Applied Ergonomics 53 (2016) 152e160

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Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo

Wrist rotations about one or two axes affect maximum wrist strength Katherine Plewa a, Jim R. Potvin b, James P. Dickey a, * a b

Joint Biomechanics Laboratory, School of Kinesiology, Western University, London, Ontario, N6A 3K7, Canada Department of Kinesiology, McMaster University, Hamilton, Ontario, L8S 4K1, Canada

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 May 2011 Received in revised form 8 September 2015 Accepted 17 September 2015

Most wrist strength studies evaluate strength about one axis, and postural deviations about that same axis. The purpose of this study was to determine if wrist posture deviations about one axis (e.g. flexion/ extension), or two axes (e.g. flexion/extension and pronation/supination), affect the strength about another axis (e.g. ulnar deviation). A custom-built instrumented handle was used to measure maximum static isometric torque exertions at 18 wrist postures (combinations of flexion/extension, radial/ulnar deviation, and pronation/supination). Ulnar deviation torques were highest when the wrist was in neutral. This pattern was not maintained for the other torque directions; the generated torque tended to be highest when the wrist posture was not neutral. The effects were similar for male and female subjects, although male subjects exerted significantly larger torques in all directions. This study illustrates that there is a complex relationship between wrist posture and maximal wrist torques. © 2015 Elsevier Ltd and The Ergonomics Society. All rights reserved.

Keywords: Wrist strength Wrist torque Wrist motion

1. Introduction Strength limits of the upper extremities are of interest to ergonomists in light of the high prevalence of injuries in the workplace. The National Institute of Safety and Health reviewed epidemiological studies and observed strong evidence showing a positive association between work that requires extreme postures and the prevalence of hand/wrist tendinitis (Bernard, 1997). Risk factors and disorders associated with hand and arm injuries reveal that awkward postures of the wrist, along with repetitive tasks, high wrist velocities and high-force exertions, are related to an increased risk of injury (Muggleton et al., 1999; Nordander et al., 2013). Ergonomists use different tools, including computer programs, when assessing and designing workplaces (Roman-Liu, 2014). HandPak (Work in Progress Ergonomics, Hamilton, Ontario, Canada) and 3DSSPP (The University of Michigan, Ann Arbor, MI) are two examples of software packages that are designed to determine recommended acceptable force and torque values for a wide variety of tasks commonly found in the workplace. These guidelines are valuable for determining the injury risk associated with tasks that

* Corresponding author. Joint Biomechanics Laboratory, TH 3156, School of Kinesiology, Western University, 1151 Richmond Street, London, Ontario, N6A 3K7, Canada. E-mail address: [email protected] (J.P. Dickey). http://dx.doi.org/10.1016/j.apergo.2015.09.005 0003-6870/© 2015 Elsevier Ltd and The Ergonomics Society. All rights reserved.

have different grips, postures, frequencies, durations and effort requirements. These programs have a number of modules for specific task demands, one of which is torque. This module applies to tasks that require the application of a torque or moment to some object that has been grasped with the hand. Acceptable limits have been separately determined for pronation, supination, flexion, extension, radial deviation and ulnar deviation exertions. These ergonomic software packages have been developed by integrating a large body of scientific research from published literature. However, the limitation with most wrist strength studies, and thus most strength predicting software, is that maximum torques about a particular axis were only measured with respect to postural deviations about that same axis (eg. the effect of wrist flexion/extension angle on wrist flexion strength; Greig and Wells, 2004; Hallbeck, 1994; Jung and Hallbeck, 2002; Marley and Thomson, 2000; Snook et al., 1999, 1995, 1997). These previous studies have not investigated the potential effects of a rotation, or rotations, about axes other than that which is being tested for strength. For example, we are not aware of published studies that have evaluated the effect of radial, ulnar, pronation or supination deviations on wrist flexion or extension strength. Wrist joint motion in one direction affects the magnitude of the range of motion (ROM) in the other directions (Li, 2002; Li et al., 2005; Marshall et al., 1999). For example, there is coupling between flexion/extension and radial deviation/ulnar deviation directions in the wrist; the ROM in one direction (flexion or extension) decreases as the wrist moves away from neutral in the

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other direction (radial or ulnar deviation; Li et al., 2005; Marshall et al., 1999). Different anthropometrics (Kivell et al., 2013) result in females having greater wrist ROM than males (Marshall et al., 1999), and may result in females adopting more extreme postures in the workplace (Chen et al., 2010; Won et al., 2009). Both grip strength and wrist torque decrease as the wrist deviates away from neutral (Dempsey and Ayoub, 1996; Jung and Hallbeck, 2002). It has been suggested that naturally coupled wrist motion affects wrist strength, and thus should be accounted for in workstation design and rehabilitation practices (Li et al., 2005). On average, the physiologic cross-sectional area of muscles is smaller in females than males; therefore, on average, they do not generate as much force as males in pinch and grips, and wrist flexion and extension (Abernethy et al., 2005; An et al., 1986; Dempsey and Ayoub, 1996; Hallbeck, 1994; Hallbeck and McMullin, 1993; Harkonen et al., 1993; Mathiowetz et al., 1985; Maughan et al., 1983; Morse et al., 2006). However, data do not currently exist related to how torque production trends vary between male and female subjects when wrist posture deviations are combined (e.g. flexed and ulnar deviated). The purpose of this study was to investigate the effects of wrist postures on wrist flexion, extension, radial deviation, and ulnar deviation torque strengths (e.g. flexion strength when the wrist is in a combined posture of pronation and ulnar deviation). We hypothesized that wrist strength about one axis will be affected by deviations about one or both of the other two axes. This is currently not being considered in most ergonomic assessments of tasks that place torque demands on the wrist and/or forearm. Furthermore, we hypothesized that male subjects will generate higher maximum wrist torques than female subjects in all directions of exertion, and that there will be similar effects of posture on maximum wrist torques for male and female subjects.

correction algorithms (Hansson et al., 2004; Sato et al., 2010), we used uncorrected goniometer outputs due to the relatively small errors, similarly to other previous studies (Finneran and O'Sullivan, 2013). Subjects moved through the full range of wrist motion in flexion (FLX), extension (EXT), radial deviation (RD) and ulnar deviation (UD) e and returned to neutral between each movement. Each trial was repeated twice to ensure consistency. Data were collected using a custom-made LabVIEW program v8.6 (National Instruments, Austin, TX, USA). Maximum values for flexion, extension, radial deviation, and ulnar deviation were measured for each subject (Table 1). Using zscores, wrist angles were calculated that would allow 99% (i.e. zscore of 2) of the sample population to comfortably achieve the four wrist angles (50
FLX, 35
EXT, 15
RD, and 20
UD). These four angles, and 45
for pronation (PRO) and supination (SUP) respectively (Matsuoka et al., 2006; O Sullivan and Gallwey, 2005), were used to define the posture matrix to establish the combinations of wrist postures for testing (Table 2). Some combinations of wrist posture were not used for testing because it was not physically possible to achieve the posture. For example, pilot subjects complained of pain while attempting to combine flexion and radial deviation. Although there were some differences in average ROM between male and female subjects, these differences were small (about 2e10
) for all directions (Table 1). By using a z-score of 2, we calculated angles that were comfortable for both male and female subjects and did not require any extreme ROM; therefore, we did not test different positions for male and female subjects. Although extreme postures are important for studying injury risk, these postures are less common during daily work activities (Keir et al., 1998).

2. Methods

A custom-built device was used to measure wrist torque (Fig. 1a). This device consisted of a standard hand-tool handle (the handle from a 21e295 e Surform® Flat File e Regular Cut Blade, Stanley Black and Decker LTD., New Britain, CT, USA). This handle had a smooth hard plastic finish and an oval cross-sectional area (12 cm circumference). The handle was attached to a force/torque transducer (Omega 160, ATI Industrial Automation, Apex, NC, USA) via a lockable ball-and-socket joint. This setup permitted easy positioning of the handle so that the desired wrist posture could be set. This apparatus was similar to other studies (Seo et al., 2008), though we did not use a wrist support and they did not measure off-axis torques. Handle positions corresponding to the desired wrist positions were based on the average ROM angles determined in the pilot testing described in Section 2.2. One control subject was used to pre-set the desired 18 handle positions using the electrogoniometer and all of the test subjects used these handle positions. The location of the wrist joint center and the orientation of the handle were digitized using a 3D digitizer (MicroScribe G2,

2.1. Subjects A total of 28 young, healthy university-aged subjects (14M, 24.3 ± 2.6 years old, 81.6 ± 12.9 kg; 14F, 24.6 ± 2.4 years old, 67.6 ± 7.4 kg) participated in this study. All subjects reported no current or previous history of hand and upper extremity pain, disorders, or carpal tunnel syndrome. All subjects were right-hand dominant, and applied torques with their right hands. 2.2. Pilot testing Ten subjects (5M, 5F) participated in pilot testing to determine maximum ROM of the wrist in flexion/extension and radial deviation/ulnar deviation directions. The maximum ROM of the right wrist was measured using an electrogoniometer (SG150, Biometrics Ltd., Ladysmith, VA, USA). Subjects stood with the shoulder slightly flexed, but with no internal/external rotation, the elbow flexed at 90
, with the forearm resting on an armrest in neutral (halfway between full pronation and full supination, thumb facing up) and the wrist in neutral flexion/extension (neither flexed nor extended), and the digits slightly extended. From this position, the electrogoniometer was placed on the posterior aspect of the hand and forearm, crossing the wrist joint in line with the 3rd digit of the hand. The voltage from the electrogoniometer in this neutral wrist posture was defined as zero. Studies have shown that flexible biaxial electrogoniometers have crosstalk errors that are related to rotations (Hansson et al., 1996, 2004); these errors are only a few degrees and are considered small in both epidemiological and physiological contexts (Hansson et al., 2004). Although some previous studies have presented

2.3. Apparatus

Table 1 Average ROM (with standard deviations) for male and female subjects during the pilot testing. These data were used to define the combinations of wrist postures for the main experiment. The greatest amount of ROM was seen for female subjects in flexion and ulnar deviation. Male

FLEX (
) EXT (
) RD (
) UD (
)

Female

Average

SD

Average

SD

76.4 58.2 27.1 34.6

11.0 7.8 4.1 8.4

82.6 60.6 25.5 43.7

14.7 13.2 5.3 7.4

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Table 2 Testing matrix for strength measurements. Table values indicate combinations of postures where maximal radial and ulnar deviation (RD/UD; red font) and flexion and extension (Flx/Ext; blue font) torques were measured. There were 8 posture combinations where only two directions were tested, and 10 postures where four directions were tested for a total of 56 testing conditions. The bolded condition was neutral about all three axes.

Fig. 1. Photos of testing setup. a) Handle is attached to load cell (beneath square plate) via a lockable ball-and-socket joint. b) Subject showing the neutral wrist posture with the arm strapped in, ready for testing.

REVWARE Systems Inc., Raleigh, NC, USA) in order to define the orientation of the wrist's orthopedic coordinate system with respect to the load cell. A custom-made LabVIEW Program (National Instruments, Austin, TX) was used to calculate force and torque about the wrist joint center using the cosine matrix based on the position of the wrist joint center and the orientation of the handle with respect to the load cell. In order to rapidly and reproducibly reposition the handle into the specific orientations, a supplementary system of two laser pointers was temporarily mounted to the handle in a registered location. The laser pointer projections corresponding to each of the handle positions were marked on the laboratory walls and were used to locate the specific handle positions between trials. 2.4. Test protocol/procedure After being informed of the protocol, all subjects signed an informed consent form (University of Western Ontario, London, ON, Canada) before beginning the study. Name, age, and gender were recorded for each subject. While standing, subjects comfortably gripped the center of the handle with their wrist in a neutral posture (Fig. 1b). Neutral posture refers to wrist positions with neutral flexion-extension and radial-ulnar deviation of the wrist joint with the third metacarpal in line with the long axis of the forearm (Delp et al., 1996; Hallbeck, 1994); this wrist posture has minimal carpal tunnel pressures and muscular activity (Fagarasanu et al., 2004; Kaljun and Dol
sak, 2012; Keir et al., 2007). The subject's wrist was centered above the load cell and their forearm rested on a high-density foam armrest; it was strapped down to stabilize the forearm and prevent subjects from

leaning or pulling with their upper body. Subjects performed 2e3 trials to familiarize themselves with exerting wrist torque in the isolated directions. Real-time feedback of generated torques was presented to subjects during each trial through a custom LabVIEW Program; this allowed subjects to isolate generated torques so that 90% of the resultant torque was exerted about the desired axis (De Looze et al., 2000; Hoffman et al., 2007; La Delfa et al., 2015). Isolating the desired torque was important since off-axis forces are common in unconstrained trials when subjects are not given feedback (Hoffman et al., 2007). The maximum effort protocol was based on the Modified Caldwell Regimen to ensure subjects sustained torque generation and did not jerk the handle (Caldwell et al., 1974; Hallbeck, 1994). Subjects slowly built up to maximum torque, sustain the maximum for 3 s, and then slowly return to a relaxed state. We required isolated torques, so the maximum flexion torque was determined when participants reduced the off axis torques to less than 10% of the total torque magnitude (Fig. 2). Two static isometric exertions in opposite directions (i.e., flexion and extension, or radial deviation and ulnar deviation) were performed at each handle position, with a 1-min rest between each handle position. One trial was performed for each direction at each handle position. There were 14 combinations of postures tested for strength about both the flexion/extension axis and the ulnar deviation/radial deviation axis (Table 2) and this resulted in a total of 56 torquedirection  posture conditions. For flexion and extension strength trials, the wrist was kept in a neutral pronation/supination (NP/S) angle and a matrix of postures combined flexion, neutral flexion/ extension (NF/E) or extension with radial deviation, neutral radial deviation/ulnar deviation (NR/U) or ulnar deviation. Of these 9 combinations, it was not possible for subjects to simultaneously

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6

5

90

4 80 3 70 2 60

Torque (Nm)

Percent Threshold (%)

100

155

Percent Threshold

1

90% Threshold Torque

50

0 0

2

4

6

8

Time (s) Fig. 2. Sample of flexion torque for one trial. The task required isolated torques but the participants were able to generate torques in other directions as the handle was fixed. Accordingly the maximum flexion torque was determined when participants reduced these off axis torques to less than 10% of the torque magnitude. We observed that the maximum flexion torque was lower in the isolated torque condition (where the percent torque (dashed line) exceeded the red 90% threshold level) compared to when the off-axis torques were not constrained.

flex and radially deviate the wrist so this posture was eliminated from testing, which was similar to a previous study (Marshall et al., 1999). In addition, flexion and extension strength trials were also performed with the wrist kept in a NR/D posture and a matrix of 9 postures combined flexion, NF/E or extension with pronation, NP/S and supination. For the radial deviation and ulnar deviation strength tests, the wrist was kept in a NP/S posture and flexion, NF/E or extension were combined with radial deviation, NR/U or ulnar deviation (except flexion/radial deviation). Finally, with a NF/E posture, pronation, NP/S and supination were combined with radial deviation, NR/U or ulnar deviation. The order of presentation of the testing conditions was randomized to control for the effects of fatigue and learning.

all female subjects was 5.46 ± 2.10 Nm, also in the ulnar deviation direction with the wrist in a neutral posture and the forearm supinated. There were a small number of trials (15 out of 1568 trials; ulnar deviation ¼ 1, radial deviation ¼ 2, flexion ¼ 4, extension ¼ 8) where subjects could not effectively generate torque in the given wrist posture. In these trials, subjects could not get a good grip on the handle to generate a maximal torque, or had trouble isolating the desired torque with minimal torque in other directions. The torque generated in each of these trials was neutral, neutral > ulnar and neutral > radial. There were 3 comparisons where supination resulted in higher flexion strength than with the neutral forearm, one where supination > pronation and one where neutral > pronation (Fig. 5). 3.2.4. Extension strength The highest strength was 3.41 ± 0.23 Nm with an extended wrist, neutral radial/ulnar deviation and a pronated forearm. The lowest strength was 1.83 ± 0.17 Nm (54% of maximum) with an extended wrist, neutral radial/ulnar deviation and a supinated forearm. Post hoc comparisons within each flexion/extension posture revealed that there were a total 5 wrist radial/ulnar deviation and forearm pronation/supination combinations that demonstrated significant differences (Fig. 6). The largest difference was observed with the posture combinations noted above, where the pronated forearm resulted in extension strength an average of 1.58 Nm (87%) higher than the supinated forearm, when the wrist was extended and in neutral radial/ulnar deviation. There were 3 comparisons where a neutral radial/ulnar posture resulted in higher extensor strength than either radial or ulnar deviated postures. There were 4 cases where the pronated posture resulted in higher strength than with a neutral forearm posture (Fig. 6). 4. Discussion

3.2.3. Flexion strength The highest strength was 4.42 ± 0.38 Nm with a neutral wrist and supinated forearm. The lowest strength was 2.59 ± 0.26 Nm (59% of maximum) with a neutral wrist and pronated forearm. Post hoc comparisons within each flexion/extension posture revealed that there were a total 5 wrist radial/ulnar deviation and

In this study, we showed that strength about flexion/extension and radial/ulnar deviation axes of the wrist depend on off-axis postures of the wrist and forearm. Both wrist flexion/extension and forearm pronation/supination postures had significant effects on both wrist radial (Fig. 3) and ulnar (Fig. 4) deviation strengths. In

Radial Deviation Strength

8

Rad Deviated Neut-Rad/Uln Uln. Deviated

7

Torque (nm)

6 5 4 3 2 1 0 Neut-Flx/Ext

Neut-Flx/Ext

Neut-Flx/Ext

Extended

Flexed

Neut-Pro/Sup

Pronated

Supinated

Neut-Pro/Sup

Neut-Pro/Sup

Fig. 3. Mean wrist radial deviation maximum torques pooled across sex for each of the 14 posture combinations (n ¼ 28). Standard error bars are provided. Significant post hoc comparisons, within each radial/ulnar deviation posture, are indicated with horizontal arrows. Solid arrows indicate comparisons within the neutral radial/ulnar posture and dashed arrows indicate comparisons with the wrist ulnar deviated.

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Ulnar Deviation Strength

8

Rad Deviated Neut-Rad/Uln Uln. Deviated

7 6

Torque (nm)

5 4 3 2 1 0 Neut-Flx/Ext

Neut-Flx/Ext

Neut-Flx/Ext

Extended

Flexed

Neut-Pro/Sup

Pronated

Supinated

Neut-Pro/Sup

Neut-Pro/Sup

Fig. 4. Mean wrist ulnar deviation maximum torques pooled across sex for each of the 14 posture combinations (n ¼ 28). Standard error bars are provided. Significant post hoc comparisons, within each radial/ulnar deviation posture, are indicated with horizontal arrows. Solid arrows indicate comparisons within the neutral radial/ulnar posture and dashed arrows indicate comparisons with the wrist ulnar deviated.

Flexion Strength

5

Flexed Neut-Flex/Ext Extended

Torque (Nm)

4

3

2

1

0 Neut-Rad/Uln

Neut-Rad/Uln

Neut-Rad/Uln

Uln Deviated

Rad Deviated

Neut-Pro/Sup

Pronated

Supinated

Neut-Pro/Sup

Neut-Pro/Sup

Fig. 5. Mean wrist flexion maximum torques pooled across sex for each of the 14 posture combinations (n ¼ 28). Standard error bars are provided. Significant post hoc comparisons, within each flexion/extension posture, are indicated with horizontal arrows. Solid arrows indicate comparisons within the neutral flexion/extension posture and dashed arrows indicate comparisons with the wrist extended.

addition, both wrist radial/ulnar deviation and forearm pronation/ supination significantly affected both wrist flexion (Fig. 5) and extension (Fig. 6) strengths. To our knowledge, these are some of the only data to demonstrate these interactions. Such interactions are currently ignored in most ergonomic analyses, as it is generally assumed that strength about one axis is independent of rotations in other axes (Greig and Wells, 2004; Hallbeck, 1994; Jung and Hallbeck, 2002; Snook et al., 1999, 1995, 1997), yet many occupational tasks involve coincident wrist rotations about more than one €gg et al., 1997). axis (Chen et al., 2010; Donoghue et al., 2013; Ha Nevertheless, it has been assumed that strength about a given wrist axis is unchanged by deviations from the neutral posture in offaxes.

4.1. Strengths in radial/ulnar deviation Significant off-axis posture effects existed in the strengths in radial/ulnar deviation; neutral flexion/extension resulted in higher radial torques than with a flexed wrist, and a supinated forearm resulted in higher torques than with a neutral forearm. Significant off-axis effects resulted in higher radial torques when the wrist was in a neutral flexion/extension posture than in a flexed or extended posture. Additionally, forearm postures in supination and pronation resulted in significantly higher torques than with a neutral forearm posture. Current ergonomic assessments could overestimate radial deviation strength if the wrist was flexed, and could underestimate strength if the forearm was supinated. Similarly,

158

K. Plewa et al. / Applied Ergonomics 53 (2016) 152e160

Extension Strength

5

Flexed Neut-Flex/Ext Extended

Torqeu (Nm)

4

3

2

1

0 Neut-Rad/Uln

Neut-Rad/Uln

Neut-Rad/Uln

Uln Deviated

Rad Deviated

Neut-Pro/Sup

Pronated

Supinated

Neut-Pro/Sup

Neut-Pro/Sup

Fig. 6. Mean wrist extension maximum torques pooled across sex for each of the 14 posture combinations (n ¼ 28). Standard error bars are provided. Significant post hoc comparisons, within each flexion/extension posture, are indicated with horizontal arrows. Solid arrows indicate comparisons within the neutral flexion/extension posture and dashed arrows indicate comparisons with the wrist extended.

ergonomic assessments could overestimate ulnar deviation strength if the wrist was either flexed or extended. Furthermore, they could underestimate strength if the forearm was either supinated or pronated.

consistent with the expected relationship between muscle length and tension. There was a complex interaction between muscle properties and joint angle; maximum muscle force and maximum joint torque do not always occur at the same joint angles (Loren et al., 1996).

4.2. Strengths in flexion 4.4. Flexion/extension e effects of grip There was a variety of off-axis effects observed with no consistent trend with regards to the effect of radial/ulnar deviations on wrist flexion strength. Where significant pronation/supination posture effects existed, supination resulted in higher flexion strength than with a neutral or pronated forearm, neutral was greater than with pronation. Thus, ergonomic assessments could provide incorrect flexion strength estimates if the wrist was deviated about the radial/ulnar axis, could underestimate strength if the forearm was supinated and overestimate strength if the forearm was pronated. When the wrist was in neutral radial/ulnar deviation and pronation/supination, we observed that the flexor torque was greatest when the wrist was extended, compared to neutral flexion/extension or flexed (Fig. 5). These results for flexion torque are consistent with the expected relationship between muscle length and tension (Delp et al., 1996; Jung and Hallbeck, 2002). As the muscle is lengthened beyond its optimal length, both the passive connective tissue forces and active muscle forces contribute to wrist torques. This effect was not observed when the wrist was deviated in radial/ulnar deviation or in pronation/supination. 4.3. Strengths in extension Neutral radial/ulnar deviation resulted in higher extension torques than with either radial or ulnar deviations, and pronation resulted in higher strength than with a neutral forearm. Thus, current ergonomic assessments could overestimate extension strength if the wrist was either radial or ulnar deviated, and could underestimate strength if the forearm was pronated. When the wrist was in neutral radial/ulnar deviation and neutral pronation/ supination, we observed that the extensor torque was smallest when the wrist was flexed, compared to neutral flexion/extension or extended (Fig. 6). These results for extension torque are not

Previous research has evaluated the effects of wrist posture on force and torque generation; flexion and extension torque follow an inverted U-shaped curve with the largest torques near neutral postures and skewing towards flexion (Delp et al., 1996; Hallbeck, 1994; Jung and Hallbeck, 2002; Morse et al., 2006). These studies evaluated wrist torque at various wrist ROM postures; however, they isolated the dedicated wrist muscles by keeping the fingers relaxed. Only one study simultaneously tested wrist torque while maintaining a grip (Morse et al., 2006). Our results do not show similar trends (Figs. 5 and 6); we do not always observe the greatest torques in a neutral posture. We observed significantly higher flexion than extension torque produced by both male and female subjects. When pooled across all postures, the extension torque was 72% of flexion torque, which was similar to other studies (Morse et al., 2006). 4.5. Coupled motion This effect of wrist posture on torque may be explained by the coupling of wrist ROM; wrist flexion is coupled with a secondary motion in ulnar deviation, and similarly, wrist extension and radial deviation are coupled together (Li et al., 2005; Marshall et al., 1999). Additionally, radial deviation ROM is facilitated by wrist extension but decreased by wrist flexion (Marshall et al., 1999). It is expected that wrist torque will be lower when the wrist is deviated from neutral, as it may be near the end of the range of motion, and the moment arm and muscle lengths may not be optimal. For example, we expect lowest radial deviation torque when the wrist is in a flexion posture. Our results support this coupling (Fig. 3) e radial deviation torque was lower when the wrist was in a flexion posture in comparison to an extension posture. Additionally, we saw

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highest radial deviation torques when the wrist was in an ulnar deviation posture (Fig. 3), and higher ulnar deviation torques when the wrist was in a radial deviation posture than an ulnar deviation posture (Fig. 4). These data illustrate that the maximum torque was larger when the wrist was in an extreme posture and the active muscle was at a longer length. Furthermore, many subjects complained that this wrist posture was difficult because they felt that their wrist posture was near the end of its ROM and they could not move in radial deviation any further, nor could they create a radial deviation torque without pain. Perhaps investigations of muscle or sarcomere lengths, and passive properties of the wrist joint, would clarify these relationships. 4.6. Postures used The wrist postures in this study were based on a pilot study of five male and five female subjects, as described in section 2.2. Although this is a relatively small number of subjects, the ROM findings are consistent with previously reported wrist deviation data (Hansson et al., 1996). 4.7. Sex effect The upper body muscular strength of females is significantly lower than males; chest and arm strength in females is about 50e60% that of males (Abernethy et al., 2005). Males have a higher muscle cross-sectional area than females, corresponding to higher strength and torque production (Friden and Lieber, 2002; Maughan et al., 1983; Morse et al., 2006). On average, females are about half to two-thirds as strong as males (An et al., 1986; Hallbeck, 1994). This study is consistent with these trends as the torque generated by female subjects in this study was significantly less than the male subjects; it was about 64% that of maximum torque generated by male subjects across all wrist postures and torque directions. ROM has an effect on torque production; one study showed that when subjects reached extremes in their ROM, they produced lower torques than when they were at less extreme postures (Hallbeck, 1994). In any of our testing postures, female subjects may be at a lower percentage of full ROM; therefore they may be at a more optimal sarcomere length to produce greater torque. However, since the difference between male and female average ROM was not very large (Table 1), the main difference in torque is likely due to PCSA differences between male and female subjects. 4.8. Effect of the 90% criterion We observed that our subjects needed practice to successfully generate the isolated torque for this experiment. Our subjects struggled with isolating the desired torque at the start of the session. After a few trials, our subjects appeared to find it easier to generate torque in the desired direction, and they appeared to generate higher maximal torques, likely illustrating aspects of motor control. The subjects had to generate more than 90% of the resultant torque in the desired direction. This may be difficult due to the synergistic properties of the digital extensor and wrist flexor muscles that contract together to increase the effectiveness of each muscle (Friden and Lieber, 2002). Substantial off-axis forces occur in unconstrained trails compared to constrained trials; furthermore, the ratio of off-axis to on-axis forces increased as a function of the magnitude of force required (Hoffman et al., 2007). This likely lowered the magnitudes of the torques in the current experiment compared to less constrained tests. For example, as illustrated within a sample trial, the maximum flexion and extension torques were reduced in the isolated torque

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condition (90% component along that axis) compared to when the off-axis torques were not considered (Fig. 2). In this example, the highest flexion torque was 5.19 Nm but this torque was only 82% in the flexion direction. When we included the 90% threshold, the highest recorded torque was only 3.32 Nm (Fig. 2). This trend was similar for all other torque directions. This constraint on isolated torques may explain why our maximal torques were lower than those reported in the literature (Delp et al., 1996; Morse et al., 2006). More specifically, our average flexion torque of 4.08 Nm (for male subjects) was considerably lower than previously reported data of 11.1 Nm (Delp et al., 1996). However, our torques were larger than those reported with a somewhat similar experimental setup where isometric wrist torques were measured using a Cybex isokinetic dynamometer (Vanswearingen, 1983). That study reported flexion strengths of 1.85 and 1.09 Nm, extension torques of 0.98 and 0.63 Nm, radial deviation torques of 1.49 and 0.81 Nm and ulnar deviation torques of 1.24 and 0.71 Nm for males and females, respectively. Additionally, our ulnar and radial deviation torque results are comparable to a recent study looking at the effect of forearm position on wrist strength (La Delfa et al., 2015). More specifically, they reported that female participants produced and average strength range of 3.9e4.5 Nm and 3.5e4.5 Nm in the ulnar and radial deviation directions respectively, which are comparable to our findings (averages of 4.5 and 3.5 Nm for ulnar and radial deviation directions, respectively). Similarly, their ulnar deviation strength range is 5.8e10.6 Nm for male participants, whereas our study showed an average male UD strength of 7.5 Nm. The differences in experimental setup, and the effect of handle grip, may be contributing to these differences between experimental findings. 4.9. Apparatus effects Some subjects reported that it was difficult to create an isolated flexion or extension torque on the handle. This difficulty appeared to be due to the fact that some subjects twisted the handle in order to create the desired flexion/extension torque, rather than performing wrist efforts. However, this twisting effort is similar to working with a screwdriver with a power grip (i.e., wrist flexion/ extension with neutral pronation/supination; Kong et al., 2007), and also hand tightening hose fittings and other maintenance operations (Imrhan and Jenkins, 1999). When we compare our results with studies that measured maximum volitional torque for screwdrivers, our results of 4.08 Nm and 2.92 Nm for flexion (for male and female subjects respectively) are higher than the previously reported data of about 3.0 Nm and 2.5 Nm (for male and female subjects) (Mital, 1986; Mital and Channaveeraiah, 1988). This demonstrates that although our wrist strength data was lower than some studies that used less constrained setups, our results have a strong real-life application, especially regarding tasks that require a power grip. 4.10. Limitations Although this study provided a detailed examination of torque efforts with off-axis wrist postures, there were several limitations. For example, the handle positions were set up based on the wrist positions of one subject; it is possible that the handle position to achieve the desired wrist positions would slightly differ between subjects. Although wrist ROM angles were defined from ten pilot subjects, we used raw goniometer data and did not use a correction algorithm for cross-talk as performed by other researchers (Hansson et al., 2004; Sato et al., 2010). Finally, it was not possible to test all combinations of wrist postures for flexion/extension and ulnar/radial deviation torques.

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5. Conclusions Maximum wrist torque differed significantly between wrist postures, sexes, and all four torque directions. Our data are consistent with trends from previous research with respect to sex, torque direction, and ROM coupling (Delp et al., 1996; Hallbeck, 1994; Jung and Hallbeck, 2002; Li et al., 2005; Marshall et al., 1999; Morse et al., 2006). This was one of the first studies to determine wrist strength for postures with rotations about more than one axis. The interactions seen with combined postures and differing torque directions show that the relationship between wrist strength and posture is complex; it depends on the magnitude of the rotations about all axes, not simply the axis of the torque. Due to the limiting effect of our handle on flexion and extension torque production, further work should be performed to evaluate the same wrist postures with different size, style and grips of handles as well as a different setup that does not involve twisting a handle. Our results are important when considering workplace injuries and ergonomic assessments. Our study is most relevant for tasks involving gripping a tool or part and creating flexion or extension torques (for example, when using tools such as screwdrivers with a power grip, hand tightening hose fittings and other maintenance operations). Therefore, the observed maximum torque values with combined posture trends should be incorporated into ergonomic software to improve task assessments and future workplace design. References Abernethy, B., Hanrahan, S.J., Kippers, V., Mackinnon, L.T., Pandy, M.G., 2005. The Biophysical Foundations of Human Movement, second ed. Human Kinetics, Champaign, IL. An, K., Askew, L., Chao, E., 1986. Biomechanics and Functional Assessment of Upper Extremities. Elsevier Science Publishers BV, North-Holland, pp. 573e580. Bernard, B.P., 1997. Musculoskeletal Disorders and Workplace Factors: a Critical Review of Epidemiological Evidence for Work-related Musculoskeletal Disorders of the Neck, Upper Extremity, and Low Back. US Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health), Cincinnati, Ohio (USA). Caldwell, L.S., Chaffin, D.B., Dukes-Dobos, F.N., Kroemer, K.H.E., Laubach, L.L., Snook, S.H., Wasserman, D.E., 1974. A proposed standard procedure for static muscle strength testing. Am. Ind. Hyg. Assoc. J. 35, 201e206. Chen, H.C., Chang, C.M., Liu, Y.P., Chen, C.Y., 2010. Ergonomic risk factors for the wrists of hairdressers. Appl. Ergon. 41, 98e105. De Looze, M., Van Greuningen, K., Rebel, J., Kingma, I., Kuijer, P., 2000. Force direction and physical load in dynamic pushing and pulling. Ergonomics 43, 377e390. Delp, S., Grierson, A., Buchanan, T., 1996. Maximum isometric moments generated by the wrist muscles in flexion-extension and radial-ulnar deviation. J. Biomech. 29, 1371e1375. Dempsey, P.G., Ayoub, M., 1996. The influence of gender, grasp type, pinch width and wrist position on sustained pinch strength. Int. J. Ind. Ergon. 17, 259e273. Donoghue, M.F., O'Reilly, D.S., Walsh, M.T., 2013. Wrist postures in the general population of computer users during a computer task. Appl. Ergon. 44, 42e47. Fagarasanu, M., Kumar, S., Narayan, Y., 2004. Measurement of angular wrist neutral zone and forearm muscle activity. Clin. Biomech. 19, 671e677. Finneran, A., O'Sullivan, L., 2013. Effects of grip type and wrist posture on forearm EMG activity, endurance time and movement accuracy. Int. J. Ind. Ergon. 43, 91e99. Friden, J., Lieber, R.L., 2002. Mechanical considerations in the design of surgical reconstructive procedures. J. Biomechanics 35, 1039e1045. Greig, M., Wells, R., 2004. Measurement of prehensile grasp capabilities by a force and moment wrench: methodological development and assessment of manual workers. Ergonomics 47, 41e58. € €gg, G.M., Oster, €m, S., 1997. Forearm muscular load and wrist angle Ha J., Bystro among automobile assembly line workers in relation to symptoms. Appl. Ergon. 28, 41e47. Hallbeck, M., 1994. Flexion and extension forces generated by wrist-dedicated muscles over the range of motion. Appl. Ergon. 25, 379e385. Hallbeck, M., McMullin, D., 1993. Maximal power grasp and three-jaw chuck pinch force as a function of wrist position, age, and glove type. Int. J. Ind. Ergon. 11, 195e206. Hansson, G., Balogh, I., Ohlsson, K., Rylander, L., Skerfving, S., 1996. Goniometer measurement and computer analysis of wrist angles and movements applied to

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