Behavioral Analysis of Mowing Workers Based on Hilbert–Huang Transform: An Auxiliary Movement Analysis of Manual Mowing on the Slopes of Terraced Rice Fields

: In the mountainous areas of Japan, the weeds on the slopes of terraced rice paddies still need to be cut by the elderly manually. Therefore, more attention should be given to maintain proper postures while performing mowing actions (especially the pre-cutting actions) to reduce the risk of accidents. Given that complex mowing actions can be decomposed into different sub-actions, we proposed a joint angular calculation-based body movement analysis model based on the Hilbert–Huang transform to analyze the pre-cutting actions. We found that the two most important sub-actions were fast pre-cutting and slow pre-cutting. Based on ﬁeld experiments, we analyzed the pre-cutting actions of workers with different experience levels and identiﬁed the factors that affected their falling risk (stability). The results showed differences and similarities in the actions’ frequency and amplitude in the sub-actions of workers with different mowing experience, conﬁrmed the inﬂuence of body characteristics (body height, etc.) on body stability, and showed that workers should pay attention to their age and ankle part while mowing. The analysis results have identiﬁed factors for the mowing workers’ training and the development of equipment for use in complicated geographical conditions.


Introduction
A ridge is generally placed on the boundary of a paddy field to prevent irrigation water from leaking out.Some weeds can grow over 2 m tall, and if vegetation management is neglected, ventilation and lighting in paddy fields will be hindered [1].In terraced rice fields, the range of ridges and slopes is wide.In Japan, paddy fields with a slope of 1/20 or more and fields, grasslands, and pastures with a slope of 15 degrees or more are defined as slope farmland [2].Because of the difficult terrain, weeds on the ridges and slopes of terraced rice paddies still need to be cut manually using a handheld petrol mower [2].The number of people engaged in agriculture has decreased from 1.755 million in 2005 to 1.226 million in 2022, during which the aging rate (the proportion of the population aged 65 years and over) has risen from 65% to 70% [3].In particular, the hilly and mountainous areas are facing remarkable depopulation and aging; people over the age of 65 are responsible for many mowing works in these areas.The farmlands in mountainous areas in Japan, especially terraced rice fields, have steep slopes and many obstacles, such as pebbles.The Japanese government also has provided financial support for mowing the ridges and slopes of the farmland in hilly and mountainous areas since 2000.However, due to the aging of the farming population, the farmers over the age of 65 in those areas have to mow the weeds on ridges and slopes by themselves.So, more research is needed to ensure the safety of the mowing works [4].
As a basic movement that requires the cooperation of whole-body muscles, the mowing action requires a combination of complex movements within a short time to complete the adjustment of body balance to prevent falls while increasing efficiency.According to a related survey [5], about 30% of accidents, including falls, slips, etc., are caused by workers' unstable posture.Improper postures can place a heavy burden on a worker's body, thus increasing the risk of accidents [6].Therefore, an in-depth analysis of mowing workers' behaviors on slopes is necessary to determine their movement characteristics.
With the development of IoT-based technologies, human body movements can already be acquired well dynamically.One common method is the use of motion capture devices, such as the Xsens MVN, to record whole-body movements via accelerometer sensors [7].However, due to the complexity of human actions, actions that appear to have the same purpose can be decomposed into different sub-actions, and these complex sub-actions may affect our analysis of the whole action.
The decomposition of motion features with varying action goals in the frequency domain has been extensively explored with human behavioral feature extraction techniques [8].However, the traditional methods of Fourier Transform (FT) [9,10] and wavelet transform [11] have been found to be challenging to interpret and not ideal for the analysis of human actions.A more appropriate method for the decomposition of realistic actions is the Hilbert-Huang Transform (HHT), a non-linear and empirical technique [12][13][14].
In our previous studies [15,16], we analyzed the mowing action on slopes by using a motion capture technique with key joint angle analysis.However, we focused on the "kinetic energy release phase" (e.g., main "cutting" action) of the mowing workers and did not pay much attention to the "kinetic energy storage phase," the preparatory action that directly affects the quality of all "main" mowing actions.
Unlike the "main" mowing action, which swings by gravity and has only one purpose (i.e., "cutting"), the "cutting preparatory (pre-cutting) action" needs to balance at least three complex purposes of "maintaining body stability," "reserving kinetic energy," and "lifting the mower (trimmer machine)".Direct analysis of the limb joint movements during worker movements alone may not be comprehensive enough to avoid accidents.Therefore, we need to analyze the sub-actions that have different purposes in this phase.
Based on the definition of "pre-cutting action," this paper proposes a framework for analyzing mowing behaviors based on HHT action decomposition and realizes an empirical decomposition of pre-cutting action based on slope mowing data captured by a high-precision multi-sensor motion capture instrument.Then, based on the combination of relevant videos and interviews, this paper compares the differences between workers' similar sub-actions and investigates the effect of the change of the workers' joint angles on their stability in different sub-actions.

Human Movement Analysis
In order to solve the problems caused by human behaviors, such as driving accidents or operational accidents, many studies have used motion capture devices to obtain and analyze human movements.For example, in our previous work, we collected and analyzed pre-braking action data from drivers with different driving experience by using a motion capture device to identify the influencing factors affecting driver behavior through a comparative analysis [7].Moreover, there are two main branches in existing motion capture, camera and image analysis-based motion capture and wearable accelerometerbased motion capture, which have their own advantages and disadvantages.
There are many optional tools for motion capture based on camera and image processing technology, such as the recently popular MediaPipe from Google [17].For example, Zhang et al. used YOLOv5 in MediaPipe and proposed a method for detecting deep squatting movements by subjects' trunk angle, hip angle, and knee angle [18].Similarly, according to the use of a camera and MediaPipe to track users' hand, head, and body movements, Ma et al. proposed a martial art posture recognition system [19].
On the other hand, many studies have used more accurate wearable accelerometers to capture human movements.Although not as convenient as camera-based motion capture, the higher accuracy gives researchers a wider scope for research.For example, Cockcroft et al. analyzed 10 bike riders' body movements via the data collected from the motion capture device called IMS.They validated the possibility of obtaining accurate outdoor exercise data from bicycles [20].Moreover, Wouda et al. experimented with running movements by comparing the differences between three different motion capture devices.The devices they used are the Xsens MVN (which is also used in this research), Plug-In Gait, and OpenSim Gait2392 [21].

Hilbert-Huang Transform
Motion capture data are difficult to analyze due to their non-stationary and non-linear properties.These motion capture data are usually collected as a multivariate signal from the complicated human body structure by putting sensors on important joints of the body in the time series.Meanwhile, HHT has shown higher performance in analyzing non-stationary and non-linear signals than FT and wavelet transform.Based on the advantage of HHT, Dong et al. [12] presented a non-linear motion analysis framework using HHT, enabling researchers to non-linearly investigate human movements in the frequency domain.The previous study showed that human movements could be decomposed into a number of distinct motion primitives, ranging from high-frequency to low-frequency components.Moreover, this method also showed that HHT could help to generate training data in deep learning-based motion methods, such as humanoid robot motion design [13,14].
In total, as shown in these previous studies, human movements could be investigated in the frequency domain after decomposing motion data into motion primitives.However, in the previous research, the motion primitives were not discussed based on a behavioral view.Therefore, in this paper, to analyze the mowing motion of workers on slopes, we perform a more in-depth motion analysis of the auxiliary movements using HHT and discuss the results based on statistical methodologies.

Definition: Mowing Motion on Slopes and Pre-Cutting Actions
To avoid confusion, the "pre-cutting" behaviors in this research is defined as the actions exhibited by a mowing worker's body before he/she releases kinetic energy to "cut the grass" by a mower (trimmer machine) during a whole mowing action.
As shown in Figure 1, pictures (1) to ( 7) are the key point slices of successive mowing actions on the same timeline.The pictures of ( 1) and ( 7) are almost identical, so (1) through (7) can be considered as a cycle of standard mowing movements.Therefore, based on the purpose of action, the entire mowing actions of the mowing worker's body can basically be divided into two stages: (1-4) lifting the mower to save kinetic energy and (5-7) swinging the mower to release kinetic energy.In contrast to our previous focus [15,16], in this study, we will focus on the actions of the "pre-cutting" part for analysis.

Device: Body Motion Capture MVN
As shown in Figure 2, in order to obtain more precise data on the physical movements of the mowing work, a wearable device, Xsens MVN, is used that can collect body joints' 3D coordinates with high precision (60 Hz).Unlike traditional camera-based indoor devices, this device can provide extended outdoor experimental support.

Device: Body Motion Capture MVN
As shown in Figure 2, in order to obtain more precise data on the physical movements of the mowing work, a wearable device, Xsens MVN, is used that can collect body joints' 3D coordinates with high precision (60 Hz).Unlike traditional camera-based indoor devices, this device can provide extended outdoor experimental support.

Device: Body Motion Capture MVN
As shown in Figure 2, in order to obtain more precise data on the physical movements of the mowing work, a wearable device, Xsens MVN, is used that can collect body joints' 3D coordinates with high precision (60 Hz).Unlike traditional camera-based indoor devices, this device can provide extended outdoor experimental support.A total of 23 joints can be collected by this device, MVN, including the spinal segments T12, T8, and tiptoe, which may not be considered in similar motion capture devices.

Method: Hilbert-Huang Transform
This section describes the HHT adopted in our proposed analysis framework.Although the mowing motion is more complex than normal human motion, most research has been limited to analysis in the time series.In contrast, a frequency-domain analysis method that accounts for periodicity is desirable because of the overlapping mowing motions.This study proposes a frequency-domain analysis method for the mowing motion A total of 23 joints can be collected by this device, MVN, including the spinal segments T12, T8, and tiptoe, which may not be considered in similar motion capture devices.

Method: Hilbert-Huang Transform
This section describes the HHT adopted in our proposed analysis framework.Although the mowing motion is more complex than normal human motion, most research has been limited to analysis in the time series.In contrast, a frequency-domain analysis method that accounts for periodicity is desirable because of the overlapping mowing motions.This study proposes a frequency-domain analysis method for the mowing motion using HHT since this method is suitable for human movements.Based on the analysis results, we discuss the relationship between the mowing motion and the decomposed motion primitives.

Hilbert-Huang Transform
The Hilbert-Huang Transform (HHT) involves two key steps to analyze signals [22].First, the original signal is subjected to Empirical Mode Decomposition (EMD) to decompose the signal into a few intrinsic mode functions (IMFs).Second, the Hilbert Transform (HT) is applied to each IMF to yield the imaginary part of the IMF.That is, HT converts a non-linear monochromatic wave signal A(t) cos(ω(t)t) into its imaginary part.This allows us to determine the instantaneous amplitude A(t) and frequency ω(t) of the signal at each point in time.The key benefit of HHT is that it provides the instantaneous frequency of the monochromatic wave at each moment.Furthermore, numerous research efforts have been made to expand EMD [23][24][25].In addition, EMD has recently been reported to be very useful for creating motion data for deep learning [12,13].
As evident from the property of EMD, HHT distinguishes itself from FT. FT decomposes signals with linear oscillating components, while HHT decomposes them into a limited number of non-linear signals and underlying trends.HHT is deemed advantageous for analyzing non-linear and non-stationary mowing motions captured by multi-channel data.
Based on the above, this study focuses on the multi-channel data of motion-captured mowing motion.We apply HHT to this data and decompose the mowing motion into IMFs, which are the decomposed motion primitives during the mowing motion.

Empirical Mode Decomposition
HHT first adopts EMD that decomposes x(t) into a finite number of IMFs ∑ n k=1 IMF Re k (t) with one residual r(t), as shown in (1) [22].Next, the decomposed IMF Re k is used as the real part, and the corresponding imaginary part IMF Im k is obtained by HT to calculate the frequency ω(t) and amplitude A(t) of x(t) [26].
The following criteria are the definition of IMFs: • The difference between the local extrema and zero crossings should not exceed one.

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The mean value of the upper and lower envelopes derived from the local extrema should equal zero.
The EMD procedure decomposes x(t) heuristically, as shown in the steps below.
Initialize IMF old (t) = r(t) and retrieve IMF Re k .(a) The envelope u(t) connecting the maxima of IMF old (t) and the envelope l(t) connecting the minima are obtained by cubic spline interpolation, and the average of u(t) and l(t) is subtracted from IMF old (t).
Repeat steps 1 and 2 until all IMFs are extracted.
As we can see from the EMD algorithm, while FT and the wavelet transform assume a basis, such as a sin function, which transforms the signal into the frequency domain, EMD decomposes the signal sequentially using the above procedure, without assuming a basis, and terminates the loop at a threshold value.As a result, the principal components are extracted as oscillations, making them suitable for analyzing and editing real-world signal waveforms with complex noise.

Multivariate Empirical Mode Decomposition
EMD itself is an empirical decomposition and does not always accurately decompose signals into monochromatic waves.It is known that decomposition accuracy can be empirically improved by decomposing as multivariate data.For example, multivariate EMD (MEMD) extended to multiple channels has been proposed [25].This study employs MEMD because human motion is sampled from multiple sensors simultaneously.
Next, we process the decomposed IMFs of mowing motions according to the previous study [27].For all decomposed IMFs, the imaginary part IMF Im k is obtained from IMF Re k by HT.The frequency ω(t) and amplitude A(t) are obtained.Weighted average frequencies and amplitudes of the decomposed IMFs can be used to analyze the biomechanical properties that is the speeds and movement ranges of each decomposed motion primitive.The motion analysis method using MEMD shows excellent performance in mowing motion analysis.Therefore, this study decomposes the mowing motion data recorded by multiple sensors into multiple channels using MEMD and analyzes them statistically for the joints that are considered major from the biomechanical view.

Joint Structure
Node graph architecture is widely adopted in different areas, such as social media network research [28,29].On the other hand, motion data are captured using multiple sensors.Therefore, the data are usually mapped into a skeleton to calculate the joint rotation angle.Because of its multiple joints, the human body has also been considered as multiple nodes connected to each other [30].Thus, in this research, we also calculate the body joint angles by considering the leg as a connected node structure.
As Figure 3 shows, the skeletal structure used in this study is rooted at the waist, and movement is described by 23 joints throughout the entire body.Each joint moves with three degrees of freedom: θ x , θ y , and θ z .During the mowing motion, each motion primitive is a simple motion, and the body joints involved in this motion move at an arbitrary angle.Thus, if one motion primitive is to be captured, it is sufficient to take the MEMD of a specific angle and look at the instantaneous frequency.Therefore, because different joint angles are used in each swing, it is necessary to decompose the entire movement into three degrees of freedom, θ x , θ y , and θ z , and calculate the Euclidean metric of the amplitude of each angle.

Mowing Motion Decomposition
MEMD is used to extract motion primitives.Motion primitives are the most basic motion units in motion analysis and automatic generation.Many studies based on motion primitives have been conducted [31].When considering the decomposed motion, confirmation by motion video is desirable.Each IMF, trend, and posture of mowing motion obtained from MEMD is written in Biovision Hierarchy (BVH) format, which describes the human body as a hierarchical skeletal model.However, in IMF motion analysis, the trend (residual), which is the time variation of the basic posture of mowing motion, must be added to reconstruct the motion.As Figure 4 shows, when decomposing the signal in MEMD, all joints and their 3-axis angles are empirically decomposed.Figure 4 demonstrates an instance of motion decomposition applied to the mowing motion.It shows that the original motion was decomposed into 10 IMFs.After adding the basic posture to each IMF, the motion primitives are obtained.By visualizing these extracted motion primitives of the mowing motion, we can see that IMF4 is the fast precutting, and IMF5 is the slow pre-cutting.Because the cutting types might differ from the samples, we classified the type of samples based on evaluations of the decomposed motion primitives (BVHs shown in Figure 4) and the averaged frequencies of them.The details of this issue will be discussed in Section 5.

Average Amplitude and Frequency
Boashsah's work has shown that if the instantaneous frequency changes sufficiently slowly [32], then the instantaneous frequency obtained by applying an HT to the EMDdecomposed IMF can be smoothed.This property has been exploited by Niu et al., and they proposed the weighted average frequency algorithm (WAFA) [33].Furthermore, recent research has shown that using WAFA to obtain each IMF's average amplitudes and frequencies could help researchers understand the physical meaning of these decomposed periodic modes [34].
The mowing motion analysis assumed in this paper calculated the average frequencies of all IMFs in the WAFA of the mowing motion.When the noise level of the mowing motion data was high, it was resolved as a non-linear wave ()cos(()) in a periodic or a cyclic multidimensional sphere.A series of mowing motions was decomposed into a single non-linear monochromatic wave (IMF).In this case, for each IMF, the amplitude () of that mode had almost zero amplitude at the time when there is no motion.When averaging in instantaneous frequencies to obtain the averaged frequency, it is necessary to multiply () by ().This algorithm also obtains the average frequency for each IMF described below.For the average amplitude, the arithmetic mean was adopted in this study.

Model: Joint Angular Calculation-based Body Movement Analysis
According to the assumption that "the vast majority of single-purpose active actions can be decomposed into different sub-actions" and given our previous research experience [15,16], we tried to propose a joint angle calculation-based comparative model for normal body movement analysis.As shown in Figure 5, for general motion capture experiments, the model has three main components: field experiments, data processing, and body movement analysis.Figure 4 demonstrates an instance of motion decomposition applied to the mowing motion.It shows that the original motion was decomposed into 10 IMFs.After adding the basic posture to each IMF, the motion primitives are obtained.By visualizing these extracted motion primitives of the mowing motion, we can see that IMF4 is the fast precutting, and IMF5 is the slow pre-cutting.Because the cutting types might differ from the samples, we classified the type of samples based on evaluations of the decomposed motion primitives (BVHs shown in Figure 4) and the averaged frequencies of them.The details of this issue will be discussed in Section 5.

Average Amplitude and Frequency
Boashsah's work has shown that if the instantaneous frequency changes sufficiently slowly [32], then the instantaneous frequency obtained by applying an HT to the EMDdecomposed IMF can be smoothed.This property has been exploited by Niu et al., and they proposed the weighted average frequency algorithm (WAFA) [33].Furthermore, recent research has shown that using WAFA to obtain each IMF's average amplitudes and frequencies could help researchers understand the physical meaning of these decomposed periodic modes [34].
The mowing motion analysis assumed in this paper calculated the average frequencies of all IMFs in the WAFA of the mowing motion.When the noise level of the mowing motion data was high, it was resolved as a non-linear wave A(t)cos(ω(t)t) in a periodic or a cyclic multidimensional sphere.A series of mowing motions was decomposed into a single non-linear monochromatic wave (IMF).In this case, for each IMF, the amplitude A(t) of that mode had almost zero amplitude at the time when there is no motion.When averaging in instantaneous frequencies to obtain the averaged frequency, it is necessary to multiply A(t) by ω(t).This algorithm also obtains the average frequency for each IMF described below.For the average amplitude, the arithmetic mean was adopted in this study.

Model: Joint Angular Calculation-based Body Movement Analysis
According to the assumption that "the vast majority of single-purpose active actions can be decomposed into different sub-actions" and given our previous research experience [15,16], we tried to propose a joint angle calculation-based comparative model for normal body movement analysis.As shown in Figure 5, for general motion capture experiments, the model has three main components: field experiments, data processing, and body movement analysis.• Field experiments: The purpose of the vast majority of human behavior comparison experiments is to find out the similarities and differences between different subject groups.Therefore, this model argues that in the field experiment stage, it is necessary to group subjects in advance for the purpose of the experiment and to collect the environmental data of the experimental site as objectively as possible.This will enhance the 3D model accuracy captured by the devices for the final human body movements.

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Data processing: After receiving the raw data of body movements, this model does not put the data into machine learning or other black-box algorithms directly.Instead, the model calculates all the joint angles of the subject's body based on their original 3D coordinates before the analysis and then selects the most relevant parts to represent their body movement characteristics data for calculation according to the experimental purpose.Based on this, the corresponding model (full or partial) will be selected, and the HHT movement decomposition method will be used to decompose the target body movements to obtain the raw overall statistics and timeseries-based graphical data for the following analysis.

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Body movement analysis: Because not all sub-actions have an obvious purpose, some minor redundant actions that are not even perceptible to humans need to be eliminated before the analysis (10 Hz filtering).In combination with the previous grouping of subjects, the model classifies all meaningful sub-actions (IMF1, IMF2,...) of the relevant data into two types (statistic data and graph data) before the between and within groups comparative analyses.Different types of sub-action data can be analyzed in different comparative ways.This will facilitate model users' interpretation and discussion of the purpose and effect of the target sub-action from different perspectives of the whole and time series.

Field Experiments
A set of experiments was conducted at Kouchi, Higashi Hiroshima, Hiroshima, Japan.A conventional U-shaped portable mower was used for the experiments.This type of mower allows the worker to hold it in both hands and mow in a prescribed direction (from left to right).
As shown in Figure 6, the sloping area has a slope angle of greater than or equal to 30 degrees, and the trigonometric tangent is expressed as a percentage of 30 degrees = 57%.The weeds are thick and have small stones hidden in them, which increases the difficulty of manual mowing and the risk of falling during the operation.All mowing workers needed to make several horizontal trips to complete the work in the area they were

•
Field experiments: The purpose of the vast majority of human behavior comparison experiments is to find out the similarities and differences between different subject groups.Therefore, this model argues that in the field experiment stage, it is necessary to group subjects in advance for the purpose of the experiment and to collect the environmental data of the experimental site as objectively as possible.This will enhance the 3D model accuracy captured by the devices for the final human body movements.

•
Data processing: After receiving the raw data of body movements, this model does not put the data into machine learning or other black-box algorithms directly.Instead, the model calculates all the joint angles of the subject's body based on their original 3D coordinates before the analysis and then selects the most relevant parts to represent their body movement characteristics data for calculation according to the experimental purpose.Based on this, the corresponding model (full or partial) will be selected, and the HHT movement decomposition method will be used to decompose the target body movements to obtain the raw overall statistics and time-series-based graphical data for the following analysis.

•
Body movement analysis: Because not all sub-actions have an obvious purpose, some minor redundant actions that are not even perceptible to humans need to be eliminated before the analysis (10 Hz filtering).In combination with the previous grouping of subjects, the model classifies all meaningful sub-actions (IMF1, IMF2,...) of the relevant data into two types (statistic data and graph data) before the between and within groups comparative analyses.Different types of sub-action data can be analyzed in different comparative ways.This will facilitate model users' interpretation and discussion of the purpose and effect of the target sub-action from different perspectives of the whole and time series.

Field Experiments
A set of experiments was conducted at Kouchi, Higashi Hiroshima, Hiroshima, Japan.A conventional U-shaped portable mower was used for the experiments.This type of mower allows the worker to hold it in both hands and mow in a prescribed direction (from left to right).
As shown in Figure 6, the sloping area has a slope angle of greater than or equal to 30 degrees, and the trigonometric tangent is expressed as a percentage of 30 degrees = 57%.The weeds are thick and have small stones hidden in them, which increases the difficulty of manual mowing and the risk of falling during the operation.All mowing workers needed to make several horizontal trips to complete the work in the area they were responsible for.(The slope height is about 10 m).All subject workers were asked to wear the motion capture suit and perform their usual mowing work.Workers' physical data was also collected before the experiments.Then, a post-test interview was conducted with each subject to identify details that could not be obtained through the device.
Agriculture 2023, 13, x FOR PEER REVIEW 10 of 22 responsible for.(The slope height is about 10 m).All subject workers were asked to wear the motion capture suit and perform their usual mowing work.Workers' physical data was also collected before the experiments.Then, a post-test interview was conducted with each subject to identify details that could not be obtained through the device.As shown in Table 1, 5 experienced mowing workers, each with more than 5 years of experience in the area, were invited to participate in the experiment.All subjects participating in the experiment were right-handed, male, in good health, and had no cognitive impairment problems.

Dataset
Eventually, 567 sets of data were collected.As mentioned above, we focused only on the pre-cutting behavior part for the analysis.Based on the pre-processing, we parsed the sub-action (IMFs) composition by the HHT decomposition method, which meant we could obtain data, including the mowing workers' movement trends, as well as the frequency and amplitude for all IMFs' body joints.
For the statistical analysis, the HHT decomposition method used in this paper divided each action according to frequency.However, according to a previous study [35], subjects would not be able to control movements with an action frequency of less than 10 Hz.This means that these movements can be considered as redundant actions and can be ignored.
Therefore, only IMFs no. 3, 4, and 5 were eligible (>10 Hz) in our research, and the measures for each IMF were used for comparative analysis.As shown in Table 2, we considered a total of 23 groups of human joints in terms of their angles and their variations, as follows.As shown in Table 1, 5 experienced mowing workers, each with more than 5 years of experience in the area, were invited to participate in the experiment.All subjects participating in the experiment were right-handed, male, in good health, and had no cognitive impairment problems. .

Dataset
Eventually, 567 sets of data were collected.As mentioned above, we focused only on the pre-cutting behavior part for the analysis.Based on the pre-processing, we parsed the sub-action (IMFs) composition by the HHT decomposition method, which meant we could obtain data, including the mowing workers' movement trends, as well as the frequency and amplitude for all IMFs' body joints.
For the statistical analysis, the HHT decomposition method used in this paper divided each action according to frequency.However, according to a previous study [35], subjects would not be able to control movements with an action frequency of less than 10 Hz.This means that these movements can be considered as redundant actions and can be ignored.
Therefore, only IMFs no. 3, 4, and 5 were eligible (>10 Hz) in our research, and the measures for each IMF were used for comparative analysis.As shown in Table 2, we considered a total of 23 groups of human joints in terms of their angles and their variations, as follows.In addition, according to the characteristics of the mowing movement, we used all joint data for the global analysis.This allowed us to obtain the skeletal data, which helped to identify the purpose of all sub-actions easily.

Data Processing and Analysis
To confirm the purpose of the different sub-actions, we compared pre-cutting actions between different mowing workers and examined the effect of whole-body joint changes on mowing stability.

HHT-based Actions Decomposition and Classification
In this study, we decomposed motion capture data into several IMFs from high frequency to low frequency by putting all the joint angles into MEMD.Table 3 shows the average frequencies of each decomposed IMF obtained from six samples.By analyzing the decomposed IMFs, we observed that two different cutting actions could be extracted from the original motion data: (a) Fast pre-cutting is performed at about 1.1 s-2.5 s (0.4 Hz-0.9 Hz), corresponding to IMF4-6.(b) Slow pre-cutting is performed at about 5 s-2 s (0.2 Hz-0.5 Hz), corresponding to IMF5-7.Underlined numbers in Table 4 show the fast pre-cutting and slow pre-cutting decomposed as different values and different motion speeds (frequency) caused by variations in the cutting styles of each person.Because all joint data had been inputted into MEMD to achieve multivariate decomposed IMFs, we could visualize the decomposed motion by mapping the IMFs to the original skeleton.As shown in Figure 7, these two different cutting actions can be clearly classified.Therefore, by investigating both different cuttings of each sample, our research can reveal the difference between beginners and experts, thus helping beginners to improve their skill of grass cutting.Our results can also provide the features as training data for risk of falling detection using deep learning by investigating the pre-cutting movements of the two extracted motion types.

Differences between Workers with Different Mowing Experience
We included a less experienced subject in our latest slope mowing experiment.Therefore, we could discover the difference in movements resulting from different mowing experience.Based on the two different pre-cutting actions (fast/slow) that we decomposed using the HHT method, the data (fast/slow pre-cutting action's frequency and amplitude) were analyzed using a non-parametric identification method, namely the independentsamples Mann-Whitney U test.In addition, more data were collected for subject number 5 than for the other subjects.Therefore, his data were not included in this section to avoid any bias.

Differences between Workers with Different Mowing Experience (Fast)
Based on the Kolmogorov Smirnov normality test and related Levene's test, we find that the data we collected did not conform to a normal distribution but generally consistent with assumption of homogeneity of variance.The independent-samples Mann-Whitney U test can be used in this case.Therefore, by investigating both different cuttings of each sample, our research can reveal the difference between beginners and experts, thus helping beginners to improve their skill of grass cutting.Our results can also provide the features as training data for risk of falling detection using deep learning by investigating the pre-cutting movements of the two extracted motion types.

Differences between Workers with Different Mowing Experience
We included a less experienced subject in our latest slope mowing experiment.Therefore, we could discover the difference in movements resulting from different mowing experience.Based on the two different pre-cutting actions (fast/slow) that we decomposed using the HHT method, the data (fast/slow pre-cutting action's frequency and amplitude) were analyzed using a non-parametric identification method, namely the independentsamples Mann-Whitney U test.In addition, more data were collected for subject number 5 than for the other subjects.Therefore, his data were not included in this section to avoid any bias.

Differences between Workers with Different Mowing Experience (Fast)
Based on the Kolmogorov Smirnov normality test and related Levene's test, we find that the data we collected did not conform to a normal distribution but generally consistent with assumption of homogeneity of variance.The independent-samples Mann-Whitney U test can be used in this case.
As shown in Table 4, for the "fast pre-cutting" action's frequency, the results showed that the experienced workers differed significantly from novices in almost all metrics.In contrast, for the "fast pre-cutting" action's amplitude, shown in Table 5, most of the indicators did not show significant differences, whereas the differences were mainly concentrated in the indicators of Rshoulder_amp, Relbow_amp, Rwrist_amp, and Lshoulder_amp.This suggests that differences in mowing experience on slopes mainly affect the speed of mowing workers' whole-body movements and have a limited effect on the overall force used.As special cases, experienced workers concentrated a higher force in the right arm (shoulder, elbow, and wrist) and left shoulder (all subjects were right-handed) to increase the efficiency of mowing.

Differences between Workers with Different Mowing Experience (Slow)
The same analysis was performed for the slow pre-cutting action's frequency and amplitude.However, the results were more complicated.As shown in Table 6, for the "slow pre-cutting" frequency, most of the indicators did not show significant differences, whereas the differences were mainly concentrated in the indicators of Chest_freq, Chest3_freq, and Chest4_freq.In contrast, for the "slow pre-cutting" action's amplitude, shown in Table 7, the experienced workers differed significantly from novices in only the metrics of Chest_amp, Chest2_amp, Chest3_amp, Chest4_amp, Rshoulder_amp, Rel-bow_amp, Rwrist_amp, Lcollar_amp, Lshoulder_amp, Lwrist_amp, and Rankle_amp.Such results suggest that, for most of the slow pre-cutting action, the difference in speed between workers with different mowing experience was not significant, but rather the difference in the force used was more pronounced.This difference was mainly in the right arm (shoulder, elbow, and wrist) and left arm (shoulder and wrist).Thus, experienced workers in the slow pre-cutting action had a similar speed as novices but had more power in their hand movements.Together with the results for fast pre-cutting, this indicates that experienced workers are more confident in using force compared to novices.
In addition, the results for the partial spinal joints (chests) showed that experienced workers had less spinal flexion, but used more force during the preparatory mowing maneuver.One possible explanation is that experienced workers move more steadily, but use force more firmly, thus ensuring the quality of the mowing.

Falling Risk Analysis
This paper focuses only on the pre-cutting part of the whole mowing action.In our previous study [14], we discussed the influence of various joint movements on worker stability in general mowing motion (cutting).Therefore, in this section, we will discuss the influence of changes in the angle of each joint and related body characteristics on the stability of workers in the pre-cutting movement to identify the safety postures that minimize the possibility of workers' falling risk.
Similar to our previous studies, which used the standard deviation of the waist joint angle to judge the risk of falling, this paper will comprehensively judge the risk of falling by taking the standard deviation of the corresponding hip joint angle of different sub-movements (fast/slow pre-cutting actions).Because the frequency and amplitude of each group of movements were respectively taken as indicators in this paper, we had four indicators that could be used as dependent variables to evaluate the impact of other movement joints on the risk of falling: SD_hip (Freq.Fast), SD_hip (Amp.Fast), SD_hip (Freq.Slow), and SD_hip (Amp.Slow).
After correlation pretreatment, we determined that the dependent variables mentioned above basically conformed to a normal distribution.Therefore, stepwise regression analysis is chosen to confirm the angle changes of each joint and the stability of the workers' relationship.5.3.1.Falling Risk Analysis for Pre-Cutting SD_hip (Freq.Fast) First, the stepwise regression method was used to calculate the influence of different joint angle changes on the dependent variable SD_hip (Freq.Fast).Because the original data of the dependent variable did not quite conform to a normal distribution, we analyzed it on the basis of logarithm (the adjusted data were close to a normal distribution).
The results (Table 8) showed that the most important factors affecting the fast precutting action were the head (Head_amp and Head_freq) and left leg part (Lhip_freq, Lankle_freq, Ankle_height, and Foot_or_Shoe_length). Specifically, the results showed that the faster the rate of change in the joints of a worker's head, the faster the frequency of vibrations in the worker's waist.One possible explanation is that to gather kinetic energy more efficiently, a worker's head acts as a stabilizing force in the fast pre-cutting action.
In addition, the results for the right leg indicated that the left leg was the power output source in the pre-cutting movement and that the faster the joint transformation speed, the higher the vibration frequency of the worker's waist.Therefore, workers may need to be careful not to overexert their left leg when performing the pre-cutting actions.
Moreover, another interesting result was that the decomposition indicated the effect of the workers' physical characteristics (Ankle_height and Foot_or_Shoe_length) on their stability.Workers with larger feet and lower ankles had fewer waist vibrations during movement.This may be explained by the steadiness of larger feet, as well as the lower center of gravity that may come with lower ankles.sub-movements, more attention needs to be paid to ankle parts than to other parts of the body to prevent falls.The results obtained in this paper suggest that these influencing factors should be emphasized in the training of mowing workers and the development of auxiliary equipment, etc.
The study had some limitations.First, the number of workers engaged in mowing was relatively small, and most of them were old.Thus, the sample size discussed in this study was a bit small.Second, this is an exploratory study, and the results may not be taken as confirmatory.
In the future, we plan to collect eye movement data of workers in the grass-cutting experiment and analyze the relationship between eye movement, body movement, and stability through data fusion.The management of the ridges and slopes for paddy fields in mountainous areas requires more time and labor than the flat areas, so the cost is relatively high.The development of a mowing model that increases efficiency while ensuring safety will also lead to a reduction of mowing costs.

Figure 1 .
Figure 1.The Pre-cutting Behaviors in this Study.

Figure 1 .
Figure 1.The Pre-cutting Behaviors in this Study.

Figure 2 .
Figure 2. Subject equipped with the motion capture device Xsens MVN.

Figure 2 .
Figure 2. Subject equipped with the motion capture device Xsens MVN.

22 Figure 3 .
Figure 3.The Joint Structure of the Mowing Motion Skeleton.3.3.5.Mowing Motion DecompositionMEMD is used to extract motion primitives.Motion primitives are the most basic motion units in motion analysis and automatic generation.Many studies based on motion primitives have been conducted[31].When considering the decomposed motion, confirmation by motion video is desirable.Each IMF, trend, and posture of mowing motion obtained from MEMD is written in Biovision Hierarchy (BVH) format, which describes the human body as a hierarchical skeletal model.However, in IMF motion analysis, the trend (residual), which is the time variation of the basic posture of mowing motion, must

Figure 3 .
Figure 3.The Joint Structure of the Mowing Motion Skeleton.

Figure 4 .
Figure 4.An Example of the Mowing Motion Decomposition.

Figure 4 .
Figure 4.An Example of the Mowing Motion Decomposition.

Figure 5 .
Figure 5. Joint Angular Calculation-based Body Movement Analysis Model.

Figure 6 .
Figure 6.An Example of the Mowing Motion Decomposition.

Figure 6 .
Figure 6.An Example of the Mowing Motion Decomposition.

Table 1 .
Details of the Experimental Subjects.

Table 1 .
Details of the Experimental Subjects.

Table 3 .
Average Frequencies of Each Decomposed IMF.