The present study investigated three different conditions, BFR during exercise (EX), BFR during rest (RE), and training only (CON), in males and examined muscle strength, muscle CSA, and pain during exercise and at rest. HIT with BFR during exercise was shown to prevent muscle hypertrophy and muscle strengthening. Other hand, HIT with BFR during rest increased muscle hypertrophy and muscle strength with fewer exercise repetitions under natural blood flow. Furthermore, BFR during rest did not exacerbate pain during exercise.
In the present study, HIT with BFR during exercise prevented training effects. Training volume was previously shown to be important for muscle hypertrophy due to training [20, 21]. It was recently reported that muscle hypertrophy could be obtained even when LIT was performed to fatigue (Sampson and Groeller 2016), and training volume was shown to be important for muscle hypertrophy regardless of the exercise intensity.
There for, muscle hypertrophy necessary a training volume regardless of exercise intensity. However, HIT with BFR during exercise did not decrease peak torque during exercise, which is considered that no fatigue to training. It is considered that the cuff pressure prevented the muscle strength. Therefore, it is presumed that necessary training volume was not secured in the BFR during exercise, it is not desirable to perform BFR during HIT. On the other hand, BFR during rest led to increased blood flow efficiency and induced muscle hypertrophy and muscle strengthening. In a previous study, training with BFR decreased repetition to fatigue [22] and similar results were obtained in the present study. It was reported that conventional LIT with BFR can lead to muscle hypertrophy and muscle strengthening more than LIT under natural blood flow conditions [19, 23–25]. These studies were performed using a protocol with certain exercise repetition (step 1, 30 reps; steps 2–4, 15 reps). However, muscle hypertrophy was equivalent when training to fatigue with BFR [26]. HIT with BFR was shown to have no effect on muscle hypertrophy and muscle strength [12, 13], and led to fatigue with few reps due to high-mechanical stress. Therefore, training to fatigue with BFR is thought to contribute greatly to training efficiency (e.g., exercise repetition, time). The training protocol used the present study indicates that it is optimal to perform BFR during rest. Furthermore, peak torque during exercise decreased BFR during rest, suggesting that BFR during rest may be useful when training to fatigue.
Muscle swelling (or cell swelling) is evaluated as the increase muscle thickness acute after exercise and is an important factor for muscle hypertrophy because it promotes muscle protein synthesis [3, 27]. In a previous study, LIT with BFR led to an increase in leg circumference after training to a greater extent that with LIT under natural blood flow conditions [23]. Training with BFR using CC or EC was performed, and CC training with BFR showed greater muscle swelling, muscle hypertrophy, and muscle strength [24]. Therefore, muscle swelling is an important factor for muscle hypertrophy, but it is unclear how important it is to induce muscle swelling. Muscle swelling is a useful index of muscle hypertrophy, but training protocols need to be evaluated in more detail.
Pain during exercise increased with BFR. In a previous study, LIT with high-pressure BFR led to greater pain during exercise than HIT [28]. It is predicted that because pain differed with BFR and exercise, ischemia–reperfusion numbness was caused by stimulation of the sensitization of transient receptor potential ankyrin 1 with active oxygen generated by reperfusion [29]. Pain during exercise is caused by the combination of many factors (e.g., oxygen, ion, protein) as well as muscle damage (Miles and Clarkson 1994). This present study found that pain was greater with BFR than pain during exercise because it was increased with BFR regardless of exercise or rest conditions. Pain was greatest during BFR with HIT to near fatigue, rather than exercise failure due to muscle fatigue; therefore, intense pain may make exercising more difficult, which makes the necessary training volume unachievable. However, BFR during rest was the same as pain during exercise, but increased muscle hypertrophy and muscle strength with decreased exercise repetitions and peak torque during exercise. Although exercise pain was the same as HIT without BFR, BFR during rest decreased the exercise repetition and peak torque required to result in muscle hypertrophy and muscle strength. Therefore, BFR during rest could be a useful method to examine the effects of BFR without inducing excessive pain.
There are some limitations to the present study. First, the time and duration of BFR differed in the EX and RE groups, which may have led to different effects. Training intensity may be changed by increasing the number, time, and pressure of BFR. However, it was reported that training effects of BFR are equal regardless of whether high or low pressure is used [18, 30]. Therefore, the present study most focused on the timing of BFR. However, it is necessary to investigate the number and duration of BFR in the future. Second, the training in this experiment involved isokinetic resistance training using a BIODEX device, and each repetition was performed at the maximum output, representing HIT. In previous study, BFR was not shown to be useful during HIT combined with BFR. However, studies in athletes have shown the effect of BFR, which may be more effective in athletes, even at high intensity. Also, there are many items to the training effect, and it is easy to only evaluate the effect on muscle hypertrophy or muscle strengthening. Therefore, a study that focuses on items and/or training intensity may be useful for BFR other than during exercise. Third, this study evaluated fatigue during exercise on peak torque, and it did not investigate torque at each repetition. However, previous studies that investigated muscle fatigue with BFR investigated muscle activation and subjective fatigue during exercise as well as and maximum voluntary contraction. Few studies have investigated peak torque during exercise. Therefore, measuring the peak torque of each repetition will allow us to investigate the effects of BFR on the body in more detail.