Study on the Kinematic characteristics of different level jumpers squatting and jumping under different ground

Objective: The purpose of this study is to compare the kinematic differences of different level jumping athletes in the process of squatting on different ground, and to reveal the biomechanical mechanism of the effect of squatting on different ground on training effect. Methods: The subjects were 11 first-class and above jumping athletes (excellent group) and 11 second-class athletes (ordinary group). The kinematic data were photographed by SONYDCRHC52E high-speed camera with a sampling frequency of 50 frames per second, and the data were analyzed by SIMI-Motion three-dimensional motion analysis system. The subjects squatted and jumped on four kinds of ground (force platform, balance pad, gymnastics pad and sand) with their hands on their hips. Results: In the intra-group analysis, it was found that the athletes of different levels squatted and jumped on four different kinds of ground, but there was no significant difference only on the balance pad and gymnastics mat (excellent group p=0.344 , ordinary group p=1 ). In the inter-group analysis, it was found that there were significant differences between different levels of athletes in balance pad ( p=0.043 ) and sand ( p=0.02 ). In the buffering stage of squatting jump, there was no significant difference in the buffering angle displacement of lower limb joints of different athletes on different ground. In the stage of pedal and extension, the angular displacement of ankle on the middle platform of the excellent group was significantly different from that of the balance pad ( p=0.014 ), gymnastics pad ( p=0.017 ) and sand ( p=0.013 ), but in the ordinary group, the ankle angular displacement of the middle platform was only significantly different from that of the gymnastics pad ( p= 0.048 ) and sand ( p=0.017 ), but not significantly different from that of the balance pad ( p=0.166 while the ordinary group squats and jumps on the ground with low elastic coefficient and unstable ground. the jumping effect is poor.

Jumping technique is very important in sports, such as volleyball blocking, high jump, long jump and so on, while squat jump is usually used to evaluate the muscle strength, explosive power and rapid development of lower limbs. Squat jump needs to quickly do the action of squatting down, and then quickly jump upward, and through the stretch reflection mechanism, elastic energy storage and release to reveal the characteristics of lengthening and shortening cycle [3] . During the period of lengthening and shortening the cycle, the elastic energy stored during the eccentric phase (i.e. stretching) is used to cause the pre-activated muscle to elongate rapidly (the eccentric phase), and then the shortening contraction begins immediately (the centripetal phase). Compared with the centripetal contraction alone, this centrifugal / centripetal contraction mechanism produces a greater contractile force [4] . As a typical exercise of lengthening and shortening periods, squat jumping is usually used to measure jumping ability in the field of sports training and research. In the squat jump test, the external factors that affect the squat jump test are easy to control, so many researchers analyze the movement through the characteristics of biomechanical and muscle mechanical parameters of human movement. Especially in the detection and evaluation of lower limb muscle strength and explosive force, and the utilization rate of elastic potential energy, the value of squat jump test is particularly important. Squat jumping is not only an index to evaluate the explosive power of athletes' lower limbs, but also one of the commonly used methods of lower limb strength training and bouncing training in the process of sports training.
In sports, sports performance depends on speed, agility and strength [8], while athletes training in different environments will have different effects. Differences in sports surface components will affect athletes' speed, agility and strength [10] . When athletes train on different sports surfaces, the athletes will have different reactions because of the different surface characteristics. for example, the different physical structure between the natural turf surface and the artificial turf surface will make the human body change the direction of the force in the jumping process, thus changing the sports ability. Meyers et al. [11] found that athletes on traditional artificial turf surfaces can run faster and change direction faster than natural turf surfaces. However, when athletes train on different ground, how the different characteristics of the surface have an impact on people still need further research, so that athletes can get as much training benefit as possible in the shortest possible time [12] . Choosing a suitable ground for squatting and jumping training is one of the key factors that affect the effect of lower limb strength training. With the training of athletes on different materials and jumping training on different floors, the negative effects are gradually highlighted. Two of them are the problems that researchers and coaches pay most attention to: first, when squatting and jumping training on hard ground, the ground will have a high impact on the joints at the moment of landing. Easily lead to bone and ligament injury [13] . Second, in some events, hard underground squatting training can not effectively improve the special sports performance of athletes, and the effect of sports transfer is not significant [14.15] . In order to solve these problems, coaches and researchers are constantly exploring new solutions to reduce the negative impact of squat and jump training of different materials. In practice, coaches arrange athletes to squat and jump in various environments, such as sand, cushion, hard ground, trampoline, water and so on [16][17][18][19][20] . At the same time, a large number of studies have compared the squatting and jumping training of different materials, but the results are not consistent. There is a great controversy as to whether the squatting and jumping training of different materials can improve the training efficiency. Different surface training will affect the performance, speed and power of the lower limb joints of the body. In the process of jumping, how the movement surface affects the lower limb joints and the influence of the movement surface on the lower limb joints is not clear. The purpose of this study is to compare the differences of lower limb joint kinematics of different level jumping athletes in the process of squatting and jumping on different sports surfaces. 1 Research methods

Object of study
The experiment selection 11 (excellent groups) and 11th athletes (excellent group) and 11 (ordinary groups) of the three levels of sports colleges, the Sports Institute of Zhejiang Normal University (ordinary group), and participate in this study, physical health, no exercise damage, sports ability Normal, recent diseases, no neuromblex, cardiopulmonary function, have a good rest before testing, the muscles have no fatigue, all subjects have filled out the informed consent, and the experiment has obtained the Zhejiang Normal University Laboratories. Approval, all methods are implemented in accordance with Zhejiang Normal University Laboratory Criteria and Ordinance, the basic information of the subject is Table 1: (1) Before this experiment, different ground recovery coefficients are tested, the test method is the sphere rebound height method, and the standard to evaluate the motion surface recovery coefficient is the sports surface test index-sphere rebound height (ASTMF2117-2010) issued by the International Association for material testing ((ASTM)). Tennis balls are used on these four sports surfaces to let tennis balls fall freely from a height of 2 meters, and each surface falls 3 times. The recovery coefficient of the sports surface is evaluated by calculating the average rebound height of tennis balls after falling 3 times. The higher the springback height, the greater the recovery coefficient of the sports surface. The elastic coefficients of floors of different materials are calculated by the formula:e = ℎ ℎ ，Hb is the bounce height of the ball and hd is the falling height of the ball. Finally, it is concluded that the recovery coefficients of the four kinds of ground are force platform > balance pad > gymnastics pad > sand.
(2)Before the experiment, the subjects were asked to warm up and jog on the treadmill at the speed of 5km/h for 10 minutes, followed by 5min dynamic stretching to move the joints and muscles of the body, especially the joints of the lower limbs, to prevent the subjects from being injured during the experiment.
( 3) The staff counted the basic conditions of the subjects, such as height, weight, age, lower limb length, thigh length, calf length, foot length, medical history and so on, affixed reflective markers on the subjects, and informed the subjects of the whole experimental process. make the subjects familiar with the whole process of the experiment.
( 4 ) In the experiment, the whole movement process of squatting jump was photographed (sampling frequency 50 frames per second). The exposure time was set and placed vertically in the positive direction of the subjects, the distance was about 5m. Before the test, the subjects first fixed the PEAK three-dimensional calibration frame (the frame coordinate X represents the left and right direction, Y represents the front and rear direction, Z represents the vertical direction) to calibrate the range of motion of the subjects. After the calibration is completed, the positions of the two cameras and various shooting conditions remain unchanged, and the distinction is suspended for each test, and the shooting frequency is 50 frames per second. At the beginning of the exercise, the frame was removed to photograph the subjects' whole squatting and jumping process. In order to accurately observe the motion track of the main joints of the human body during video analysis, reflective markers were affixed to the center of the main joints of the subjects (see Table 2). Table 2  2 (5)Use the SonyDCRHC52E high-speed camera and Kistler dynamic system to collect kinematics and kinetics during completion of the squat. Subjects have completed the squat jump in different materials on the laboratory platform, and in the laboratory with three-dimensional performance platform, the angle between the main optical axes of the two camera is about 90 degrees, and 2 sets. SONYDCRHC52E digital camera (sampling frequency 25 frames / sec), camera effective shooting range is about 5m long, 2.5m wide and 2.5m high. Within the effective shooting range, a Swiss Kistler dynastucting station (contact area 90 * 60 * 10cm) is placed on the ground, and the sampling frequency is 1000 Hz (Fig. 1).
The subject stands on the hands of the hands on the hands of the hands, the double-footed is more width. When listening to the password of the staff, in turn, squat three times in four terrestrials, four terrestrial tests in the same The day is completed, and each ground test is 10s, each ground test interval is 1 min.

Statistical analysis
Using Excel, SPSS21.0 data processing software sorted the effective data taken, the subject 3 test data takes the average of 3 test data for each floor after data processing, all measurement parameters The data is expressed in average (x) ± standard deviation (SD); the difference in the kneeling of the lower extremities during different levels of different levels of different levels of different levels of different levels of different levels, significant comparison Method, significant level P <0.05.

Jumping height
As can be seen from Figure 1, the difference between different levels of athletes in force, balance pad, gymnastic pad, and sand is found using repeated measurement variance analysis. Different level athletes are discovered under four different grounds in the group analysis. Jump, there is no significant difference in balance mat and gymnastics (excellent group P = 0.344, ordinary group P = 1); discovery different level athletes in balance pad (P = 0.043) and sand (p = 0.02) Differences were significant.  Table 4 Excellent group and ordinary group There is no significant difference in the knee angle displacement group (P> 0.05), but the hip angular displacement of different athletes in the gymnastics is subjected to a significant difference (P = 0.045), and the difference between groups), excellent group The ankle angular displacement on the tower is significantly different from the balance pad (P = 0.014), gymnastics pad (p = 0.017) and sand (P = 0.013), but found in ordinary groups The joint angular displacement is only significantly different from the gym (P = 0.048) and sand (P = 0.017), and there is no significant difference in the balance pad (P = 0.166).     Table 5); can be seen from    places. There is only differences in the stretching in terms of angular displacement and maximum angular velocity.
At present, the research on squat jump in our country is mostly focused on the influence of arm swing on take-off effect, the comparative analysis of mechanical characteristics of different sports after squat jump training and the influence of stretching action on squat jump ability before exercise. You Yonghao et al. [7] analyzed the biomechanical characteristics of young men with or without arm swing in the form of squat jump, and mainly revealed the action mechanism of arm swing in squat jump from the aspects of longitudinal jump buffer stage, pedal extension stage and maximum pedal extension buffer time. Squat jumping is an indispensable sports skill in many sports, and the success or failure of jumping events in sports largely depends on the ability of athletes' lower limbs. This is why many studies analyze squatting movements from a physical point of view, in order to find the influencing factors that can not be distinguished by the naked eye in the jumping process, so as to improve the competitive ability of athletes.
Through the experiment, it is found that the surface recovery coefficient has a great influence on the jumping height, and the vertical jumping height of the human body can reflect the athletes' longitudinal jumping ability and explosive power. The recovery coefficients of four kinds of ground are tested before the experiment, and the results are force platform > balance pad > gymnastics pad > sand. Previous studies have found that the greater the surface recovery coefficient, the higher the jump height, but in the test of the ordinary group, the height of jumping on the balance pad in the ordinary group is lower than that on the gymnastics pad. The reason may be that the instability of the balance pad makes the athletes lose more energy during take-off. However, this situation did not occur in the excellent group, which may be due to the better physical control and coordination ability of the elite group athletes, and less energy loss during the jump.
When jumping on the unstable balance pad, the ground will deform to a certain extent in the pedal and extension stage, which makes it difficult for the force of the joint in the final exertion stage to advance along the vertical direction, and finally leads to the decrease of the jump height. It has also been reported that jumping in the sand absorbs almost 100% of the energy generated by the impact [23] . Therefore, it is likely that the energy generated by the muscles in the jump buffer stage has a great effect on the squat jump, so the surface reaction force is small and the jump height is low. The characteristics of energy absorption in the sand will increase the contraction time of the lower limb muscles of the squatting jump and excite the leg extensor before contraction.
therefore, the squatting jump in the sand does more work than on the hard ground. Some studies have found that the squat height (28.8cm) on the hard surface is significantly higher than that on the sand (24.8cm), which may be related to the instability and low stiffness of the sand surface, which reduces the take-off speed of squatting jump [24] . The angular displacement of different level athletes on different ground is shown in figure 2 and figure 3. The angular displacement of lower limb joint of elite athletes is higher than that of ordinary athletes, and the angular displacement of lower limb joint sand is higher than that of other grounds. there was no significant difference in the angular displacement of hip joint and knee joint during the pedal and extension stage of squatting jump in figure 3. However, when squatting and jumping on the gymnastic mat, there were significant differences in hip angular displacement among different athletes (inter-group difference). In the excellent group, the ankle angular displacement on the force platform was significantly different from that of balance pad, gymnastics pad and sand, but it was found in the ordinary group. the ankle angular displacement of the middle force platform in the ordinary group was only significantly different from that of the gymnastics pad and sand, but there was no significant difference with the balance pad. Some studies have found that when jumping on a hard surface, the range of motion of the hip and knee joint is smaller than that in the sand [25] . In addition, the study of Tilp et al. [26] shows that the flexion of the knee joint in the pedal and extension stage of the volleyball spike is significantly higher in the sand than on the indoor wooden floor, indicating that the flexion range of the soft surface is higher than that of the hard surface.
The reason for this difference is that the ground produces a higher reaction force during the jump, and the flexion angle of the joint increases in order to avoid skidding and maintain balance. Therefore, the range of motion of the lower limb joint will increase with the increase of the ground reaction force. Therefore, the lower limb joints on the softer ground will bend more than other surfaces, but the buckling range of sand is higher than that of other surfaces. This difference can be understood as when squatting and jumping in the sand, the mobility and instability of the sand lead to a decrease in balance, which will preserve the balance of the body by increasing the curvature of the joints of the lower extremities.
The sequence of human lower limb joint activity is as follows: the muscle groups of

Conclusion
(1) Differently face the general group, and in the ground, the maximum angular velocity of the ankle joint in the ordinary group is small, indicating that the ordinary group of ankle muscles is more elastic coefficient. The low ground rapid contraction ability is poor; (2) When the surface is extracted, the excellent group is good for physical control and coordination, and the joint energy loss is low, so that the jump effect is better; When the ordinary group is subjected to a lower elastic coefficient and unstable ground, the jump effect is poor. Figure 1 Work ow of the study. Upon introduction of pGL3-5'UTR vector, relative luciferase activity increased more than 10 times to the equivalent amount of pGL3-promoter. Note that the SARS-CoV-2 derived 5'UTR synergistically activated translation in an intracellular factor-dependent manner. Relative luciferase activity was calculated against the activity of endogenous pRL-RSV serving as control (100%). The Mann-Whitney U test was used for statistical signi cance (p <0.05).

Figure 3
RNA-immunoprecipitation. (a) Prediction of luciferase mRNA-5'UTR fusion. Two optimal 28 nt regions gRNAs targeting the luciferase RNA were designed -gRNA1 and gRNA2. (b) gRNA-dCas13 mediated RNA immunoprecipitation of host RNA bound to 5'UTR sequences. RT-qPCR was performed to determine the accumulation of immunoprecipitated 5'UTR sequence (Top) and luciferase RNA (bottom) via gRNA-dCas13 mediated targeting of luciferase RNA. The host RNA bound to 5'UTR was immunoprecipitated and the enrichment factor was calculated as 2 (-ΔΔCt [RIP / background]). The mean and standard error of the three independent experiments is shown. Statistical analysis was performed using a two-sided paired t-test. * Indicates a signi cant difference from the control sample at a value of p <0.05. and A549L1Cv) that were signi cantly enriched (p <0.05) compared to the control (293L2GL3 or A549L2GL3). 293T-XIST is an RNA-immunoprecipitation sample serving as a negative control using HEK293T cells expressing lncRNA XIST-targeting guide RNA and dCas13. The genes enriched by Control-293 vs 293T-L2-CoV2, Control-A549 vs A549-L1-CoV2, and Control-A549-L2-CoV2 are human intracellular factors that interact with 5'UTR derived from SARS-CoV-2. (c) Genes enriched by Control-293 vs 293T-L2-CoV2 dataset were ranked according to their differential enrichment and the resulting enriched GO terms are visualized using a DAG graphical representation with color-coding re ecting their enrichment degree.  Overexpression of 5'UTR in SARS-CoV-2 alters genes expression involved in the mevalonate pathway in HEK293T and A549 cells. (a) 1 ug of pGL3-promoter and pGL3-5'UTR vectors were introduced into cells cultured in 6 well plates (0.3 x 106). Expression of lipid metabolism factors from total RNA was measured. Overexpression of 5'UTR affected the expression of ACAA and HMGCS. Results are shown as mean ± SD, and Student's t-test was used as a test of signi cance. (b) Outline of metabolic pathway of Acetyl-CoA by overexpression of 5'UTR of SARS-CoV-2 and inhibitory mechanism of HMG-CoA reductase inhibitors like statins. (c) 1 ug of pGL3-promoter and pGL3-5'UTR vectors was introduced in cells cultured in 24 well plates. HMGCoA reductase inhibitors were added to the culture solution at a concentration range of 1nM, 10nM, 100nM and 1,000 nM. Following incubation for 48 hours, the luciferase activity was measured. To assess the effect on the 5'UTR, the pGL3-promoter and luciferase activity from each adjacent well into which the pGL3-5'UTR vector was introduced were subtracted. The internal control used was the sum of each value. The results were shown as mean ± SD and were tested for signi cance by Student's t-test. The internal control used was the sum of the respective values. Results were shown as mean ± SD, and Student's t-test examined signi cance.