Investigation of precision task demands on workers’ muscular activities,heart rate,and perceived exertion rating

Abstract

BACKGROUND:

Manual material handling (MMH) tasks significantly contribute to musculoskeletal disorders in manual workers.

OBJECTIVES:

In this study, we assessed the physical factors of precision lifting tasks that affect muscular activities (electromyography, EMGs), heart rate, and perceived exertion rating in ten healthy male workers aged 25–35 years, while considering the safety aspect of the common types of safety footwear.

METHODS:

The independent variables that were assessed are as follows: 1) lifting method (precise and inexact), 2) lifting frequency (one and four liftings/min), and 3) type of safety footwear worn by the worker (light, medium, or heavy). The response variables data, represented by EMG signals, for four muscular activities (biceps brachii, deltoid, trapezius, and erector spinae), heart rate, and perceived exertion were analyzed using a three-factor within-subjects design.

RESULTS:

The results showed that wearing heavy safety shoes increases the effort used with precise lifting methods in trapezius and erector spinae muscular activities. We also observed that the heart rate and perceived exertion increased rapidly at four lifts/min compared to one lift/min, regardless of the lifting method.

CONCLUSIONS:

The advantages of choosing appropriate safety footwear must be carefully assessed before replacing the conventional working safety shoes.

Keywords

Precision tasks manual materials handling lifting method lifting frequency electromyography safety shoes

1 Introduction

Manual material handling (MMH) tasks require humans to lift, lower, carry, move, pull, or push materials [1]. Work-related risk factors such as MMH, improper posture, and repetitive and forceful exertions significantly cause lower back pain (LBP) and work-related musculoskeletal disorders (WMSDs). In 2015, the Bureau of Labor Statistics stated that WMSDs accounted for 31% (356,910 cases) of the total cases of injuries among manual industrial workers, from which 17.3% injuries were caused due to LBP, and required a median of 7 days to recover before returning to work [2 –6]. The measures taken to reduce these risk factors by developing new interventions to reduce LBP have considerably decreased WMSDs and injury rates in workers, thereby reducing the loss of working hours [5].

Similar precision in activities was observed on aircraft, trains, and ships, which includes carefully and precisely placing materials such as high-priced artworks on shelves or babies in bassinets. Stambolian et al. [7] examined biomechanical stresses associated with placing avionics boxes in restricted spaces in aerospace vehicles. Although many studies have been conducted on lifting activities, only a few have examined the stress associated with placing a box in restricted or limited spaces [7 –9]. However, none of these studies have determined limitations of the lifting capabilities of workers based on the maximum acceptable weight per lift.

In addition, lifting activities mainly depend on the functionality of the musculoskeletal system, which is a bundle of skeletal muscle fibers surrounded by connective tissues that move particular segments of the body. Electromyography (EMG) is used to measure the muscle responses and electrical activities produced by skeletal muscles. Kim et al. [10] reported that the active muscle mass involved in exercises is highly correlated with an individual’s maximum aerobic capacity. Milanović et al. [11] demonstrated that both males’ and females’ physical activity levels and adaptive functional decline were equal due to the aging process. Differences were found between the young and elderly were due to decreased muscle strength in the upper and lower limbs, body fat percentage, agility, flexibility, and endurance [12]. Lee et al. [13] reported that the lifting height significantly affects the worker’s preferred lifting height, which was approximately 30% lower than their respective maximum acceptable height.

Additionally, a few studies from different disciplines investigated the impact of the type of footwear on lifting tasks. Fortenbaugh et al. [14] and Park et al. [15] demonstrated that wearing shoes with different heel heights affected the lifting capabilities and posture of the lifter. However, there are no reports on the effect of wearing shoes while lifting a load in limited or restricted spaces.

Lifting frequency is essential in determining a worker’s lifting capacity. An increase in the lifting rate results in an increased workload, which requires the worker to apply additional metabolic energy to perform the task [16]. Périard et al. stated that the cardiovascular response (i.e., heart rate) to dynamic exercises can be quantified objectively based on the maximal functional capacity (i.e., maximum aerobic capacity) [17]. Most psychophysical studies have concluded that the maximum acceptable lifting weight decreases with increased lifting frequency [18, 19]. Similarly, Tzu-Hsien et al. found that the maximum acceptable lifting weight for two and four lifts per minute decreased by 7.4% and 16.1%, respectively [20].

Furthermore, Wu et al. [21] conducted a study revealing that the maximum acceptable weight lifted by a worker decreased significantly with an increase in the lifting rate. In this study, we considered the lifting frequencies as 1 and 4 lifts per minute, which has been used by several researchers [22 –24]. The perceived exertion rating represents subjective tools for evaluating personnel’s opinions on the executive effort, and several manual material handling studies employ this rating measurement method [25 –28].

Although previous studies have reported the significant relationship between EMGs, maximum acceptable weight of lift (MAWL), workplace criteria, and lifting frequencies, only a few considered the factor of wearing safety shoes [24 , 29]. However, no studies have reported the effects of safety shoes during precision lifting. Therefore, in this study, we evaluated the changes in EMGs, heart rate (breaths/min), and perceived exertion rating of workers associated with MAWL while wearing different kinds of safety shoes under different lifting frequencies.

2 Methods

2.1 Participants

Ten male workers with a mean age (SD) of 29.70 (3.34) years, mean weight (SD) of 72.20 (7.21) kg, mean height of 167.30 (7.13) cm and mean BMI of 25.79 (2.03) kg/m² were recruited from the university workers population. The participants had no back and lower limb disorders, heart disease symptoms, or breathing complications. All participants willingly acknowledged the purpose and experimental procedures and signed consent forms written in their native languages, mainly Urdu and Hindi, and approved by the Scientific Research Director–Ethics Committee, under ethical code #E-18-3752. In addition, the principal investigator also informed participants about their rights to withdraw from the study at any given time.

2.2 Instrumentation and measurement

2.2.1 Anthropometric data

The height of the workers standing upright in front of a wall, heels touched, was measured in centimeters. A balance scale (Seca 708, with a range of 0.1–200 and an accuracy of 0.1 kg) was used to measure the weight of workers in kilograms, wearing lightweight clothing and no footwear. Body mass index (BMI) was calculated as: BMI = mass (kg) / height² (m)². Furthermore, the Siber Hegner GPM anthropological instrument was used to measure anthropometric dimensions, such as the fixed anthropometry (0–2100 mm with a straight probe and curved measurement branches) and fiberglass belts (Dean, 0–1500 mm), of the workers, similar to the previous studies [30, 31].

2.2.2 Maximum acceptable weight of lift (MAWL)

MAWL, which is the maximum weight that a worker can repeatedly decrease or increase based on their perception, has been psychologically determined to not exhaust in any given lifting conditions [32 –34]. The maximal lifting capacities were measured before the maximum acceptable lifting [35]. Two lifting loads, a heavy box (∼95% of the workers’ maximal lifting capacity) and a light box (∼33% of the workers’ maximal lifting capacity) were pre-set for the participants, who were randomly assigned a light or heavy box and instructed to make weight adjustments to increase or decrease its mass for 15 min to determine the MAWL. Then, each participant was allowed to continue for another 5 min to consider the MAWL measurements for data analysis. At the end of each session, a minimum of 5 min rest time was provided to the participants to ensure recovery. The mass adjustments and MAWL values were not shared with the participants.

2.2.3 Perceived Exertion Rating (PER)

Borg et al. [36] developed a perceptual exertion rating scale to measure the intensity of work performed at levels 6 through 20, considering these numbers are related to the phrases used to assess how participants perceive the effort exerted during the activity. For example, 6 denotes “nothing at all” and represents a very light workload, whereas 20 denotes “maximum exertion” and represents a very high workload. The PER was placed on a wall facing the participants during the experiment. The employed experimental procedure was similar to the same procedure applied in Al-Ashaik et al. [24].

2.2.4 Electromyography signal response (EMG)

Electromyography (EMG) is a graphical method (global EMG) for recording muscle action potential and analyzing its properties using surface electrodes [37]. In this study, we collected the surface electromyography from four muscle groups, that is, the biceps, deltoid, trapezius, and erector spinae, using a pair of bipolar disposable Ag/AgCl surface electrodes distanced at 2.5 cm from each other, and prepared using standard electrode preparation procedure (shaving and cleaning the skin using alcohol) [38, 39]. The EMG signals were recorded at time intervals of 0.1 s using the ME6000 system in average mode and Ag/AgCl surface electrodes. The signals were differentially amplified and filtered using a band-pass filter (at frequencies ranging from 20–500 Hz, A/D converted and sampled at 1000 Hz).

2.2.5 Heart rate measurement

Heart rate is directly proportional to the workload and lifting frequency [23, 32]. The heart rate of participants was measured and monitored using a polar monitor (Polar Kemple Co., Finland) by inserting it tightly using a conductive gel and placing it on the sternum, for which the participant’s skin was cleaned.

2.3 Experimental variables

The independent variables assessed in this study are as follows: 1) the lifting method (precise and non-precise), 2) lifting frequencies (1 lift/min and 4 lifts/min), and 3) three types of safety shoes (light, medium, and heavy safety shoes).

2.3.1 Lifting method

We used two different lifting methods, the precise and non-precise lifting methods. The precise lifting method was used when the box was placed in a limited space on a wooden frame on a shelf, as shown in Fig. 1.a, whereas the non-precise lifting method was used when the box was to be placed anywhere on the shelf, as shown in Fig. 1.b.

Fig. 1

Lifting methods (a) Precise lifting, and (b) Non-precise lifting methods.

2.3.2 Lifting frequency

Lifting frequencies of 1 and 4 lifts/min were applied in this study, considering these frequencies were examined in similar studies, and hence, the analysis results can be compared [18 , 40].

2.3.3 Types of safety shoe

Three types of safety shoes designed by Shelterall Co., Italy, were used in this study, which complies with the regulations of the Saudi Standard Specification No. SASO/ISO 20345/2011 [41]. The shoes were classified as light-weight, medium, or heavy-duty safety shoes. The light-weight safety shoes, weighing 0.9 kg a pair, were similar to the customary leather shoes, comprising a full leather double seal, rubber sole, steel toe cap, and padded collars. Likewise, the medium safety shoe, weighing 1.05 kg a pair, was manufactured using full leather and comprised of a polyurethane molded sole, low-top steel toe cap, and a double-density padded collar. Lastly, the heavy safety shoe, weighing 1.45 kg a pair, was manufactured using wax full-grain leather and comprised of a polyurethane molded sole, high cut double steel toe, and double density padded collar [24 , 43]. The principal investigator trained the participants to wear the most reasonable pair of safety shoes based on the different sizes of research facilities where the shoes would be utilized.

2.4 Experimental design

A within-subjects design with three independent variables was used to determine the effects of the lifting method, lifting frequency, and type of safety shoe on muscular responses, cardiac responses, and perceived exertion rating. This study evaluated twelve experimental conditions considering the combinations of sequences between two levels of lifting methods, two levels of lifting frequencies, and three levels of safety shoe types. The test sequences were randomly assigned. Once the normality of the data was confirmed using the Shapiro-Wilk test and Mauchly’s Test of Sphericity, ANOVA statistical analyses were performed using the SPSS software (version 22; IBM, Armonk, NY, USA). Significant differences among the data were considered statistically significant (p < 0.05). Finally, the Duncan test was used to distinguish significant differences among the levels of the shoe type factor. The average of all collected data for both runs was used for statistical analysis.

2.5 Experimental procedures

A participatory plan was established after acquiring each participant’s consent. Additionally, the anthropometric data of participants and the maximum amount of weight that the participant could lift in one lift were measured. To familiarize participants with the experimental devices, procedures, protocols, a brief demonstration of lifting the box and adjusting its weight when placing inside the frame on the shelf was provided. On the first day of the experiment, each participant was trained and familiarized with the experimental protocol followed by manual lifting sessions. The experimental treatments were executed randomly.

All participants were asked to avoid strenuous physical actions before the experimental sessions and have a night of regular sleep. Additionally, all participants were asked to wear their working uniforms and an assigned suitable size pair of safety shoes used in the experiment.

At the beginning of each session, Ag/AgCl solid adhesive pre-gelled surface electrodes were placed following standard procedures on the arm, shoulder, neck, and back of the participants to collect the EMG signal response [44]. The electrodes were placed longitudinally in the direction of the muscle fibers, at the midpoint of the palpated muscle nearly halfway between the motor endpoint and the distal part of the muscle

Furthermore, the participant’s strength was measured to calculate the maximum EMG signal response in a specific session. Each MVC test lasted for 9 s (3 s in between was used to keep the MVC constant). The experimenter constantly used verbal encouragement to increase muscle contraction capacity in a loud and strong tone, which generally reduces due to lack of motivation and inhibitory effects [45]. Three trials with 3 min rest time between each trial were performed to relieve muscle fatigue. The measurement procedures were standardized according to body posture, verbal instructions, and encouragement and performed by the investigator [46, 47].

Furthermore, a participant was asked to walk on the treadmill and raise the wooden box under the guidance of the experimenter and helper, who adjusted the lifted weight by lowering the wooden box from the participant’s shoulder to their knuckles. The helper stood on the ground beside the shelves, allowing him to lower the box from shoulder to knuckle height, as shown in Fig. 1.a. If the participant would show any signs of musculoskeletal dysfunction or asked the experimenter to end the study, the test would be terminated. In both runs with different start weights, it was observed that during the weight adjustment, each participant asked the helper to either increase or decrease the weight until their desired weight was achieved (maximum acceptable weight). The participant used to raise a two-handle box with weights (40×60×22 cm).

Each participant was given 15 min to decide their maximum acceptable weight for the lifting speeds of 1 or 4 lifts/min using either one of the two lifting methods (precise and non-precise) and wearing any one of the different shoe types. Participants were encouraged to adjust their lifted weights, starting with a random choice of box’s weights. Finally, each participant was asked to continue lifting the weights for another 5 min using the maximum acceptable weight without making any further adjustment changes in the final session to track and evaluate the physiological responses. When the participant’s heart rate exceeded the maximum recommended limit before the session ended, the experiment was terminated (maximum heart rate = 220 –participant’s age). After a recuperation period of 5 min, a second-round was completed. Only one treatment with two runs was carried per day for each participant, and by the end of each session, the participants were asked to remain under observation for 5 min to monitor and report their recovery heart rate. Following each treatment, the participants orally rated their perceived rate of exertion.

3 Results

3.1 Perceived exertion rating (PER) using borg analysis

The ANOVA results showed that the three independent variables—the lifting method (LM), F (1,9) = 12.041, p < 0.007, where η²= 0.572, the lifting frequency (LF), F (1,9) = 134.745, p < 0.000, where η² = 0.937, the shoe type (ST), F (2,18) = 16.571, p < 0.000, where η²= 0.648, and a two-way interaction between LM and ST, F (2,8) = 4.733, p < 0.022, where η² = 0.345, significantly affected the PER, as shown in Table 1. At first, the PER was significantly higher at 4 lifts/min (mean, [SD] 16.62 [0.26]) than 1 lift/min (mean, [SD] 10.88 [0.32]), as shown in Fig. 2.a. Moreover, the PER was significantly higher in the precise lifting method wearing all three types of safety shoes (mean [SD] = 13.60 [0.35] for lightweight, mean [SD] = 13.80 [0.36] for medium, and mean [SD] = 15.25 [0.33] for heavy weight shoes) as compared to the non-precise lifting method (mean (SD) = 11.90 [0.41] for lightweight, mean [SD] = 12.80 [0.38] for medium, and mean [SD] = 15.15 [0.34] heavy weight shoes), as shown in Fig. 2.b.

Table 1
ANOVA test results for PER determined by the subject

Source SS DF MS F-value p-value Partial eta squared

LM^† 26.133 1 26.133 12.041 0.007 0.572

Error 19.533 9 2.170

LF^† 986.133 1 986.133 134.745 0.000 0.937

Error 65.867 9 7.319

ST^† 132.200 2 66.100 16.571 0.000 0.648

Error 71.800 18 3.989

LM X ST^† 12.867 2 6.433 4.733 0.022 0.345

Error 24.467 18 1.359

Source	SS	DF	MS	F-value	p-value	Partial eta squared
LM^†	26.133	1	26.133	12.041	0.007	0.572
Error	19.533	9	2.170
LF^†	986.133	1	986.133	134.745	0.000	0.937
Error	65.867	9	7.319
ST^†	132.200	2	66.100	16.571	0.000	0.648
Error	71.800	18	3.989
LM X ST^†	12.867	2	6.433	4.733	0.022	0.345
Error	24.467	18	1.359

^† LM, LF, and ST represent lifting method, lifting frequency, and shoe type, respectively.

Fig. 2

Effect of lifting frequency and lifting method on the PER based on the shoes types. a. Effect of lifting frequency on the PER, and b. Effect of lifting method and shoe types on the PER. Bar errors represent the standard deviations. ^*p < 0.05, ^***p < 0.000.

3.2 Heart rate (beats/min) analysis

The ANOVA results also showed that only one independent variable, that is, the lifting frequency (LF), F(1, 8) = 16.664, p < 0.004, where η² = 0.676) significantly affected he heart rate (beats/min) of the participants. Additionally, the heart rate (beats/min) was significantly higher at 4 lifts/min (mean, [SD] 106.58 [2.91]) than at 1 lift/min (mean, [SD] 95.17 [2.37]).

3.3 Electromyography (EMG) signal response analysis

The muscle groups examined in this study included the biceps brachii, deltoid, trapezius, and erector spinae muscles. The EMG signal responses achieved were normalized (EMG signal acquired in a specific session divided by EMG associated with maximum overall strength on the same day) and used in this study.

3.3.1 Analysis of the biceps brachii muscle

The lifting frequency significantly affected the normalized mean square root of the biceps brachii muscle (% MVC), F(1, 9) = 48.331, p < 0.000, where η² = 0.843), as shown in Table 2. The % MVC of the muscle at 4 lifts/min (mean [SD] = 12.87 [1.90]) was significantly than at 1 lift/min (mean, [SD] 5.50 [0.90]).

Table 2
ANOVA results for muscles responses

Source SS DF MS F-value p-value Partial Eta Squared

Biceps brachii muscle

LF^† 1650.354 1 1650.354 48.331 0.000 0.843

Error 307.320 9 34.147

Deltoid muscle

LF† 1421.454 1 1421.454 21.034 0.001 0.700

Error 608.205 9 67.578

LF X ST† 198.305 2 99.152 3.824 0.041 0.298

Error 466.764 18 25.931

Erector spinae muscle

LF† 7844.618 1 7844.618 17.183 0.003 0.656

Error 4108.838 9 456.538

LM X LF X ST† 623.452 2 311.726 4.582 0.025 0.337

Error 1224.464 18 68.026

Trapezius muscle

LM X LF† 68.044 1 68.044 6.100 0.036 0.404

Error 100.396 9 11.155

Source	SS	DF	MS	F-value	p-value	Partial Eta Squared
Biceps brachii muscle
LF^†	1650.354	1	1650.354	48.331	0.000	0.843
Error	307.320	9	34.147
Deltoid muscle
LF†	1421.454	1	1421.454	21.034	0.001	0.700
Error	608.205	9	67.578
LF X ST†	198.305	2	99.152	3.824	0.041	0.298
Error	466.764	18	25.931
Erector spinae muscle
LF†	7844.618	1	7844.618	17.183	0.003	0.656
Error	4108.838	9	456.538
LM X LF X ST†	623.452	2	311.726	4.582	0.025	0.337
Error	1224.464	18	68.026
Trapezius muscle
LM X LF†	68.044	1	68.044	6.100	0.036	0.404
Error	100.396	9	11.155

LM, LF, and ST represent lifting method, lifting frequency, and shoe type, respectively.

3.3.2 Analysis of the deltoid muscle group

The ANOVA results indicated that two independent variables, that is, the lifting frequency ((LF), F (1, 9) = 21.034, p < 0.001, where η² = 0.700) and the two-way interaction between lifting frequency (LF) and shoe type (ST) (F(2, 18) = 3.824, p < 0.041, where η² = 0.298), significantly affected the normalized mean square root of the deltoid muscle (% MVC), as shown in Table 2. The results showed that the % MVC of the deltoid muscle was significantly higher at 1 lifts/min while wearing the lightweight safety shoes as compared to the medium and heavy weight safety shoes (mean [SD] = 10.74 [0.97], mean [SD] = 7.90 [1.04], and mean [SD] = 8.01 [1.21], respectively), as compared to 4 lift/min, where the % MVC was significantly higher while wearing heavy shoes as compared lightweight and medium safety shoes (mean [SD] = 18.23 [3.11], mean [SD] = 14.72 [1.66], and mean [SD] = 14.31 [2.09], respectively), as shown in Fig. 3.

Fig. 3

Effect of lifting frequency on % maximum voluntary contraction (MVC) of Deltoid Muscle according to the shoe type. Bar errors represent the standard deviation. ^*p < 0.05.

3.3.3 Analysis of the erector spinae muscle

ANOVA results showed that three independent variables, that is, the lifting frequency ((LF), F (1,9) = 17.183, p < 0.003, where η² = 0.656), three-way interaction between lifting method (LM) and lifting frequency (LF), and the shoe type (ST), (F (2,18) = 4.582, p < 0.025, where η² = 0.337) significantly affected the normalized mean square root of the erector spinae muscle (% MVC), as shown in Table 2. The results signified that the % MVC was significantly higher in the precise lifting method at 4 lifts/min while wearing lightweight, medium, and heavy safety shoes (mean [SD] = 29.49 [5.29], mean [SD] = 30.66 [6.43] and mean [SD] = 54.93 [16.82], respectively) than at 1 lift/min while wearing lightweight, medium, and heavy safety shoes (mean [SD] = 17.81 [3.46], mean [SD] = 19.28 [3.04], and mean [SD] = 27.07 [11.16], respectively), as shown in Fig. 4. Similarly, the % MVC of the erector spinae muscle in the non-precise lifting method was slightly more significant at 4 lifts/min while wearing lightweight, medium, and heavy safety shoes (mean [SD] = 47.14 [13.96], mean [SD] = 27.02 [4.91], and mean [SD] = 43.42 [14.96], respectively) as compared to 1 lift/min while wearing light, medium, and heavy safety shoes (mean [SD] = 25.30 [10.43], mean [SD] = 18.58 [4.04], and mean [SD] = 27.61 [8.53], respectively), as shown in Fig. 4.

Fig. 4

Effect of lifting method and lifting frequency according to the shoe type on the % MVC of the Erector Spinae Muscle response. Bar errors represent the standard deviations. ^*p < 0.05.

3.3.4 Analysis of the trapezius muscle group

ANOVA results showed that two independent variables, that is, the two-way interaction between the lifting method (LM) and lifting frequency (LF) (F (1, 9) = 6.100, p < 0.036, where η²= 0.404) significantly affected the normalized mean square root of the trapezius muscle (% MVC), as shown in Table 2. The % of MVC of the trapezius muscle was significantly higher in the precise lifting method at 4 lifts/min and 1 lift/min (mean [SD] = 17.62 [3.95], and mean [SD] = 11.76 [4.97], respectively) as compared to that of the non-precise lifting method at 4 lifts/min and 1 lift/min (mean [SD] = 12.29 [2.09], and mean [SD] = 9.44 [3.34], respectively), as shown in Fig. 5.

Fig. 5

Effects of lifting method and lifting frequency on the trapezius muscle response. Bar errors represent the standard deviations.^*p < 0.05.

4 Discussion

The main objective of this study was to investigate the effects of the lifting method, lifting frequency, and types of safety shoes on the heart rates, muscular activities, and perceived exertions of manual workers. The hypotheses of this study stated that the lifting frequency, lifting methods (precise and non-precise), and wearing safety shoes significantly increased the heart rate, muscular activity, and perceived exertion. Thus, the increase in heart rate and muscular activities could increase work stress and decreased lifting capabilities. Conversely, the increase in perceived exertion could be due to a specific safety shoe type along with an increase in work stress.

In this experiment, the PER was significantly higher when the frequency of lifting was high (4 lifts/min), approximately 52.77% higher than that at 1 lift/min, as an index of work intensity. The literature in [21] supports the frequency of lifting results of this experiment. Furthermore, the results also agree with that in studies conducted by Al-Ashaik [24] and Ghaleb et al. [27], where participants experienced low perceived exertion (mean (SD), 10.88 (0.32)) at 1 lift/min as compared to that at four lifts/min(mean (SD), 16.62 (0.26)). However, in this study, the PER was significantly affected by the other independent variables, lifting method, and the type of safety shoe. A higher PER was reported when participants wore heavy safety shoes in both lifting methods and wore lightweight or medium safety shoes in precise lifting method. Again, these results are in agreement with the previous results described in the literature [24, 27].

Considering the precise lifting method required lifting and placing objects in restricted spaces, an increase in accuracy was most likely due to the slow placing movement at the destination, which increased the holding time, and hence, participants accepted lighter loads. Therefore, this study has proved that the lifting method, type of safety shoe, and lifting frequency affected the participants’ heart rate. Furthermore, the results showed that increasing the lifting frequency from 1 lift/min to 4 lifts/min can increase the mean heart rate (beats/min) by 11.99%. These results are in agreement with that of [24 , 40].

The precise lifting method significantly increased the % MVC of the trapezius muscle activity compared to the non-precise lifting method, which agrees with the findings reported by Sadler et al. [48]. Additionally, the % MVC of the erector spinae muscle activity was significantly higher in the precise lifting method for both lifting frequencies and increased by 43.37% at 4 lift/min, which agrees with Sadler’s findings et al. [48]. This indicates that the precise lifting caused higher musculoskeletal system stress as the lifting frequency increased compared to the non-precise lifting method. In addition, the precise lifting method significantly increased the % MVC of the erector spinae muscle activity when wearing heavy shoe type than lightweight and medium shoe types, as compared to the non-precise lifting method. This finding was in agreement with the findings of Al-Ashaik et al. [38].

Moreover, this study indicated that muscular activation in the anterior deltoid muscle was greater when wearing heavy safety shoes at a pace of four lifts per minute than light safety shoes at one lift per minute. As a result, lifters should avoid wearing heavy safety shoes during high-frequency lifting and lightweight or medium safety shoes during low-frequency lifting. Additionally, while wearing different safety shoes, the % MVC of anterior deltoid muscle activation was considerably higher at high lifting frequencies than at low lifting frequencies. The percent of the MVC of biceps brachii muscular activity at four lifts/min was significantly increased by 134% compared to one lift/min. This finding corroborates with that of Ghaleb et al. [27], who proved that a high percentage of the MVC of the biceps brachii muscle was associated with a high-frequent lifting.

5 Limitations

To understand and interpret the results of this study, some considerations should be clarified. The number of participants in the experiment was relatively small (n = 10), which is typical for most ergonomic laboratory studies. The repeated measurements acquired from the subjects gave the statistical power sufficient to protect the biologically significant differences between the different effects from type II errors, considering the small number of subjects’ limited anthropometry variability, which may have influenced the results. Furthermore, this study focused on freestyle lifting techniques and was not limited to a specific controlled lifting technique. The biomechanical stress in joints can be affected differently depending on the lifting condition. Although the potential limitations have a limited impact on current outcomes, the need to investigate other lifting methods, box positions, frequency of lifts, and safety shoe types persists.

6 Conclusions

The results of this study have verified that the precise lifting method, high lifting frequency, and heavy safety shoe type significantly influenced the increase in PER, EMG signal responses, and heart rate. Therefore, this study can guide manual workers in industries and service sectors to perform lifting activities using different lifting methods and to wear safety shoes. Furthermore, this study demonstrated that replacing regular heavy safety shoes with suitably selected shoes at some workplaces can significantly decrease the PER needed to lift objects in restricted spaces. However, the benefits should be carefully evaluated before replacing the existing safety shoes. Understanding the effects of safety shoes, lifting methods, and lifting frequencies on the EMG signals, heart rate, and PER can help industrial designers, researchers, ergonomists, and safety professionals make informed decisions during task planning regarding selecting appropriate safety shoes for activities.

Based on the results of the study, the following recommendations have been made for future research to improve precision placement in manual tasks for different lifting frequencies and using different lifting methods, while wearing different types of safety shoes: 1) Investigating the lifting capabilities under extreme environmental conditions wearing different types of safety shoes, and using different task variable values, other than those evaluated in this study (lift duration, lifting style, lifting object weight, box size, etc.), 2) Investigating the biomechanical, physiological, and psychophysical approaches simultaneously to determine the individual handling capabilities under different environmental conditions wearing different types of safety shoes, 3) Investigating the mechanism behind the effect of precision on the maximal lifting capacity and maximal acceptable weight of lift, and 4) Investigating the different manual tasks involving precision, considering precision affects all lifting assessment tools.

Data availability

The data used to support the findings of this study is available from the corresponding author upon reasonable request.

Conflicts of interest

The authors declare that there are no conflicts of interest.

Footnotes

Acknowledgments

The authors fully acknowledge all who helped the researchers in this work.

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