Effects of tool grip span,workpiece orientation,moving direction,and working height on two-handed torque strength and subjective measures

Abstract

BACKGROUND:

Using both hands is regularly needed for force/torque exertions in many activities, especially when using hand tools.

OBJECTIVE:

To investigate the effects of tool grip span, workpiece orientation, moving direction, and working height on two-handed wrist U/R deviation torque strength, usability, comfort, and discomfort while using locking pliers.

METHODS:

Participants (n = 22) took part in an experimental study which evaluated the effects of tool grip span (4.5–6.5 cm), workpiece orientation (transverse/sagittal), moving direction (clockwise (CW)/counterclockwise (CCW)), and working height (shoulder/elbow/knuckle) on two-handed wrist U/R deviation torque, (dis)comfort, and usability while using locking pliers.

RESULTS:

The results showed no significant effect of tool grip span on wrist U/R deviation torque strength, but the locking pliers with 4.5 cm handle grip span led to more comfort and better usability. The two-handed wrist U/R deviation torque strengths were significantly higher in sagittal plane than in transverse plane, and in CW direction than in CCW direction. The highest values of two-handed wrist U/R deviation torque strength in sagittal and transverse planes were exerted in knuckle and elbow heights, respectively.

CONCLUSION:

The findings can be used to develop guidelines and recommendations with regard to daily and occupational activities which require the use of both hands for force exertion with manual hand tools.

Keywords

Hand tools locking pliers two-handed torque usability wrist torque

1 Introduction

Upper extremity is one of the most important part of the body to accomplish central nervous system commands [1]. A systematic review study indicated the annual incidence and the prevalence of upper extremity musculoskeletal disorders worldwide ranged from 0.08% to 6.3% and from 0.14% to 21.9% in the working population, respectively [2]. In light of the high prevalence of work-related injuries, studies on upper extremity strength limits are of interest to ergonomists [3]. Human strength data, as ergonomic guidelines, are used for force and posture assessments as well as for product and workstation design. These data are also used to make sure that the required muscle force when using a product or doing a task does not exceed the users’ capabilities. These data can be used by ergonomists and health professionals to prevent local fatigue and cumulative diseases among workers [4, 5].

Occupational and daily activities often require applying high levels of forces and moments by the upper extremities [6, 7]. The risk of occupational injuries is higher when the torque demand of the task exceeds torque capacity of an individual [8]. Regardless of mechanization, still the primary interface between work and many operators are manual hand tools [9]. Manual hand tools such as screwdrivers, wrenches, knives, pliers, and hammers are frequently used in daily and occupational activities. Ergonomically designed hand tools can lead to reduced discomfort, biomechanical stresses, and musculoskeletal risks and improved usability, efficiency and performance [10 –12]. To evaluate the ergonomic design of hand tools, consideration of both subjective and objective measures is really important [12, 13].

The human hand’s muscular strength largely depends on exerting force with one (dominant) or two hands [14]. A majority of previous studies have investigated hand torque strength using just one (dominant) hand, while force exertion with both hands may be needed in actual working conditions. For instance, when a higher level of muscular strength is required for a task, the use of both hands allows better control and task performance than one-handed performance. So far, several studies have examined two-handed torque exertion for evaluation of pulling/pushing tasks [15], lifting/lowering tasks [16], backhand stroke motions [17], and hand wheels [18, 19]. However, to the authors’ knowledge, there is no study which evaluates two-handed torque strength exertion when using manual hand tools. More specifically, we were interested in evaluating two-handed torque strength exertion in wrist ulnar/radial (U/R) deviation direction. The wrist U/R deviation torque strength is required, for example, for pulling or pushing a handle to turn it around a central axis. Locking pliers are one example of hand tools used for the wrist U/R torque exertions in repair or maintenance tasks. Although locking pliers are typically one-handed operated tool, the use of both hands is required for better task control or higher torque exertions when using this tool. In addition, it was aimed in this study to examine how the design of a handle (e.g., hand tool grip span) and workstation configurations influence the wrist U/R torque exertions with both hands. The findings from research on this topic can help to model the exerted wrist U/R torque strength with both hands on the tool handles in different workstation configurations. To address this issue, the present study was conducted to evaluate the tool handle design and workstation configurations involving two-handed wrist U/R deviation torque strength exertion using locking pliers as an exemplar manual hand tool by taking into account both objective and subjective measures. Therefore, the aim of study was to investigate the effects of tool grip span, workpiece orientation, moving direction, and working height on two-handed wrist U/R deviation torque strength, usability, comfort, and discomfort while using locking pliers.

2 Materials and methods

2.1 Participants

Healthy male students from the Tabriz University of Medical Sciences, aged between 18 and 35 years, defined as inclusion criteria, were invited by an advertisement placed on a notice-board in the university. Former history of neurologic or inflammatory disorders or upper limb injuries were determined as exclusion criteria. Based on a pilot study on 5 participants, study variables (tool grip span, workpiece orientation, moving direction, and working height) and the resultant two-handed wrist U/R deviation torque strengths, with a confidence level of 95% and 80% power, the sample size was calculated to be 22 participants. The participants’ ages ranged from 19 to 35 years (mean (SD) = 23.9 (3.8) years). All participants were right-handed, none were professionally involved in working with manual hand tools, and all were healthy with no history of upper extremity musculoskeletal injuries (based on self-report data). All participants signed a written consent form before participation. Participants’ anthropometric indices are summarized in Table 1. These anthropometric dimensions were measured and examined in the study, because they may influence the torque strength exertions [9]. The ethical review committee of Tabriz University approved the study protocol.

Table 1
Anthtopometric characteristics of study participants (n = 22)

Characteristics Mean (SD) Range

Age (year) 23.9 (3.8) 19–35

Stature (cm) 170.0 (10.2) 151.0–185.0

Weight (kg) 70.7 (15.3) 45.1–100.4

Arm length (cm) 34.4 (2.4) 30.5–38.9

Forearm length (cm) 26.3 (2.7) 21.6–30.2

Hand length (cm) 17.9 (1.4) 15.5–20.0

Hand width (cm) 8.1 (0.7) 6.8–9.8

Wrist width (cm) 5.6 (0.5) 4.8–6.7

Wrist depth (cm) 3.9 (0.5) 3.2–4.9

Thumb length (cm) 6.6 (0.7) 4.9–7.5

Thumb width (cm) 1.9 (0.2) 1.5–2.2

Thumb depth (cm) 1.58 (0.2) 1.3–1.9

Index finger length (cm) 7.2 (0.6) 6.1–8.2

Index finger width (cm) 1.8 (0.2) 1.4–2.1

Index finger depth (cm) 1.6 (0.2) 1.3–1.9

Middle finger length (cm) 7.6 (0.7) 6.5–8.8

Middle finger width (cm) 1.8 (0.2) 1.5–2.2

Middle finger depth (cm) 1.7 (0.2) 1.3–2.0

Ring finger length (cm) 7.0 (0.6) 5.8–8.1

Ring finger width (cm) 1.7 (0.2) 1.3–2.3

Ring finger depth (cm) 1.6 (0.2) 1.3–1.9

Little finger length (cm) 5.7 (0.5) 4.9–6.6

Little finger width (cm) 1.5 (0.2) 1.1–2.0

Little finger depth (cm) 1.3 (0.2) 1.0–1.7

Characteristics	Mean (SD)	Range
Age (year)	23.9 (3.8)	19–35
Stature (cm)	170.0 (10.2)	151.0–185.0
Weight (kg)	70.7 (15.3)	45.1–100.4
Arm length (cm)	34.4 (2.4)	30.5–38.9
Forearm length (cm)	26.3 (2.7)	21.6–30.2
Hand length (cm)	17.9 (1.4)	15.5–20.0
Hand width (cm)	8.1 (0.7)	6.8–9.8
Wrist width (cm)	5.6 (0.5)	4.8–6.7
Wrist depth (cm)	3.9 (0.5)	3.2–4.9
Thumb length (cm)	6.6 (0.7)	4.9–7.5
Thumb width (cm)	1.9 (0.2)	1.5–2.2
Thumb depth (cm)	1.58 (0.2)	1.3–1.9
Index finger length (cm)	7.2 (0.6)	6.1–8.2
Index finger width (cm)	1.8 (0.2)	1.4–2.1
Index finger depth (cm)	1.6 (0.2)	1.3–1.9
Middle finger length (cm)	7.6 (0.7)	6.5–8.8
Middle finger width (cm)	1.8 (0.2)	1.5–2.2
Middle finger depth (cm)	1.7 (0.2)	1.3–2.0
Ring finger length (cm)	7.0 (0.6)	5.8–8.1
Ring finger width (cm)	1.7 (0.2)	1.3–2.3
Ring finger depth (cm)	1.6 (0.2)	1.3–1.9
Little finger length (cm)	5.7 (0.5)	4.9–6.6
Little finger width (cm)	1.5 (0.2)	1.1–2.0
Little finger depth (cm)	1.3 (0.2)	1.0–1.7

2.2 Design of the experiment

Based on the results of a recent review study, there is no consensus on the optimal diameter of hand tools while exerting one-handed wrist ulnar/radial deviation, wrist flexion/extension, and forearm supination/pronation torque strengths [9]. Additionally, there was no similar study evaluating the two-handed ulnar/radial deviation torque strength using hand tools. Therefore, locking pliers in this study were mainly chosen based on the commonly available tools on the market and considering a range for comparison purposes. Thus, three locking pliers were used in the study (Table 2). One of them was a traditional type, which is commercially available and is usually used by industrial workers (handle grip span = 4.5 cm). Two other prototypes were constructed by modifying the handle grip span of traditional locking pliers to 5.5 cm and 6.5 cm. All locking pliers had the same length of 25 cm. Another independent variable was workpiece orientation with two levels (sagittal and transverse planes). Third independent variable was moving direction (clockwise (CW) and counterclockwise (CCW) conditions). Working height was the fourth independent variable with three levels: shoulder height, elbow height, and knuckle height. There were four dependent variables including: 1) two-handed wrist U/R deviation torque strength (Nm), 2) subjective assessment of upper limb discomfort, 3) subjective evaluation of comfort, and 4) usability, which was defined as the extent to which a system, product, or service can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use [20].

Table 2
Characteristics of the tools used in the study

Handle grip span (cm) Length (cm) Weight (g) Appearance

4.5 25 535

5.5 25 580

6.5 25 638

Handle grip span (cm)	Length (cm)	Weight (g)
4.5	25	535
5.5	25	580
6.5	25	638

2.3 Procedure

At first, all participants became familiar with the goal and procedures of the experiment and they were told that they had full right to withdraw at any point of the experiment. The participants’ age and anthropometric dimensions (measured using caliper, stadiometer, and scale) were recorded. A total of 36 treatment combinations (3 prototype locking pliers with different handle grip spans × 2 workpiece orientations × 2 moving directions × 3 working heights) were conducted in two sessions (two days) for each participant. The order of examinations was randomized to control the effects of fatigue and learning. All the experiments were carried out in an ergonomics lab by the same investigator. All trials were conducted from 8 am to 1 pm in order to control circadian rhythm.

2.4 Measurement of the dependent variables

To measure torque strength, an isometric dynamometer was designed and fabricated with the ability to measure pure torque at the three anatomical planes (transverse, sagittal, and frontal planes) (Fig. 1). This set-up consisted of mechanical, electrical, and software sections. In this device, TM Automation Instruments Co., Ltd. was used as a single axis torque sensor mounted between self-align bearings. Advantech® (USB-4704-AE) with a maximum sampling rate of 4800 Hz was used to convert analog output signals to digital data (A/D). To calibrate the dynamometer in the range of±200 Nm (CW and CCW), least-square linear regression was used to find optimal gain and optimal offset [21]. To identify the device resolution, the minimum detectable change (MDC) and the standard error of measurement (SEM) for observations were determined. These values were 0.56 Nm and 0.20 Nm for the CCW and 0.42 Nm and 0.15 Nm for the CW directions, respectively.

Fig. 1

Torque meter (a); an example of torque measuring setup for measurement of the wrist U/R torque strength in sagittal plane.

All participants performed practice trials prior to actual tests to become familiar with the study. All postures and hand positions were controlled in the experiments. All trials were conducted in standing position. To maintain the same situation for torque exertions, the participants were asked to adopt their preferred posture (e.g., the same distance between their legs, and between their body and the dynamometer) at the start of the experimental session and to maintain that position uniformly as far as possible throughout the trials. As torque strength is heavily dependent on the tool length, hand postures (e.g., positioning of the participants’ hands on the tool handle) were kept constant as far as possible throughout the trials. For this, participants exerted their maximal torque strength with their dominant hand, with their non-dominant hand acted as a support for the dominant hand for better control of the task and exertion of additional torque on the tool handle. They were asked to follow this uniform hand position as far as they could during the trials. Maximal isometric torque exertion followed the standard recommended protocol: the participant increased his force gradually over a 2–3 s period, exerted his maximum force for 3 s (steady-state of exertion), and then gradually reduced it to zero over a 2–3 s period [22]. Data were sampled at 20 Hz. Two repetitions for each treatment combination with sufficient rest allowance (2–3 min) were recorded, and they considered valid if they were within 10% of each other [22, 23]. The average value from the steady-state portion of two exertions was used for data analysis. No visual/verbal feedback or encouragement was given to the participants [22].

Upper limb discomfort was assessed by a body map together with a Likert scale from 0 = no pain to 5 = severe pain [12]. Hand tool comfort was evaluated using comfort questionnaire for hand tools (CQH) developed by Kuijt-Evers et al. [24]. The CQH has 17 questions with Likert scale responses from 1 to 7 (completely disagree to completely agree). This scale has also a main query with a Likert scale response from 1 = the most uncomfortable to 7 = the most comfortable. The cross-cultural adaptation and also translation of the English version of CQH into Persian was done according to the recommended guidelines [25]. Two independent Persian ergonomists who were proficient in English language translated the questionnaire from English to Persian in forward translation step. Afterwards, two professional Iranian translators carried out the back translation to English. In qualitative assessment, a panel of 10 experts (ergonomists and psychometricians) proved its content validity after some minor revisions. The Cronbach’s alpha (0.890), CVI (>0.75) and CVR (>0.62), and also impact score (>1.5) for almost all queries were acceptable. Higher scores of CQH indicate more comfort. The comfort and discomfort scales were completed after finishing each 36 experimental conditions during the two test sessions. The mean values of different handle grip spans were computed for subsequent analyses.

System usability scale (SUS) consisting of ten questions with 5-point Likert scale responses ranging from 1 = completely disagree to 5 = completely agree was used for evaluating usability [26]. A validated Persian version of SUS was used [27]. This scale gives a score from 0 to 100, with higher scores showing better usability. The SUS questionnaire was filled out after finishing each experimental condition.

2.5 Data analysis

SPSS Statistics v.25 software (IBM Corp., Armonk., NY, USA) was used for analysis (descriptive and inferential statistics). The upper extremity anthropometric dimensions (shown in Table 1) were considered in models as control variables via three principal components (PCs). Principal component analysis (PCA) was used to reduce the number of upper extremity anthropometric indices (presented in Table 1), with greatest variance extracted. The components with eigenvalues higher than 1 were kept in the models [28]. Mixed model analysis of variance (ANOVA) was used for evaluation of the main effects of tool grip span, workpiece orientation, moving direction, and working height. Interaction effects among the independent variables were also included in the models. Restricted Maximal Likelihood (REML) method was used for estimating the parameters in the model. Covariance structure within repeated measurements was selected as first order autoregressive (AR1) based on Akaike Information Criteria (AIC). Analyses were followed by Sidak post hoc on adjusted means to explore the effects in more detail. Non-parametric Friedman test was used to examine upper limb discomfort ratings. P-values < 0.05 were considered as statistical significance for all tests.

3 Results

3.1 Two-handed wrist U/R deviation torque strength

Mean (SD) and range of the exerted two-handed wrist U/R deviation torque strength with each prototype locking pliers in different experimental conditions are shown in Table 3. The results of the post hoc tests are shown in Table 4.

Table 3
Torque strength (Nm) by each locking pliers with different grip spans in different workpiece orientations, moving directions, and working heights

Locking pliers Workpiece Moving Working Mean (SD) Range

orientation^† direction^‡ height

4.5 cm handle grip span 31.1 (18.9) 7.7–107.5

S CW Shoulder 33.4 (17.3) 15.6–70.8

S CCW Shoulder 24.4 (18.4) 7.7–78.8

S CW Elbow 38.7 (21.0) 13.9–100.0

S CCW Elbow 34.7 (18.5) 12.5–88.1

S CW Knuckle 44.7 (24.9) 13.0–102.0

S CCW Knuckle 42.9 (24.4) 15.1–107.5

T CW Shoulder 24.1 (14.0) 8.1–53.1

T CCW Shoulder 22.61 (1.3) 8.9–48.9

T CW Elbow 29.6 (15.8) 10.1–68.6

T CCW Elbow 26.9 (12.8) 10.8–53.0

T CW Knuckle 27.0 (16.4) 9.4–60.6

T CCW Knuckle 25.5 (13.0) 8.6–49.6

5.5 cm handle grip span 31.0 (19.0) 5.1–116.7

S CW Shoulder 34.3 (19.2) 16.1–86.4

S CCW Shoulder 27.2 (19.2) 5.1–79.3

S CW Elbow 37.6 (19.4) 15.6–89.6

S CCW Elbow 34.7 (17.6) 9.8–77.0

S CW Knuckle 44.4 (23.7) 14.7–98.3

S CCW Knuckle 44.6 (28.7) 12.5–116.7

T CW Shoulder 24.0 (13.4) 7.5–54.0

T CCW Shoulder 21.9 (11.3) 10.8–44.3

T CW Elbow 28.9 (16.0) 9.7–64.1

T CCW Elbow 25.3 (11.2) 10.4–46.8

T CW Knuckle 27.4 (16.4) 7.1–64.0

T CCW Knuckle 24.8 (12.5) 10.5–47.2

6.5 cm handle grip span 30.6 (18.3) 7.2–109.5

S CW Shoulder 34.6 (17.9) 16.0–71.4

S CCW Shoulder 27.8 (16.5) 9.0–68.5

S CW Elbow 35.4 (19.1) 12.8–76.7

S CCW Elbow 38.4 (21.6) 14.5–78.3

S CW Knuckle 40.6 (22.1) 15.9–100.6

S CCW Knuckle 45.8 (26.3) 10.6–109.5

T CW Shoulder 22.1 (10.8) 7.9–41.8

T CCW Shoulder 22.0 (10.9) 9.5–47.2

T CW Elbow 26.3 (14.0) 9.7–55.5

T CCW Elbow 25.5 (11.9) 12.4–48.8

T CW Knuckle 25.4 (14.7) 7.2–56.1

T CCW Knuckle 25.3 (12.1) 9.9–49.9

Locking pliers	Workpiece	Moving	Working	Mean (SD)	Range
4.5 cm handle grip span				31.1 (18.9)	7.7–107.5
	S	CW	Shoulder	33.4 (17.3)	15.6–70.8
	S	CCW	Shoulder	24.4 (18.4)	7.7–78.8
	S	CW	Elbow	38.7 (21.0)	13.9–100.0
	S	CCW	Elbow	34.7 (18.5)	12.5–88.1
	S	CW	Knuckle	44.7 (24.9)	13.0–102.0
	S	CCW	Knuckle	42.9 (24.4)	15.1–107.5
	T	CW	Shoulder	24.1 (14.0)	8.1–53.1
	T	CCW	Shoulder	22.61 (1.3)	8.9–48.9
	T	CW	Elbow	29.6 (15.8)	10.1–68.6
	T	CCW	Elbow	26.9 (12.8)	10.8–53.0
	T	CW	Knuckle	27.0 (16.4)	9.4–60.6
	T	CCW	Knuckle	25.5 (13.0)	8.6–49.6
5.5 cm handle grip span				31.0 (19.0)	5.1–116.7
	S	CW	Shoulder	34.3 (19.2)	16.1–86.4
	S	CCW	Shoulder	27.2 (19.2)	5.1–79.3
	S	CW	Elbow	37.6 (19.4)	15.6–89.6
	S	CCW	Elbow	34.7 (17.6)	9.8–77.0
	S	CW	Knuckle	44.4 (23.7)	14.7–98.3
	S	CCW	Knuckle	44.6 (28.7)	12.5–116.7
	T	CW	Shoulder	24.0 (13.4)	7.5–54.0
	T	CCW	Shoulder	21.9 (11.3)	10.8–44.3
	T	CW	Elbow	28.9 (16.0)	9.7–64.1
	T	CCW	Elbow	25.3 (11.2)	10.4–46.8
	T	CW	Knuckle	27.4 (16.4)	7.1–64.0
	T	CCW	Knuckle	24.8 (12.5)	10.5–47.2
6.5 cm handle grip span				30.6 (18.3)	7.2–109.5
	S	CW	Shoulder	34.6 (17.9)	16.0–71.4
	S	CCW	Shoulder	27.8 (16.5)	9.0–68.5
	S	CW	Elbow	35.4 (19.1)	12.8–76.7
	S	CCW	Elbow	38.4 (21.6)	14.5–78.3
	S	CW	Knuckle	40.6 (22.1)	15.9–100.6
	S	CCW	Knuckle	45.8 (26.3)	10.6–109.5
	T	CW	Shoulder	22.1 (10.8)	7.9–41.8
	T	CCW	Shoulder	22.0 (10.9)	9.5–47.2
	T	CW	Elbow	26.3 (14.0)	9.7–55.5
	T	CCW	Elbow	25.5 (11.9)	12.4–48.8
	T	CW	Knuckle	25.4 (14.7)	7.2–56.1
	T	CCW	Knuckle	25.3 (12.1)	9.9–49.9

^†S = Saggital, T = Transverse. ^‡ CW = Clockwise, CCW = Counterclockwise.

Table 4

Differences (p-values)^† between wrist U/R deviation torque exertions in various conditions

Pairwise comparisons	Sagittal plane		Transverse plane
	CW	CCW	CW	CCW
Shoulder/elbow	0.117	0.000	0.002	0.027
Shoulder/knuckle	0.000	0.000	0.076	0.103
Knuckle/elbow	0.000	0.000	0.568	0.941

^†Significant differences are shown in bold. CW = Clockwise, CCW = Counterclockwise.

The results of PCA showed that the first three PCs were critical factors with eigenvalues of 12.85, 2.87, and 1.04, respectively. These accounted for 61.2%, 13.7%, 4.9% (totally 79.8%) of the upper extremity anthropometric variations. The first PC (F = 12.29; p = 0.003) and third PC (F = 7.409; p = 0.014) were significantly related to the two-handed exerted torque, indicating that the linear combination of anthropometric indices were related to the two-handed exerted wrist U/R deviation torque and were adjusted for, accordingly.

A significant main effect of workpiece orientation (F(1, 7) = 400.38; p < 0.001), moving direction (F(1, 7) = 13.06; p < 0.001), and working height (F(2, 7) = 75.83; p < 0.001) was found for the maximal two-handed wrist U/R deviation torque strength. The effect of tool grip span was not statistically significant. Maximal two-handed wrist U/R deviation torque strength was significantly higher in anatomical sagittal plane (mean [±SE] = 36.9 [±2.4]) than in transverse plane (mean [±SE] = 25.2 [±2.4]) (p < 0.001). Additionally, higher torque strength was found for the CW direction (mean [±SE] = 32.1 [±2.5]) in comparison with the CCW direction (mean [±SE] = 30.0 [±2.5]) (p < 0.001). Moreover, maximal wrist U/R deviation torque exertions, in decreasing order, were recorded in knuckle (mean [±SE] = 35.2 [±2.5]), elbow (mean [±SE] = 31.6 [±2.5]), and shoulder (mean [±SE] = 26.4 [±2.5]) heights (p < 0.001).

Significant interactions were also found among the variables. Interaction between workpiece orientation and working height (p < 0.001) indicated that in sagittal plane, exerted torque was higher in knuckle height, while in transverse plane, the highest torque was exerted in elbow height. The interaction between moving direction and working height (p < 0.05) showed that the highest torque was exerted in knuckle height in both CW and CCW directions.

There was significant interaction among workpiece orientation, moving direction, and working height (p < 0.01) (Table 4). The torque exerted in sagittal plane, CW direction, and knuckle height (mean [±SE] = 44.1 [±2.6]) was significantly higher than that exerted in sagittal plane, CW direction, and shoulder height (mean [±SE] = 33.9 [±2.6]) (p < 0.001) and in elbow height (mean [±SE] = 36.9 [±2.6]) (p < 0.001). The torque exerted in sagittal plane, CCW direction, and knuckle height (mean [±SE] = 45.2 [±2.6]) was significantly higher than those exerted in sagittal plane, CCW direction, and shoulder height (mean [±SE] = 26.2 [±2.6]) (p < 0.001) and in elbow height (mean [±SE] = 35.6 [±2.6]) (p < 0.001). Moreover, maximal wrist U/R deviation torque exertion in sagittal plane, CCW direction, and elbow height (mean [±SE] = 35.6 [±2.6]) was higher than that exerted in sagittal plane, CCW direction, and shoulder height (mean [±SE] = 26.2 [±2.6]) (p < 0.001). Exerted torque in transverse plane, CW direction, and elbow height (mean [±SE] = 28.2 [±2.6]) was higher than exerted torque in transverse plane, CW direction, and shoulder height (mean [±SE] = 23.3 [±2.6]) (p < 0.01). Additionally, exerted torque in transverse plane, CCW direction, and elbow height (mean [±SE] = 25.8 [±2.6]) was higher than exerted torque in transverse plane, CCW direction, and shoulder height (mean [±SE] = 22.1 [±2.6]) (p < 0.05).

3.2 Upper extremity discomfort

The mean severity of discomfort ratings for various parts (A = Small finger distal phalanx; B = Ring finger distal phalanx; C = Middle finger distal phalanx; D = Index finger distal phalanx; E = Small finger proximal and middle phalanges; F = Ring finger proximal and middle phalanges; G = Middle finger proximal and middle phalanges; H = Index finger proximal and middle phalanges; I = Palm area; K = Hypothenar; L = Thenar area and thumb proximal phalanx; M = Thumb distal phalanx; O = Arm; P = Forearm; Q = Wrist) of the dominant (right) upper extremity are shown in Fig. 2. The highest reported discomfort was in the thenar area and thumb proximal phalanx (part L). The highest discomfort ratings were generally recorded for the locking pliers with 6.5 cm handle grip span, although the results showed no significant differences among the locking pliers.

Fig. 2

Mean severity of upper extremity discomfort while using three different locking pliers (Error bars show standard errors). Upper extremity areas: A = Small finger distal phalanx; B = Ring finger distal phalanx; C = Middle finger distal phalanx; D = Index finger distal phalanx; E = Small finger proximal and middle phalanges; F = Ring finger proximal and middle phalanges; G = Middle finger proximal and middle phalanges; H = Index finger proximal and middle phalanges; I = Palm area; K = Hypothenar; L = Thenar area and thumb proximal phalanx; M = Thumb distal phalanx; O = Arm; P = Forearm; Q = Wrist.

3.3 Comfort

The locking pliers with 4.5 cm handle grip span was rated significantly more comfortable than the other two prototypes (p < 0.001). The CQH mean scores for the locking pliers with 4.5 cm, 5.5 cm and 6.5 cm handle grip spans were 77.4, 55.9, and 42.7, respectively.

3.4 Usability

According to the results of mixed model ANOVA, the mean SUS score for three different prototypes of locking pliers were significantly different. The highest SUS score was obtained for the locking pliers with 4.5 cm handle grip span (mean [±SE] = 74.2 [±2.2]). This was followed by the locking pliers with 5.5 cm handle grip span (mean [±SE] = 56.7 [±3.3]) and the one with 6.5 cm handle grip span (mean [±SE] = 45.4 [±3.1]).

4 Discussion

Most available data on human hand strength, particularly when using manual hand tools, are related to studies which have considered strength exertions using just one hand, while there are some cases in which both hands are needed to exert force on manual hand tools. The aim of this study was to investigate the effects of tool grip span as one of the important aspects of tool handle design and workpiece orientation, moving direction, and working height as workstation design features on two-handed wrist U/R deviation torque strength and subjective measures using locking pliers. The results showed no significant effect of tool grip span on wrist U/R deviation torque strength exertions, although the locking pliers with 4.5 cm handle grip span was found to be the best option in terms of comfort and usability. The exerted two-handed wrist U/R deviation torque strengths were significantly higher in sagittal plane than in transverse plane, and in CW direction than in CCW direction. The highest levels of two-handed wrist U/R deviation torque strength in sagittal and transverse planes were recorded in knuckle and elbow heights, respectively. In both CW and CCW directions, the highest level of two-handed torque strength was recorded in the knuckle height.

The findings showed no significant difference in the exerted two-handed torque strength for the three different prototypes of locking pliers. The same findings were found for the discomfort assessments. In contrast, the most comfortable locking pliers was the one with 4.5 cm handle grip span. The results of usability assessments also showed that the locking pliers with 4.5 cm handle grip span had a higher mean SUS score than the other two prototypes. Therefore, it appears that the locking pliers with 4.5 cm handle grip span tends to be the best option among the examined locking pliers in exerting two-handed torque exertion.

One of the main findings was that the exerted two-handed torque strength was significantly higher in sagittal plane compared to transverse plane. This is similar to the findings reported by Dianat et al. [12], who found that the wrist U/R deviation torque strength exerted in sagittal plane was higher than that exerted in transverse plane. Remarkably, they examined the one-handed wrist U/R deviation torque strength using spanner wrenches, which required the same type of wrist torque strength exertion as locking pliers in this present research. These findings suggest that regardless of one- or two-handed wrist U/R deviation torque exertion, strength exertion in sagittal plane is preferable to that in transverse plane. The implication of these findings for ergonomic practice is that, as far as the workstation design is concerned, workstations that require wrist U/R deviation torque exertion in the sagittal plane tend to be preferable to those that need wrist U/R deviation torque strength exertion in the transverse plane.

The wrist U/R deviation torque strength exertion in CW direction was found to be higher than that in CCW direction. The mean wrist U/R deviation torque strength exertion in CCW direction was approximately two-thirds (25.1 Nm vs. 36.9 Nm = 68%) of that exerted in CW direction. This finding may suggests that radial deviators exerted more torque strength than ulnar deviators. This is in agreement with the findings reported by Vanswearingen [29], who measured wrist muscle strength and found that radial deviators produced greater statically and dynamically torque force than did ulnar deviators.

Although it was expected that the torque exertions would be highest at elbow height due to the neutral position of the hand, forearm and trunk, the results revealed that the two-handed wrist U/R deviation torque exerted in sagittal plane was highest at knuckle height. This was followed by torque exertions at elbow and shoulder heights, respectively. Nevertheless, the highest level of wrist U/R deviation torque strength exertion in transverse plane was recorded in elbow height, followed by the knuckle and shoulder heights. The effects of workpiece orientation and working height on torque strength can possibly be explained by the fact that posture affects muscle force in two physiological conditions: 1) while joint angles change, muscles undergo length changes that affects their force output; 2) by changing joint angles, moment arms vary and the torque output changes consequently [1]. These findings highlight the importance of consideration of interactions between workpiece orientation and working height in designing workstations that require wrist U/R deviation torque exertion using manual hand tools.

This study revealed that anthropometric characteristics were related to the two-handed wrist U/R deviation torque strength. Studies of upper extremity anthropometrics associated with using hand tools are underway. The results of the relationship between torque strength and individuals’ anthropometric parameters are somehow infrequent and support various arguments. No significant association was reported by Dianat et al. (2017) between wrist U/R deviation torque strength and hand size [12]. However, positive relationships between hand length and hand breadth and wrist twisting torque were demonstrated in some prior studies [30, 31]. It is noteworthy that the type of torque exertion (one- or two-handed forearm supination/pronation, wrist flexion/extension, and wrist ulnar/radial deviation torque) has to be considered.

Although this study examined the design of locking pliers handle and workstation layout, the findings assist in developing guidelines using various hand tools while exerting two-handed wrist U/R deviation torque strength. The generalizability of the findings in this study should be considered in light of several limitations. First, the study participants were college students, who were not professional users of manual hand tools. Therefore, further studies may be required to explore the generalizability of these findings in actual working contexts among industrial workers professionally involved in manual hand tool use. In addition, the study participants were recruited from male volunteers. As females’ muscular strength is generally between 49% and 75% of males’ strength depending on the investigated muscle group and type of evaluated strength and also differences in their neuromuscular operations, muscle mass, even cultural behaviors [32], therefore caution should be given when generalizing the research findings to people in general. Employing various tool handle configurations and workstation layouts (e.g. different tool grip spans, workpiece orientations, and working height) while exerting two-handed wrist U/R deviation torque strength are also recommended for future researches to generalize the results. Moreover, to explore the influence of using one or two hands on the wrist U/R deviation torque strength, conducting studies using one and two hands with the same experimental conditions is recommended. In addition, other types of two-handed muscular strength such as wrist flexion/extension torque strength or hand grip strength using different manual hand tools are also worth investigating. Additionally, some investigators have noted simultaneous consideration of grip strength and wrist flexion/extension torque strength and their interaction in their experimental protocol [33]. Similar protocols can be recommended for future studies investigating the grip strength and two-handed torque strength exertions using manual hand tools. Additionally, the current work evaluated isometric (static) strength. Therefore, repetitive and also dynamic strengths are worth investigating for future research. To make better decisions about the optimum hand tool design and workstation layout, carrying out physiological tests, for instance, muscle electromyography (EMG) and endurance strength is recommended as well.

5 Conclusions

Many daily and occupational activities, such as the use of manual hand tools, involve the use of both hands for force exertion. This study evaluated the effects of tool grip span of locking pliers and workstation design features (workpiece orientation, moving direction, and working height) on two-handed wrist U/R deviation torque strength and subjective measures. The findings indicated no significant effect of tool grip span on wrist U/R deviation torque strength, but the locking pliers with 4.5 cm handle grip span caused more comfort and better usability. Higher levels of two-handed wrist torque exertions were recorded in sagittal plane and in CW direction. Interactions between workstation design features have potential implications for the design of tasks involving wrist U/R deviation torque exertion with manual hand tools.

Ethical approval

The ethical review committee of the Tabriz University approved the study protocol (Code no. 61420).

Informed consent

All participants signed a written consent form before participation.

Conflict of interest

The authors have no conflict of interest to report.

Footnotes

Acknowledgments

This article was part of thesis conducted to fulfill the requirements for a PhD degree from the Tabriz University of Medical Sciences (Code no. 61420).

Funding

No funding was received for this study.

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Effects of tool grip span,workpiece orientation,moving direction,and working height on two-handed torque strength and subjective measures

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1 Introduction

2 Materials and methods

2.1 Participants

Table 2 Characteristics of the tools used in the study Handle grip span (cm) Length (cm) Weight (g) Appearance 4.5 25 535 5.5 25 580 6.5 25 638

2.4 Measurement of the dependent variables

3 Results

3.1 Two-handed wrist U/R deviation torque strength

3.4 Usability

4 Discussion

5 Conclusions

Ethical approval

Informed consent

Conflict of interest

Footnotes

Acknowledgments

Funding

References

Table 2
Characteristics of the tools used in the study

Handle grip span (cm) Length (cm) Weight (g) Appearance

4.5 25 535

5.5 25 580

6.5 25 638