Effects of ischemic preconditioning on local hemodynamics and isokinetic muscular function

Abstract

BACKGROUND:

The ergogenic effect of ischemic preconditioning (IPC) has been widely approved, but the mechanisms underlying the beneficial effects are still not fully clarified.

OBJECTIVE:

The aim of this study was to examine the effects of IPC on human isokinetic muscular function and hemodynamics during exercise.

METHODS:

In a counterbalanced, crossover study, 14 healthy non-athletic males (26.0 $\pm$ 3.5 years) performed isokinetic muscle strength and endurance tests of the dominant leg on an isokinetic dynamometer, preceded by either IPC on bilateral thighs (3 $\times$ 5-min compression/5-min reperfusion cycles at 50 mmHg greater than the participant’s systolic blood pressure) or SHAM (10 mmHg) intervention. Participants underwent strength testing by performing three maximum isokinetic knee extensions and flexions at the angular velocities of 30 ${}^{\circ}\cdot$ s ${}^{-1}$ , 150 ${}^{\circ}\cdot$ s ${}^{-1}$ , and 270 ${}^{\circ}\cdot$ s ${}^{-1}$ . An endurance test was also conducted over 30 repetitions at 180 ${}^{\circ}$ /s. Hemodynamics of the vastus lateralis muscle were monitored before and after the interventions and during exercise tests by near-infrared spectrometer (NIRS).

RESULTS:

Resting total hemoglobin significantly increased after IPC ( $p=$ 0.048, $d=$ 0.15). During the endurance testing, the oxygen uptake was significantly improved after IPC as shown by the change of oxygenated hemoglobin, deoxygenated hemoglobin, and oxygen saturation, at small to moderate effect size. However, both muscular strength and endurance were found unchanged.

CONCLUSION:

IPC improves the local oxygenation status without altering maximal muscle strength and endurance in young non-athletic males.

Keywords

Tourniquets muscular strength muscle oxygenation

1. Introduction

Ischemic preconditioning (IPC), which consists of non-lethal episodes of ischemia and reperfusion via a pressure cuff on skeletal muscles, provides protection of muscle cells in the heart and other organs against ischemia-reperfusion injury [1]. Notably, this technique is different from intermittent pneumatic compression, which applies a non-ischemic pressure to facilitate lymphatic and venous return [2]. Previous studies have reported that IPC enhances the maximal and whole-body performance by improving peripheral muscle function and central cardiovascular adaptation [3, 4, 5, 6, 7]. Moreover, IPC enhances cycling, swimming and running performance in people who are either physically active or non-athletes [8, 9, 10]. Nevertheless, a meta-analysis study [11] concluded that IPC has beneficial effect on aerobic (ES $=$ 0.51) and anaerobic (ES $=$ 0.23) exercise, and another recent systematic review [12] also demonstrated that IPC could be beneficial to exercise performance, especially in healthy amateur aerobic exercisers.

Despite the reported ergogenic effect, the mechanisms underlying the beneficial effects are still not fully clarified. Animal studies indicated that IPC might increase intramuscular adenosine triphosphate-sensi-tive potassium channels and adenosine levels for a protective effect on circulation and improved blood flow in muscle [13, 14]. Studies also indicated that IPC might affect muscle metabolism during reperfusion by increasing phosphocreatine production and improving muscle function after reperfusion in both humans and rats [15, 16, 17, 18]. Furthermore, IPC optimizes local blood flow and muscle contraction in humans [10, 19, 20]. The increased blood flow could contribute to the removal of lactate metabolites [21, 22]. Enhanced blood flow could also improve muscle contraction efficiency by increasing oxygen delivery and excitation-contraction coupling [23]. Muscle perfusion, oxygen uptake, and repeated force capacity have been shown to increase in strength-trained athletes after IPC [20].

Oxidative metabolism is a crucial determinant of exercise [10]. Ischemic preconditioning could reduce the oxygen saturation of the arm during muscle ischemia and increase oxygen saturation during reperfusion. Previous studies indicated that oxygen consumption at the muscle level increases after IPC [10, 15, 20]. However, oxygen saturation was found unchanged in the other arm and the thigh of the same side during IPC [7]. Similar results were found for IPC applied on bilateral thighs [8] or unilateral upper arm [9]. Briefly, IPC may affect oxygen saturation and accelerate the oxygen uptake only in the intermittent ischemia limb.

Accordingly, it is important to identify the hemodynamics during exercise and muscular functions following IPC. Although recent works attempted to determine the relationship between the ergogenic effect of IPC and muscle function, limited data are available on the role of muscular deoxygenation status during exercise after IPC [10, 20, 24]. In the present study, we aimed to examine the effects of IPC applied to bilateral thighs on isokinetic muscular strength and endurance of lower limbs in young non-athletic males and use near-infrared spectrometer (NIRS) to keep monitoring blood flow and oxygenation status during exercise. We hypothesized that IPC would enhance local muscular function by improving blood flow and oxygenation in lower limbs.

Figure 1.

Protocol of this study. IPC (ischemic preconditioning): 3 $\times$ 5 min lower limb cuff occlusion, SHAM: cuff pressure only 10 mmHg.

2. Methods

2.1 Participants

Based on the results of deoxygenated hemoglobin from the previous study [19], a sample size of 13 is required for detecting a difference between groups for a two-sided analysis with $\alpha=$ 0.05 and $\beta=$ 0.8 [25]. A recruitment flyer was posted in the campus and a self-reported health questionnaire was applied to recruit healthy non-athletic male volunteers. Fourteen healthy non-sedentary, non-smoking male volunteers (age: 26.0 $\pm$ 3.5 years, height: 174.6 $\pm$ 7.9 cm, weight: 73.6 $\pm$ 7.5 kg, thigh skinfold: 13.6 $\pm$ 4.0 mm) were recruited and asked to maintain daily physical activity during the experimental periods. Based on their medical history, participants were free of health problems and did not take any medicines within three months before the study. Their mean systolic blood pressure, diastolic blood pressure and resting heart rate were 122.1 $\pm$ 9.0 mmHg, 63.9 $\pm$ 7.3 mmHg, and 69.6 $\pm$ 9.8 beats $\cdot$ min ${}^{-1}$ , respectively. All participants provided written informed consent before participation. The study was approved by the Institutional Review Board of the Taoyuan General Hospital, Taiwan with the IRB number of TYGH102007.

2.2 Experimental design of the study

All participants refrained from consuming alcohol, caffeine, and additional nutritional supplements 24 hours before testing and strenuous exercise three days before testing. A counterbalanced, crossover study was designed. Since the participants were healthy non-athletic adults, the experimental procedures were modified by reducing the intensity of warm-up from the generally employed protocol designed for athletes [19, 20]. Participants reported to the laboratory twice and performed the same tests on both days. Before the intervention, systolic blood pressure was measured. Participants sat on chairs with the arms supported horizontally, palms turned upward and both the cubital fossa and the upper arm at the level of the mid-sternum. After IPC or SHAM intervention, participants rested for 5 min and then performed a 5-min warm-up on a treadmill at a velocity of 7 km $\cdot$ h ${}^{-1}$ [19, 20, 26]. Then the isokinetic muscular strength and endurance exercise tests were conducted. The experiments were performed at least seven days apart at the same time of the day [19, 20]. Figure 1 shows the schema of the study.

2.3 Ischemic preconditioning and SHAM interventions

IPC technique was modified from previous studies that induced the exercise performance benefits with the application of the lowest pressure in non-athletic adults [4]. Briefly, IPC was performed with the participants in the supine position to block the blood of the femoral artery. The occlusion cuffs (14.2 cm wide, Spirit ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}{\textregistered}}$ P-106NXL $+$ P-107ADT, Taiwan) were positioned on the anterior midline of the thigh, midway between the proximal border of the patella and the inguinal crease as well as inflated to 50 mmHg greater ( $\sim$ 170 mmHg) than the participant’s systolic blood pressure. The intervention involved three cycles of 5-min occlusion of blood flow and 5-min reperfusion on bilateral thighs by tourniquets. For the SHAM intervention, participants followed an identical protocol, but the cuff was inflated to only 10 mmHg (which did not alter blood inflow).

Figure 2.

Isokinetic muscular strength after IPC or SHAM interventions at the angular velocities of (A) 30 ${}^{\circ}$ $\cdot$ s ${}^{-1}$ , (B) 150 ${}^{\circ}$ $\cdot$ s ${}^{-1}$ , and (C) 270 ${}^{\circ}$ $\cdot$ s ${}^{-1}$ .

2.4 Isokinetic exercise tests

A Biodex isokinetic dynamometer (Biodex System Pro 3, USA) was used to assess the knee-joint isokinetic peak torque according to the operating manual. The dynamometer was corrected for gravity before each test. Participants were placed in the dynamometer seat at an angle of 90 ${}^{\circ}$ at the hip joint. Participants were secured to the dynamometer with a strap stabilizing the upper body and the pelvis to isolate the muscle groups of interest and to avoid any upper-body contribution to the torque of interest. Participants performed three maximum isokinetic knee extension and flexion over a range of motion of 90 ${}^{\circ}$ at 3 randomly ordered angular velocities of 30 ${}^{\circ}\cdot$ s ${}^{-1}$ , 150 ${}^{\circ}\cdot$ s ${}^{-1}$ , and 270 ${}^{\circ}\cdot$ s ${}^{-1}$ [27], after six repetitions of standardized warm-up/familiarization and a 5-min rest; the maximal torque data were recorded. Standardized strong verbal encouragements were given during the movements and the subjects were blinded to the angular velocity of the test. The resting period between the angular velocities was two min. After 5 min of the strength test, participants performed six repetitions at an angular velocity of 180 ${}^{\circ}\cdot$ s ${}^{-1}$ to familiarize themselves with the isokinetic endurance test and then rested for 5 min. The formal isokinetic endurance test was assessed with 30 repetitions. The peak torque, mean power, total work, and muscular endurance index were measured. The endurance index was calculated as the (mean torque of the final five times/means a torque of the first five times) $\times$ 100% [28].

2.5 Tissue oxygenation

A frequency-domain tissue NIRS (Oxiplex TS Oximete ISS, Champaign, IL, USA) was used to detect blood perfusion of the exercising muscle. The sensor of the NIRS was applied over the vastus lateralis muscle of the dominant leg in the supine and sitting positions before and after IPC or SHAM interventions and during continuous 30-repetition knee flexions and extensions, respectively. Oxygenated hemoglobin (O ${}_{2}$ Hb) and deoxygenated hemoglobin (HHb) were detected by NIRS before, immediately after the muscular strength tests and during the muscular endurance test. The concentration of total Hb (tHb) was calculated as the sum of O ${}_{2}$ Hb and HHb concentrations. The proportion of oxygen saturation (SaO ${}_{2}$ ) was calculated as O ${}_{2}$ Hb/tHb. The $\Delta$ O ${}_{2}$ Hb, $\Delta$ HHb, and $\Delta$ SaO ${}_{2}$ represented the change in variables compared with baseline values.

2.6 Statistical analysis

Data were analyzed using SPSS 17.0 software (SPSS, Chicago, IL, USA). Shapiro-Wilk test was performed to examine the distribution normality. Paired t-tests or Wilcoxon signed-rank test were used to assess differences in NIRS variables (O ${}_{2}$ Hb, HHb, tHb, SaO ${}_{2}$ , $\Delta$ O ${}_{2}$ Hb, $\Delta$ HHb, and $\Delta$ SaO ${}_{2}$ during the isokinetic endurance test), and to compare the variables of the isokinetic exercise test with IPC and SHAM intervention; the effect size of the observed differences was calculated as Cohen’s $d$ . The significance level was set at $p<$ 0.05; the magnitude of effect size was classified as $d\leqslant$ 0.4 is small, 0.4 $<$ $d\leqslant$ 0.8 is moderate, and $d>$ 0.8 is large [29]. All data are presented as the means $\pm$ standard deviation (SD).

3. Results

Except for $\Delta$ HHb at 60 s after endurance testing and the endurance index, all the other variables were normally distributed.

3.1 Isokinetic muscular strength and endurance

Figure 2 presents data for isokinetic muscle strength in healthy males after IPC or SHAM interventions at the test velocities. Isokinetic muscular strength did not differ between IPC and SHAM interventions for the three angular velocities. The mean power, total work, and endurance index with 30 repetitions of extension at 180 ${}^{\circ}\cdot$ s ${}^{-1}$ in IPC and SHAM intervention groups were presented in Fig. 3. Muscular endurance also showed no difference between interventions.

Figure 3.

Isokinetic muscular endurance performance of (A) total work, (B) mean power, and (C) endurance index after IPC or SHAM interventions.

Figure 4.

NIRS parameters before and after IPC or SHAM interventions. *: significantly changed compared with before intervention ( $p<$ 0.05).

Figure 5.

NIRS parameters during continuous 30-repetition knee flexions and extensions after IPC or SHAM interventions.

Figure 6.

Change in NIRS parameters during continuous 30-repetition knee flexions and extensions after IPC or SHAM interventions. *: significantly different compared with IPC intervention ( $p<$ 0.05).

3.2 NIRS variables

The tHb significantly increased after IPC intervention (57.6 $\pm$ 18.9 vs 55.2 $\pm$ 17.6 $\mu$ M, $p=$ 0.048, $d=$ 0.15). However, the tHb after SHAM intervention, as well as O ${}_{2}$ Hb, HHb, and SaO ${}_{2}$ in both treatments did not change (Fig. 4). O ${}_{2}$ Hb, HHb, tHb, and SaO ${}_{2}$ during the isokinetic muscular strength and endurance tests at the same time points did not differ between IPC and SHAM interventions (Fig. 5). Nevertheless, the changes in NIRS variables during isokinetic muscular endurance test after IPC intervention were significantly larger than SHAM intervention for $\Delta$ SaO ${}_{2}$ at 10 s, 20 s, and 30 s (IPC vs SHAM, $-$ 14.5% $\pm$ 4.8% vs $-$ 11.5% $\pm$ 6.0%, $p=$ 0.032, $d=$ 0.55; $-$ 17.9% $\pm$ 5.3% vs $-$ 15.1% $\pm$ 6.6%, $p=$ 0.036, $d=$ 0.47; $-$ 17.4% $\pm$ 5.3% vs $-$ 14.4% $\pm$ 6.0%, $p=$ 0.013, $d=$ 0.53, respectively); $\Delta$ HHb at 30 s (IPC vs SHAM, 12.4 $\pm$ 7.9 vs 10.3 $\pm$ 6.9 $\mu$ M, $p=$ 0.030, $d=$ 0.28); $\Delta$ O ${}_{2}$ Hb at 10 s and 20 s (IPC vs SHAM, $-$ 11.2 $\pm$ 5.3 vs $-$ 8.4 $\pm$ 5.3 $\mu$ M, $p=$ 0.007, $d=$ 0.53; $-$ 12.9 $\pm$ 6.2 vs $-$ 10.0 $\pm$ 5.9 $\mu$ M, $p=$ 0.035, $d=$ 0.48, respectively) (Fig. 6).

4. Discussion

The present study focuses on the isokinetic muscular function of lower extremity and local hemodynamics after IPC in healthy non-athletic male, excluding peripheral muscle function from whole-body performance and minimizes the impact of central cardiovascular adaptation. Our results showed that acute IPC with pressure of 170 mmHg before isokinetic knee extension and flexion stimulated the local oxygenation status without influencing local muscular function.

In accordance with previous studies, we observed higher blood perfusion and faster oxygen consumption locally after IPC [10, 20, 24, 30]. One study demonstrated that the blood levels of nitric oxide (NO) metabolites are elevated after IPC [31]. Because NO is secreted by endothelial cells following shear str-ess [32], which occurs with reperfusion of IPC, the beneficial effect of IPC on vascular function appears to be regulated by NO. Notably, since ischemic preconditioning does not alter cardiac performance [3, 4, 5, 10], vascular adaptation is considered to occur peripherally but not systemically.

Previous studies indicated that IPC alters oxygen saturation of the treated arm during muscle ischemia and reperfusion and suggested that IPC is an important element in warm-up procedures in preparation for maximal exercise [7, 9]. Correspondingly, the peak and average force of maximum voluntary knee extension tend to increase after IPC [20]. Furthermore, ischemic preconditioning also improves endurance performance. Time to exhaustion in severe-intensity cycling exercise enhanced during the work-to-work test after IPC [10], and the time to task failure was longer in local muscular endurance [24]. Exercise economy and time-trial performance are also improved in endurance athletes [33]. In contrast, although IPC was found to improve performance in both aerobic and anaerobic exercises with a large metabolic requirement [11], we did not find any improvement in isokinetic muscular strength or endurance. Since the beneficial effects of IPC may depend on the intensity, duration, and type of exercise, which is not clarified, and individual variation may exist [34], the mode of exercise test, i.e., isokinetic exercise, in our study may not fully represent the ergogenic effect of IPC. Besides, although the type of warm-up intervention and exercise testing that we chose were different from the conventionally employed IPC procedures [19, 20] and, may have led to different experimental results, this process is indispensable before performing a maximum exercise testing [27]. Specifically, in this study, we applied a relatively low-intensity walking warm-up, which may reduce such bias.

In addition, the circumference of the thigh, limb composition, and limb mass may affect the pressure applied to the underlying soft tissue [23]. In our study, we measured the systolic blood pressure of the arm with participants in a seated position and then applied IPC to the thigh with participants in a supine position. Although the systolic blood pressure is significantly higher in the supine than sitting position [35, 36], the pressure in the legs is also higher than that in the arms [37]. Therefore, we considered individual differences in systolic blood pressure to modify IPC protocol [4]. Previous studies showed that full femoral occlusion would not occur [17] and the effects of IPC could be induced in relative hypoxia with systolic blood pressure plus 15 mmHg [9]. Although the resting concentration of tHb is elevated in our study, which indicates the validity of IPC, we still suggest that occlusion pressure with IPC should be based on femoral blood pressure in the supine position when applied to the lower limbs in a future study to decrease the systematic error. Besides, as a plateau of HHb is a sign of occlusion, we also suggest future researchers to keep monitoring the hemodynamics during IPC.

Furthermore, the minimal pressure of IPC to obtain the ergogenic effect is still equivocal. Our finding is accompanied with the emerging evidence that ischemia pressure should be set from 200 to 220 mmHg to obtain ergogenic effects. The present study also suggests that applying 50 mmHg greater than systolic blood pressure (i.e. $\sim$ 170 mmHg in this study) might be insufficient to induce the proposed metabolism and performance benefits despite the positive hemodynamic effects [12, 28, 38, 39, 40]. On the contrary, a systematic review [41] suggested that 15 mmHg above systolic blood pressure could benefit the endurance performance in elite swimmer, and numerous studies applying IPC with the same pressure protocol as present study indicated the improvement in exercise tests of predominantly lactic anaerobic and aerobic capacity. Nevertheless, the protocol of IPC plays a crucial role in ergogenic effects. The present study employed the 3 $\times$ 5-min episodes of IPC as indicated in previous studies [4, 6]. However, one study reviewing 39 research studies suggested that 4 $\times$ 5-min episodes may represent the optimal dose to improve exercise performance [38]. Further studies are needed to optimize the time of ischemia and reperfusion as well as occlusion pressure of IPC protocol.

5. Conclusion

In summary, the present study argues against the effect of ischemic preconditioning on isokinetic muscular function, which revealed that muscular strength and endurance do not reflect the beneficial efficacy of IPC on the local muscle perfusion and oxygen uptake. Our results provide further insight into the underlying mechanism of the ergogenic effect. However, the benefits of IPC on different muscular functions and the empirical mechanism should be investigated in the future.

Footnotes

Acknowledgments

The authors would like to thank all subjects for participating and helping in this study.

Conflict of interest

The authors have no conflict of interest to declare. No funding was received for this study.

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