To increase muscle bulk and strength, high-intensity resistance training must be performed with a mechanical load greater than 65% of one-repetition maximum (1RM) (17). However, such intensive loads cannot often be applied for frail participants or for deconditioned or injured athletes in a clinical setting (24).
In recent years, several lines of study have provided compelling data showing that resistance exercise with blood flow restriction (BFR), which has been widely applied as an exercise prescription for sports athletes, results in favorable and short-term training effects despite using a lower workload (1,2,21,28). This training method could be performed safely (2,21) and has even seemed effective in early rehabilitation after an injury or surgery in athletes and clinical cases (18,20,25,29).
Several studies have demonstrated that stimulation of metabolism with muscle contraction in response to BFR exercise is important to obtain muscle bulk and increases in strength (4,12,26,27). However, we have previously observed that a considerable number of participants could not get sufficient muscular stress during low-intensity resistance exercise with BFR. There was a wide range of individual differences in the muscular metabolic responses to this type of exercise (26,27). It could be speculated that those differences might be due to training status and muscular physiological characteristics. We therefore hypothesized that BFR exercise might not provide the same effects in different types of athletes, specifically sprinters and endurance runners. The purpose of the present study was to compare physiological responses during low-intensity resistance exercises with BFR between sprinters and endurance runners by 31P magnetic resonance spectroscopy (MRS), which can elucidate intramuscular energetic metabolism and muscle fiber recruitment.
Twelve trained male participants (sprinters, n = 6; endurance runners, n = 6) took part in the study. The average personal best times for the sprinters in the 100-m sprint, 110-m hurdle, and 400-m sprint were 10.90, 14.82, and 48.29 s, respectively. The average personal best time for the endurance runners in the 5-km run was 15 min 45 s. All participants were healthy and without orthopedic or cardiovascular diseases. Informed consent was obtained from all participants. The study was approved by the ethics committee of Hokusho University (HOKUSHO-SPOR: 200704).
Anthropometric measurements were performed in all participants after participants abstained from exercise training and vigorous physical activities of for at least 24 h. The measurement was performed from 6:00 to 8:00 p.m. Blood pressure was evaluated at sitting rest, and the average levels of two sequential measurements were used. The muscle cross-sectional area (MCA) of the plantar flexor group, which is composed of the medial gastrocnemius, lateral gastrocnemius, soleus, flexor hallucis longus, tibialis posterior, flexor digitorum longus, peroneus longus, and peroneus brevis, was evaluated by magnetic resonance imaging (9).
All participants performed a unilateral plantarflexion exercise under four conditions: two resistance exercises without BFR and two BFR protocols. The two resistance exercises without BFR were low-intensity exercises at 20% of 1RM (L) and high-intensity exercises at 65% of 1RM for 2 min (H). The 2 BFR protocols were as follows: 20% of 1RM with BFR for 2 min (L-BFR), and prolonged exercise in L-BFR for 3 min (prolonged BFR). The exercise intensities used in the present study have been assigned to the level of low-intensity resistance training. The plantarflexion exercises were carried out at 30 repetitions per minute, and the weight was lifted 5 cm above the ground. Throughout the experiments, we carefully monitored the lifting height (5 cm) and repetitions (30 per minute) using a ruler and metronome, respectively, to avoid variance within and between participants. The 1RM was determined to be a successful concentric-only contraction on the same plantarflexion apparatus equipped with a magnetic resonance device. Participants were instructed to lift the load through the range of motion to prevent assistance from any other body part (e.g., the thigh). The 1RM trials were designed using increments of 10 kg until 60%–80% of the perceived maximum. The load was then gradually increased by 1- to 5-kg weights until lift fail, during which the participant was not able to maintain proper form or to completely lift the weight. The last acceptable lift with the highest possible load was determined as 1RM. BFR pressure was set as 130% of resting systolic blood pressure (6,26,27). Participants performed exercises under all four conditions in random order, on 2 d separated by at least 1 wk. Before each protocol, we confirmed the recovery in altered intramuscular phosphocreatine (PCr) and pH to baseline levels. BFR was carried out using a pneumatic rapid inflator (E-20 rapid cuff inflator; Hokanson, Bellevue, WA) with an 18.5-cm–wide pressure cuff placed around the right thigh. The cuff was inflated for 10 s before the exercise protocol and promptly released after the exercise was completed. The real-time cuff pressure was monitored digitally and precisely maintained during exercise.
Participants lay in the supine position on an original apparatus equipped with a magnetic resonance device, and the right foot was attached to the pedal by a Velcro strap. 31P MRS was performed using a 55-cm-bore 1.5-T superconducting magnet (Magnetom Vision VB33G; Siemens, Erlangen, Germany). An 80-mm surface coil was placed under the muscle belly of the right gastrocnemius. Shimming was adjusted using the proton signal from water. Spectra of high-energy phosphate were acquired at a pulse width of 500 µs, a transmitter voltage of 20 V, and a repetition time of 2000 ms. The spectra were obtained at rest and every 30 s during exercise. Each spectrum consisted of an average of eight scans during 16 s before each time point. For the spectral postprocessing procedure, the standard software package (LUISE; Siemens) was applied. After Fourier transformation, Gauss filtering in the time domain, and phase and baseline correction of the spectral raw data, the peaks were identified and semiautomatically fitted for the integrals under the curve in the frequency domain. The intramuscular PCr millimolar concentration assumed that PCr millimolar concentration + creatine concentration = 42.5 mM (13) and supposed that the inorganic phosphate (Pi) concentration is equal to the creatine concentration (16). Intramuscular pH was calculated from the chemical shift of Pi relative to PCr. When distinct Pi splitting was shown, the pH was calculated by standardizing the obtained individual pH on the basis of peaks corresponding to each Pi (19).
Calf muscle oxygenation was assessed by near-infrared spectroscopy (NIRS) using a tissue oximeter (NIRO-200; Hamamatsu Photonics, Hamamatsu, Japan) during L and L-BFR in six randomly selected participants (sprinters = 3, endurance runners = 3). The theory behind the NIRS approach and the reliability to measure tissue O2 saturation has been reported (22). The NIRO-200 provides tissue O2 saturation data as tissue oxygenation index (TOI = oxyhemoglobin (HbO2)/total hemoglobin, expressed in percentage). The TOI value reflects predominantly the mean of arteriolar, capillary, and venular O2 saturations, with a minor contribution from myoglobin (22). After exercise begins, HbO2 saturation is depleted from the stable baseline (100%) and then reaches a plateau (0%) indicating the balance of oxygen demand and supply in muscle tissue. After exercise is completed, the HbO2 saturation increases until it reaches a plateau. Data sampled every 1 s were fed into a personal computer and saved as a file, and this result showed an average of 5 s.
Measurement of peak oxygen uptake.
Peak oxygen uptake (peak V˙O2) was determined with participants exercising on a treadmill using a modified Bruce protocol. Peak V˙O2 was identified by satisfying more than two conditions, as follows: HRmax (bpm) is 220 − age ± 10, respiratory gas exchange ratio is more than 1.2, and leveling off is observed. Respiratory gas analysis was performed with a breath-by-breath apparatus (Vmax; SensorMedics®, Yorba Linda, CA). The anaerobic threshold (AT) was determined by the V-slope method (3).
As an index of perceived effort, the RPE was evaluated with the modified Borg 10-point scale. The RPE was taken immediately after the exercises were finished.
The values are presented as means ± SE in the table, text, and figures. Intergroup comparisons of a single measurement were performed using the Student’s unpaired t-test in sprinters and endurance runners. Comparisons of metabolic measures at the end of exercise among protocols were examined by one-way ANOVA. Interaction effects (group × time) were examined by two-way ANOVA with repeated measures. Post hoc comparisons were examined by the Bonferroni test. The relationship between variables was examined by linear regression analysis by the Pearson test. The comparisons of split Pi appearance among exercise conditions were performed by a χ2 test. The level of significance was set at P < 0.05. All statistical tests were performed using SPSS 17 (SPSS 17.0 for Windows software, Chicago, IL).
The 1RM in sprinters was significantly greater than that in endurance runners, and the loads of 20% and 65% of 1RM in sprinters were greater than those in endurance runners, but no differences between the two groups were observed in MCA or 1RM/MCA. Peak V˙O2 and AT in endurance runners were significantly greater than those in sprinters. The training frequency per week in endurance runners was significantly greater than that in sprinters. The resistance training time per week for sprinters was significantly greater than that for endurance runners (Table 1).
Intramuscular energetic metabolism during BFR exercises.
Figure 1 shows the time courses of PCr during exercises carried out under L and L-BFR conditions. There was no significant difference in the average intramuscular PCr or pH levels at rest in any of the protocols (PCr = 38.0 ± 0.2 mM (sprinters) vs 37.6 ± 0.2 mM (endurance runners), pH = 7.01 ± 0.00 vs 7.00 ± 0.00). PCr in L was significantly lower in sprinters than in endurance runners (Fig. 1A), whereas in L-BFR, that in endurance runners was significantly lower than that in sprinters (Fig. 1B). Also, there were significant interactions in the L and L-BFR conditions (P < 0.001). Although PCr in H tended to be lower in sprinters compared with endurance runners, the difference was not statistically significant (25.7% ± 4.1% vs 32.7% ± 3.3%). There was not a significant interaction under the H condition. Similarly, PCr at the end of prolonged BFR exercise in endurance runners tended to be lower than that in sprinters (32.0% ± 3.2% vs 24.6% ± 1.4%, P = 0.062). There was a significant interaction under the prolonged BFR condition (P < 0.001). The time courses of intramuscular pH were very similar to those of PCr. However, the difference between the two groups did not reach statistical significance in any condition possibly because of the wide individual variation and large SD (Fig. 2). Interestingly, under the L-BFR protocol, the decrease in PCr at the end of exercise was positively correlated with peak V˙O2 in all participants (Fig. 3), whereas the PCr decrease at the end of exercise under the L protocol was not significantly correlated with peak V˙O2 (r = −0.357, P = 0.255). There were no correlations between the degree of metabolic response and the 1RM or BFR pressure.
We next examined metabolic responses under various conditions for each of the groups (Fig. 4). Intragroup comparisons showed PCr decreases at the end of all exercise protocols in both sprinters (Fig. 4A) and endurance runners (Fig. 4B). In both groups, PCr decreases in L-BFR, H, and prolonged BFR were significantly greater than those in L. Although endurance runners showed similar PCr decreases in H and L-BFR, in sprinters, the decreases were significantly greater in H than in L-BFR. In endurance runners, PCr decreases in prolonged BFR were significantly greater than those in L-BFR. Moreover, those in prolonged BFR tended to be greater than those in H (P = 0.093). In contrast, PCr decreases in sprinters during the long-duration protocols were less than those in H. Furthermore, PCr decrease in prolonged BFR tended to be greater than that in L-BFR in sprinters (P = 0.051). PCr decreases might be enhanced by increasing exercise time in both groups.
Fast-twitch fiber recruitment at the end of exercises.
Although recruitment of fast-twitch (FT) fibers, as evaluated by Pi splitting (19), was observed during L-BFR, H, and prolonged BFR, none was observed during L. The incidence of Pi splitting was greater in H and prolonged BFR than in L-BFR in all participants (L-BFR = 33.3% vs H = 100% and prolonged BFR = 83.3%, P < 0.01). Sprinters and endurance runners showed a similar incidence of Pi splitting in each comparable protocol (L-BFR = 33.3% vs 33.3%, H = 100% vs 100%, prolonged BFR = 83.3% vs 83.3%).
Oxygenation levels in calf muscle.
Averaged TOI as oxygenation levels in calf muscles during exercises for the last 30 s in L tended to be greater in sprinters than in endurance runners (80.1% ± 7.9% vs 96.2% ± 2.7%, P = 0.188), whereas in L-BFR, these values were similar between sprinters and endurance runners (11.8% ± 1.7% vs 14.8% ± 4.7%). There was a significant interaction in L (P < 0.001), whereas there was not a significant interaction in L-BFR. Therefore, the degree of BFR in exercising muscle was similar in both athletes (Fig. 5).
No significant differences in RPE at the end of exercise between sprinters and endurance runners were observed under any of the conditions used (L = 2.7 ± 0.3 vs 2.3 ± 0.6, L-BFR = 6.5 ± 0.7 vs 8.0 ± 0.5, H = 8.3 ± 0.5 vs 8.5 ± 0.5, prolonged BFR = 9.2 ± 0.4 vs 8.8 ± 0.5).
We elucidated the intramuscular energetic metabolism during resistance exercises with a BFR protocol in sprinters and endurance runners. The results showed that muscular metabolic stress as indicated by PCr and intramuscular pH decrease during exercises with BFR is significantly greater in endurance runners than in sprinters. In addition, the muscular stress during low-intensity BFR exercises was found to be equal or superior to that during high-intensity exercise without BFR in endurance runners but not in sprinters; in contrast, during exercises without BFR, sprinters showed greater muscular metabolic stress than endurance runners. These results indicate that endurance runners could more effectively be subjected to muscular stress by applying BFR during resistance exercise than could sprinters. Although this training method has been used for various athletes in rehabilitation after injury and surgery, a uniform procedure might not have the same successful effects in all individuals, especially in sprint-type athletes.
The different response of intramuscular metabolic stress during resistance exercise with BFR between sprinters and endurance runners could be explained in several different ways. First, there is a difference in aerobic capacity between sprinters and endurance runners. Endurance runners naturally have a higher aerobic capacity than sprinters. Higher aerobic capacity is associated with higher oxygen delivery to the exercising muscle, which might mean a larger blood flow dependence in endurance runners. Therefore, there might be a greater disturbance of BFR in energetic metabolism during exercise in endurance runners compared with sprinters. In fact, a strong correlation was found between PCr decrease and peak V˙O2 in the present study (Fig. 3). Concordantly, Clark et al. (5) have suggested that the higher the endurance capacity with natural blood flow, the greater the reduction in endurance time with BFR.
The second possible explanation is the different muscular histological properties of the two different athlete groups. Endurance runners have more slow-twitch (ST) fibers (7,10,11), which results in a higher volume of mitochondria and oxidative enzymes (e.g., citrate synthase, succinate dehydrogenase), a higher capillary density (7,14,30), and higher myoglobin levels (8,31) compared with sprinters, also leading to higher oxygen dependence. In contrast, sprinters have more FT fibers, which have more glycolytic enzymes (e.g., lactate dehydrogenase, phosphofructokinase) (7), and can carry out multifaceted anaerobic adenosine triphosphate productions even under the BFR condition. Similarly, Ingemann-Hansen et al. (15) have reported that a higher ST fibers ratio was associated with a greater decline of muscle strength under an ischemic condition.
In the present study, there was no difference in the FT fiber recruitment as detected by splitting Pi between the two groups. As mentioned above, sprinters with a greater percent of FT fiber might show a greater incidence of splitting Pi during resistance exercise. On the other hand, endurance runners were more stressed during resistance exercise with BFR compared with sprinters, which led to accelerated Pi splitting. Thus, both factors could conflictingly contribute to no significant difference in Pi splitting.
The present study has some limitations. First, we did not examine the muscle fiber type composition by muscle biopsy. However, we carefully selected the participants by their typical competition records and training status. It is well known that elite sprinters have a predominance of FT fibers, whereas endurance runners have a predominance of ST fibers (7). Second, it is possible that the supplemented effects in intramuscular metabolic stress in response to BFR might differ by MCA of the attachment site of the cuff and exercising muscle, but the calf MCA and thigh circumference did not influence change in intramuscular energetic metabolism because there was no correlation in the present study.
In conclusion, the results of the present study suggest that muscular stress during resistance exercise with BFR differs between sprinters and endurance runners. Supplementation of BFR might provide sufficient effects on resistance training in endurance runners but not in sprinters. Therefore, the present study suggests that more attention should be paid to creating an effective prescription in resistance exercise with BFR (Fig. 4).
This work was supported by grants from the Hokusho University Northern Regions Lifelong Sports Research Center, the Meiji Yasuda Life Foundation of Health and Welfare, and the Descente and Ishimoto Memorial Foundation for the Promotion of Sports Science.
The authors thank Dr. Takayuki Sako, Japan Women’s University, for help with advisement and technical assistance in NIRS measurement. They also thank Mr. Takayuki Kosuge and Mr. Takashi Sato, Hokusho University, for assistance in data collection. The authors also thank the study participants.
The authors have no disclosures to declare.
The author contributions are as follows: S.T., T.S., M.O., T.K., N.M., M.H., T.Y., K.H., and M.T. performed the experiments; S.T. and K.O. designed and planned the study; S.T., K.O., T.S., N.M., and M.H. prepared the article; S.K. and H.T. supervised the study; and all authors interpreted data.
The results of the present study do not constitute endorsement by the American College of Sports and Medicine.
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