2 h (TT2) after the end of the last IPC bout. The 5-km TT was conducted on a cycle ergometer (Lode Excalibur Sport Ergometer, Lode B.V., the Netherlands) by the same researcher in a constant environment (23°C ± 0.5°C; humidity, 35% ± 5%) at the same time of the day (for each subject) and with the same hypoxia. After a 10-min self-paced warm-up, the subjects performed the 5-km TT (remaining seated) as quickly as possible. Resistance was set with an alpha of 0.4 W·min−1 and was performed in the “linear mode” until participants accumulated 125 (kJ). The only available feedback was the distance covered each 500 m. During the interval between TT1 and TT2 (~1 h), the subjects rested on a bed inside the chamber. The subject’s dietary intake profile was documented in the first session then replicated during the second session (16,17). The same procedure was conducted with food or water consumption during the recovery period between TT1 and TT2.
Subjects were blinded about any hypoxia information because this is potential bias in hypoxic studies (18) and performance data until the end of the research, i.e., no information about time to complete the 5 km, heart rate (HR) values, and blood lactate (19). Coffee (or caffeine products), tea, and alcohol intake were prohibited as well as strenuous exercise for 48 h before testing.
Perceived recovery status, muscle soreness, and discomfort
Before each session, the participants indicated a score on a perceived recovery scale (20) (from 0 to 10 arbitrary units [AU]), regarding relative physical recovery, and a score of delayed-onset muscle soreness (DOMS) on a visual analog scale (0 to 10 AU). The participants indicated the level of general muscle soreness (nontrauma related) in the legs region during movement. The rate discomfort related to IPC/SHAM was also recorded with the same visual analog scale used in the DOMS procedure.
Perception of conditions and expectancy (belief)
After the last visit, to check any possible placebo or nocebo effect (19), the subjects answered the following questions: (a) Did you notice any difference between the two pressure cuff conditions (yes/no)? (b) If yes, what was the difference, considering the cuff conditions (i.e., higher pressure IPC, lower pressure SHAM)? (c) Did you expect any effect on your performance (no effect/positive influence/negative influence)? After which, participants were informed about the hypoxia level used.
Muscle oxygenation measurements
Muscle oxygenation was assessed using the near-infrared spectroscopy (NIRS) technique well described elsewhere (9). The NIRS device (PortaLite, Artinis®, Zetten, the Netherlands) was used to measure changes in muscle oxygenation by placing a sensor containing three light source transmitters (each with two wavelengths of 760 and 850 nm) at 30, 35, and 40 mm distance to the receiver, on the vastus lateralis muscle of the right thigh, at 1/3 distance from the top of the patella to the greater trochanter. The PortaLite was placed and attached with double-sided tape and then wrapped with tension against the leg to reduce movement during exercise. Permanent pen was used to mark the position, and images were taken to reproduce the placement in subsequent visits. Measurements included a standard differential path length factor of 4.0 for the vastus lateralis as there is a lack of any clear standard value during exercise (21) and that manufacturer recommends 4.0. Changes in concentrations of oxyhemoglobin (O2Hb), deoxyhemoglobin (HHb), total hemoglobin (tHb), and tissue saturation index (TSI, %) were obtained. All signals were continuously recorded during IPC/SHAM and TT with a sampling frequency of 50 Hz and were down sampled at 10 Hz for further analysis (Oxysoft 3.0.53, Artinis). A fourth-order low-pass zero-phase Butterworth filter (cutoff frequency, 0.2 Hz) was applied during analysis to reduce artifacts and to smoothen from pedal–stroke perturbations (22). For analysis of preconditioning, data were expressed as a normalization to the resting baseline (average of the last 60 s of stage − average of the last 30 s of a 2-min resting supine baseline). For the TT analysis, data were normalized as the difference between the average value during the TT and the average of last 30 s of the initial warm-up stage at 100 W, except for absolute TSI values, which correspond to the absolute value during the TT. The NIRS results were assessed with this normalization to compare a change between dynamic movements as the baseline (warm-up vs TT); as to our knowledge, resting baseline data have not shown reproducibility (23). Furthermore, the day-to-day variation in these parameters is approximately 8%–9% (24,25).
The ΔTSI (%) of IPC stages represents the average of changes (min–max) in TSI after cuff release over the three IPC stages. The ΔTSI of the TT was calculated as the change in the maximum value within the first 5% and the minimum value within the last 5% of each TT during the IPC condition only, as a stable nonchanging signal was present in the SHAM condition.
IPC and placebo protocols
After a 10-min supine rest in a quiet, temperature-controlled room (inside the hypoxic chamber - FiO2 16%), IPC/SHAM was performed on a bed using bilateral arterial occlusion. To avoid a placebo effect (19), subjects were informed about the testing of two external pressure conditions and that both could improve performance. To prevent a nocebo effect, the subjects were informed that IPC/SHAM would cause no harm, despite discomfort related to the maneuver (19). Occlusion cuffs (Model SC12D, 13 × 85 cm; Hokansson, Bellevue, WA) connected to a rapid cuff inflation system (E20/AG101 Rapid Cuff Inflation System, D.E. Hokansson) were placed at the proximal upper leg and inflated to 220 mm Hg for 5 min (5,12). The IPC was repeated three times bilaterally, with each ischemic episode separated by 5 min of reperfusion with no pressure (i.e., 3 × 5 min occlusion/5 min reperfusion). In the SHAM (another session), subjects received an identical protocol, but the cuff was inflated to 20 mm Hg (12) during the “occlusion phase.” The rationale for using this specific IPC protocol was as follows: (a) it was used to meet the threshold of ischemic stimulus (i.e., at least 4 min) (26), and (b) the same IPC protocol has been used successfully in a study involving exercise performance and moderate hypoxia (12). The 5-km TT was performed with at least 72 h between sessions to respect a potential late effect from IPC, called the second window, and a maximum of 1 wk to prevent fitness alterations (14).
Blood lactate, pulse oxygen saturation, and HR measurements
Capillary blood samples (5 min before and 2 min after each 5-km TT) were collected from the right earlobe and analyzed for blood lactate concentration using a portable device (Lactate Scout; EKF Diagnostics, GmbH, Leipzig, Germany). During the 5-km TT, HR was monitored (Polar RS 400, Kempele, Finland) and pulse oxygen saturation (SpO2) was measured by noninvasive oximetry (8000Q2 Sensor; Nonin Medical Inc., Amsterdam, the Netherlands) from the left earlobe.
Statistical analysis
Data are presented as mean ± SD. The Shapiro–Wilk test was applied to verify the normality of the data. The Wilcoxon test was used for nonnormality data: rating of perceived recovery, DOMS, and discomfort. Paired t-tests were executed for data that passed the normality test: delta (absolute and relative) performance (s) in TT1 and TT2. For all other variables, which involved moment comparisons (TT1 vs TT2) and treatment (IPC vs SHAM), a two-way ANOVA followed by a post hoc Tukey’s test was used. The oxygenation measurements were analyzed using a linear mixed model with participant as the random effect: 1) the IPC/SHAM stages were analyzed using the average of the three stages of IPC/SHAM with fixed effect of condition (IPC vs SHAM); 2) the TT was analyzed using fixed effects of condition (IPC vs SHAM) and TT (TT1 vs TT2). Analyses were executed using R (R Core team 2017, Foundation for Statistical Computing, Vienna, Austria) and nlme4 (27). Least-squares means for mixed models (library lsmeans (28)) using the Tukey method were used to obtain the contrasts. The statistical significance was set at 0.05. Effect size (Cohen d) was calculated (mean difference divided by the pooled standard deviation) to determine the practical relevance (only in performance data if P < 0.05 was found) and classified as trivial (<0.2), small (>0.2–0.6), moderate (>0.6–1.2), large (>1.2–2.0), and very large (>2.0). Pearson correlation coefficients were calculated between the acute hyperemic responses after cuff release during the three preconditioning stages during the IPC condition and the differences in the initial and end values during the TT SHAM and IPC. Analyses other than oxygenation parameters were conducted using GraphPad® (Prism 6.0, San Diego, CA).
RESULTS
Attendance and belief
None of the subjects had complications with the IPC procedure at (normobaric) hypoxia, and all of them correctly noticed the difference between high and low cuff pressures. Five subjects expected no effect from IPC/SHAM, six expected negative effects from IPC (three of these performed worse in IPC), and two subjects expected positive effect from IPC in the TT performance, but neither of these two performed better in IPC.
Perceived recovery status, muscle soreness, and discomfort
The rating of perceived recovery did not differ (P > 0.999, Wilcoxon test) between IPC (8.3 ± 0.9 AU) and SHAM (8.3 ± 0.9 AU). The rating of DOMS did not differ (P = 0.289, Wilcoxon test) between IPC (1.1 ± 1.2 AU) and SHAM (1.5 ± 1.8 AU). IPC presented a higher (P < 0.001, Wilcoxon test) rating of discomfort (3.3 ± 1.6 AU) than SHAM (0.3 ± 0.4 AU).
Blood lactate, SpO2, HR, and RPE responses
Table 1 shows that blood lactate and SpO2 did not differ (P > 0.05) between IPC and SHAM in all time points. Blood lactate increased (P < 0.05) in pre-TT2 versus pre-TT1, and the increment postexercise was lower (P < 0.05) in TT2 versus TT1. SpO2 decreased (P < 0.05) during TT1 and during TT2 in comparison with preexercise baseline (Table 1). HR responses and RPE scores did not differ (P > 0.05) between IPC versus SHAM during TT1 and TT2 (Table 1).
TABLE 1: Physiological responses and vastus lateralis normalized oxygenation values from the first and second 5-km TT (TT1 and TT2, respectively) for placebo (SHAM) and IPC conditions, and normalized oxygenation values from the average of the three 5-min preconditioning stages.
Performance data
There was no difference between IPC and SHAM for the time to complete TT1 (P = 0.598) and TT2 (P = 0.977) (Fig. 2A). IPC exhibited similar (P = 0.381) time in TT2 (580 ± 117 s) when compared with TT1 (565 ± 112 s), but SHAM performed slower (P = 0.011, d = 0.27) in TT2 (584 ± 115 s) in comparison with TT1 (554 ± 112 s) (Fig. 2A). There was no difference between IPC and SHAM for mean power output (Fig. 2C; TT1 P = 0.468; TT2 P = 0.962). However, IPC showed similar (P = 0.360) mean power output in TT2 (227 ± 56 W) when compared with TT1 (232 ± 58 W), whereas SHAM showed lower (P = 0.005, d = 0.23) mean power output in TT2 (225 ± 54 W) in comparison with TT1 (238 ± 59 W) (Fig. 2C).
FIGURE 2: Representation of the average and individual TT duration (s) (A), percent difference in TT duration (%) (B), and mean power output (W) for both TT1 and TT2 in placebo (SHAM) and IPC conditions (n = 13) (C). Data are presented as mean ± SD; n = 13. A, *Difference (P = 0.011, d = 0.27) between SHAM TT1 and SHAM TT2; B. #Difference (P = 0.017, d = 0.76) between SHAM and IPC; C. *Difference (P = 0.005, d = 0.23) between SHAM TT1 and SHAM TT2.
Peak power and average power output over the final minute of the TT did not differ (P > 0.05) between IPC and SHAM during both TT1/TT2. Two-way ANOVA interaction and main effects were as follows: F3, 36 = 0.2310, P = 0.874 (peak power W); F3, 36 = 1.390, P = 0.261 (last minute mean power, W); and F3, 36 = 0.51, P = 0.672 (last minute peak power, W). Decrement in performance (from TT1 to TT2) was lower after IPC (15 ± 19 s/2.5% ± 2.7%) than SHAM (30 ± 26 s/5.2% ± 4.2%): paired t-test P = 0.017, d = 0.68 for s and d = 0.81 for %. Paired t-test revealed IPC resulted in a lower (−6 ± 6 W/−2.5% ± 2.7%) decrement in TT2 mean power (in relation to TT1) in comparison with SHAM (−13 ± 12 W/−5.1% ± 4.2%): P = 0.018, d = 0.81 for W and P = 0.015, d = 0.76 for %.
Muscle oxygenation measurements
The oxygenation values for the IPC stages as well as both TT1 and TT2 are shown in Table 1. Individual and mean [O2Hb], [HHb], and [tHb] responses during TT1 and TT2 are displayed in Figure 3, as well as a trace of the average of all individuals NIRS values throughout TT in Figure 4. As expected, all muscle oxygenation responses differed during the IPC/SHAM stages (P < 0.001 for all) with [HHb] being higher in IPC whereas [O2Hb], [tHb], and TSI were lower. During the TT, main effects demonstrated lower [O2Hb] during TT2 than TT1 (P = 0.005). Further, there were greater concentrations of total hemoglobin ([tHb]) in IPC than SHAM (P = 0.031) and greater [tHb] in TT1 than TT2 (Table 1, Fig. 3). There was a trend present for an interaction for [O2Hb] between condition and TT (F = 4.05, P = 0.07), demonstrating lower [O2Hb] in TT2 than TT1 during IPC, and there was greater concentration of IPC than SHAM during TT1 (Table 1).
FIGURE 3: Individual representation (n = 13) of average values normalized to the last 30 s of a warm-up stage at 100 W [O2Hb] (A), [HHb] (B), and [tHb] during TT1 and TT2 in placebo (SHAM) and IPC conditions (C). Mean ± SD. #P < 0.05, significant main effect of condition, IPC different from SHAM. **P < 0.01, significant main effect of TT, TT1 different from TT2.
FIGURE 4: Representation of the average of all participants’ NIRS values normalized the last 30 s of a warm-up stage at 100 W over each 1% of the TT for placebo (SHAM) and IPC conditions. O2Hb, HHb, and tHb (A and B); TSI for TT1 and TT2 (C and D, respectively).
During the IPC condition, there was a significant correlation between average change in TSI of IPC stages and average change in TSI during TT1 (r = 0.760, P = 0.011), as shown in Figure 5, but not during TT2 (r = 0.285, P = 0.425). Furthermore, there were no correlates of either of these changes in TSI (preconditioning stages or TT) with performance time or power output, nor percent changes in performance time or power output.
FIGURE 5: Representation of the relationship between average changes (min–max) in TSI amplitude after cuff release during three IPC stages and average changes (min–max) of TSI during TT1 during the IPC experimental condition.
DISCUSSION
This study is unique because it evaluated both acute and delayed effects of IPC applied at hypoxia on two consecutive 5-km cycling TT with ~1 h of rest in between in hypoxia. Our main finding is that although IPC did not improve performance (immediately or 2 h later), it prevented a performance decrement after previous exhaustive exercise (~2 h post-IPC). IPC maintained the performance of a second TT similar to the first one, whereas SHAM resulted in a significantly deteriorated performance. This might have practical application in sports with successive maximal-intensity exercise bouts performed in altitude (e.g., cross-country ski team sprint or biathlon single mixed relay). The underlying ergogenic mechanism might arise from increased blood volume and greater oxygen extraction observed during the TT in IPC but not in SHAM.
Performance and physiological indicators
Although IPC did not improve performance in TT1 and TT2, SHAM exhibited a significant performance decrease from TT1 to TT2 that was not present in IPC (either expressed in watts or seconds, with small to moderate effect sizes). This result suggests a delayed positive effect from IPC, especially because TT1 performance was not affected (Fig. 2). Our finding is in line with a study that showed swimming performance improvement starting after ~2 h and lasting for at least 8 h after IPC, suggesting a time-dependent effect of IPC on sprint swimming performance at sea level (16). The authors speculated that IPC could release substances (e.g., adenosine, bradykinin, reactive oxygen species) that need time to be signaled and to reach the target tissues benefiting the exercise performance (16). Future studies should examine these mechanisms as well as other ones likely influenced by hypoxia as fiber-type recruitment, endothelial responses, specific fatigue pattern, substrate use, or excess postexercise oxygen consumption.
To our knowledge, the present study is the first one to investigate the effects of IPC on two successive TT performed in hypoxia. To date, two studies presented contradictory findings in a “single” 5-km TT performance at moderate hypoxia. Although Paradis-Deschenes et al. (12) reported an improved (~1.5%) performance in endurance cyclists, Wiggins et al. (29) found no IPC effect on the 5-km TT performance despite an improved O2 extraction (↑ [DeoxyHb+Mb]), but only at a moderate-intensity workload after IPC. Of interest is that these authors confirmed the ergogenic effect of IPC on a 5-km TT performed in normoxia. Beyond the differences between these two studies (IPC applied bilaterally in a normoxic environment (12) vs alternatively in hypoxia (29); performance of the subjects is ~50 s faster in the 5-km TT in the study of Paradis-Deschenes et al. (12) compared with the study of Wiggins et al. (29)), these data overall suggest that the effects of IPC are likely intensity dependent and that well-trained cyclists may have optimized their O2 extraction at high intensity. Our data support this explanation (i.e., ~80 s slower in the 5-km TT1 than (12)) because we did not report any performance enhancement in the first TT when compared with SHAM.
There was no difference between IPC and SHAM in lactate at any measurement points (before and after; in absolute or relative delta values). Lactate pre-TT was higher in TT2 than that in TT1, likely because of an insufficient time (~1 h) to return to baseline values, which require ~90 min at sea level (30).
The arterial blood O2 saturation did not differ between IPC and SHAM either before or during TT1 and TT2. There are a few studies investigating the effects of IPC on SpO2 at hypoxia, and the results appear dependent on the level of induced blood hypoxemia (31). In our study, the untrained participant’s hypoxemia was moderate (mean values during the exercise ranging from 90.5% to 92%). In these conditions, our results are well in line with the responses observed in trained cyclists performing a 5-km TT while breathing 18% FiO2 (~1200 m altitude equivalent) and reaching mean SpO2 of 90.5% (12). When the same cyclists performed at 15.4% FiO2 (~2400 m altitude equivalent), they suffered severe hypoxemia (SpO2 82.8%–83.7% with lowest values below 80%), and only then did IPC offer a protection that was conducive to TT performance enhancement (12). The likelihood to observe ergogenic effects derived from the IPC maneuver, therefore, probably increases with the severity of arterial hypoxemia. Along this line of reasoning, when participants exercised maximally between 3560 and 4342 m terrestrial altitude, IPC also significantly reduced the pulmonary artery systolic pressure and increased SpO2 from 75.3% to 80.3% at rest (32). Taken together, IPC appears to elicit ergogenic effects especially when the cardiorespiratory system is highly taxed and/or when the exercise stimulus poses a physiological threat to the organism. Although mechanisms are still unknown, IPC likely altered ventilation/perfusion matching, alveolar ventilation, and O2 diffusion.
The values of HR obtained here (Table 1) confirmed that the effort for both TT1 and TT2 was near maximal (~92% of HRmax), reinforcing the validity of the present study. Mean and peak HR responses were not affected by IPC. Local (legs) and overall RPE values (Table 1) did not differ between IPC/SHAM, corroborating studies performed at sea level, in intermittent (19) and resistance exercises (33), in running 5-km TT (11), but not at hypoxia (12,32).
Oxygenation responses
As expected, there was a greater concentration of total hemoglobin ([tHb]) in the IPC condition than SHAM, as well as lower [tHb] in TT2 compared with TT1. It has been previously shown that [tHb] may be interpreted as tissue blood volume (34), although interpretation should be cautioned as this is not a measurement of skeletal muscle blood flow. Until now, the only way to accurately measure actual tissue blood flow is by using a dye tracer with venous occlusion plethysmography immediately after completion of exercise (35). This greater blood volume during the IPC condition is a consequence of the well-described hyperemic effect of IPC, a mechanism reported since the 1950s (5). Reactive hyperemia consists of an increased blood flow upon reperfusion of the tissue after a phase of vascular occlusion, likely due to the release of nitric oxide within blood vessels. This restriction and reperfusion action undoubtedly stresses the vascular reactivity and blood flow regulation, as recently suggested during repeated sprint exercise with blood flow restriction (36). Reactive hyperemia has been suggested as an important mechanism involved in IPC and has been reported (37) in many previous studies (38), but not in all (5). The present results during the TT may partially reflect a kind of vascular response from the IPC, where there was greater deoxygenation during the IPC stages than the control (Table 1).
As suggested earlier, the IPC-derived hyperemia and increased oxygen delivery to active muscles could alleviate detrimental hypoxia-induced effects. Further, the relationship found between change in TSI during preconditioning stages and TT1 of the IPC condition suggests that improved peripheral vascular function from IPC can be transferred to subsequent increased oxygen delivery to the exercising muscle. Together, this improved vascular function could contribute to alleviating the detrimental effects of hypoxia. However, the presence of this relationship did not transfer to a performance outcome in the current study.
The concentration of tHb also involves the amount of active muscle mass and desaturation level (even if not observed here) because this circulatory adaptation is mediated by the sympathetic nervous system to regulate blood flow via the muscle metaboreflex (39).
The present study also demonstrated that oxyhemoglobin concentrations ([O2Hb]) were lower in TT2 than TT1, especially for IPC. This may suggest that there was an increased oxygen use (lower [O2Hb]) during the TT when IPC was performed before exercise, as previously demonstrated with greater HHb concentrations during TT (12) and other modes of exercise (9,29). Surprisingly, this present study did not find any differences during the TT for HHb. These results must be taken with caution because we only observed trends and not clear statistical differences.
We found no differences between IPC and SHAM for both recovery and DOMS status before commencing the testing, ensuring primary control in a crossover study. Not surprisingly, all subjects correctly noted the difference in cuff pressure between the protocols. Because the placebo or nocebo effects may be present in IPC studies (5,33), we asked the subjects what they expected regarding performance for IPC/SHAM. The results were heterogeneous: almost 50% expected negative influence from IPC probably because of the local discomfort (higher in IPC vs SHAM). The current IPC discomfort score (3.3 ± 1.6, out of 10 AU) is lower than that reported previously (6.3 ± 0.5 AU) at sea level in men (40).
Of further importance is that the NIRS measurements should be interpreted with caution, as the NIRS signal is unable to detect differences between hemoglobin (in blood) and myoglobin (in muscle) content, which could influence the rate of oxygenation (35). Because the analysis of NIRS was performed using a dynamic baseline after the IPC stages, it has likely influenced the data.
CONCLUSION
IPC performed at simulated moderate hypoxia generated a hyperemic effect, mitigating the performance decrement between two consecutive TT performed in hypoxia. However, the effect was not effective enough for inducing a performance enhancement at either 25 min or 2 h after the IPC application in comparison with SHAM. The present data however suggest that a large arterial desaturation level is required for inducing a clear performance enhancement in endurance exercise performed in hypoxia.
Dr. Mota was supported by the Swiss National Science Foundation (IZK0Z3_172424). The authors thank the participants for their efforts and commitment to this study. In addition, the authors extend a thank you to the reviewers for their valuable suggestions. The authors have no conflicts of interest, source of funding, or financial ties to disclose and no current or past relationship with companies or manufacturers who could benefit from the results of the present study. The authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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