Considering the important role of lower extremity muscle strength in daily activities and sports, as well as a more suitable exercise mode for clinical skeletal muscle rehabilitation patients, the elderly individuals with muscular atrophy and other special populations, this study chose low-intensity (25% 1RM) squat training as the exercise intervention method. The applications of this method could have important implications for sports rehabilitation.
Studies on the process of muscle strength growth have shown that early strength growth (0–10 weeks) in the early phase is due to increased neuromuscular adaptation, while strength growth in the later period (after 10 weeks) is related to muscle hypertrophy[23]. In this study, BFRT combined with low-intensity squat training was used to induce a significant muscle hypertrophy after 6 weeks, proving that BFRT combined with low-intensity resistance training seems to produce muscle hypertrophy effect similar to that of the traditional high-intensity resistance exercise, which is consistent with the results of previous studies. At present, the research on the effect of EMS is mostly focused on muscle strength [10, 24, 25], and there are few studies on the effect of muscle volume. In this study, it was found that 6 weeks of EMS combined with low-intensity squat training could promote the volume of RF, but the improvement effect was significantly less than that of the BFRT group and CMB group. This may be related to the intensity of electrical stimulation and the intervention cycle selected in this experiment. To avoid the discomfort caused by electric current to the subjects, the minimum threshold value (75 Hz), which can effectively stimulate the increase in muscle strength, was selected in this study. In terms of the combination of BFRT and EMS, Gorgey et al. found that the acute effects of EMS + BFRT may suggest that an increase in flow mediated dilation may partially contribute to skeletal muscle hypertrophy, and BFRT combined with EMS could be a strategy to increase skeletal muscle size [26]. Natsume et al. found in animal experiments that BFRT combined with EMS can induce muscle hypertrophy more than BFRT alone, resulting in increased protein expression levels related to muscle hypertrophy and accumulation of metabolites, as well as promoting muscle fatigue and increasing gastrocnemius muscle mass [27]. The results of this study showed that the combined BFRT and EMS could induce muscle volume changes more than EMS alone, which was consistent with previous studies, but the specific mechanism needs to be further studied.
Increased recruitment of muscle fibers is thought to be one of the underlying neural mechanism that causes muscle strength to increase [28]. However, our study found that combined BFRT and EMS was significantly better than BFRT alone in promoting muscle activation, and BFRT combined with low-intensity squat training did not seem to produce the same increase in muscle activation as the traditional high-intensity training[8, 9]. The main factor by which BFRT combined with low intensity squat training improves muscle strength may be the muscle hypertrophy effect, while neuromuscular adaptation has little effect. Yamanaka et al. showed that compared with low-intensity BFRT (30% 1RM), the training effect of high-intensity training (75% 1RM) and combined training (2 days low-intensity BFRT + 1 day high-intensity training) was mainly due to neural adaptation [29]. Therefore, it was suggested that BFRT can be used in combination with traditional high-intensity training to produce more effective stimulation of muscle volume and neuroadaptation. Previous studies have found that BFRT can cause nerve numbness and other adverse reactions [8]. Therefore, we believe that the failure of BFRT to enhance neuromuscular adaptability may be related to the limitation of nerve conduction while restricting blood flow. At present, no study has explicitly investigated the effect of BFRT combined with EMS on the electrical activity of muscles. The results of this study found that compared with low-intensity training + BFRT, low-intensity training combined with BFRT + EMS had a better effect on the increase in muscle activation. The combination of BFRT and EMS can simultaneously achieve effective improvement of muscle volume and neuromuscular adaptation. Training combined with EMS can compensate for the weakness of BFRT limiting the increase in muscle fiber recruitment, and low-intensity training is safer than high-intensity training. In addition, studies have shown that after BFRT, the concentration of inorganic phosphate in muscle increases significantly, resulting in more obvious peripheral fatigue [30]. After BFRT, inorganic phosphate in type I and type II muscle fibers was rapidly consumed, and EMS increased muscle fiber recruitment to maintain power output. This may also be a possible mechanism for increased muscle activation in the CMB group.
The role of BFRT in promoting muscle strength has been confirmed by a large number of studies [31, 32]. The results confirmed that BFRT combined with low-intensity squat training can improve the muscle strength of subjects who exercise irregularly, and it was believed that the main reason for the improvement of the lower limb muscle strength caused by BFRT combined with low-intensity squat training may be related to the significant increase in muscle volume.
The improvement of muscle strength depends not only on the increase in muscle volume, but also on the increase in motor unit recruitment. EMS can improve the innervation of the central system and coordinate the agonistic and antagonistic muscles, thus promoting the recruitment of motor units, activating more muscle fibers, and enhancing muscle strength [33]. The mechanism by which EMS promotes muscle strength is closely related to the intensity and frequency of stimulation. Cunha et al. found that 8 weeks of EMS training (70 Hz) can effectively improve volleyball players' lower limb muscle strength and explosive power [34]. Toth et al. pointed out that EMS (400 µs, 50 Hz) can prevent quadriceps atrophy after ACL surgery and improve the maximum muscle strength of patients [35]. Pantović et al. showed that EMS and traditional high-intensity resistance strength training had the same training effect in improving muscle strength [36]. Our study found that 6 weeks of EMS combined with low-intensity squat training effectively improved lower limb muscle strength, and the intervention effect was similar to that of BFRT combined with low-intensity squat training, which may be related to more significant muscle activation in the EMS group. In terms of the combination of BFRT and EMS, Gorgey et al. found that the combination of BFRT and EMS can effectively improve the wrist strength of patients with incomplete quadriplegia [26]. Natsume et al. showed that the combination of BFRT and EMS can significantly increase the maximum knee extension strength of young men with irregular exercise habits [27]. Slysz et al. found that 6 weeks of EMS combined with BFRT training effectively improved the leg lift strength of lower limbs [17]. The above scholars all believe that the increase in muscle strength is mainly related to muscle hypertrophy. However, the results of this study showed that when BFRT was used, whether or not EMS was used together had a significant influence on muscle strength. Similarly, when using EMS, whether BFRT was combined with EMS also had a significant influence on the muscle strength. This suggests that the increase in muscle strength caused by BFRT and EMS is related to neuromuscular adaptation and muscle hypertrophy.
The current study has some limitations. First, the sample of this study was restricted to young, healthy men. Thus, we acknowledge that our findings may not apply to other populations. In future studies, patients with musculoskeletal diseases can be recruited as research subjects to further verify the clinical application value. Second, due to the large number of measurements and subjects involved in this experiment, all measurements were only collected before and after intervention. Therefore, future studies can appropriately focus on certain major measurements and exclude individual differences between subjects through the measurement of different time.