Working Posture and the Center of Mass Assessment While Starting a Chainsaw: A Case Study among Forestry Workers in Croatia

Abstract

Motor-manual work with a chainsaw is still an irreplaceable occupational activity in forest harvesting operations in Croatia and in many other countries. Chainsaw workers are exposed to different risk factors, such as noise and vibrations, heavy load lifting, awkward and preflexion postures, repetitive movements, etc. The working posture and the assessment of the center of mass plays an important role in understanding the exposure of forest workers to postural and occupational risks, either as a part of the entire work process or as a part of an individual element of the work technique. Therefore, the aim of this research was to investigate the impact of three different chainsaw starting methods (from the ground, between the knees, and “drop starting”) on the postural load of the chainsaw worker and its association with personal and occupational factors. The field portion of data collection was conducted in the second and third quarters of 2022. For trunk posture assessment (n = 74), the rapid entire body assessment (REBA) tool was used, and for the center of mass measurement (n = 28), the Xsens MVN Link motion capture suit was used, a relatively new technology that allows data collection in real field conditions. Descriptive and inferential statistical methods were used to verify and analyze the field data. Based on the obtained results, almost 50% of the sampled chainsaw workers ignored safety instructions and preferred “drop starting” a chainsaw. The results also showed that there is a significant difference between the three starting methods in the REBA score, as well as in the number of segments scored during the assessment. The obtained mean REBA score for all three methods is between 4 and 7, placing all methods at a medium level of MSD risk. Regarding the analysis of the Center of Mass (CoM) vertical deviation, the statistical difference is noticeable for the body mass index (BMI) group, the group related to experience with the chainsaw, and the age and height group in relation to three methods for starting a chainsaw. The safest method, which implies starting a chainsaw from the ground, resulted also as the riskiest in terms of postural strain, which, cumulatively over the time variable, can contribute to health problems in forest workers. In conclusion, related to the future process of professional training development for chainsaw workers, an integral part of the training must contain the most optimal postural-movement habits that need to be defined by further kinematic research in forestry.

1. Introduction

The forestry industry, particularly logging operations, is characterized as the most dangerous, physically demanding, and dirty work compared to all other sectors [1,2,3]. Regarding logging operations in forestry, many countries still rely on motor-manual work with a chainsaw because of different field and stand characteristics, the implementation of different wood extraction systems, or the unavailability of modern technologies in underdeveloped countries. In Croatia, approximately 95% of wood is felled and processed motor-manually [3], and the main problems with achieving a sufficiently high level of safety during logging operations are the inadequate training of forest workers for working with chainsaws and the use of inadequate work techniques [4]. In the last decade, numerous authors investigated the issue of motor-manual work regarding the current ergonomic condition of chainsaws in non-professional use [5], workload, exposure to noise, and risk of musculoskeletal disorder in poplar clear cuts [6], and time consumption and productivity in manual tree felling with a chainsaw [7].

During forestry work, the anatomical structures of the worker’s body are exposed to static and dynamic “strains” that result in biomechanical stress. According to some researchers, while working in logging operations, workers are exposed to a high level of physiological [8] and physical stress and possible musculoskeletal disorders [9]. In addition, musculoskeletal disorders (MSDs) can occur as a result of acute trauma (slip, accident, fall, blow, etc.) or cumulatively as a result of long-term exposure to static and/or dynamic stress. In contrast to an acute injury, overexertion syndrome is of a chronic nature because repeated trauma over time overcomes the tissue’s ability to regenerate (tendons, bones, cartilage, mucous membranes, or muscles) [10]. Further, musculoskeletal disorders often appear in the form of pain and limited mobility in limbs or parts of the body [11] and can therefore have major implications for the general health of an individual, i.e., work safety in high-risk industries such as logging operations in forestry. The exposure to vibration and extreme temperatures is also a contributing factor [12,13], and as such, motor-manual work in forestry can often result in the development of MSDs. There are incorrect work techniques and recurring musculoskeletal pain that might cause reflex posture changes, leading to distortions of body coordination and stability, thus increasing the risk of accidents [14].

The second important factor from the aspect of safety in forestry, regarding work with a chainsaw, is a worker’s Center of Mass (CoM) position. The CoM is a position defined relative to an object or system of objects. It is the average position of all the parts of the system, weighted according to their masses [15]. The estimation of the human CoM motion is essential in ergonomics [16,17], sports [18], clinical practice [19,20], etc. since it provides quantitative data regarding the risk of imbalances and postural impairments in people. The estimation of the forestry worker’s CoM motion in real time field conditions, from the aspect of working technique, allows monitoring the deficit of postural control and potential prevention of unsafe conditions such as the occurrence of minor or serious injuries at work. The aforementioned is especially possible by analyzing each element of the work technique for the chainsaw workers at logging operations.

The first element of working technique in logging operations is starting a chainsaw. In practice, every day workers use one of three methods of starting the chainsaw: (a) the “from the ground” method; (b) the “between the knees” method; and (c) the “drop starting” method. In field conditions, every chainsaw worker must start a chainsaw several times a day, and sometimes the starting procedure lasts several seconds, especially if the chainsaw is cold, putting the operator in an awkward position and exposing him to physical stress for a longer time. Starting a chainsaw several times a day over the years can cumulatively affect the health condition of chainsaw workers in the form of the appearance of musculoskeletal disorders and/or immediately, through the aspect of worker safety, i.e., the occurrence of injury due to a worker’s fall or cut that occurs when the CoM exceeds the base of support.

The purpose of current research is to evaluate postural load during the three mentioned methods of starting a chainsaw using the REBA (rapid entire body assessment) tool with an assumption of significant differences between them and between the respondent’s body mass index (BMI), working experience with the chainsaw, age, and height. The second goal of the research is related to the application of state-of-the-art inertial measurement units developed by Xsens Technologies to explain the differences in the vertical deviation of the chainsaw worker’s CoM calculated from kinematic data and the comparison of the obtained results against the applied method of starting the chainsaw and some descriptive variables such as the respondent’s body mass index (BMI), experience with the chainsaw, age, and height of the worker.

2. Materials and Methods

In logging operations, workers use professional chainsaws with a total weight of over 6 kg. Starting a chainsaw is an important step in chainsaw handling. When starting a chainsaw, the chain brake is always to be engaged. The Occupational Safety and Health Administration (OSHA) proposes two safe methods of starting a chainsaw [21]. The first method is starting a chainsaw from the ground by placing the left hand on the front handle while pressing it down, putting a right foot into the back handle, and pressing down to firmly secure the chainsaw on the ground, and finally, pulling a starter grip with the right hand until it starts (Figure 1A). The second method is to start a chainsaw firmly secured between the knees while holding the front handle with the left hand, keeping the arm straight, and pulling a starter grip with the right hand until it starts (Figure 1B). Some workers ignore safety measures and choose to start a chainsaw using the so-called “drop start” method. This means that they use the jerking motion of a chainsaw going down, held by the left hand by the front handle, while simultaneously pulling a starter grip with the right hand until it starts (Figure 1C). Drop-starting a chainsaw can potentially cause injuries, especially if a chain brake is not engaged, because the chainsaw is not properly secured and can pivot around the front handle.

The field measurement among chainsaw workers was conducted directly at the forest sites on the territory of the entire Republic of Croatia during the second and third quarters of 2022, in accordance with the territorial distribution of the regional forest administrations within the formal structure of Hrvatske šume Ltd., Zagreb, Croatia (state forestry company) [22]. All sampled workers used the Stihl brand of chainsaw, while the model of the chainsaw differed from respondent to respondent (MS 661 in 25% of cases, MS 461 in 21% of cases, MS 462 in 47% of cases, and MS 500i in 7% of cases). For working posture assessment, a random stratified sample was used for sampling chainsaw workers employed by the company Hrvatske šume Ltd. and chainsaw workers employed by private forestry contractors (16 groups consisting of forest administrations, i.e., forest offices, and an additional three groups consisting of licensed timber harvesting contractors). The proportion for each stratum was calculated based on the total number of workers, and the number of workers to be sampled within each stratum was calculated based on the proportion. The resulting number of sampling workers within each stratum was selected by a simple random sample, where all members of the stratum have an equal chance of being selected, using the “Research Randomizer” calculator. Within the stratum, each worker was numbered in alphabetical order by surname (from 1 to N) and selected using the above calculator (e.g., within the first stratum, worker numbers 3, 8, 14, etc.). At the same forest sites for the CoM measurement on the chainsaw workers, an intentional (quota) sample was used because of the personal judgment of the researcher due to health reasons and the technical limitations of the used measuring equipment. Namely, due to health reasons (transmission of COVID-19 or hepatitis virus), the choice of the respondent due to wearing a motion capture Lycra suit on the almost naked body and under the work suit was reduced to one worker who is willing to participate in the research and due to anthropometric values can wear the same suit that contains measuring sensors. During the field measurement across the Republic of Croatia, the researcher considered the representation of workers according to age group and work experience in both sampled employees groups.

The study was approved by the Ethical Committee of the Faculty of Forestry and Wood Technology of Zagreb University (protocol code 251-72-06-20-1, 22 January 2020), and the same was accepted by the Management Board of the Croatian Science Foundation. Before the field survey and data collection, all workers who were sampled received information about the purpose of the research and agreed to participate in the measurements.

2.1. Technique for a Working Posture Assessment

All sampled postures were evaluated using the ErgoFellow 3.0 software^® (FBF Sistemas LTDA, Belo Horizonte, State of Minas Gerais, Brazil) through the REBA observation technique. A total of 74 workers on the job were recorded using a GoPro Hero 8 video camera. The obtained video material was viewed in the VLC media player^® (VideoLAN Organization, Paris, France). Frames that showed typical postures when starting a chainsaw were extracted using the print screen function. One frame per worker was extracted and analyzed using the REBA [23] tool in Ergofellow 3.0 software. The extracted image per subject represents the position in which the worker visually deviates the most from the natural N-pose. In REBA, tool points are assigned in five categories, and the final result is a total REBA score that is used to classify observed working posture into one of five levels of MSD risk. The minimum score is 1—negligible risk—and the maximum is 15—very high risk. Group A consists of points assigned to the neck, trunk, and legs, totaled according to Hignett and Mcatamney [23]. Group B consists of points assigned to the lower arm, upper arm, and wrist, also subtotaled according to Hignett and Mcatamney [23].

2.2. The Center of Mass Measurement with the Motion Capture Suit

For all sampled chainsaw workers, the CoM was measured with the Xsens MVN Link motion capture suit and analyzed with the MVN BIOMECH software^® (Xsens, MVN Analyzer Pro, Version 2021.2.0, Enschede, The Netherlands). The Xsens MVN motion capture suit is an easy-to-use system for full-body human motion capture that consists of 17 miniature inertial 3DOF Orientation Tracker (MTx) sensors. The MTx is an inertial and magnetic measurement unit and comprises 3D gyroscopes, 3D accelerometers, and 3D magnetometers, which are chained together to the Xbus Masters. The Xbus Master synchronizes all sensor sampling, provides sensors with power, and handles wireless communication with the PC or laptop [24]. The sampling rate was set at 240 HZ for the optoelectronic system. Based on measurement and data processing as part of the obtained kinematic model of the human body, the center of the mass estimation is based on the work of Zatsiorsky et al. [25] and Dempster [26]. Previous studies have confirmed the reliability and validity of the Xsens kinematic suit for analyzing different kinematic data in scientific disciplines [27,28,29].

A total of 28 chainsaw workers on logging were measured with the Xsens MVN Link motion capture suit and parallelly recorded using a GoPro Hero 8 video camera. Prior to field measurement, basic anthropometric parameters (

Table 1. Anthropometric parameters from the chainsaw workers participating in the study (n = 28).

Body Dimensions	Mean	Std. Deviation	Min	Max
* Body height (cm)	181.54	5.59	171.00	192.00
Shoe length (cm)	30.77	1.17	28.00	34.00
* Shoulder height (cm)	149.67	6.93	138.00	170.00
Shoulder width (cm)	44.83	2.79	39.00	49.00
Elbow span (cm)	91.05	5.70	82.00	105.00
Wrist span (cm)	143.39	6.27	131.00	155.00
Arm span (cm)	182.41	6.52	168.00	195.00
* Hip height (cm)	104.55	5.94	89.00	120.00
Hip width (cm)	34.34	2.96	27.00	41.00
* Knee height (cm)	54.48	2.84	49.00	60.00
* Ankle height (cm)	11.75	0.83	10.00	14.00

* marked dimensions are 3 cm above the actual values due to shoe sole thickness.

) were measured for each worker and a calibration measurement was performed using N-pose [30]. Descriptive anthropometric values of the sampled workers are shown in Table 1.

2.3. Data Processing and Statistical Analysis

All the data processing was done off-line, for sampled postures using the ErgoFellow 3.0 software and for CoM using the MVN BIOMECH software. Primary processed data was entered and systematized in the software package Microsoft Excel^®, and further data analysis was performed using statistical software: Statistica^® (TIBCO Software Inc., version 14, Palo Alto, CA, USA) and SPSS^® (IBM-SPSS Inc., version 28, Armonk, NY, USA).

The primary data processing was performed with a descriptive statistical analysis that included calculating and displaying the main characteristics of the sampled chainsaw workers. The age of the respondents was divided into two groups (group 1 = 39 years or less, and group 2 = 40 years or more). The chainsaw work experience in the logging industry was divided into three groups (group 1 = 5 years or less; group 2 = from 6 to 15 years; group 3 = 16 years or more). The height of the respondents was divided into two groups (group 1 = 179 cm or shorter and group 2 = 180 cm or taller), and the body mass index was also divided into three groups (group 1 = 24.99 or less; group 2 = from 25 to 29.99; and group 3 = 30 and more).

For each variable, an appropriate test of distribution normality and homogeneity of variance was made (Shapiro-Wilk’s and Levine’s test), on the basis of which the following data processing was performed. T-tests or an alternative nonparametric Mann-Whitney U test were performed for the posture REBA score and the CoM value to determine if sampled chainsaw workers were statistically similar regarding age and height in terms of the three methods for starting the chainsaw. Furthermore, a one-way ANOVA or alternative nonparametric Kruskal-Wallis test was used to test the differences between the defined groups for chainsaw work experience and body mass index versus posture REBA score and the CoM values.

3. Results

3.1. A Postural REBA Load Assessment When Starting a Chainsaw

For postural load assessment, the field sampling of chainsaw workers (N = 74) employed by the company Hrvatske šume Ltd. and by a private contractor in forestry across the Republic of Croatia was done during the second and third quarters of 2022. In

Table 2. General information from the chainsaw workers participating in the postural load study (N = 74).

Descriptive Variables	Mean	Std. Deviation	Min	Max
Height (cm)	180.81	7.00	164	200
Weight (kg)	90.26	15.16	59	140
Age (years)	35.92	9.45	21	58
Body Mass Index	27.55	4.13	19.28	36.80
Working experience with a chainsaw (years)	8.52	7.65	0.10	35

, the general descriptive information of the respondents is shown. The displayed values in Table 2 (such as age, work experience, height, etc.) corresponded well with the distribution of the general population. Regarding the body mass index, 44.60% of sampled workers are in the overweight category, and 25.70% of workers are in the obese category.

Additionally, the Shapiro-Wilk test did not prove normality distribution for all used variables (p-value less than 0.05). As a result, for further analysis, a nonparametric Mann-Whitney U test or Kruskal-Wallis H test was used. The total REBA score, individual body segment points, and subtotal of groups A and B were tested against three methods of starting the chainsaw using the Kruskal-Wallis H test (

Table 3. Testing the postural load difference between the chainsaw starting methods using the Kruskal-Wallis H test.

Variable	REBA Score	Trunk	Legs Add	Group A	Upper Arm	Group B
X2	26.99	8.45	31.49	18.02	13.99	13.99
df	2	2	2	2	2	2
p-value	<0.001	0.015	<0.001	<0.001	<0.001	<0.001

). Testing results and statistically significant differences are shown in Table 3.

In the three methods of starting a chainsaw using the Mann-Whitney U post-hoc test (

Table 4. The Mann-Whitney U test results for chainsaw worker postural load assessment.

Method	Variable	REBA Score	Trunk	Legs Add	Group A	Upper Arm	Group B
A-B	U-value	66.00	124.50	- - -	112.00	143.00	143.00
	Z-value	−3.55	−2.315	- - -	−2.324	−2.371	−2.371
	p-value	<0.001	0.021	- - -	0.020	0.018	0.018
A-C	U-value	74.00	169.50	76.50	88.50	227.50	227.50
	Z-value	−4.725	−2.942	−5.282	−4.257	−2.958	−2.958
	p-value	<0.001	0.003	<0.001	<0.001	0.003	0.003
B-C	U-value	233.00	- - -	173.00	- - -	- - -	- - -
	Z-value	−2.765	- - -	−4.385	- - -	- - -	- - -
	p-value	0.006	- - -	<0.001	- - -	- - -	- - -

), it was determined that the mean REBA score in method A (starting the chainsaw from the ground) (M = 7.12; Md = 7.00; N = 17) is significantly different from method B (starting the chainsaw between the knees) (M = 5.36; Md = 6.00; N = 22) and method C (chainsaw drop start) (M = 4.46; Md = 4.00; N = 35). The mean REBA score in method B (starting the chainsaw between the knees) is also significantly different from group method C (chainsaw drop start) (Table 4). For the variable trunk (Table 4), when evaluating the postural load, it was determined that the points obtained in method A (starting the chainsaw from the ground) (M = 3.88; Md = 4.00; N = 17) are significantly different from method B (starting the chainsaw between the knees) (M = 3.45; Md = 4.00; N = 22) and method C (chainsaw drop start) (M = 3.40; Md = 3.00; N = 35). Regarding individual variables, a statistically significant difference was obtained for additional legs (Table 4) between methods A-C and B-C. For the variable upper arm (Table 4), a statistically significant difference was obtained between methods A-B and A-C. When testing the difference between group A and the other two methods of starting a chainsaw (Table 4), it was determined that method A (M = 5.76; Md = 6.00; N = 17) is significantly different from method B (M = 5.00; Md = 5.00; N = 22) and method C (M = 4.57; Md = 4.00; N = 35). However, when testing the difference between group B and the three methods of starting a chainsaw (Table 4), it was determined that method A (M = 5.24; Md = 5.00; N = 17) is significantly different from method B (M = 5.00; Md = 5.00; N = 22) and method C (M = 5.00; Md = 5.00; N = 35).

The examination of the differences in the REBA score against descriptive variables (such as age, height, body mass index, etc.) did not result in statistically significant differences except for the type of employer (U = 386.50; Z = −2.620; p = 0.009). A statistically significant difference was obtained between chainsaw workers employed by the company Hrvatske šume Ltd. (M = 5.70, Md = 5.00, N = 50) and chainsaw workers employed by a private forestry contractor (M = 4.58, Md = 4.00, N = 24).

3.2. The Centre of Mass Analysis When Starting a Chainsaw

The descriptive values for work experience with a chainsaw and age (

Table 5. General information from the chainsaw workers participating in the kinematic study (N = 28).

Descriptive Variables	Mean	Std. Deviation	Min	Max
Height (cm)	178.96	5.78	168.00	189.00
Weight (kg)	85.18	9.73	59.00	100.00
Age (years)	35.00	9.81	21.00	56.00
Body Mass Index	26.56	2.52	20.42	29.75
Working experience with a chainsaw (years)	8.63	6.79	0.30	24.00
Minor injuries at work (n)	0.61	1.13	0.00	5.00
Severe injuries at work (n)	0.14	0.36	0.00	1.00

) regarding workers participating in the kinematic study (N = 28) are almost the same as in the REBA postural load study (Table 2). On the other hand, in the kinematic study, the values related to height, mass, and body mass index of the sampled workers (Table 5) are somewhat lower due to the limitation of the possibility of wearing the subject’s lycra suit with sensors.

The CoM vertical deviation, when taking a position to perform a certain work element in the production process, represents an important link regarding the risk of imbalances and postural impairments in workers. Performing a particular work element is additionally risky when using or starting a hand-operated machine tool, such as a chainsaw (Figure 2). Accordingly, difference testing was performed between the CoM vertical deviation and two descriptive variables (Figure 3 and Figure 4) on a sample of 43,377 recorded frames (1 frame represents 14 milliseconds of data recording).

The test results and statistically significant differences are shown in Figure 3 and Figure 4. For three methods of starting a chainsaw, using the Mann-Whitney U post-hoc test, it was determined that the mean CoM vertical deviation (Figure 3) in method A (starting the chainsaw from the ground) (M = 0.8754; Md = 0.8629; N = 12,357) is significantly different from method B (starting the chainsaw between the knees) (M = 1.0645; Md = 1.0585; N = 14,399) and method C (chainsaw drop start) (M = 1.0534; Md = 1.0531; N = 16,621). The mean CoM vertical deviation in method B (starting the chainsaw between the knees) is also significantly different from method C (chainsaw drop start) (Figure 4). Regarding groups for chainsaw work experience (Figure 4), by using a post-hoc U test, it was determined that the mean CoM vertical deviation in group 1 (up to 5 years) (M = 0.9368; Md = 0.8938; N = 15,261) is significantly different from that in group 2 (from 6 to 15 years) (M = 1.0388; Md = 1.05331; N = 20,581) and group 3 (16 and more years) (M = 1.0590; Md = 1.0808; N = 7535). The mean CoM vertical deviation in group 2 (from 6 to 15 years) is also significantly different from group 3 (16 and more years) (Figure 4).

The CoM vertical deviation was also compared versus defined age groups, height groups, and body mass index groups (

Table 6. The Mann-Whitney U test results for CoM vertical deviation.

Variable	U-Value	Z-Value	p-Value
Age	1.395	−36.093	0.00
Hight	1.559	−60.680	0.00
Body mass index	1.366	−24.344	0.00

). Using the Mann-Whitney U test, a statistically significant difference was found for all three descriptive variables (Table 6). In relation to age, younger chainsaw workers had lower values of the CoM vertical deviation (M = 0.993; Md = 1.027; N = 32,119) than those aged 40 and over (M = 1.044; Md = 1.039; N = 11,258). Regrading height, shorter chainsaw workers also had lower values of the CoM vertical deviation (M = 0.983; Md = 1.019; N = 21,054) than those with a height of 180 centimeters and above (M = 1.028; Md = 1.060; N = 22,323). Within the indicators of body mass index, chainsaw workers with normal weight had a higher value of the CoM vertical deviation (M = 1.013; Md = 1.067; N = 9678) in regard to overweight workers (M = 1.004; Md = 1.027; N = 33,699).

4. Discussion

The objective of this research was to investigate the impact of three different chainsaw starting methods on the postural load of the chainsaw worker and its association with personal and occupational factors. Additionally, the second objective was to investigate the CoM trajectory against the applied methods of starting the chainsaw in real field conditions.

Certain limitations should be considered when interpreting the results of this study. In future analysis, the sample should consist of a larger number of participants equally represented against personal and occupational factors, especially in the part of the CoM trajectory research. The second part of the study regarding the Center of Mass is focused on the mean value of only one indicator (vertical deviation) when performing all tasks (such as bending down and standing up while picking up a chainsaw, starting a chainsaw) within one working element. A better approach would be to repeat the measurement with the inclusion of a larger number of indicators in controlled conditions with a clear demarcation of tasks as in other conducted research [20,31]. Secondly, related to the problem of maintaining balance, in addition to the CoM, it is important to include a wider kinematic analysis of the application of force and changes in joint angles while performing the working technique element of starting a chainsaw. Lastly, the physical presence of the researcher when conducting field measurements may have influenced the worker in the selection of the chainsaw starting method in relation to everyday practice.

Regarding risk assessment for musculoskeletal disorders in forestry, some research has used the REBA method [32,33] and the other Nordic Musculoskeletal Questionnaire (NMQ) [34,35,36], which target the entire work process and not a single element of the work technique. In this research, the focus is on the single element of the work technique relating to starting a chainsaw. The Kruskal-Wallis H test showed that there is a significant difference between the three starting methods in the REBA score as well as in the number of segments scored during the assessment. A post-hoc Mann-Whitney U test also pointed to significant differences in REBA scores between observed method pairs (A-B; A-C; B-C), confirming the fact of three very different working postures.

When comparing methods A and B, a significant difference was observed in trunk, group A, upper arm, and group B segments where the A method received higher points. When using method A, a worker must bend his legs and back significantly, he also has to reach further with his hands to grab the front handle and starter grip. However, when using the B method, the worker has to only slightly bend his back and legs in order to secure the chainsaw, which is also closer to his upper extremities, and thus, he does not have to reach far. In two of the methods, A and C, there is a significant difference in trunk and legs; group A, upper arm; and group C, segments. Method A received higher points in the observed segments of the assessment. Mostly neutral work posture while using method C results in lower points attributed to individual segments of the assessment. Method B received higher points in the legs plus segment, which refers to the knee joint angle, in comparison to method C. When using method C, the worker’s legs are almost perfectly straight in contrast to method B, where a worker must position himself in a half squat to secure the chainsaw between the knees and consequently expose himself to higher postural strain. A higher deviation of the worker from his neutral N-pose is resulting in higher obtained points in individual body segments and, consequently, a higher risk of MSDs. To firmly secure a dangerous tool, workers must abandon the N-pose while doing it, and that’s why the safe chainsaw starting methods are being evaluated with higher points per segment. While immediate safety in motor-manual felling and processing is a primary objective, it comes at the cost of awkward postures and possible future injuries. In relation to the type of employer, workers employed by the company Hrvatske šume Ltd. had a higher REBA result value due to the fact that in 88.24% of cases (15 respondents) they used the A method for starting the chainsaw compared to workers employed by a private contractor.

In forestry operations, there are no specific studies examining the impact of the CoM trajectory against the applied methods of starting the chainsaw and its association to personal and occupational factors of chainsaw workers. The review of literary sources of the subject matter is related to the CoM trajectory research in skiing [37], in walking tests [38], in the evaluation of biomechanical risks [39,40], etc. Considering that the measurement was made as part of the daily work process in real field conditions without the possibility of defining the micro-tasks within the working element of starting the chainsaws, it was not possible to investigate the CoM trajectory in detail as in the laboratory test research of Germanotta et al. [31] or Najafi et al. [20]. In this study, through the analysis of the CoM vertical deviation, the statistical difference is noticeable for the body mass index (BMI) group, the group related to experience with the chainsaw, and the age and height group in relation to three methods for starting a chainsaw. Shorter and younger workers with the least length of service with a chainsaw achieved lower values for CoM vertical deviation due to the fact that they more often used the A method of starting the chainsaw from the ground, which is the riskiest from a postural analysis and cumulatively over the years can contribute to the development of MSDs in chainsaw workers. On the other side, older and overweight workers achieved a higher value for CoM vertical deviation due to the fact that they more often used the C method and the B method of starting a chainsaw. A higher CoM vertical deviation implies that the worker is retaining a close-to-neutral position by avoiding back bending or kneeling, which would surely result in a lower CoM vertical deviation.

5. Conclusions

According to the study results and the set research objectives, the following conclusions are drawn. The mean REBA score for all three methods is between 4 and 7, placing all methods at a medium level of MSD risk. In addition to the previous theoretical and practical aspects, the safest A method of starting a chainsaw from the ground is also the riskiest in terms of postural strain and, in the long term, one of the factors that can contribute to the development of MSDs in forestry workers. However, kinematic research of the CoM related to maintaining balance suggests the lowest vertical deviation for the A method and closely similar values for the B and C methods of starting the chainsaw. Although a statistically significant difference between the methods of starting a chainsaw was proven based on the REBA score and CoM vertical deviation, the C method, which is considered unsafe, resulted in the least postural strain. In conclusion, from the safety and postural aspects, the “between the knees” method is the most optimal for use when starting a chainsaw in professional forestry. Given that the configuration of the terrain on forest work sites is extremely diverse with many different obstacles, especially in the recuperation of forests affected by natural disasters such as wind, ice, or snow, it is necessary to make workers aware of and educated about the potential risks when practicing a particular way of starting a chainsaw. Further, another important item from the ergonomic aspect is related to the process of professional training of chainsaw workers, where an integral part of the training should include and contain, within the performance of individual elements of the work technique, the most optimal postural-movement habits that need to be defined by further kinematic research in forestry.