The purpose of this study was to investigate the effects of PAP induced by different squat amplitudes on the squat jump, as well as the interaction effects of squat amplitude, stimulus load, and PAP duration. Squat jump performance before and after the back squat was analyzed to evaluate the effects of different "load-squat angle" combinations of back squat stimuli on the squat jump. It was found that the squat jump performance was directly proportional to the magnitude of the back squat knee angle and the load of the back squat, i.e., the effects of different back squat knee angles on the squat jump performance were 120° knee angle > 90° knee angle > 60° knee angle (Table 2).
There are two possible explanations for this. The first explanation is that a larger knee angle results in a larger absolute mass for the same proportion of 1RM. The data in Table 1 show that the subjects' mean 1RM for the 120° knee angle back squat was 220.7 ± 26.9kg, the mean 1RM for the 90° knee angle back squat was 186.7 ± 16.8kg, and the mean 1RM for the 60° knee angle back squat was 111.0 ± 14.8kg. At the same 75%, 85%, and 95% 1RM loads, the absolute mass of the 120° knee angle back squat was nearly twice that of the 60° knee angle back squat, resulting in a higher actual muscle load. Previous studies have similarly found that the quadriceps muscle produces a maximum isometric contraction moment when the knee is in the 60° flexion position (120° knee angle).[39].
The present study also found that the strength of PAP depends on the intensity of the stimulus load. In Table 3, other things being equal, PAP induced at 95% 1RM loading improved deep squat jump performance more than 85% and 75% 1RM, J.A. METTLER et al[10] also found that different intensities of MVC were able to induce different levels of PAP, and that overall the greater the intensity of the stimulus, the better the PAP induced, K.A. TILL[11] applied different methods to induce PAP in soccer players, and the results showed that continuous knee-hold jumps with low intensity failed to induce PAP in the subjects, in the load-fatigue relationship, greater load intensity is required to induce PAP, but the amount of load is not better; too little load intensity reduces peak PAP, while too much load increases fatigue recovery time and prevents the muscle from recovering adequately during peak PAP[16]. Therefore, you must do your best to increase the weight of the barbell when performing the back squat, but the number of repetitions of the back squat must be controlled in order to achieve a higher peak PAP.
The second explanation is that the knee angle of the 120° back squat is closest to the knee angle of the squat jump, and the muscle groups stimulated overlap the most with those used in the squat jump. The method used in this study for squat jumping was to bend the knee joint to 120° and hold it, and then make a maximal effort to jump up when the command was heard, so the 120° back squat knee angle is closest to the contraction torque when performing the jump, and the back squats of the three knee angles have some similarities in the muscle groups used to generate the force, but there are also some differences.
The quadriceps are the main muscle group that plays a key role in both deep and shallow squats; they are located on the front of the thigh and are responsible for knee flexion. In back squats at all three knee angles, the quadriceps take the main load and help lift the body. At 120 °, the squat is flat and the range of motion is small, so relatively few muscle groups need to be involved, and the quadriceps are the most heavily used muscle group during knee extension at this knee angle. However, in squat ranges up to deeper squats (knee angles ≤ 90°), the posterior leg muscle groups such as the semitendinosus, biceps femoris, and semimembranosus are used more frequently for balance and support, and the gluteal muscle groups such as the gluteus maximus and gluteus medius play a greater role in the deep squat, especially when pushing back to a standing position, where they provide additional strength and stability. The deep squat involves a greater range of motion and therefore requires the core muscles to maintain core stability. The rectus abdominis, psoas, and deeper abdominal and back muscles play a supportive and protective role for the spine in the deep squat.
Previously some researchers have conducted some studies on the effect of joint contraction angle on PAP. N. MIYAMOTO et al[31] used 3 different angles of ankle flexion and extension angles (normal angle, 20° dorsiflexion and 20° plantarflexion) to apply 10 s maximum voluntary contraction (MVC) stimulus to the ankle joints of the subjects, it was found that the PAP produced by the gastrocnemius muscle was significantly stronger than that produced by the soleus muscle after MVC at the normal angle and 20° of dorsiflexion, whereas no significant difference was found between the PAP produced by the gastrocnemius muscle and that produced by the soleus muscle at 20° of plantarflexion, suggesting that the angle of contraction of a particular joint plays an important role in the effect of the PAP on that joint when stimulated; J.I. ESFORMES et al [22] applied two types of back squat stimuli, parallel squat (thighs parallel to the floor) and quarter squat (thighs at a 45° angle to the floor), and tested the subjects with a counter movement jump at the 10th min after the stimuli, the results showed that PAP was induced after both stimuli, but the parallel squat induced stronger PAP than the quarter squat. The above studies suggest that the angle of the joint stimulated by precontraction has a direct effect on the strength of the subsequently induced PAP, which is related to the degree of muscle extension when the joint is at different angles as well as the means of testing.
Observation of the data revealed that there was individual variability in the time at which the PAP effect peaked after the stimulation was performed, but in the overall distribution was in the 6th-12th minute, and in terms of the mean values mostly peaked at the 9th minute (Fig. 2,Table 4). The appearance of the peak muscle PAP after precontractile stimulation showed some delayed characteristics due to the fatigue effect, where the muscle produces PAP after the induced stimulus along with the induction of fatigue, and fatigue dominates for a period of time after the induced stimulus. Therefore, the subjects' motor performance does not improve immediately after the inducing stimulus, and as fatigue recovers, the effect of PAP gradually prevails and motor performance improves and gradually peaks[17]. L.P. KILDUFF et al[18] applied a back squat stimulus with a load intensity of 3RM to the subjects and tested the subjects' vertical jump performance at different time points after the stimulus, the results showed that the subjects' vertical jump performance was higher than that of other time points at the 8th and 12th min after the stimulus, and the results of the test in all groups of the present experiment were similar to it.
From the observation of the experimental results, the present study proposed the conjecture that there is a limit to the enhancement of explosive power by PAP. In the present study, three loads of 75%1RM, 85%1RM and 95%1RM were used, and previous studies have concluded that the greater the intensity, the better the stimulation effect, from the results of this study, the effect of different knee angle back squats on squat jump performance was 120°>90°>60°, so theoretically the best squat jump performance should be obtained after knee angle 120°-95%1RM back squat, However, the actual test results showed that the best combination was 120°-85%1RM, 120°-95%1RM, and 90°-95%1RM, after all three combinations of back squat stimulation, peak squat jump heights of 42–43 cm were achieved, but the stimulus effect of the 95% 1RM combination was very close to that of the 85% 1RM when the knee angle was the same as 120° (P = 0. 892, ES = 0.04), and the stimulus effect of the 120° knee angle combination was very close to that of the 90° knee angle combination when the load was equal to 95% 1RM (P = 0.760, ES = 0.08). The results of these three comparisons contradict previous studies that have shown that the higher the knee angle and the higher the load, the better the performance. Perhaps the reason for this is that the 120°-85% 1RM knee angle and the 90°-95% 1RM knee angle have already reached the limits of squat jump performance, and it is difficult to further increase the intensity of the load or increase the amplitude of the squat to further improve, but further experiments are needed to validate this conjecture.
This experiment demonstrated that using a back squat with a larger knee angle to induce PAP in subjects was more effective in increasing the subject's squat jump height compared to a smaller knee angle. However, there are limitations to this study. First, there are two possible reasons why this study designed three knee angles, 60°, 90°, and 120°, and found that 120° stimulation was more effective than the other two; the first is that the absolute weight of the 1RM for the 120° back squat is greater than that of the other two, and the other is that the joint angle used for the 120° knee angle back squat is closest to the joint angle used for the squat jump, so these two aspects need further investigation in subsequent studies; Secondly, this study used squat jump performance as an indicator to verify the effect of back squatting at different joint angles, but the jumping process in high jump, long jump and other events during competition is a dynamic process, so whether the results of this study are applicable to dynamic jumping needs to be verified; finally, the subjects in this experiment consisted only of male athletes, so it is not sure whether it is suitable for female athletes.