# Analysis of Biomechanical Parameters of Martial Arts Routine Athletes’ Jumping Difficulty Based on Image Recognition

Authors: Mengjie Qiao; Shibiao Dong
Journal: Computational Intelligence and Neuroscience
 (2022)

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## Abstract

This paper uses image segmentation technology to examine the biomechanical parameters of martial arts routine athletes’ whirlwind legs and backflips, two difficult jumping sports. The successful completion of the whirlwind leg, a typical martial arts jumping difficulty, during the buffer period of the take-off stage, the left knee angle flexion, the drop of the body’s center of gravity, and the drop of the horizontal speed of the center of gravity are all significantly correlated, so it is only necessary to grasp airborne altitude and speed from landing. The 720-degree cyclone foot has a flying height of 0.470.11 m, which is 4 cm higher than the 540-degree cyclone foot (0.430.11 m). The antigravity of the last foot is greater than about 1.3 kgf/kg of the left foot during the run-up stage, which allows for a higher rotational angular velocity and completion of the 720-degree difficulty of the whirlwind foot. As a result, it is crucial to pay attention to how you step with your right foot. In the backflip, the coordination of the two legs and the upper body is crucial. The right foot’s effective braking can help to increase the body’s rising angle. The trunk inclination angle in the flying stage is between 110° and 120°, the knee angle is between 60° and 70°, and the angle between the two legs is between 35° and 35°. When lifting off the ground and landing, the tibialis anterior muscle discharge is greater than the gastrocnemius muscle discharge, which helps to maintain the balance between the fulcrums. As a result, it is necessary to let the non-supporting leg fall first in order to achieve the goal of a smooth landing.


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## Body

## 1. Introduction

With the advancement of the athletes’ competitive level, competitive martial arts is gradually evolving in the direction of “high, difficult, beautiful, new, stable, and refined.” The movement difficulty in competitive martial arts routine competition is increasing, and the movement between the movements is becoming more difficult. Athletes must complete difficult movements delicately, steadily, and with high quality in order to achieve excellent competition results, especially as the trend of changing connections becomes more complex. These difficult movements include not only the fast rotation of the trunk around the sagittal and coronal equiaxes during the jumping process, but also the stable balance of the connection after the difficult jumping movements have been completed [1]. The difficulty of the movements from fast movement to sudden stillness and stable support, as well as the high-difficulty and graceful posture shown by the jumping movements, adds color to the viewing of competitive martial arts routines. On the other hand, as sports science and technology advance, and training quality improves, the gap between athletes’ physical fitness and competitive level narrows [2]. The success of any action has a significant impact on the evaluation of the quality of the athlete’s entire set of actions and the final score, especially when the level and strength are similar. In competitive martial arts routine competitions, the error rate of athletes’ jumping movements is quite high, and athletes of any level have certain deficiencies in the completion time and quality of their balance movements. Therefore, the mastery and stable performance of jumping movements are also the decisive criteria for judging the level of athletes’ competitive level.In order to make the technical practice of the athlete’s balance movement more scientific and effective, this paper has a preliminary understanding of the balance movement by visiting relevant experts, watching on-site, or watching martial arts competitions and other aspects. According to factors such as the frequency of occurrence, difficulty level, and share of balance movements in martial arts competitions in recent years, two movements of backflip and whirlwind leg were selected for analysis. Using image segmentation technology to analyze the biomechanics method reveals its movement law. It provides a reference for improving the rationality and optimization of the balance movements of martial arts routine athletes, so as to solve the technical difficulties of balance movement practice encountered in daily training, so as to achieve the purpose of improving the athletes’ competitive level and refining and perfecting technical movements.Analysis systems based on modern digital image processing technology [3, 4] are becoming increasingly important in the process of analyzing martial arts routines. Because of its intuitive and rapid characteristics, it can effectively overcome the drawbacks of traditional martial arts teaching, assist learners in quickly mastering the fundamentals of martial arts movements, and attract an increasing amount of attention from martial arts enthusiasts. The research on identification, detection, and analysis technology in the jumping process has a lot of potential due to the unique movement mode of martial arts routines. A set of Wushu routine movement analyzer systems is developed in this paper using image recognition technology [5–7] on the basis of video acquisition and video processing. Through the front-end camera and the capture card, the system collects and records the martial arts routine motion video into the specified file, adds a variety of line drawing analysis tools to the video playback and comparison process, and uses graphics and lines to aid in the action video analysis. This paper investigates a set of image recognition algorithms that use the background difference method to grayscale captured image sequences, extract edges, median filtering, and binarization, based on the premise that basic video acquisition and analysis functions can meet the needs of martial arts teaching. The movement trajectory of the jumping process is finally obtained after processing; using the frame-to-frame difference method to compare the change trend of the two frames before and after, the analyzer system can assist martial arts practitioners in correcting swinging and jumping movements, and cooperate with the later developed man-machine interface and simple file management module, which makes the analysis of high-difficulty jumping movements in martial arts more convenient. The system meets the requirements for analyzing and using high-difficulty jumping movements in martial arts after a large number of field experiments.The innovation of this paper: this paper analyzes the actual needs of martial arts routine athletes training and the particularity of martial arts high-difficult jumping movements. Image recognition technology can intelligently identify the details of martial arts athletes’ jumping movements and gradually analyze the biomechanical parameters of each environment.The structure of the article: first, we introduce some international researches on the analysis of the physical force of sports movements based on image recognition technology; the biomechanical parameters of the whirlwind leg and backflip in the jumping action are analyzed; finally is a summary of the full text.


## 2. Related Work

In today’s social production and life, the application value produced by the combination of video processing technology and image recognition technology is more and more concerned by the majority of scientific researchers, and the fields involved are also very wide. Human motion detection and tracking system is mainly used to deal with image sequences including moving human body. Motion detection extracts foreground motion regions from the background from an image sequence. Motion detection is the basis for the classification and tracking of moving objects, as well as the analysis and understanding of moving human movements. The processing results at this stage directly affect the effect of subsequent processing. With the continuous progress of the development of sports science and technology, the technical research on jumping movements also tends to use electronic and digital scientific instruments for analysis and research [8]. Tripodi et al. [9] mainly focused on the study of its “backward jumping” action, using close-range dynamic stereo photogrammetry as the research method, and using the Aijie motion image analysis system to analyze technical movements, respectively. The speed change, the center of gravity displacement change, and the torso displacement change have been comprehensively studied. The analysis results show that the strength of the legs is the key factor affecting the stability of the back jump. The small muscle group of the right supporting leg of the martial arts routine athletes is weak, and strength training should be strengthened [9]. Li and Zhang [10] also used this method to study the jumping movements of “sidekicking and holding the feet upright” of martial arts routine athletes. The technical indicators such as the swinging leg, the opposite side arm of the swinging leg, the speed of the center of gravity, and the displacement of the center of gravity are selected as parameters, and the swing speed of the swinging leg and the opposite side arm of the swinging leg in the X-axis direction is changed. The maximum swing speed of the swing leg is slightly greater than the swing of the opposite arm of the swing leg, but when it swings over the central axis of the body, the swing speed of the arm is faster than the speed of the leg. At the same time, reach the best jumping state to complete the action. In addition, it can be seen from the comparison curve of the center of gravity speed and displacement that martial arts routine athletes change evenly in the center of gravity speed during the entire movement process, which lays the foundation for their stable jumping support [10]. Through a series of data analysis, Mastalerz et al. [11] concluded that, in the practice of martial arts jumping movements based on muscle strength, attention should be paid to strengthening the coordinated development of upper and lower limb muscle strength and the flexibility of the legs [11]. In the study by Tagaev et al. [12], the kinematics comparative analysis of the sidekicks and the upright jumps of the martial arts routine athlete and Zhang Yanan, the two champions, was found in the comparison. Sex is closely related to the swing speed of the right arm and the swing sequence of the two [12]. Strengthening the rapid arm swing exercises to form a rapid cooperation between the arms and the legs can greatly enhance the stability of the jumping action. In the study of other martial arts jumping movements, Lewis et al. [13] conducted a technical analysis of the sea-exploring jumping and lying-fishing jumping movements in swordsmanship routines to clarify the principle of maintaining the stability of jumping movements. Find the difficult points of the jumping action. It calculates and draws the coordinates of the center of gravity of each link of the body and the coordinates of the total center of gravity of the body with reference to the Japanese body model of Hideji Matsui to evaluate the stability of the jumping action. At the same time, it also pointed out the main muscle groups involved in the movement of the sea-exploring jumping movement and studied the stability regulation of the flexion and extension muscle groups of the body on the sea-exploring jumping [13]. In Atalay et al.’s [14] article on the muscle EMG analysis of Sanda’s sidekick action, the EMG test was performed on seven muscles including the rectus femoris, medial vastus, biceps femoris, and semitendinosus, which are mainly involved in the movement of the contralateral kick action. Analysis and research show that the time sequence and sequence of muscle force exerted by excellent Sanda athletes in the sidekick action are relatively close and evenly concentrated, while the time and sequence of muscle force exerted by ordinary athletes when they complete the same action are uneven, uniform, and scattered. The importance of strengthening muscle coordination exercises during training was emphasized [14]. The attacking leg of the whipping technique was studied simultaneously by three-dimensional kinematics analysis and surface electromyography measurement to explore the movement speed of each link of the attacking leg, the contraction mode of the main muscle groups, and the order of force at the attacking moment [15]. The study found that when the knee angle of the attacking leg presents about 140° in the preparation moment of the whip leg attack, it is beneficial to move quickly on the ring and maintain the angle of each joint to achieve better results. Through the analysis of the electromyography tester, it was found that, in the stage of knee flexion and leg raising, sartorius, tensor fascia lata, biceps femoris, and gastrocnemius participated in the muscle contraction, and the rectus femoris EMG contributed the most, and the tensor fascia lata muscle contributed the most. It is the smallest, but it is mainly done by the tensor fascia lata in the stage of widening the knee [15].With the combination of martial arts and computer technology, more and more martial arts-related equipment has introduced information technology. However, professional golf swing analysis systems are still relatively rare in the world, and the difficulty of jumping martial arts routines using image recognition technology is even rarer, and some martial arts routine sports coaching software that simply uses video processing and analysis technology is available in some countries. The particularity of Wushu sports determines the special requirements for equipment of Wushu sports teaching analysis system. The existing teaching system has shortcomings such as low software intelligence, poor image quality, single video angle, etc., and there is currently no professional golf course. The swing analysis system cannot well complete the Wushu sports teaching task. Therefore, the analysis of the jumping difficulty of Wushu routine athletes based on image recognition in this paper is very meaningful for the research in this field.


## 3. Image Segmentation-Based Feature Extraction Method for Jumping Action

This paper proposes a method to calculate the period based on the geometric shape of the signal. Before extracting the signal, the original signal should be median-filtered to make the period clearer. It is worth pointing out that, due to the median filtering, horizontal lines may appear in the signal. Since the minimum point of the signal is required to extract the period, when a horizontal line appears, there may be several consecutive identical minimum points, which will interfere with determining the distance between each minimum point, so the horizontal line in the signal should also be removed. The lowest point of each cycle appears in cycles, and the cycle in which the lowest point appears can be regarded as the cycle of the signal [16]. The steps to determine the cycle of the signal are shown in Figure 1.Figure 1
Flowchart of the cycle algorithm.Motion regions are extracted by thresholding pixel-based temporal differences between adjacent images in successive image sequences [17]. The early temporal difference method is to use the difference of two adjacent images to obtain the motion area, as shown in(1)Dkx,y=fkx,y−fk−1x,y,where fkx,y is the gray value at the midpoint x,y in the k-th frame and fk−1x,y is the grayscale value at the midpoint x,y in the k−1-th frame. The Dk binarization of the difference result is shown in(2)Rxx,y=1,Dkx,y>T,0,Dkx,y≤T.Pixels with values of 1 and 0 correspond to the foreground (moving object area) and background (non-moving area), respectively [18]. The differenced D also includes the change of the scene between two consecutive frames. This change is composed of many factors, including the movement of the target, lighting, shadow, noise, etc. It can be considered that the change of the moving target is obvious. Set a threshold T; when the difference of a certain pixel value in the difference is greater than the given threshold T, the pixel is considered as a foreground pixel; otherwise, it is considered as a background pixel. The interframe difference method is the simplest method to detect changes between adjacent frame images. It directly compares the difference in gray value of corresponding pixels in two or three consecutive frames of images in a video sequence and then sets a threshold to extract motion regions in sequence images [19, 20]. The main advantages of the interframe difference method for target detection are that the algorithm is simple to implement, the programming complexity is low, and it is easy to realize real-time monitoring. Based on the adjacent frame difference method, since the time interval between adjacent frames is generally short, the method is generally less sensitive to changes in scene light. The most basic interframe difference method can detect changes in the scene and extract the target, but in practical applications, the result of the interframe difference method is not very accurate, and it is difficult to obtain an accurate description of the target area [21].The background subtraction method is a motion detection method that uses the difference between the current image and the background image to extract the motion area, and it is also a widely used method at present [22], which can generally provide complete data of the target. The principle of the background difference method is to use the current image and the background model to differentiate and then threshold to obtain the moving object. In practical application, it is difficult to get the background directly from the monitored area because of the movement of objects or the change of environment, such as the moving target becoming a part of the background, the local background becoming a moving target, etc. Therefore, it is very necessary to acquire, reconstruct, and update the background [23].The background difference method assumes that the background is stationary, so the background does not change with the number of frames. First, the differenceDk between the current fk and the background bk is obtained by using(3)Dkx,y=fkx,y−bkx,y.Then, the difference is also binarized according to formula (2). Whether the threshold T is selected accurately or not directly affects the quality of the binary dipper. If the threshold T is selected too high, the area determined as a moving target in the binary value will be fragmented; on the contrary, if the threshold T is selected too low, a lot of noise will be introduced. The most common method for selecting the threshold T is to use the grayscale histogram with double peaks or multiple peaks and select the grayscale value at the bottom of the valley between the two peaks as the threshold value. The background subtraction method is simple to calculate and fast, and the obtained result directly gives the position, size, shape, etc. of the target, so that a complete and accurate description of the moving target area can be obtained [24].


## 4. Case Study of Jumping Action

### 4.1. Analysis of Biomechanical Parameters of Cyclone Foot Movement

Approach, take-off, air, and landing make up the 720-degree cyclone foot. When starting, the right foot is buckled in, and the right leg is bent and squatted and quickly pushes the ground, while the left leg swings to the left and the body is twisted counterclockwise; when taking off, the right foot is buckled in, and the right leg is bent and squatted and quickly pushes the ground. After the right leg completes the straight swing and the body’s center of gravity is near the highest point, the left hand taps the right sole, the two legs are completed in the air, the limbs are kept vertical, and the arms are close to the body. After rotating in the air for nearly two weeks, spread the limbs and slow down the rotation speed, touch the ground with both feet, take the horse step or drop the vertical fork, and finish the landing action. The subjects for this study were seven martial arts routine athletes. The average vertical distance between the right swing leg and the lowest shoulder point was 0.180.16 m after the subjects completed the strike in the air. The results are shown in Figure2.Figure 2
Kinematic parameters of the cyclone foot at 720°.During the flying stage, the subjects completed the 720-degree cyclone foot with an average flying height of 0.47 ± 0.11 m, which was 4 cm higher than the 540-degree (0.43 ± 0.11 m) cyclone foot. This is a comparison of the overall samples that completed theC-level and B-level jumping difficulty movements of the cyclone feet. The paired t-test was used to find that the same athletes completed the C-level and B-level jumping difficulty movements of the cyclone feet, respectively, and the flying heights of the two were not significant. It can be considered that the same athlete has no difference in the height of the air when completing the B-level and C-level jumping difficulty movements of the whirlwind foot, and the main reason is that the rotation speed in the air is increased. The comparison of kinematic parameters of cyclone feet with different difficulties is shown in Figure 3.Figure 3
Comparison of kinematic parameters of cyclone feet with different difficulties (mean value).The average maximum ground reaction force of the left foot in the final step of the approach stage is 2.151.03 kgf/kg body weight, while the average maximum ground reaction force of the right foot in the final step is 3.500.76 kgf/kg body weight, which is significantly greater than the starting braking. The action of the left foot indicates that the last step’s ground reaction force should be high in order to help raise the air’s height. The turning moment around the body’s longitudinal axis is large, allowing for a higher rotational angular velocity and completing the whirlwind foot’s 720-degree difficulty. As a result, it is important to pay attention to how you walk with your right foot.The EMG signal strength of the vastus medialis muscle is the strongest in the take-off and landing stages; the EMG signal of the vastus lateralis muscle is stronger in the take-off and landing stages; the EMG signal of the rectus femoris in the take-off stage is average, and there is a discharge signal in the flight stage, which is consistent with the discharge time of the adductor magnus. It shows that this stage is the clicking action when flying, and there is also an EMG signal when landing; the gastrocnemius discharge time is the earliest, because the right leg has a certain angle with the ground when taking off, and the triceps of the calf keeps the ankle joint through isometric contraction. At a certain joint angle, the ground reaction force is transmitted through the calf to the thigh. In training, attention should be paid to the exercises of calf strength, especially the isometric strength exercises to strengthen the calf.Athletes who use one foot to perform a 720-degree take-off with a whirlwind foot should obtain a certain degree of rotation as much as possible before leaving the ground; that is, the human body has already rotated part of the body before it is in the air, so as to improve the height of the air and complete the degree of rotation. The training methods for theB and C-level jumping difficulty of the whirlwind foot include the click of the legs together, the legs together, and the landing technique. In the flying stage, when the center of gravity of the human body rises close to the highest point, the click must be completed, and the time from take-off to the completion of the click should be as short as possible, so as to achieve the corresponding degree of leg rotation in the flying stage as soon as possible. Athletes choose horse stance in the dynamic and static connection of Changquan and Nanquan. Although individual athletes avoid landing with both feet, foot movement, or jumping in sequence, they focus on one leg in order to stabilize the movement. In the action quality score, points are deducted if the squatting legs are not level and the distance between the feet is too small. Therefore, the 720-degree stance of the whirlwind foot must strengthen the awareness of movement norms and training requirements. Figures 4 and 5 show the kinematic parameter diagrams of the athlete’s front extension.Figure 4
Kinematic parameters of the athlete’s front extension.Figure 5
Kinematic parameters of the athlete’s front extension.It can be seen from Figure4 that the left knee angle of the seven athletes is (119.74 ± 20.83)° at the lowest point of the center of gravity of the body. At this time, the left leg is firmly supported by the inside of the foot and actively stretches, and the knee angle begins to increase. When the right foot hits the ground, the angle is (139.26 ± 14.17)°, when the left foot is off the ground, the knee angle is (164.93 ± 4.19)°, the kicking range is (45.19 ± 21.48)°, and the vertical velocity of the center of gravity is (1.91 ± 0.35) m/s. After inspection, the left knee extension during the front extension period is significantly positively correlated with the vertical speed of the center of gravity when the left foot leaves the ground, indicating that the greater the left knee extension during the front extension period, the higher the vertical speed of the center of gravity when the left foot leaves the ground. In addition, the active stretch action of the left leg can quickly push the hip and body weight forward, which is of great significance for obtaining a larger impulse during take-off and quickly stepping on the right leg to the take-off point. The horizontal distance of the body’s center of gravity moving forward and the horizontal speed of the center of gravity were (0.18 ± 0.08) m and (2.21 ± 0.81)  m/s, respectively, during the forward stretch. After testing, the horizontal distance of the body’s center of gravity moving forward during the forward stretch is significantly correlated with the horizontal speed of the center of gravity when the left foot is off the ground, indicating that the more fully the left leg is stretched during the forward stretch, the greater the horizontal distance of the body’s center of gravity forward, and the more it is beneficial to obtain a larger horizontal velocity of the center of gravity when the left foot is off the ground. 1, 2, 5, and 6 give full play to the left leg’s positive role during the forward stretch period, allowing the right leg to quickly transition to the stretch action and the early swing of the left leg after stepping on the take-off point. The main reason for the disparity among athletes is that the left leg is not actively stretched, which is extremely detrimental to improving center of gravity speed and maintaining good body posture when taking off. Figure 6 depicts the swing speed of the two arms during the front kicking and extension period.Figure 6
The variation characteristics of the swing speed of the athlete’s arms during the forward extension period.It can be seen from Figure6 that the swing of the left and right arms shows different changing laws during the front kicking and extension period. From the lowest point of the center of gravity to the moment when the right foot hits the ground, the swing speeds of the left and right arms continue to increase. When the right foot hits the ground, the swing speeds of the left and right arms are (8.64 ± 1.47) and (9.33 ± 1.73)  m/s, respectively. Based on the previous analysis, it can be seen that, from the moment of the lowest point of the center of gravity to the moment when the right foot hits the ground, the athlete pushes the center of gravity forward through the active stretch of the left leg and the accelerated swing of the two arms, creating a favorable situation for the right leg to move towards the take-off point actively and quickly. condition. 1, 2, 5, and 6 have more sufficient kicking and stretching actions, and the swing speed of the left and right arms is higher than that of other athletes, which indicates that the athletes such as Zhou can make full use of the swing of the arms to take-off better than other athletes. After the right foot touched the ground, the swing speed of the left arm continued to increase. When the left foot was off the ground, the swing speed reached the maximum (10.21 ± 2.24)  m/s, while the swing speed of the right arm showed a decreasing trend. After kicking off the ground, the swing speed of the right arm drops to (7.01 ± 1.51) m/s. After inspection, when the left foot leaves the ground, the swing speed of the left arm has a significant positive correlation with the horizontal speed of the center of gravity at this moment. It shows that the greater the swing speed of the left arm, the greater the horizontal speed of the center of gravity when the left foot leaves the ground. It can be seen that, from the moment when the right foot hits the ground to the time when the left foot leaves the ground, the horizontal speed of the center of gravity of the body is maximized by the rapid swing of the left arm, and the body balance is maintained by reducing the swing speed of the right arm.


### 4.2. Analysis of Biomechanical Parameters of Backflip

The backflip consists of four steps: run-up, take-off, flying, and landing. In the run-up stage, try to keep the balance of the body and run in a straight line; in the take-off stage, the right heel brakes on the ground, the upper body is leaned back, and the left leg is raised and the left leg swings up. Raise your legs and swing your knees to your chest, and tuck your legs in the air; in the landing stage, after your body has completed a 360-degree backward rotation in the air, land both feet on the ground at the same time, landing in a single butterfly step. In this paper, 6 martial arts athletes are selected for action analysis, and key information is proposed based on image segmentation technology. The torso anteversion is the angle between the torso and the supporting leg. The inclination angle of the non-supporting leg is the angle between the non-supporting leg and the horizontal position as shown in Figure7.Figure 7
The angle of each link of the backflip.The angle of the ankle joint of the supporting leg is controlled at about 90° when the backflip is stable, which means that, in the leaning-over balance, the body’s center of gravity is moving forward. In the upright position, there is no discernible difference; however, the center of gravity is projected onto the support legs, making balance easier to control. The backflip’s torso inclination angle is between 110° and 120°. Because the action requirements of each link differ, one requires raising the chest while the other requires leaning forward. As a result, the trunk’s forward inclination angle becomes the primary basis for the two groups’ balance. The inclination of the non-supporting leg of the backflip, on the other hand, is found to be the same as the inclination of the supporting leg using statistical technical parameters of image segmentation. When the fulcrum stays the same, the forward probe of the torso causes the center of gravity to move forward, resulting in a change in the gravitational distance and an increase in the gravitational arm. The non-supporting leg compensates for the change in gravitational distance due to the forward movement of the center of gravity and ensures that the total external moment is zero in order to maintain the balance position with the torso in the air. The non-supporting leg’s horizontal angle decreases, increasing the moment arm between the non-supporting leg and gravity and achieving the goal of maintaining balance and stability. As a result, the backflip’s non-support leg angle is slightly reduced, as shown in Figure8.Figure 8
Muscle IEMG of backflip test of martial arts athletes.In the process of observing the support leg movement of the backflip action, the support leg of the backflip action is all the pelvis flexed at the hip joint and is in a forward tilted position. The support leg is always in the upright phase. The support leg is always in the upright phase. In both balanced stabilization movements, the torso and non-supporting legs are rotated counterclockwise to a horizontal position about the hip joint. In this state, the rectus femoris muscle contracts concentrically, the biceps femoris muscle contracts eccentrically, and the moment arm of the rectus femoris muscle is shorter than the moment arm of the biceps femoris muscle, and the movement balance is maintained under the condition of equal muscle force distance, so the discharge of the rectus femoris is greater than that of the biceps femoris. The calf is flexed at the ankle joint, the tibialis anterior muscle performs eccentric contraction, the gastrocnemius muscle contracts concentrically, and the muscle arm of the tibialis anterior muscle is much smaller than that of the gastrocnemius muscle, so the discharge of the tibialis anterior muscle is more than that of the gastrocnemius muscle. The gluteus maximus and gluteus medius do eccentric contraction in the hip flexion movement in the bending part, and the muscle arm of the gluteus medius is smaller than that of the gluteus maximus, so the discharge of the gluteus medius is more than that of the gluteus maximus. Figure9 shows the angle of the body part of the backflip buckle leg link.Figure 9
Body part angle of backflip.It can be seen from the figure that the ankle joint angle of the support leg becomes smaller in the stable stage of the backflip than in the upright stage, because the bending of the support leg causes the body’s center of gravity to drop and move forward. At this time, the angle of the ankle joint decreases with the forward movement of the center of gravity, which increases the projected area of the center of gravity on the support surface, which is beneficial to control the stability of the center of gravity. Buckling balance requires the support leg and thigh to be kept as level as possible. According to the principle of parallel lines, the range of the knee angle is controlled between 60 and 70° to compensate for the increased moment arm due to hip flexion and squatting, so that the action can be balanced between the fulcrums. The trunk leans forward from the hip joint, so that the head is close to the vertical line of the center of gravity, which increases the balance and stability and makes the movement more graceful and stretched. The angle between the two legs is between 35 and 50°, which is to balance the component forces on the left and right sides of the action, so that the combined external force is zero, so as to achieve the purpose of landing smoothly. The swing speed and swing sequence of the non-supporting leg and the heterolateral arm have a great relationship with the stability of the movement. The swing of the opposite side arm should be prior to the swing of the non-supporting leg, which is conducive to improving the side swing speed, shortening the time to complete the action, and enhancing the stability of the balance while improving the quality of the action.


## 4.1. Analysis of Biomechanical Parameters of Cyclone Foot Movement

Approach, take-off, air, and landing make up the 720-degree cyclone foot. When starting, the right foot is buckled in, and the right leg is bent and squatted and quickly pushes the ground, while the left leg swings to the left and the body is twisted counterclockwise; when taking off, the right foot is buckled in, and the right leg is bent and squatted and quickly pushes the ground. After the right leg completes the straight swing and the body’s center of gravity is near the highest point, the left hand taps the right sole, the two legs are completed in the air, the limbs are kept vertical, and the arms are close to the body. After rotating in the air for nearly two weeks, spread the limbs and slow down the rotation speed, touch the ground with both feet, take the horse step or drop the vertical fork, and finish the landing action. The subjects for this study were seven martial arts routine athletes. The average vertical distance between the right swing leg and the lowest shoulder point was 0.180.16 m after the subjects completed the strike in the air. The results are shown in Figure2.Figure 2
Kinematic parameters of the cyclone foot at 720°.During the flying stage, the subjects completed the 720-degree cyclone foot with an average flying height of 0.47 ± 0.11 m, which was 4 cm higher than the 540-degree (0.43 ± 0.11 m) cyclone foot. This is a comparison of the overall samples that completed theC-level and B-level jumping difficulty movements of the cyclone feet. The paired t-test was used to find that the same athletes completed the C-level and B-level jumping difficulty movements of the cyclone feet, respectively, and the flying heights of the two were not significant. It can be considered that the same athlete has no difference in the height of the air when completing the B-level and C-level jumping difficulty movements of the whirlwind foot, and the main reason is that the rotation speed in the air is increased. The comparison of kinematic parameters of cyclone feet with different difficulties is shown in Figure 3.Figure 3
Comparison of kinematic parameters of cyclone feet with different difficulties (mean value).The average maximum ground reaction force of the left foot in the final step of the approach stage is 2.151.03 kgf/kg body weight, while the average maximum ground reaction force of the right foot in the final step is 3.500.76 kgf/kg body weight, which is significantly greater than the starting braking. The action of the left foot indicates that the last step’s ground reaction force should be high in order to help raise the air’s height. The turning moment around the body’s longitudinal axis is large, allowing for a higher rotational angular velocity and completing the whirlwind foot’s 720-degree difficulty. As a result, it is important to pay attention to how you walk with your right foot.The EMG signal strength of the vastus medialis muscle is the strongest in the take-off and landing stages; the EMG signal of the vastus lateralis muscle is stronger in the take-off and landing stages; the EMG signal of the rectus femoris in the take-off stage is average, and there is a discharge signal in the flight stage, which is consistent with the discharge time of the adductor magnus. It shows that this stage is the clicking action when flying, and there is also an EMG signal when landing; the gastrocnemius discharge time is the earliest, because the right leg has a certain angle with the ground when taking off, and the triceps of the calf keeps the ankle joint through isometric contraction. At a certain joint angle, the ground reaction force is transmitted through the calf to the thigh. In training, attention should be paid to the exercises of calf strength, especially the isometric strength exercises to strengthen the calf.Athletes who use one foot to perform a 720-degree take-off with a whirlwind foot should obtain a certain degree of rotation as much as possible before leaving the ground; that is, the human body has already rotated part of the body before it is in the air, so as to improve the height of the air and complete the degree of rotation. The training methods for theB and C-level jumping difficulty of the whirlwind foot include the click of the legs together, the legs together, and the landing technique. In the flying stage, when the center of gravity of the human body rises close to the highest point, the click must be completed, and the time from take-off to the completion of the click should be as short as possible, so as to achieve the corresponding degree of leg rotation in the flying stage as soon as possible. Athletes choose horse stance in the dynamic and static connection of Changquan and Nanquan. Although individual athletes avoid landing with both feet, foot movement, or jumping in sequence, they focus on one leg in order to stabilize the movement. In the action quality score, points are deducted if the squatting legs are not level and the distance between the feet is too small. Therefore, the 720-degree stance of the whirlwind foot must strengthen the awareness of movement norms and training requirements. Figures 4 and 5 show the kinematic parameter diagrams of the athlete’s front extension.Figure 4
Kinematic parameters of the athlete’s front extension.Figure 5
Kinematic parameters of the athlete’s front extension.It can be seen from Figure4 that the left knee angle of the seven athletes is (119.74 ± 20.83)° at the lowest point of the center of gravity of the body. At this time, the left leg is firmly supported by the inside of the foot and actively stretches, and the knee angle begins to increase. When the right foot hits the ground, the angle is (139.26 ± 14.17)°, when the left foot is off the ground, the knee angle is (164.93 ± 4.19)°, the kicking range is (45.19 ± 21.48)°, and the vertical velocity of the center of gravity is (1.91 ± 0.35) m/s. After inspection, the left knee extension during the front extension period is significantly positively correlated with the vertical speed of the center of gravity when the left foot leaves the ground, indicating that the greater the left knee extension during the front extension period, the higher the vertical speed of the center of gravity when the left foot leaves the ground. In addition, the active stretch action of the left leg can quickly push the hip and body weight forward, which is of great significance for obtaining a larger impulse during take-off and quickly stepping on the right leg to the take-off point. The horizontal distance of the body’s center of gravity moving forward and the horizontal speed of the center of gravity were (0.18 ± 0.08) m and (2.21 ± 0.81)  m/s, respectively, during the forward stretch. After testing, the horizontal distance of the body’s center of gravity moving forward during the forward stretch is significantly correlated with the horizontal speed of the center of gravity when the left foot is off the ground, indicating that the more fully the left leg is stretched during the forward stretch, the greater the horizontal distance of the body’s center of gravity forward, and the more it is beneficial to obtain a larger horizontal velocity of the center of gravity when the left foot is off the ground. 1, 2, 5, and 6 give full play to the left leg’s positive role during the forward stretch period, allowing the right leg to quickly transition to the stretch action and the early swing of the left leg after stepping on the take-off point. The main reason for the disparity among athletes is that the left leg is not actively stretched, which is extremely detrimental to improving center of gravity speed and maintaining good body posture when taking off. Figure 6 depicts the swing speed of the two arms during the front kicking and extension period.Figure 6
The variation characteristics of the swing speed of the athlete’s arms during the forward extension period.It can be seen from Figure6 that the swing of the left and right arms shows different changing laws during the front kicking and extension period. From the lowest point of the center of gravity to the moment when the right foot hits the ground, the swing speeds of the left and right arms continue to increase. When the right foot hits the ground, the swing speeds of the left and right arms are (8.64 ± 1.47) and (9.33 ± 1.73)  m/s, respectively. Based on the previous analysis, it can be seen that, from the moment of the lowest point of the center of gravity to the moment when the right foot hits the ground, the athlete pushes the center of gravity forward through the active stretch of the left leg and the accelerated swing of the two arms, creating a favorable situation for the right leg to move towards the take-off point actively and quickly. condition. 1, 2, 5, and 6 have more sufficient kicking and stretching actions, and the swing speed of the left and right arms is higher than that of other athletes, which indicates that the athletes such as Zhou can make full use of the swing of the arms to take-off better than other athletes. After the right foot touched the ground, the swing speed of the left arm continued to increase. When the left foot was off the ground, the swing speed reached the maximum (10.21 ± 2.24)  m/s, while the swing speed of the right arm showed a decreasing trend. After kicking off the ground, the swing speed of the right arm drops to (7.01 ± 1.51) m/s. After inspection, when the left foot leaves the ground, the swing speed of the left arm has a significant positive correlation with the horizontal speed of the center of gravity at this moment. It shows that the greater the swing speed of the left arm, the greater the horizontal speed of the center of gravity when the left foot leaves the ground. It can be seen that, from the moment when the right foot hits the ground to the time when the left foot leaves the ground, the horizontal speed of the center of gravity of the body is maximized by the rapid swing of the left arm, and the body balance is maintained by reducing the swing speed of the right arm.


## 4.2. Analysis of Biomechanical Parameters of Backflip

The backflip consists of four steps: run-up, take-off, flying, and landing. In the run-up stage, try to keep the balance of the body and run in a straight line; in the take-off stage, the right heel brakes on the ground, the upper body is leaned back, and the left leg is raised and the left leg swings up. Raise your legs and swing your knees to your chest, and tuck your legs in the air; in the landing stage, after your body has completed a 360-degree backward rotation in the air, land both feet on the ground at the same time, landing in a single butterfly step. In this paper, 6 martial arts athletes are selected for action analysis, and key information is proposed based on image segmentation technology. The torso anteversion is the angle between the torso and the supporting leg. The inclination angle of the non-supporting leg is the angle between the non-supporting leg and the horizontal position as shown in Figure7.Figure 7
The angle of each link of the backflip.The angle of the ankle joint of the supporting leg is controlled at about 90° when the backflip is stable, which means that, in the leaning-over balance, the body’s center of gravity is moving forward. In the upright position, there is no discernible difference; however, the center of gravity is projected onto the support legs, making balance easier to control. The backflip’s torso inclination angle is between 110° and 120°. Because the action requirements of each link differ, one requires raising the chest while the other requires leaning forward. As a result, the trunk’s forward inclination angle becomes the primary basis for the two groups’ balance. The inclination of the non-supporting leg of the backflip, on the other hand, is found to be the same as the inclination of the supporting leg using statistical technical parameters of image segmentation. When the fulcrum stays the same, the forward probe of the torso causes the center of gravity to move forward, resulting in a change in the gravitational distance and an increase in the gravitational arm. The non-supporting leg compensates for the change in gravitational distance due to the forward movement of the center of gravity and ensures that the total external moment is zero in order to maintain the balance position with the torso in the air. The non-supporting leg’s horizontal angle decreases, increasing the moment arm between the non-supporting leg and gravity and achieving the goal of maintaining balance and stability. As a result, the backflip’s non-support leg angle is slightly reduced, as shown in Figure8.Figure 8
Muscle IEMG of backflip test of martial arts athletes.In the process of observing the support leg movement of the backflip action, the support leg of the backflip action is all the pelvis flexed at the hip joint and is in a forward tilted position. The support leg is always in the upright phase. The support leg is always in the upright phase. In both balanced stabilization movements, the torso and non-supporting legs are rotated counterclockwise to a horizontal position about the hip joint. In this state, the rectus femoris muscle contracts concentrically, the biceps femoris muscle contracts eccentrically, and the moment arm of the rectus femoris muscle is shorter than the moment arm of the biceps femoris muscle, and the movement balance is maintained under the condition of equal muscle force distance, so the discharge of the rectus femoris is greater than that of the biceps femoris. The calf is flexed at the ankle joint, the tibialis anterior muscle performs eccentric contraction, the gastrocnemius muscle contracts concentrically, and the muscle arm of the tibialis anterior muscle is much smaller than that of the gastrocnemius muscle, so the discharge of the tibialis anterior muscle is more than that of the gastrocnemius muscle. The gluteus maximus and gluteus medius do eccentric contraction in the hip flexion movement in the bending part, and the muscle arm of the gluteus medius is smaller than that of the gluteus maximus, so the discharge of the gluteus medius is more than that of the gluteus maximus. Figure9 shows the angle of the body part of the backflip buckle leg link.Figure 9
Body part angle of backflip.It can be seen from the figure that the ankle joint angle of the support leg becomes smaller in the stable stage of the backflip than in the upright stage, because the bending of the support leg causes the body’s center of gravity to drop and move forward. At this time, the angle of the ankle joint decreases with the forward movement of the center of gravity, which increases the projected area of the center of gravity on the support surface, which is beneficial to control the stability of the center of gravity. Buckling balance requires the support leg and thigh to be kept as level as possible. According to the principle of parallel lines, the range of the knee angle is controlled between 60 and 70° to compensate for the increased moment arm due to hip flexion and squatting, so that the action can be balanced between the fulcrums. The trunk leans forward from the hip joint, so that the head is close to the vertical line of the center of gravity, which increases the balance and stability and makes the movement more graceful and stretched. The angle between the two legs is between 35 and 50°, which is to balance the component forces on the left and right sides of the action, so that the combined external force is zero, so as to achieve the purpose of landing smoothly. The swing speed and swing sequence of the non-supporting leg and the heterolateral arm have a great relationship with the stability of the movement. The swing of the opposite side arm should be prior to the swing of the non-supporting leg, which is conducive to improving the side swing speed, shortening the time to complete the action, and enhancing the stability of the balance while improving the quality of the action.


## 5. Conclusions

The results of image segmentation technology’s biomechanical analysis of all martial arts high-difficulty jumping movements show that, whether the whirlwind foot jumps with one foot or two, the two feet should form a certain angle with the approaching route, which is conducive to the rotation in the air and the sound of the strike. The movement should be performed while the center of gravity is rising. The faster the run-up speed, the better in the warm-up phase. When the right foot touches the ground during the forward stretch phase of take-off, the sole of the foot is buckled inward, and the right foot pushes and stretches in a timely manner, with almost no concessional work. The range of leg extension is greater, and the key technology is the “orange pendulum combination” of the two legs and upper limbs. When the martial arts routine athletes slam their legs together in the vacant stage, the body’s center of gravity rises, approaching the highest point of the body’s center of gravity. The key to achieving a stable stance on the ground is to stretch the limbs as soon as possible after completing the rotation in the powder, in order to increase the human body’s rotational inertia. When taking off, the backflip should increase the amplitude and speed of the upper body slanting downwards, and the possibility of adjusting the position of the left foot when the left foot is pushing the ground should be minimized. It is crucial to get the body and the running route to form a certain angle, and the backflip’s running braking technology and group body technology are crucial. In terms of image processing technology, although motion detection-related algorithms can extract the motion trajectory of the jumping action process, it is still very difficult to recognize the hands and feet of the system. It is necessary to consider the operation time of the system and ensure the accuracy of the detected target. In future research, you can try to add some advanced algorithms of image matching and perform template matching for hands and feet.



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*Source: 6341852-2022-06-08.xml*