# Detection Method of Limb Movement in Competitive Sports Training Based on Deep Learning

Authors: Chunlin Qin; Shenglu Huo
Journal: Journal of Mathematics
 (2022)

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## Abstract

Traditional methods have the problems of insufficient accuracy and slow speed in human posture detection. In order to solve the above problems, a limb movement detection method in competitive sports training based on deep learning is proposed. The force change parameters of sports limb movements in the process of sports are computed to achieve the detection of limb movements in competitive sports training, and the limb movement characteristics in competitive sports training are extracted using a deep learning algorithm. The experimental results show that the limb movement detection method based on deep learning in competitive sports training has significantly higher detection accuracy and faster speed.


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## Body

## 1. Introduction

With the deepening of the research on artificial intelligence, the application direction of artificial intelligence technology has become more and more extensive. The detection of human posture is an important application direction [1]. It has a wide range of applications and rich application value in the fields of behavior detection, video capture, and computer graphics. An enhanced depth learning approach is presented based on the depth learning method, with the goal of increasing detection speed, reducing the number of parameters, and shrinking the model size to make practical application easier [2, 3]. The main modifications of the original model are as follows: speed and detection accuracy. The weight of the trained model is trimmed to compress the model and improve the detection speed. In addition, this paper also designs and tests a set of sit-ups based on the proposed improved open model. The hardware structure is simple, the human posture detection speed is fast, and the detection accuracy is high and has strong practicability.


## 2. Detection of Deep Learning of Limb Movements in Competitive Sports Training

### 2.1. Recognition of Limb Movement Characteristics in Competitive Sports Training

Human motion recognition is mainly composed of data acquisition and data analysis. Data acquisition is composed of multiple sensor nodes and data receiving equipment, which is used to complete the acquisition of human motion signal data. Data analysis is mainly composed of various analysis algorithms integrated in PC. The deep learning method is essentially a dual parallel convolution network model [4]. It uses two convolution networks at the same time. One convolution network locates the position of key parts of the human body from the image, and the other convolution network is used to connect candidate key parts to form limbs [5]. Then, the results of the two convolution networks are summarized for pose assembly to complete the detection of human pose in the image. Obviously, the construction of this dual parallel convolution network will greatly consume computing resources [6]. Therefore, in the implementation, the deep learning method, first, uses a convolution network to preliminarily extract image features and then inputs the features into the dual parallel convolution network for subsequent processing, which is equivalent to merging the lower part of the dual parallel convolution network into a convolution network to save computing resources.Figure1 shows the detection process of the deep learning method, which is the visualization of the process. First, the VGG-19 network 9 is used to extract the bottom features of the input image, as shown in Figure 1, which is the output of some convolution layers [7]. The underlying characteristics are then fed into two concurrent convolution networks, one of which generates the confidence graph using the nonmaximum suppression procedure. Depth picture sequences have shown to be very valuable in the development of quick 3D human skeletal joint estimation. Many depth sensors with high sample rates and inexpensive prices have lately been launched, owing to the fast development of depth-sensing technology [8]. Table 1 gives some common depth sensor information. The depth image sequence with high resolution and high sampling rate provided by these sensors can provide accurate and sufficient information for 3D human skeleton joint estimation.Figure 1
Processing flow of the deep learning model.Table 1
Common depth sensor equipment.
Sensor nameDescribeSOD stereo cameraLightweight depth sensor based on passive stereoDepthSense 336Short-range depth sensor embedded with a color camera and microphoneKinect sensor S1Depth sensor, embedded with a color camera and infrared transmitterKinect sensor S2Embedded with a color camera and microphoneIntel RealSense 3D CameraEmbedded with a color camera and projectorAus Xtion PRO LiveInfrared sensor embedded with a color camera and microphoneI softMotion capture system based on KinectThe behavior and motion capture platform is composed of an attitude and heading reference module and a computer. It can capture the motion of the human trunk, upper and lower arms, and large and small legs [9]. When the platform starts working, the attitude and heading reference module uploads the attitude data captured by human behavior at the frequency of 30 Hz through the wireless network, receives the data through the computer, and receives the data sent by the attitude and heading reference module by address query. In order to reduce the volume and electromagnetic wave interference, it is necessary to enhance the independence of the platform. The extended Kalman filter is used to collect the data in real time, conduct data fusion after denoising, capture the attitude angle, angular velocity, and acceleration, and then send the data to the upper computer through the wireless network module. The data are processed by the upper computer program and transformed into the signal form [10]. The Kalman filter is used to collect the original data, and the relative attitude angle is obtained through the conversion relationship between Euler angle and quaternion. The acceleration is collected and calculated the absolute attitude angle by using the direction cosine conversion of the magnetic field between the geographic coordinate system and the Kalman filter coordinate system, as shown in Figure 2.Figure 2
Schematic diagram of body gravity direction.It can be seen from Figure2 that G represents the gravity direction, and the XYZ axis represents the coordinate system. When the attitude and heading reference module rotates around the x-axis and y-axis, the included angle between the two axes and the gravity direction g will produce a certain angle with the rotation. When the module rotates around the z-axis, the z-axis and the gravity direction G are collinear, and the included angle will not change, so it needs to be compensated by the magnetometer [11]. When using the magnetometer to calculate the z-axis attitude, the data are divergent and cannot be used for a long time. However, the accelerometer is vulnerable to vibration factors and has poor dynamic performance. Therefore, it is necessary to fuse the data generated by the accelerometer to obtain a stable and accurate attitude angle. The filter gain of the accelerometer is updated in real time according to the vibration intensity to ensure the dynamic response characteristics of the attitude and heading reference module and quickly eliminate the steady-state error [12]. The Euler angle is utilized to determine the tilt angle roll and pitch angle based on the three-axis acceleration processed by the Kalman filter, and the yaw angle is derived based on the tilt compensation magnetometer value. The acceleration value is read and treated it according to normal orthogonalization procedures. The Euler angle conversion matrix is used to determine the high-precision attitude angle based on the state vector [13]. Through the tilt angle, the action signal of human walking and running behavior can be captured, the pitching angle can capture the action signal of human static behavior, and the yaw angle can capture the action signal of human jumping and squatting behavior. According to the design requirements of the software function, combined with the actual chip function, the embedded software program is compiled. The software design includes the software design of data acquisition node and data receiving gateway. The working mode can be divided into working mode I (real-time USB data communication), working mode II (nonreal-time USB data communication), and working mode III (nonreal-time network port data communication). The program flow chart is shown in Figure 3.Figure 3
Collection processes of action characteristics of human motion nodes.After connecting the information collection device with the computer through the USB interface, open the corresponding serial port and set the baud rate to 115200. In order to solve the problem that the refresh rate of reading inertial sensor data does not match that of manikin animation, a thread for reading inertial sensor data and a thread for reading attitude data are created [14]. The thread of the reading inertial sensor reads the original data from the inertial sensor, which is composed of a three-axis accelerometer, three-axis magnetometer, and three-axis gyroscope, and then uses CRC16 to verify the data. After verification, the data fusion algorithm is used to fuse the data into attitude data and then read the attitude data. The thread reads the attitude data from the queue and uses the attitude data to drive the dimensional manikin.


### 2.2. Numerical Calculation of Force Changes of Limb Movements in Competitive Sports Training

The human skeleton joint data are obtained based on deep learning, and it is necessary to remove the original skeleton joint coordinate data of human motion from the Kinect V spatial coordinate systema′,b′,c′ which map to the human body spatial coordinate system ox,y,z. The human body coordinate system ox,y,z takes the center of gravity of the human body, that is, the spine base node as the origin o, and the right in front of the human body as the positive direction of the z-axis. Let P a0′, b0′, and c0′ be the 3D coordinate of the joint point under the equipment spatial coordinate system, ox,y,z be the coordinate of P corresponding to the human spatial coordinate system ox,y,z, then according to the translation and rotation transformation of the 3D graphics spatial coordinates.(1)ox,y,z=Va′,b′,c′1a00010bc0−10−a0′−b0′c0′1.Get(2)s=kcosaz0′−z′x′−x0′m=wtancy′−y0′z0′−z′n=psinbx0′−x′y′−y0′,where k, w, and p, respectively, correspond to the coordinate quantities of “spine base” node o in Kinect Gr′ spatial coordinate system. Lr′ represents the rotation angle of the human body relative to xoy plane, and a∈−70,+70 can be obtained by calibrating specific joint points. Since “hile” and “upright” are symmetrical relative to Yl′ and X1′ axis, these two points are selected as calibration points. Assuming that their coordinates in oxyz coordinate system are (x, y) and (x, y), respectively, then(3)α=arctanGr′−X1′Lr′−Yl′.The captured human behavior data may have outliers. The existence of outliers will affect the overall classification results, resulting in inaccurate classification results. Therefore, it is necessary to detect and eliminate outliers first [15]. Outlier detection approaches include statistics-based methods, clustering-based methods, and some unique methods. We employ interquartile distance to locate outliers in this article. Although the interquartile distance represents the degree of dispersion of each variable in statistical data, it is more trustworthy statistical data [16]. As indicated in the table, all values are sorted in the sample from small to big and then split the data into quartiles with three points, which are quartiles (Table 2).Table 2
Quartile.
QuartileAliasPosition in the sampleA Quartile (Q1)Small quartileNumber of 26%B Quartile (Q2)MedianNumber of 55%C Quartile (Q3)Big four digitsNumber of 80%The calculation method of the upper bound is shown in(4)U=aQ1−Q2−Q3k.The calculation method of the lower bound is shown in(5)D=C+kB−AWhen the value ofK in the formula is 1.5, it is the moderately abnormal data detected; when the value of K is 3, extreme abnormal data are detected. In this paper, moderate abnormal data are used to detect outliers; that is, the value of w is 1.5. Observing the captured human behavior data shows that the angle change of limbs changes obviously in different movements, and the angle change can be defined by the Euler angle. The key to the success of behavior recognition is to select good features [17]. The quality of feature selection greatly impacts the results of behavior recognition. Human behavior characteristics include the following: 1. average value; 2. maximum value; 3 minimum value; 4. standard deviation; 5. variance; 6. median value; 7. extreme difference; 8. slope absolute energy. When extracting features, the eigenvalues sampled in a dimension of the attitude Euler angle of an inertial sensor node at all times of an action are calculated, respectively.Average value: it reflects the overall situation of a group of data, as shown in(6)μ=∑i=1naiDn−w.The standard deviation describes the average of the distance that each data in the sample deviate from the sample average, as shown in(7)s=U1μn∑i=1na−12.After extracting behavior features, redundant features will increase computational complexity. Reducing features can reduce computational complexity and improve accuracy. However, feature reduction is not blind deletion of some features [18]. Blind deletion of features will lose a lot of information because when analyzing data samples, data analysis is often isolated rather than comprehensive [19]. Therefore, it is necessary to use the method of reasonable feature reduction to reduce the features and try not to lose the information of the original features to achieve the purpose of the comprehensive analysis of all features. The characteristic covariance matrix is found, as shown in (8)S=1sD−μ2U−μ+λ.Here,λ is the average of the eigenvectors. The eigenvectors are given accordingly. The first k principal components are selected, and the value of K can be determined by(9)s+pλ−1≥K−η.Here,η is the energy loss rate. The features after dimensionality reduction according to the projection matrix are calculated, as shown in(10)YMK=AK−h−S.When the loss rate is 0.08, 60 principal components can meet the requirements, so that the dimension of the eigenvector is reduced from 270 to 60.


### 2.3. Realization of Body Movement Detection in Competitive Sports Training

In the process of human motion capture, because the inertial sensor is self-sufficient and has no external reference point, it cannot obtain spatial displacement information, so it is necessary to use positioning technology to obtain displacement information in the process of human motion capture [20]. Wide-area positioning and short-range positioning are two types of wireless positioning technologies. Wide-area positioning is one of several types of positioning technology, which also includes satellite and mobile positioning. WLAN, RFID, UWB, Bluetooth, and ultrasonic are all examples of short-range location technology. The accuracy and positioning scale of various positioning modes are shown in Table 3.Table 3
Accuracy and scale of various positioning modes.
Positioning modePositioning accuracy (m)Positioning scaleGPSAbout 11 metersOutdoor wide areaMobile positioningAbout 11 metersOutdoor wide areaInfrared, ultrasonic1–11 mOutdoor local areaWLAN, Bluetooth1–11 mIndoorUltra wideband0.1–1 mIndoorThere are two methods for attitude calculation. The first is to integrate the gyroscope into the inertial sensor. Although the gyroscope has a good dynamic response, the error will increase with the increase of time; the other method is to calculate the attitude through magnetometer and acceleration. Although the dynamic response is poor, it will not produce the cumulative error. Therefore, gyroscope, magnetometer, and accelerometer have complementary characteristics in the frequency domain [21]. The attitude calculation steps of the inertial sensor are shown in Figure 4 In the process of inertial sensor attitude data fusion, first, the quaternion is calculated according to the initial state of the inertial sensor, and then the gravity vector and magnetic line of force are inversely deduced to obtain the data of accelerometer and magnetometer, and normalized. The matrix is multiplied and then summed, then the proportional-integral controller is used to adjust the data, and finally, the quaternion is updated.Figure 4
Attitude calculation flow of the motion inertial sensor.Human motion detection requires high accuracy, and most cases are carried out indoors. According to Table2, UWB positioning technology has better performance and accuracy and is more suitable for indoor positioning than other positioning technologies [22]. In the process of motion change, using the motion estimation method based on image gray change to extract local feature points involves camera motion and produces large errors. Therefore, to accurately obtain the target motion estimation, it is necessary to eliminate the impact of camera motion [23]. The optical flow field obtained from the gray image is the sum of the local motion of the actual human body and the motion of the camera. The calculation formula is as follows:(11)δ=Sr⟶z+Sc⟶−YMK.Here,S represents the optical flow field; Sr represents the actual local motion of the human body; Sc indicates camera motion. Decompose the above three vectors into x and Y axes in turn to obtain(12)Sx=Srx+Scx,Sy=Sry+Scy.Of all moving points in a local area in the same imageSrx and Scx, the components in different directions of camera movement are consistent. As long as the component is removed, the actual motion component can be obtained, and then the background motion can be found and the scene motion information can be calculated [24]. The range of motion of each human joint is limited, but the human joint tree composed of multiple joints can make unlimited actions, as shown in Table 4. By limiting the range of motion of each joint in the human joint model, the detected human action can be more in line with the action of normal people.Table 4
Degrees of freedom and range of motion of some joints of the human skeleton model.
Joint nameJointDegree of freedomActivity range limitNeckChest30–90°Left shoulderChest20–180°Right shoulderChest30–180°Left elbowLeft arm30–150°Right elbowRight arm20–150°Left sideWaist30–130°Right sideWaist20–130°Left kneeThigh30–145°Right kneeThigh20–145°Left ankleA lower leg30–90°Right ankleA lower leg30–90°When detecting human motion, first the inertial sensor node to the specified joints of the body is bound, such as big arm, small arm, thigh, and lower leg [25]. The specific joints on which inertial sensor nodes are bound can be determined according to the actions of which joints need to be collected. Increasing the number of inertial sensor nodes can improve the accuracy of action detection and make the detected human actions more accurate.


## 2.1. Recognition of Limb Movement Characteristics in Competitive Sports Training

Human motion recognition is mainly composed of data acquisition and data analysis. Data acquisition is composed of multiple sensor nodes and data receiving equipment, which is used to complete the acquisition of human motion signal data. Data analysis is mainly composed of various analysis algorithms integrated in PC. The deep learning method is essentially a dual parallel convolution network model [4]. It uses two convolution networks at the same time. One convolution network locates the position of key parts of the human body from the image, and the other convolution network is used to connect candidate key parts to form limbs [5]. Then, the results of the two convolution networks are summarized for pose assembly to complete the detection of human pose in the image. Obviously, the construction of this dual parallel convolution network will greatly consume computing resources [6]. Therefore, in the implementation, the deep learning method, first, uses a convolution network to preliminarily extract image features and then inputs the features into the dual parallel convolution network for subsequent processing, which is equivalent to merging the lower part of the dual parallel convolution network into a convolution network to save computing resources.Figure1 shows the detection process of the deep learning method, which is the visualization of the process. First, the VGG-19 network 9 is used to extract the bottom features of the input image, as shown in Figure 1, which is the output of some convolution layers [7]. The underlying characteristics are then fed into two concurrent convolution networks, one of which generates the confidence graph using the nonmaximum suppression procedure. Depth picture sequences have shown to be very valuable in the development of quick 3D human skeletal joint estimation. Many depth sensors with high sample rates and inexpensive prices have lately been launched, owing to the fast development of depth-sensing technology [8]. Table 1 gives some common depth sensor information. The depth image sequence with high resolution and high sampling rate provided by these sensors can provide accurate and sufficient information for 3D human skeleton joint estimation.Figure 1
Processing flow of the deep learning model.Table 1
Common depth sensor equipment.
Sensor nameDescribeSOD stereo cameraLightweight depth sensor based on passive stereoDepthSense 336Short-range depth sensor embedded with a color camera and microphoneKinect sensor S1Depth sensor, embedded with a color camera and infrared transmitterKinect sensor S2Embedded with a color camera and microphoneIntel RealSense 3D CameraEmbedded with a color camera and projectorAus Xtion PRO LiveInfrared sensor embedded with a color camera and microphoneI softMotion capture system based on KinectThe behavior and motion capture platform is composed of an attitude and heading reference module and a computer. It can capture the motion of the human trunk, upper and lower arms, and large and small legs [9]. When the platform starts working, the attitude and heading reference module uploads the attitude data captured by human behavior at the frequency of 30 Hz through the wireless network, receives the data through the computer, and receives the data sent by the attitude and heading reference module by address query. In order to reduce the volume and electromagnetic wave interference, it is necessary to enhance the independence of the platform. The extended Kalman filter is used to collect the data in real time, conduct data fusion after denoising, capture the attitude angle, angular velocity, and acceleration, and then send the data to the upper computer through the wireless network module. The data are processed by the upper computer program and transformed into the signal form [10]. The Kalman filter is used to collect the original data, and the relative attitude angle is obtained through the conversion relationship between Euler angle and quaternion. The acceleration is collected and calculated the absolute attitude angle by using the direction cosine conversion of the magnetic field between the geographic coordinate system and the Kalman filter coordinate system, as shown in Figure 2.Figure 2
Schematic diagram of body gravity direction.It can be seen from Figure2 that G represents the gravity direction, and the XYZ axis represents the coordinate system. When the attitude and heading reference module rotates around the x-axis and y-axis, the included angle between the two axes and the gravity direction g will produce a certain angle with the rotation. When the module rotates around the z-axis, the z-axis and the gravity direction G are collinear, and the included angle will not change, so it needs to be compensated by the magnetometer [11]. When using the magnetometer to calculate the z-axis attitude, the data are divergent and cannot be used for a long time. However, the accelerometer is vulnerable to vibration factors and has poor dynamic performance. Therefore, it is necessary to fuse the data generated by the accelerometer to obtain a stable and accurate attitude angle. The filter gain of the accelerometer is updated in real time according to the vibration intensity to ensure the dynamic response characteristics of the attitude and heading reference module and quickly eliminate the steady-state error [12]. The Euler angle is utilized to determine the tilt angle roll and pitch angle based on the three-axis acceleration processed by the Kalman filter, and the yaw angle is derived based on the tilt compensation magnetometer value. The acceleration value is read and treated it according to normal orthogonalization procedures. The Euler angle conversion matrix is used to determine the high-precision attitude angle based on the state vector [13]. Through the tilt angle, the action signal of human walking and running behavior can be captured, the pitching angle can capture the action signal of human static behavior, and the yaw angle can capture the action signal of human jumping and squatting behavior. According to the design requirements of the software function, combined with the actual chip function, the embedded software program is compiled. The software design includes the software design of data acquisition node and data receiving gateway. The working mode can be divided into working mode I (real-time USB data communication), working mode II (nonreal-time USB data communication), and working mode III (nonreal-time network port data communication). The program flow chart is shown in Figure 3.Figure 3
Collection processes of action characteristics of human motion nodes.After connecting the information collection device with the computer through the USB interface, open the corresponding serial port and set the baud rate to 115200. In order to solve the problem that the refresh rate of reading inertial sensor data does not match that of manikin animation, a thread for reading inertial sensor data and a thread for reading attitude data are created [14]. The thread of the reading inertial sensor reads the original data from the inertial sensor, which is composed of a three-axis accelerometer, three-axis magnetometer, and three-axis gyroscope, and then uses CRC16 to verify the data. After verification, the data fusion algorithm is used to fuse the data into attitude data and then read the attitude data. The thread reads the attitude data from the queue and uses the attitude data to drive the dimensional manikin.


## 2.2. Numerical Calculation of Force Changes of Limb Movements in Competitive Sports Training

The human skeleton joint data are obtained based on deep learning, and it is necessary to remove the original skeleton joint coordinate data of human motion from the Kinect V spatial coordinate systema′,b′,c′ which map to the human body spatial coordinate system ox,y,z. The human body coordinate system ox,y,z takes the center of gravity of the human body, that is, the spine base node as the origin o, and the right in front of the human body as the positive direction of the z-axis. Let P a0′, b0′, and c0′ be the 3D coordinate of the joint point under the equipment spatial coordinate system, ox,y,z be the coordinate of P corresponding to the human spatial coordinate system ox,y,z, then according to the translation and rotation transformation of the 3D graphics spatial coordinates.(1)ox,y,z=Va′,b′,c′1a00010bc0−10−a0′−b0′c0′1.Get(2)s=kcosaz0′−z′x′−x0′m=wtancy′−y0′z0′−z′n=psinbx0′−x′y′−y0′,where k, w, and p, respectively, correspond to the coordinate quantities of “spine base” node o in Kinect Gr′ spatial coordinate system. Lr′ represents the rotation angle of the human body relative to xoy plane, and a∈−70,+70 can be obtained by calibrating specific joint points. Since “hile” and “upright” are symmetrical relative to Yl′ and X1′ axis, these two points are selected as calibration points. Assuming that their coordinates in oxyz coordinate system are (x, y) and (x, y), respectively, then(3)α=arctanGr′−X1′Lr′−Yl′.The captured human behavior data may have outliers. The existence of outliers will affect the overall classification results, resulting in inaccurate classification results. Therefore, it is necessary to detect and eliminate outliers first [15]. Outlier detection approaches include statistics-based methods, clustering-based methods, and some unique methods. We employ interquartile distance to locate outliers in this article. Although the interquartile distance represents the degree of dispersion of each variable in statistical data, it is more trustworthy statistical data [16]. As indicated in the table, all values are sorted in the sample from small to big and then split the data into quartiles with three points, which are quartiles (Table 2).Table 2
Quartile.
QuartileAliasPosition in the sampleA Quartile (Q1)Small quartileNumber of 26%B Quartile (Q2)MedianNumber of 55%C Quartile (Q3)Big four digitsNumber of 80%The calculation method of the upper bound is shown in(4)U=aQ1−Q2−Q3k.The calculation method of the lower bound is shown in(5)D=C+kB−AWhen the value ofK in the formula is 1.5, it is the moderately abnormal data detected; when the value of K is 3, extreme abnormal data are detected. In this paper, moderate abnormal data are used to detect outliers; that is, the value of w is 1.5. Observing the captured human behavior data shows that the angle change of limbs changes obviously in different movements, and the angle change can be defined by the Euler angle. The key to the success of behavior recognition is to select good features [17]. The quality of feature selection greatly impacts the results of behavior recognition. Human behavior characteristics include the following: 1. average value; 2. maximum value; 3 minimum value; 4. standard deviation; 5. variance; 6. median value; 7. extreme difference; 8. slope absolute energy. When extracting features, the eigenvalues sampled in a dimension of the attitude Euler angle of an inertial sensor node at all times of an action are calculated, respectively.Average value: it reflects the overall situation of a group of data, as shown in(6)μ=∑i=1naiDn−w.The standard deviation describes the average of the distance that each data in the sample deviate from the sample average, as shown in(7)s=U1μn∑i=1na−12.After extracting behavior features, redundant features will increase computational complexity. Reducing features can reduce computational complexity and improve accuracy. However, feature reduction is not blind deletion of some features [18]. Blind deletion of features will lose a lot of information because when analyzing data samples, data analysis is often isolated rather than comprehensive [19]. Therefore, it is necessary to use the method of reasonable feature reduction to reduce the features and try not to lose the information of the original features to achieve the purpose of the comprehensive analysis of all features. The characteristic covariance matrix is found, as shown in (8)S=1sD−μ2U−μ+λ.Here,λ is the average of the eigenvectors. The eigenvectors are given accordingly. The first k principal components are selected, and the value of K can be determined by(9)s+pλ−1≥K−η.Here,η is the energy loss rate. The features after dimensionality reduction according to the projection matrix are calculated, as shown in(10)YMK=AK−h−S.When the loss rate is 0.08, 60 principal components can meet the requirements, so that the dimension of the eigenvector is reduced from 270 to 60.


## 2.3. Realization of Body Movement Detection in Competitive Sports Training

In the process of human motion capture, because the inertial sensor is self-sufficient and has no external reference point, it cannot obtain spatial displacement information, so it is necessary to use positioning technology to obtain displacement information in the process of human motion capture [20]. Wide-area positioning and short-range positioning are two types of wireless positioning technologies. Wide-area positioning is one of several types of positioning technology, which also includes satellite and mobile positioning. WLAN, RFID, UWB, Bluetooth, and ultrasonic are all examples of short-range location technology. The accuracy and positioning scale of various positioning modes are shown in Table 3.Table 3
Accuracy and scale of various positioning modes.
Positioning modePositioning accuracy (m)Positioning scaleGPSAbout 11 metersOutdoor wide areaMobile positioningAbout 11 metersOutdoor wide areaInfrared, ultrasonic1–11 mOutdoor local areaWLAN, Bluetooth1–11 mIndoorUltra wideband0.1–1 mIndoorThere are two methods for attitude calculation. The first is to integrate the gyroscope into the inertial sensor. Although the gyroscope has a good dynamic response, the error will increase with the increase of time; the other method is to calculate the attitude through magnetometer and acceleration. Although the dynamic response is poor, it will not produce the cumulative error. Therefore, gyroscope, magnetometer, and accelerometer have complementary characteristics in the frequency domain [21]. The attitude calculation steps of the inertial sensor are shown in Figure 4 In the process of inertial sensor attitude data fusion, first, the quaternion is calculated according to the initial state of the inertial sensor, and then the gravity vector and magnetic line of force are inversely deduced to obtain the data of accelerometer and magnetometer, and normalized. The matrix is multiplied and then summed, then the proportional-integral controller is used to adjust the data, and finally, the quaternion is updated.Figure 4
Attitude calculation flow of the motion inertial sensor.Human motion detection requires high accuracy, and most cases are carried out indoors. According to Table2, UWB positioning technology has better performance and accuracy and is more suitable for indoor positioning than other positioning technologies [22]. In the process of motion change, using the motion estimation method based on image gray change to extract local feature points involves camera motion and produces large errors. Therefore, to accurately obtain the target motion estimation, it is necessary to eliminate the impact of camera motion [23]. The optical flow field obtained from the gray image is the sum of the local motion of the actual human body and the motion of the camera. The calculation formula is as follows:(11)δ=Sr⟶z+Sc⟶−YMK.Here,S represents the optical flow field; Sr represents the actual local motion of the human body; Sc indicates camera motion. Decompose the above three vectors into x and Y axes in turn to obtain(12)Sx=Srx+Scx,Sy=Sry+Scy.Of all moving points in a local area in the same imageSrx and Scx, the components in different directions of camera movement are consistent. As long as the component is removed, the actual motion component can be obtained, and then the background motion can be found and the scene motion information can be calculated [24]. The range of motion of each human joint is limited, but the human joint tree composed of multiple joints can make unlimited actions, as shown in Table 4. By limiting the range of motion of each joint in the human joint model, the detected human action can be more in line with the action of normal people.Table 4
Degrees of freedom and range of motion of some joints of the human skeleton model.
Joint nameJointDegree of freedomActivity range limitNeckChest30–90°Left shoulderChest20–180°Right shoulderChest30–180°Left elbowLeft arm30–150°Right elbowRight arm20–150°Left sideWaist30–130°Right sideWaist20–130°Left kneeThigh30–145°Right kneeThigh20–145°Left ankleA lower leg30–90°Right ankleA lower leg30–90°When detecting human motion, first the inertial sensor node to the specified joints of the body is bound, such as big arm, small arm, thigh, and lower leg [25]. The specific joints on which inertial sensor nodes are bound can be determined according to the actions of which joints need to be collected. Increasing the number of inertial sensor nodes can improve the accuracy of action detection and make the detected human actions more accurate.


## 3. Analysis of Experimental Results

On the windows 8x64 operation of Intel Core (TM) i7-4790k and 16 GB memory, Kinect for windows sdk v2.0 and visual studio 2013 are used as development tools to build an analysis platform with WPF, on which the key-frame extraction function of the action sequence is realized. Because there is currently no single standard for evaluating key-frame extraction outcomes, eye observation and manual comparison analysis are the primary methods of assessment. For action key-frame extraction, three exemplary action sequences are chosen from the provided motion dataset. Table5 shows the particular details of the action sequence. The complexity of the movement is mainly related to the body parts involved in the movement; that is, it is related to the action types in Table 5.Table 5
Experimental dataset.
Action semanticsAction representationFrame rate (frames/sec)Total framesAction typeAction complexityLurkO35419The legsSimpleStop the systemP35425Upper limbSimpleStill thingsQ35345Whole bodyComplexOn the windows 8 × 64 operation of Intel Core (TM) i7-4790k and 16 GB memory, the human motion recognition experiment based on deep learning is completed based on Visual Studio 2013 c + + language. The experiment uses 10 movements of motion datasets, with a total of 3750 samples for training and recognition. Table6 shows the average length and action type of each action. For each action sample, 90% of the action samples are selected for training each time, and the remaining 10% of the samples are used as the test set for 10 fold 10 times crossvalidation.Table 6
Action set information.
Action labelABCDEFGHIJTypeUpper limbThe legsUpper limbThe legsThe legsThe legsThe legsComprehensiveComprehensiveComprehensiveAverage length (frame)160352261261275231265398332335In this paper, the human motion recognition method based on deep learning has achieved a 100% recognition rate for most actions in the motion dataset, 97% minimum recognition rate, and 99.5% average recognition rate, so it has a high recognition rate. Simultaneously, Table7 shows that this approach has a high recognition rate for three distinct kinds of activities and no evident recognition trend, indicating that it is resilient. However, because of the high complexity of the deep learning algorithm, the average recognition time of 10 actions by the human action recognition technique based on deep learning is 148 ms, and the action recognition speed is poor, as shown in the table. This paper replaces the feature extraction model VGG-19 in the initial stage of an open pose with the lightweight network model MobileNetV2, introduces the weight and penalty term into the final loss function, and uses MobileNetV2 to process images on the local experimental platform to improve the accuracy and efficiency of attitude estimation. Although the model does not achieve the expected improvement in accuracy, the total parameters of the model are reduced. Table 7 shows the results of the MPI dataset. According to the results, it can be seen that the improved model structure can meet the experimental requirements of this paper.Table 7
Experimental results of the competitive sports dataset.
MethodHeadShoulderElbowWristHipsKneeAnkleMapDeep cut79.573.561.350.256.351.945.658.6Open pose92.386.578.866.576.568.862.575.5This paper’s method94.588.575.573.379.571.260.977.8Learning rate is a very important parameter in the decline of the deep learning gradient. The size of the batch determines the direction of gradient descent and the rate of convergence. If the batch size is set too much, the convergence speed is large, but there may be a local optimal solution if it is set too much. According to the above discrete points of human behavior data of static, walking, running, jumping, squatting standard deviation, skewness, peak value, and correlation coefficient collected fromX, Y, and Z axes, in the process of extended Kalman filter (EKF) simulation, 100 groups of data are intercepted from each data for iterative optimization and correction, and the traditional identification method is compared with this method. The results are shown in Figure 5.Figure 5
Value analyses of human motion behavior recognition data components by two methods.It can be seen from Figure5 that the activity component interval taken by the method described in this paper is basically consistent with the interval of behavior standard value. Compared with the traditional methods, the training and detection method proposed in this paper has higher accuracy in the process of practical application and fully meets the research requirements.


## 4. Conclusion

Human motion analysis has always been an important research issue in the field of human-computer interaction. It is also widely used in many fields, such as intelligent monitoring, motion analysis, entertainment games, and computer animation. After studying and analyzing the research status and relevant theoretical basis of human motion analysis at home and abroad, this paper carries out motion analysis research based on human skeleton sequence. Due to the high-frequency sampling of sensor equipment, there are a large number of redundant frames in the motion pose sequence. Therefore, the work of this paper mainly focuses on the motion pose feature description, motion key pose frame extraction, motion classification, and recognition and application, so as to achieve the research goal of limb motion detection.



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