Analysis of the change characteristics of the thigh muscle groups’ maximum activation before and after the compression squat exercise
The results in Table 2 show that the RMSMVC values of the rectus femoris, vastus medialis, and vastus lateralis decreased after compression removal. The RMSMVC values of the rectus femoris changed significantly after continuous compression (P < 0.05), while those of the vastus medialis and vastus lateralis showed no significant change (P > 0.05). Most previous studies showed that after the implementation of blood flow restriction training, the maximum activation of active muscles restricted by blood flow had a downward trend. Loenneke et al. [18] performed compression and non-compression one-knee stretching exercises with the quadriceps as the main active muscle group in 16 healthy adult males and found that the MVC value of the quadriceps muscle decreased after the compression and non-compression leg exercises. Umbel et al. [26] found that the MVC value of centripetal contraction decreased by 9.8%, and the MVC value of centrifugal contraction decreased by 3.4% after 24 h of unilateral compression knee stretching exercise and returned to normal after 96 h, which may be related to delayed onset muscle soreness. Fatela et al. [27] performed compression knee stretching exercises with different blood pressure limits in 14 adult males and found that the vastus medialis and vastus lateralis’ RMSMVC values decreased significantly after exercise under 60 and 80% BFR relative blood pressure limits. This study found that compared with knee stretch exercise, the pressurized squats exercise can lead to reduced quadriceps activation; additionally, compared with the intermittent exercise continuous pressurized squats exercise it led to more reduction, which may be linked to activity of the muscle cell metabolites caused by excessive accumulation of acidosis.
In addition, the results in Table 2 also showed that the RMSMVC values of the biceps femoralis and semitendinosus increased after the compression squatting program, and there was a significant change after the interval training. However, the RMSMVC value of the gluteus maximus decreased and significantly changed after continuous pressure interval exercise. The results showed that the RMSMVC of the posterior thigh muscle showed an increasing trend after compression squat exercises. Yasuda et al. [28] also found that the activation contribution rate of the triceps brachii as an antagonistic muscle increased from 40–60% after 30%-1RM multi-joint compression bench press exercise, suggesting that compression exercise can improve the maximum independent activation rate of antagonistic muscles and thus effectively develop the strength of antagonistic muscle group compared with non-compression multi-joint exercise. The results also showed that RMSMVC values of the biceps femoralis and semitendinosus increased significantly after blood flow restriction training. This indicated that blood flow restriction training can provide an optimal acidic environment to activate fast muscle fibers in the posterior thigh muscle group and induce more muscle fibers to activate.
Analysis of changes in the activation degree of thigh muscle groups before and after compression squat exercises
Many studies have confirmed that the RMS value of active muscles increases during low-intensity compression exercises. After the exercises, the RMS values of the anterior and posterior thigh muscle groups were higher than those before compression exercises. During continuous compression exercises, vastus lateralis, biceps femoralis, and semitendinosus were significantly improved. After the compression band was removed, the RMS values of the anterior and posterior thigh muscle groups decreased compared with the RMS values of the period before the compression at the end of the implementation of the compression program but were higher than the RMS values before compression.
The above results show that after the implementation of the compression program, the activation of the thigh’s anterior and posterior muscle groups increased. It is mainly caused by the acidic environment resulting from the accumulation of metabolites in the muscle, which can generate more type II muscle fibers, thus increasing the average amplitude of the electromyographic signal on the surface [29]. Fatela et al. [27] also found that rectus femoris and vastus medialis activation changed with external blood pressure limitation during compression knee stretching exercises. Acute external pressure stimulation increased the RMS values of the rectus femoris and vastus medialis; nevertheless, the vastus medioris and rectus femoris responded differently to different external pressure stimulations. Vastus medialis activation was significantly increased in 60 and 80% BFR external pressure restriction conditions before blood flow restriction training (post-1) compared to pre-2. The activity of the rectus femoris increased significantly only under 80% RFR, suggesting that higher blood pressure restriction can induce a reflex increase in vastus medialis and rectus femoris activation. The results also showed that the vastus lateralis, biceps femoralis, and semitendinosus had similar characteristics of change before and after compression with continuous and intermittent practice under constant external pressure limits and that the vastus lateralis, biceps femoralis, and semitendinosus, interval exercises significantly increased vastus medialis activation. In addition, the results in Table 3 show that the activation of the gluteus maximus was significantly reduced after bandaging. It was comparable to that after bandaging at the end of the treatment and did not return to pre-compression levels after intermittent depressurization. Sun et al. [30] concluded that low-intensity compression and hard pull exercises can increase the activation degree of the distal muscles with limited blood flow and the proximal muscles with no limited synergistic function. This study showed that low-intensity compression squats reduced the activation of the gluteus maximus, suggesting that different compression exercises had different effects on the activation of unrestricted proximal coordination muscles.
Analysis of changes in fatigue degree of thigh muscle groups before and after compression squat exercises
Indicators reflecting muscle fatigue are usually expressed by the MF, and the decrease in the MF value is strongly correlated with the decrease in the swing rate of the muscle bridge [31]. Place et al. [32] confirmed that the degree of intramuscular acidity and the reduction of Ca2+ absorption by the sarcoplasmic reticulum are the main factors affecting the decline in muscle contraction function. The results in Table 4 show that the MF values of the rectus femoris, vastus medialis, and vastus lateralis decreased before the end of compression training in both continuous compression mode and intermittent compression mode, and there were significant differences after continuous compression mode. It is suggested that the primary muscle in the front group of the thigh is the main force-generating muscle group, which leads to excessive accumulation of metabolites such as lactic acid in the muscle during the weight-bearing exercise in the continuous pressure mode. This results in acidity changes, reduced Ca2+ absorption by the sarcoplasmic reticulum, and ultimately, accelerated muscle fatigue.
After a short rest of 1 min after decompression, the MF values of the three muscles in the front thigh group have recovered to the pre-compression level. This indicates that the functional decline of the muscle caused by the two programs at 50% AOP level is temporary, and it can be restored to the level before compression after a short rest with decompression. In addition, Pierce et al. [15] pointed out that 60%AOP of continuous compression knee extension exercise can cause a significant reduction in the vastus lateralis and rectus femoris’ MF values. Neto et al. [33] also performed a set of 80%1RM high-intensity compression squats with 60%AOP blood pressure limitation, and the results showed that the MF values of the vastus medialis and vastus lateralis decreased by 18.5 and 18.2%, respectively. Previous studies have shown that blood flow restriction training induces fatigue mainly by stimulating protein synthesis of the Akt/mTOR signaling pathway, and the decrease in MF values is sensitive to biochemical changes in type II muscle fibers [34].
Combined with the above results, it can be concluded that neuromuscular fatigue is affected by the intermittent mode and the external pressure-limiting intensity. A higher pressure-limiting intensity will cause greater fatigue and slower recovery speed. In addition, Table 4 shows that the MF values of the biceps femoralis and semitendinosus increased before pressure removal at the end of pressure training in both modes and returned to the level before pressure removal 1 min later. This also suggests that both continuous and intermittent modes of load-bearing compression squat exercise can largely promote the activation of the posterior thigh muscle group, ensure that the neuromuscular component is at a suitable level of excitement, ensure the high synchronization of motor neuron discharge, and then promotes the reflex improvement of contraction force. In addition, the two modes of weight-bearing compression exercise had different effects on muscle fatigue in the unrestricted area. The gluteus maximus MF decreased significantly after continuous compression exercise but recovered shortly after decompression, while intermittent compression exercise had less effect on the gluteus maximus MF. It does not cause fatigue in the gluteus maximus.
Analysis of the change characteristics of the activation degree of lower limb muscle groups in each set of compressive weight-bearing squat exercises
As shown in Fig. 2, anterior thigh muscle group activation was higher during all four sets of compression squats than before the experiment, suggesting that the rectus femoris, vastus medialis, and vastus lateralis were activated to varying degrees during each set of compression. The acidic environment created after the application of pressure stimulation can attract more fast muscle fibers to participate in the activity so that the discharge frequency of fast muscle motor units gradually increases. The RMS value of the rectus femoris in the third and fourth sets was significantly higher than that in the initial set. Vastus medialis RMS values were significantly higher in the second set of continuous mode practice and the third set of intermittent mode practice than at the initial stage (P < 0.05), and the vastus lateralis values were significantly higher in the fourth set of both modes of practice than at the initial stage (P < 0.05). Counts et al. [35] found that the degree of activation of the biceps brachii during an elbow flexion exercise was not affected by external limit blood pressure. High-and low-limit blood pressure had similar adaptive effects on muscle hypertrophy, strength, endurance, and other aspects, but high-limit blood pressure would produce higher discomfort. In addition, Dandel et al. [36] found that adding BFR to interval of high-intensity elbow flexion training could not promote the activation of the biceps brachii and cause muscle hypertrophy. It has been suggested that blood flow restriction during low-intensity squat exercises can promote muscle activation and cause a hypertrophic response. The results of this study, similarly to previous studies, showed that continuous and intermittent exercises of the upper and lower extremities with moderate intensity blood pressure limitation could effectively improve the activation degree of the prime muscle and reduce discomfort.
Figure 2 shows that biceps femoralis and semitendinosus activation was significantly higher in the third and fourth sets than at the beginning of the weight-bearing exercises. However, activation of the gluteus maximus increased in the first set. Activation in the second, third, and fourth sets gradually declined, indicating that both continuous and intermittent blood flow restriction training will not only improve the thigh before the group of the original dynamic degree of muscle activation, but also improve the state of no pressure and rarely participate in and mobilization of the stocks after the muscle group, such as antagonist muscle activation degree. This can improve the thigh muscle group compared to flexion and extension and help prevent sports injuries. In addition, Abe et al. used 20% 1RM for compression squats, believing that, due to the low training intensity and the unrestricted position of the gluteus maximus, such load would not cause gluteus maximus hypertrophy [37]. However, there may be synergies between the thigh and buttocks in squat exercises. This study showed that the gluteus maximus was complementary, and its activation increased significantly when the anterior thigh group was relatively low during pressure exercise. In the second, third, and fourth exercise sets, the activity of the anterior thigh muscles gradually increased, while the activity of the gluteus maximus gradually decreased. This may be because the gluteus maximus extends the hip while the quadriceps muscle mainly extends the knee. The two muscles may be complementary in function to some extent as the adjacent active muscles of the important lower limb motor chain and the gluteus maximus may recruit more motion units to supplement the quadriceps deficit. This demonstrates that 50% AOP compression during weight-bearing squat training can induce more significant muscle group activation in the front and back of the legs, while gluteus maximus stimulation was not significant for hip extension. The authors believe that the main reason is that the compression site mainly plays a major role in lower limb blood flow occlusion but has little effect on the gluteal muscle. At the same time, lightweight squatting exercises may not significantly affect the gluteus maximus, and further extensive hip extension or increased weight-bearing strength to 85% or more may induce greater gluteus maximus activation.
Limitations
Due to the limitations of research conditions, considering the daily training of sports teams, the accumulated effects of the athletes' other physical and technical training may have interfered with the subsequent tests to a certain extent and affected the accuracy of the acquired EMG data. In addition, the study did not measure the blood concentrations of lactic acid, creatine phosphate, or other blood metabolites or the participants’ subjective physical effort level. To meet the need for training practice, future studies should combine the methods of electric index blood physiological and biochemical indicators and subjective indicators, which could comprehensively evaluate the mechanism and effect of blood flow restriction training .