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Flexion and extension forces generated by wrist-dedicated muscles over the range of motion
Applied Ergonomics 199425(6)379--385
Flexion and extension forces generated by wrist-dedicated muscles over the range of motion M. S. Hallbeck Industrial and Management Systems Engineering Department, Center for Ergonomics and Safety Research, University of Nebraska, Lincoln, NE 68588--0518, USA
An experiment was performed t o evaluate t h e relationships among active range of motion (ROM), gender, wrist position and direction of force exertion in their effects on the magnitude of static force exerted by the wrist-dedicated muscles in wrist flexion and extension. This study employed 60 right-hand-dominant subjects (30 male, 30 female) between 20 and 30 years of age, all reporting no prior wrist injury and good to excellent overall physical condition. The ROM of each subject was used to determine the number of wrist positions evaluated for static maximal voluntary forces generated in wrist flexion and extension while they were instructed to relax their fingers; thus only the six wrist-dedicated muscles were employed in the exertion. The ANOVA procedure showed gender, wrist position, direction of force exertion, and the wrist position interaction with direction to have significant effects upon maximal force exertion. Females averaged 76.3% of the mean male flexion force _and 72.4% for extension. On average, extension forces were found to be 83.4% of those generated by flexing the wrist-dedicated muscles.
Keywords:wrist-dedicatedmuscles,maximalvoluntaryforce, flexion-extensionforce measurement Forces that can be generated by the upper limb have been measured by orthopaedic surgeons, ergonomists, and occupational and physical therapists. Wrist flexion and extension forces measured externally at the hand are a combination of forces acting between the forearm and hand, generated by exertions of agonistic and antagonistic muscle groups both within the hand (intrinsic) and the forearm (extrinsic) and by wristdedicated (carpi) muscles. Many factors influence the magnitude of force that can be generated, including: type of prehension, digits involved, digital posture, wrist posture, arm posture, overall body posture, subject age, gender, hand tested, anthropometric dimensions, direction of force exertion, instructions to the subjects, day, and motivation. Review of literature
The number and strength of individual digits involved in the force generation affect the magnitude of external force (Berg et al, 1988). The digital, wrist, arm and whole body posture also affects the magnitude of force (Bunnell, 1944; Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975; Terrell and Purswell, 1976; Pryce, 1980; Brand, 1985; Putz-Anderson, 1988; Hallbeck and McMullin, 1991; McMullin and Hallbeck, 1991). The effect of wrist position on external force has been demonstrated in
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both pinch and power grasp prehension. Any deviation from neutral decreases the external force measured, with flexion causing a larger decrement than extension (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975; Ten'ell and Purswell, 1976; Pryce, 1980; Mathiowetz et al, 1985b; Putz-Anderson, 1988; Woody and Mathiowetz, 1988; HaUbeck and McMullin, 1991, 1993; McMullin and Hallbeck, 1991; ZeUers et al, 1992). Gender has been found to affect the amount of force generated in power grasp and pinch (Roebuck et al, 1975; Chaffin and Andersson, 1984; An et al, 1986; Sanders and McCormick, 1986). Female grip strength has been found to be approximately 50-60% of male grip strength (Kellor et al, 1971; An et al, 1983; Mathiowetz et al, 1985a). Females can also be expected to exert less pinch force than males. The three-jaw chuck pinch strength that females can exert has been reported as 50% (McArdle et al, 1986), 60--63% (Kellor etal, 1971), 67% (Grand jean, 1982; Mathiowetz et al, 1985a), 70% (Berg et al, 1988), and 74% (An et al, 1983; Hallbeck and McMullin, 1991, 1993; McMullin and Hallbeck, 1991) that of males. In addition, the direction of the force (flexion or extension) has been shown to affect force magnitude generation. Flexion force generation is reported to be greater than that for extension (Thompson, 1981; Norkin and Levangie, 1983; Brand, 1985). The findings
0003-6870/94/060379-07~) 1994Butterworth-HeinemannLtd
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
of studies by Jorgensen et al (1989) and Hallbeck et al (1990) indicated that the direction of power grasp force relative to the hand also appears have a significant effect on the magnitude of the force exerted. Wrist-dedicated muscles
Static positioning of the hand and wrist requires balanced action of many synergistic muscle sets (Brand, 1985). The muscles that control the wrist serve two functions in the hand. They provide the fine adjustment of the hand to its functioning position and, once this position is achieved, they stabilize the wrist to provide a stable working platform for the hand and digits (Hazelton et al, 1975). If wrist movement is considered in relation to digit movement, two independent actions are found to be possible. When the wrist-dedicated muscles stabilize the joint, the extrinsic and intrinsic digit muscles can alter the position of the digit(s). Conversely, when digit posture is stabilized, the wrist can be positioned over a large range of motion (ROM) (Flatt, 1961). Muscles whose tendons merely cross the wrist (secondary wrist movers) en route to the digits cannot be relied upon for their wrist-moving and stabilizing characteristics, as wrist function may at times conflict with their primary digit function(s) (Brand, 1985). This dual innervation may be bypassed when subjects are instructed to extend the wrist with maximal effort while their fingers are loose and flaccid (Ketchum et al, 1978). Wrist position
Wrist position affects finger position (Pryce, 1980; Woody and Mathiowetz, 1988). Grip strength is greater in the extended wrist than in the flexed wrist, owing to increased length of the flexor tendons (Linscheid and Chao, 1973). The angles at which the extrinsic digital flexors approach the digits change with wrist position (Bunnell, 1944; Brand, 1985). The capacity of the musculotendinous units to generate force is dependent in part upon the effective functional length (Hazelton et al, 1975). Thus the highest grasp or pinch strength would occur where the length-tension relationship is maximized. Digit, hand, wrist and elbow position influence peak pinch and grip strengths (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975; Pryce, 1980; Mathiowetz et al, 1985b; Woody and Mathiowetz, 1988; Hallbeck and MeMullin, 1991; McMullin and Hallbeck, 1991). As wrist position deviates from normal, both pinch and power grasp forces decrease with a larger decrement in flexion than extension (Hallbeck and McMullin, 1991, 1993); however, the effect of wrist position on the wrist-dedicated muscles was not found in the literature. Gender
Human skeletal muscle can generate approximately 3-4 kg of force per square centimetre of cross-sectional muscle (Astrand and Rodahl, 1986). Males possess a greater amount of muscle area than females and therefore they can generate a greater amount of force (McArdle et al, 1986). On average, female strength has been found to be approximately two-thirds that of males; however, there is a large overlap in gender
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strength distributions, and there are many females stronger than males (Roebuck et al, 1975; Chaffin and Andersson, 1984). Female grip strength has been found to be approximately 50-60% of male grip strength by several authors (Kellor et al, 1971; An et al, 1983; Mathiowetz et al, 1985a), but was found to be 74% that of males by McMullin and Hallbeck (1991). Females can also be expected to exert less pinch force than males. The three-jaw chuck pinch strength that females can exert has been reported as 50% (McArdle et al, 1986), 60-63% (Kellor et al, 1971), 67% (Grandjean, 1982; Mathiowetz et al, 1985a), 70% (Berg et al, 1988), 74% (An et al, 1983), and 78% that of males (Hallbeck and McMullin, 1991, 1993). An el al, (1986) found that females could generate 60% of the torque that males could generate in maximal wrist flexion and extension. Measured flexion a n d extension forces
The 'strength' of each muscle/tendon unit has been studied in cadavers. However, the contraction combinations that control the wrist movement mechanism cannot be examined without internal innervation; thus these 'strengths' are of little use to the industrial ergonomist (Gilford et al, 1943; Brand, 1985). The forearm complex has been optimized for strength in flexion, not extension; the hand contains more flexors than extensors, and they are more powerful (Thompson, 1981). When the physical capacity of the wrist muscles is assessed, the wrist flexors have more than twice the capacity of the extensors (Norkin and Levangie, 1983). Only one study was found in the literature that measured torque generation in wrist flexion and extension; it employed static measurement of the intrinsic and extrinsic digital muscles and the wristdedicated muscles (An et al, 1986). In this study An et al (1986) found that on average the flexion torque was twice that of extension. This finding supports previous findings in the literature. Summary
The forces that can be generated by the wrist-dedicated muscles alone have not been studied. Force generation has been hypothesized to be a function of the musculature employed, wrist position, gender, and direction of force exertion (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975, Roebuck et al, 1975; Pryce, 1980; Thompson, 1981; Norkin and Levangie, 1983; Chaffin and Andersson, 1984; Brand, 1985; Mathiowetz et al, 1985a; An et al, t986; Sanders and McCormick, 1986; Woody and Mathiowetz, 1988; Hallbeck and McMullin, 1991, 1993; McMullin and Hallbeck, 1991). To evaluate the effect of these variables on force generation of the wrist-dedicated muscles, a study was performed, with flexion and extension forces measured at a number of wrist positions for both male and female subjects.
Method Apparatus
A flexion-extension force measurement apparatus (FEFMA) was designed to measure the force generated in flexed and extended wrist positions. The basic
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
elements of the FEFMA were adapted from a similar device employed by the Mayo Clinic Biomechanics Laboratory (An et al, 1986). The FEFMA consisted of a box that secured the forearm and ensured constant upper limb posture, a pair of force transducers, and a handle (proximal to the metacarpophalangeal joint) connected to the force transducers. The forearm was secured in the metal box between adjustable pads, which held the distal forearm midway between pronation and supination. The proximal forearm was positioned by packing the area between the walls of the box and the forearm with foam padding. The restraint ended far enough proximal to the wrist to allow full range of motion, and the padding was not so tight as to affect the muscular contraction nor to obstruct blood flow. This padded restraint box secured the forearm into a standardized posture and allowed no appreciable motion of the forearm during the measurements. By restraining the forearm, the measured output force was only that force which could be generated within the forearm. In addition, this restraint system would limit the effect of upper arm isometric contraction on the measured force and limit shoulder force (with a 'stiffened' wrist), which may artificially inflate the measured force at large angles of wrist flexion or extension. The handle structure consisted of two plates designed such that one plate rested on the palm and the other on the dorsal side of the hand, both proximal to the metacarpophalangeal joint. The plate separation was adjusted for each subject so that maximal force exertion was not limited by hand slippage or by pain. The entire handle was padded with a thin layer of foam to cushion pressure points along the metacarpals to prevent slippage and pain. The dorsal side of the handle had a distal extension plate, which was parallel to the long axis of the third metacarpal. Force generated in the wrist was transmitted via turnbuckles from the handle to either of the force transducers (LeBow load cells, 136 kg capacity, Model no. 3397). These load cells were mounted on a pivoting arm to keep the line of force application perpendicular to the load cell, as well as perpendicular to the long axis of the third metacarpal, for each measurement position. This pivoting arm was fixed in each measurement position. One force transducer measured extension, the other flexion force, in each position. The transducer voltage output was amplified and converted into a digital signal utilizing a Metrabyte ® A/D converter. This signal was then stored on an IBM PC, as a function of time. Subjects
Sixty right-hand-dominant subjects (30 male and 30 female) between 20 and 30 years of age voluntarily participated in this study contingent upon positive palpation of the palmaris longus muscle of the subject. Each subgroup was kept as homogeneous as possible by limiting the age range and selecting only right-handdominant subjects having all six wrist-dedicated (carpi) muscles. Procedure The subjects read and signed an informed consent agreement, which included the statement that they
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were not limited by any orthopaedic dysfunction, especially within the upper limb. Range o f motion. The active range of motion (ROM) in flexion and extension for a no-load condition was measured by the experimenter using a plastic goniometer. The active R O M was defined as the angle included between extreme active flexion and extension that could be achieved with no discomfort. Force measurements. Each subject was seated in a chair with the upper body in a standardized upright position. This position specified that the upper arm of the zubject should be relaxed, hanging with no shoulder abduction, the elbow joint flexed with a 90° included angle. After the active R O M was measured on an individual, a series of positions were located at 15° intervals, beginning at the neutral position, up to and including the extreme flexion and extension points. Thus the number of wrist positions tested was dependent upon each individual's ROM. Fifteen degree increments were selected, in lieu of percentages of each individual R O M , for ease of comparison and future use in design of equipment. The hand and arm of each subject were then placed in the F E F M A for force measurements. After securing the forearm of the subject midway between pronation and supination, the hand was inserted into the handle. Flexion and extension forces generated by only the wrist-dedicated muscles (with the digits relaxed) were measured in each of the wrist positions. To eliminate any order effects on the force measurements, the wrist position order was randomized by computer. Once the wrist was positioned in a specific wrist position in the FEFMA, the subject was instructed to maximally flex or extend the wrist, using only those muscles directly affecting the wrist. Each was told to relax the fingers, not to stiffen, flex, or hyperextend the digits as Ketchum et al (1978) had instructed, and subjects were visually monitored by the experimenter to verify relaxed fingers during the exertion. The relaxation minimized recruitment of the digital muscles and thus the force was generated by the wrist-dedicated muscles. A photograph of a subject performing the exertion is shown in Figure 1. The force generation measurements followed a modified Caldwell regimen (Caldwell et al, 1974) developed by the Virginia Polytechnic Institute and State University Industrial Ergonomics Laboratory in 1987. This protocol, modified for the smaller number of both muscle groups and fibres involved, called for a slow build-up to peak force (MVC), sustaining that peak for 3 s, with a slow return to a relaxed state (Berg et al, 1988). During the 3 s sustained exertion phase, the measured peak force was required to lie within a _+10% band about the average sustained force for each exertion as defined by Caldwell et al (1974). If this requirement was not met, the static exertion was performed again until this criterion was met. Two static force exertion measurements (flexion and extension) were taken at each position; a 1 min rest was provided between each exertion as suggested by Chaffin (1975). The time required to reposition the wrist was at least 1 min; thus fatigue should not have affected the MVC values.
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
generated in flexion and extension were utilized in further analyses. This lack of significance also illustrates that the exertion can be considered a static exertion, where the average force is not significantly different from the peak; thus the _+10% criterion was effective To evaluate all possible effects of gender, wrist position (13 levels, between 90° flexion and 90° extension), and direction of force exertion (flexion and extension) on the average force that was generated, an A N O V A procedure was performed. The ANOVA summary table (Table 1) shows that gender, wrist position, direction, and the interaction of wrist position and direction effects were statistically significant.
Gender Females were able to exert less force than males in all wrist positions, as shown in Table 2. Females averaged 76.3% of the male flexion force and 72.4% for extension. When the forces were summed over all wrist positions and both directions, average female force exertion was 74.2% that of males.
Wrist position Post-hoc testing was performed on wrist position, and the Tukey test results are shown in Table 3. Mean force~ at wrist positions grouped by vertical lines do not differ significantly at et = 0.05. Flexed wrist positions Table 1 Force ANOVA summary table
Figure 1 Photograph of flexion-extension force measurement apparatus (FEFMA)
Experimental design To verify that a 'static' exertion was achieved by the testing procedure, the peak value in the 3 s window and average forces over the 3 s exertion, for both flexion and extension, were compared using the critical ratio test. The result of no significant difference between peak and average force would verify the testing procedure for static exertion; thus average forces could be utilized in further analyses. The measured static force for the 60 subjects was the dependent variable for an analysis of variance (ANOVA) procedure. The independent variables for the 2 × 30 X 13 X 2 mixedfactor full-factorial design were gender, subjects, wrist position (WP) and direction of force exertion respectively. There were 13 levels of WP corresponding to the 13 possible positions from 90° flexion to 90° extension in 15° increments. At each WP, each subject performed static flexion and extension exertions, which were the two levels of the direction factor. Post-hoc (Tukey) tests were performed on all significant effects.
Results Comparisons between the peak value within the 3 s exertion window and the average of the 3 s exertion forces were performed for flexion and extension forces, and the combination of flexion and extension forces. There was no significant difference between the mean and peak force for either flexion or extension. Therefore, only the averages of the 3 s exertion forces
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Source
df
Between subject G e n d e r (G) Subjects (S/G)
MS
F
103 984.9 4 853.7
21.42 0.0001
58 12 12 597 1 1 58 12 12 597
5 675.8 98.9 161.9 38 689.3 30.9 650,0 3 457.6 122.7 89.7
35.06 0.000l 0.61 0.8339
1
Within subject Wrist position (WP) W × G W × S/G Exertion direction (D) D × G D × S/G WP × D WP × D × G WP × D × S/G Total
P
59.52 0.0001 0.05 0.8279 38.56 0.0001 1.37 0.1764
1361
Table 2 Male and female flexion and extension forces by wrist position Male force (N)
Female force (N)
Wrist
position
Flexion Extension Flexion Extension
90 ° flexion 75 ° flexion 60 ° flexion 45 ° flexion 30 ° flexion 15° flexion 0 ° (neutral) 15 ° extension 30 ° extension 45 ° extension 60 ° extension 75 ° e x t e n s t o n 90 ° extension Avg over all WP Avg over WP and dir
104.24 68.54 101.05 58.86 76.99 51.79 81.60 64.62 81.06 66.77 76.36 64.44 73.12 63.16 74.78 68.35 73.21 68.26 65.74 60.35 57.45 52.24 47,54 54.29 60,55 76.87 73.15 62.29 67.72
75.23 45.49 79.89 46.12 58.05 40.72 59.76 46.49 63.16 47.68 57.15 45.56 53.53 45.40 58.25 46.90 54.01 47.01 48.28 42.31 43.06 37.46 39.11 40.46 47.05 59.03 55.42 45.05 50.23
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
had higher mean force exertions than extension. The surprising finding is that extreme flexion wrist positions showed higher force magnitudes than those in a neutral wrist position.
Direction The average extension force was found to be significantly different from the average flexion force for both males and females. The A N O V A results have been further summarized by tabulating the average flexion force, extension force, the ratio of extension to flexion and the exact probabilities that the flexion and extension forces exertions have the same mean at each wrist position (Table 4). At each wrist position a significant difference was shown between average flexion and extension forces. In all but the two extreme extension wrist positions, flexion force was greater than extension force. The mean flexion force was 62.09 N, significantly higher than the extension force of 52.73 N. Thus extension is 84.9% of flexion force overall. Male flexion forces over all wrist positions were 73.24 N and extension was 62.29 N, with extension representing 85.1% of flexion. Female flexion forces over all wrist positions were 55.42 N and extension was 45.05 N, with extension representing 81.3% of flexion.
Interaction between wrist position and direction The interaction of wrist position and direction of force exertion was significant for the dependent variable of force exerted. Thus, a post-hoc test was performed for flexion force by wrist position (Table 5) and a second text was performed for extension force by wrist position (Table 6). The order of the magnitudes of force differ by direction of force exertion. As shown in Table 5, flexion force in flexed wrist positions was significantly better than in most extended wrist positions. Extension force did not have such a clear pattern, with larger groupings and a different wrist position order, as shown in Table 6. The mean forces at each wrist position (categorized by gender, Table 2) were calculated and plotted for flexion and extension. These values were then plotted separately for males and females, with Figures 2 and 3 for flexion and extension respectively. A distinct pattern over the range of wrist positions is seen. The pattern for flexion differs from that of extension; yet the male and female data form the same pattern, which may account for the non-significant interaction of gender with position. Discussion a n d conclusions The mean, over subjects, of the 3 s average force did not significantly differ from the peak force within the
Table 3 Post-hoe test for force by wrist position
Wrist position
Mean force
Table 5 Post-hoc test for flexion forte by wrist position
Grouping
(N) 90 ° flexion 75* flexion 30 ° flexion 45* flexion 15" extension 15" flexion 30 ~ extension 90 ° extension 0 ° (neutral) 60 ~ flexion 45 ° extension 60° extension 75 ° extension
76.92 70.07 64.67 63.12 62.07 60.88 60.37 59.21 58.80 56.62 54.17 47.08 44.95
I
Wrist position
Mean force
90 ° flexion 75 ° flexion 30° flexion 45 ° flexion 60° flexion 15 ° flexion 15° extension 0~ (neutral) 30 ° extension 45 ° extension 90~ extension 60 ° extension 75 ° extension
93.69 88.71 72.11 70.68 67.18 66.76 66.52 63.33 63. I0 57.01 52.37 49.70 43.02
Grouping
!
Vertical lines indicate that there is not a significant (p ~< 0.05) difference between means
Table 4 Flexion and extension forces, their ratio, and probability of difference between flexion and extension
Vertical lines indicate that there is n o t a significant (p ~< 0.05) different between means Table 6 Post-hoc test for extension force by wrist position Wrist position
Average force
Mean force
Grouping
t~
(N) Wrist
lmeitlon
Flexion Extension Ext/fiex%
90 ~ flexion 75 ° flexion 60~ flexion 45 ° flexion 30 ~ flexion 15 ° flexion 0 ° (neutral) 15 ° extension 30~ extension 45 ° extension 60 ~ extension 75 ° extension 90 ~ extension
93.69 88.71 67.18 70.68 72.11 66.76 63.33 66.52 63.10 57.01 49.70 43.02 52.37
60.16 51.43 46.06 55.56 57.22 55.00 54.28 57.62 57.64 51.33 44.46 46.88 66.06
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64.2% 58.0% 68.6% 78.6% 79.4% 82.4% 85.7% 86.6% 91.3% 90.0% 89.5% 109.0% 126.1%
p 0.0061 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0055 0.0003 0.0001 0.0014 0.0001
90 ° 90 ° 30° 15° 30° 45 °
extension flexion extension extension flexion flexion 15 ° flexion 0° (neutral) 75 ° flexion 45 ° extension 75 ° extension 60° flexion 60 ° extension
66.06 60.16 57.64 57.62 57.22 55.56 55.00 54.28 51.43 51.33 46.88 46.06 44.46
Vertical lines indicate that there is not a significant (p ~< 0.05) difference between m e a n s
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
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0 Fgo 05 F60 Fi5 F30 F15 N/:U Ei5 E30 El5 E~'E-75 EgO Wrist Position (Degrees) f --m'- Male ----A~Female l
Figure 2 Plot of male and female flexion forces by wrist position 110"I
and physical condition (self-reported as good to excellent). Prior research (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Linscheid and Chao, 1973; Hazelton et al, 1975; Pryce, 1980; Mathiowetz et al, 1985b; Woody and Mathiowetz, 1988; Hallbeck and McMuilin, 1991, 1993; McMullin and Hallbeck, 1991) shows the effect of joint angle upon MVC force for grip and pinch. Thus, significance of wrist position for force generation was expected. As each joint position changes, each bone comprising that joint shifts with respect to others, displacing all tendons and ligaments in contact with the bone (Weber, 1988). Wrist position may affect the magnitude of forces that can be generated in the carpus by changing the tendon length and moment arms of the bony configuration. The pinch and grasp findings cited above lead to the hypothesis that neutral or slightly extended wrist positions would allow a greater force exertion than flexed wrist positions. However, the p o s t - h o c test on the main effect of wrist position on mean force illustrates that, in general, mean forces in flexed wrist positions are significantly higher than those performed with an extended wrist. The direction of the force was also expected to be a significant factor in the force-generation magnitude. Flexion forces were hypothesized to be greater than those of extension, as the distal upper limb contains more flexors than extensors and they are more powerful (Thompson, 1981). Ketchum et al (1978), Brand et al (1981), Norkin and Levangie (1983), Brand (1985), and A n et al (1986) found that flexion forces were twice those generated in extension. These flexion and extension force values were for hand strength, a combination of wrist and digital muscles. There are three wristdedicated flexors and three extensor muscles, and while the flexors have a larger physiological cross-sectional area, it is not twice the area of the extensors (Brand, 1985). Thus the wrist-dedicated muscles, when acting alone, may be more balanced than those of the hand. This study demonstrates a smaller difference between flexion and extension forces than those in previous studies. The mean over all wrist positions demonstrates the extension forces are 85.6% of those in flexion for males and 81.2% for females. The effect of the interaction of wrist position and direction on force exertion has not been found reported in the literature. Conceptually, one would assume that flexion force in a moderate to extremely flexed wrist position would be less than in a neutral or slightly extended position. This reduction of flexion force could be due to limitation of the contractile mechanism at extremely flexed wrist positions and a passive (recoil) mechanism aiding the flexion force in moderate to extreme extended wrist positions. The opposite would be assumed to be true for extension force: namely, higher extension forces at neutral to flexed wrist positions than in extremely extended wrist positions. This was not found to be the case, as demonstrated in Tables 5 and 6 for flexion and extension forces respectively. One explanation for the high forces at extreme postures is forearm movement with a 'stiffened' wrist. The apparatus was designed to minimize the movement potential, but deformation of the skin over the forearm may have allowed enough movement to cause significant force differences. However, visual examination of
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0 FgO F75 F60 F~I5 F30 FlS NEU EiS ~ E,4.5E60 E'75 EgO Wrist Position (Degrees) { --=- Male -~- Female ] Figure 3 Plot of male and female extension forces by wrist position 3 s window for either males or females. This finding was an artefact of the static testing procedure. The static force measure was rejected if there was a single peak point outside a + 10% band around the 3 s average for that exertion. As the peak and average forces were not significantly different, these forces could be termed a static exertion. Thus one of the underlying assumptions of this study was confirmed to be statistically true. The ANOVA procedure demonstrates that the main effects of gender, wrist position and force exertion direction are statistically significant, as is the wrist position interaction with direction. Based upon the literature, these results were expected. It has been stated that female strength is significantly lower than that of males. Female forces range between half and two-thirds of that which can be generated by males (Roebuck et al, 1975; An et al, 1986). The average forces measured in this study are significantly different for males and females. The female forces generated in this study are 76.3% and 72.4% of those generated by males in flexion and extension respectively. The subject selection may have influenced these results, as well as the subjects' age (20-30 years of age)
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
the raw data revealed that all subjects followed a very similar pattern for force over the wrist positions. It is unlikely that all subjects were able to enhance their performance in this manner. However, arm movement within the fixture does not explain the findings in the less extreme postures. If one looks at the effect of wrist position on flexion force exerted, even disregarding extreme wrist positions, flexed wrist positions allow higher force exertion than in neutral and significantly higher forces than in most extended wrist positions. This pattern is totally reversed for extension forces. In most extended wrist positions, again disregarding extreme wrist positions, extension force is higher than in a neutral wrist position and significantly higher than in most flexed wrist positions, but not significantly. Thus further study of this phenomenon is warranted. In summary then, the results of this study are similar to findings of other researchers for the main effects of gender and direction of force exertion. The main effect of wrist position on combined flexion and extension forces utilizing the wrist-dedicated muscles had not previously been studied, and yielded results that differed from those expected. This contradiction of expectation was even more apparent when the wrist position-exertion direction interaction was examined. Further study of this interaction is necessary in future experiments to explain these findings. Acknowledgements I would like to thank Dr K. H. E. Kroemer who acted as major professor for this dissertation work, as well as supporting me during my stay at VPI&SU. I would also like to acknowledge the members of my committee: Drs M. M. Ayoub, R. D. Dryden, R. C. WiUiges and H. L. Snyder. References An, K. N., Clmo, E. Y. and Asken, L. J. 1983 'Functional assessment of upper extremity joints' in IEEE, Frontiers of Engineering and Computers in Health Care IEEE, pp 136-139 An, K. N., Askew, L. J. and Clmo, E. Y. 1986 'Biomechanics and functional assessment of upper extremities' in Karwowski, W. (ed.) Trends in Ergonomics~Human Factors II1 Elsevier Science Publishers, pp 573-580 Anderson, C. T. 1965 'Wrist joint position influences normal hand function' Unpublished masters thesis, University of Iowa, Iowa City, IA ~strand, P. O. and Rodahl, K. 1986 Textbook of Work Physiology 3rd edn, McGraw-Hill, New York Berg, V. J., Clay, D. J., Fathallah, F. A. and Higginbotham, V. L. 1989 'The effects of instruction on finger strength measurements: applicability of the Cadlwell regimen' in Aghaeadeh, F. (ed.) Trends in Ergonomics~Human FactorsVElsevier Science Publishers, pp 191-198 Brand, P. W. 1985 Clinical Mechanics of the Hand C. V. Moshy, St Louis, MO Brand, P. W., Beach, R. B. and Thompson, D. E. 1981 'Relative tension and potential excursion of muscles in the forearm and hand' J Hand Surg 6(3), 209--219 Bunneil, S. 1944 Surgery of the Hand 2nd edn, J. B. Lippincott, Philadelphia, PA Caldwell, L.S., Chaflln, D . B . , Dukes-Dobos, F . N . , Kroemer, K. H. E., Laulmch, L. L., Snook, S. H. and Wasserman, D. E. 1974 'A proposed standard procedure for static muscle strength testing' Am lnd Hyg Assoc J 35(4) 201-206 Chaffin, D. B. 1975 'Ergonomics guide for the assessment of human static strength' Am lnd Hyg J 36, 505-511 Chaflin, D. B. and Andersmn, G. 1984 Occupational Biomechanics Wiley and Sons, New York, NY
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Flatt, A . F . 1961 'Kinesiology of the hand' Instructional Course Lectures, 18, 266-281 Gilford, W . W . , Bolton, B.M. and Lambrinudi, C. 1943 'The mechanism of the wrist joint, with special reference to fractures of the scaphoid' Guy's Hospital Report, 92, 52-59 Grant[jean, E. 1982 Fitting the Task to the Man: An Ergonomic Approach Taylor & Francis, London Halibeck, M. S. and MeMuHin, D. L. 1991 'The effect of gloves, wrist position, and age on peak three-jaw chuck pinch force: a pilot study' in Proc Human Factors Society 35th Annual Meeting Human Factors Society, Santa Monica, CA, pp 753-757 Hallbeek, M. S. and MeMulfin, D. L. 1993 'Maximal power grasp and three-jaw chuck pinch force as a function of wrist position, age, and glove type' lnt J lnd Ergonomics 11(3), 195--206 Halibeck, M. S., Stonegpher, B. L., Cochran, D. L., Riley, M. W. and Bishu, R. R. 1990 'Hand-handle coupling and maximum force' in Proc Human Factors Society 34rd Ann Mtg Human Factors Society, Santa Monica, CA, pp 800-804 Hazelton, F. T., Smidt, G. L. Flatt, A. E. and Stephens, R. I. 1975 'The influence of wrist position on the force produced by the finger flexors' J Biomech 8, 301-306 Jorgensen, M. J., Riley, M. W., Coehran, D. J. and Bishu, R. R. 1989 'Maximum forces in simulated meat-cutting tasks' in Proc Human Factors Society 33rd Ann Mtg Human Factors Society, Santa Monica, CA, pp 641--645 Kelior, M., Frost, J., Siiherberg, N., Iverson, I. and Cummings, R. 1971 'Hand strength and dexterity' Am J Occup Ther 25 (2), 77-83 Ketchum, L. D., Brand, P. W., Thompson, D. and Poeack, G. S. 1978 'The determination of moments of extension of the wrist generated by muscles of the forearm' J Hand Surg 3(3), 205-210 Kraft, G. H. and Detels, P. E. 1972 'Position of function of the wrist' Phys Med Rehabil 53, 272-275 Linscbeid, R. L. and Chan, E. Y. 1973 'Biomechanical assessment of finger function in prosthetic joint design' Orthop Clinics North Am 4(2), 317-330 Mathiowetz, V., Kashman, N., Volland, G., Weber, K., Dowe, M. and Rogers, S. 1985a 'Grip and pinch strength: normative data for adults' Arch Phys Med Rehabil 66, 16-21 Mathiowetz, V., RenneBs, C. and Donahoe, L. 1985b 'Effect of elbow position on grip and key pinch strength' J Hand Surg 10A(5), 694-697 McArdle, W . D . , Katch, F . I . and Katch, V.L. 1986 Exercise Physiology: Energy, Nutrition, and Human Performance 2nd edn Lea and Febignr, Philadelphia MeMuliln, D. L. and Hallbeck, M. S. 1991 'Maximal power grasp force as a function of wrist position, age, and glove type: a pilot study' in Proc of Human Factors Society 35th Annual Meeting Human Factors Society, Santa Monica, CA, pp 733-737 Norkin, C. C. and Levangie, P. K. 1983 Joint Structure and Function: A Comprehensive Analysis F. A. Davis, Philadelphia, PA Putz-Anderson, V. 1988 Cumulative Trauma Disorders: A Manual for Musculoskeletal Diseases of the Upper Limbs Taylor & Francis, London Pryce, J. C. 1980 'The wrist position between neutral and ulnar deviation that facilitates the maximum power grip strength' J Biomech 13, 505-511 Roebuck, J . A . , Kroemer, K. H. E. and Thomson, W . G . 1975 Engineering Anthropometry Methods Wiley and Sons, New York, NY Skovly, R. C. 1967 'A study of power grip strength and how it is influenced by wrist joint position' Unpublished masters thesis, University of Iowa, Iowa City, IA Terrell, R. and Purswell, J. L. 1976 'The influence of forearm and wrist orientation on static grip strength as a design criterion for hand tools' in Proc Human Factors Society 20th Ann Mtg Human Factors Society, Santa Monica, CA, pp 28-32 Thompson, D. E. 1981 'Biomechanics of the hand' Perspect Comput 3(3), 12-19 Weber, E . R . 1988 'Wrist mechanics and its association with ligamentous instability' in Lichtman D. M. (ed.) The Wrist and its Disorders W. B. Saunders, Philadelphia, PA, pp 41-53 Woedson, B. 1981 Human Factors Design Handbook McGraw-Hill, New York, NY Woody, R. and Mathiowetz, V. 1988 'Effect of forearm position on pinch strength measurements' J Hand Ther 1(3), 124-126 Zellers, K. K., Gandini, C. and Loveless, T. 1992 'The effect of arm position on pinch strength measurements: a pilot study' in Kumar, S. (ed.) Advances in Industrial Ergonomics and Safety IV Taylor & Francis, London, pp 709-712
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Flexion and extension forces generated by wrist-dedicated muscles over the range of motion M. S. Hallbeck Industrial and Management Systems Engineering Department, Center for Ergonomics and Safety Research, University of Nebraska, Lincoln, NE 68588--0518, USA
An experiment was performed t o evaluate t h e relationships among active range of motion (ROM), gender, wrist position and direction of force exertion in their effects on the magnitude of static force exerted by the wrist-dedicated muscles in wrist flexion and extension. This study employed 60 right-hand-dominant subjects (30 male, 30 female) between 20 and 30 years of age, all reporting no prior wrist injury and good to excellent overall physical condition. The ROM of each subject was used to determine the number of wrist positions evaluated for static maximal voluntary forces generated in wrist flexion and extension while they were instructed to relax their fingers; thus only the six wrist-dedicated muscles were employed in the exertion. The ANOVA procedure showed gender, wrist position, direction of force exertion, and the wrist position interaction with direction to have significant effects upon maximal force exertion. Females averaged 76.3% of the mean male flexion force _and 72.4% for extension. On average, extension forces were found to be 83.4% of those generated by flexing the wrist-dedicated muscles.
Keywords:wrist-dedicatedmuscles,maximalvoluntaryforce, flexion-extensionforce measurement Forces that can be generated by the upper limb have been measured by orthopaedic surgeons, ergonomists, and occupational and physical therapists. Wrist flexion and extension forces measured externally at the hand are a combination of forces acting between the forearm and hand, generated by exertions of agonistic and antagonistic muscle groups both within the hand (intrinsic) and the forearm (extrinsic) and by wristdedicated (carpi) muscles. Many factors influence the magnitude of force that can be generated, including: type of prehension, digits involved, digital posture, wrist posture, arm posture, overall body posture, subject age, gender, hand tested, anthropometric dimensions, direction of force exertion, instructions to the subjects, day, and motivation. Review of literature
The number and strength of individual digits involved in the force generation affect the magnitude of external force (Berg et al, 1988). The digital, wrist, arm and whole body posture also affects the magnitude of force (Bunnell, 1944; Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975; Terrell and Purswell, 1976; Pryce, 1980; Brand, 1985; Putz-Anderson, 1988; Hallbeck and McMullin, 1991; McMullin and Hallbeck, 1991). The effect of wrist position on external force has been demonstrated in
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both pinch and power grasp prehension. Any deviation from neutral decreases the external force measured, with flexion causing a larger decrement than extension (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975; Ten'ell and Purswell, 1976; Pryce, 1980; Mathiowetz et al, 1985b; Putz-Anderson, 1988; Woody and Mathiowetz, 1988; HaUbeck and McMullin, 1991, 1993; McMullin and Hallbeck, 1991; ZeUers et al, 1992). Gender has been found to affect the amount of force generated in power grasp and pinch (Roebuck et al, 1975; Chaffin and Andersson, 1984; An et al, 1986; Sanders and McCormick, 1986). Female grip strength has been found to be approximately 50-60% of male grip strength (Kellor et al, 1971; An et al, 1983; Mathiowetz et al, 1985a). Females can also be expected to exert less pinch force than males. The three-jaw chuck pinch strength that females can exert has been reported as 50% (McArdle et al, 1986), 60--63% (Kellor etal, 1971), 67% (Grand jean, 1982; Mathiowetz et al, 1985a), 70% (Berg et al, 1988), and 74% (An et al, 1983; Hallbeck and McMullin, 1991, 1993; McMullin and Hallbeck, 1991) that of males. In addition, the direction of the force (flexion or extension) has been shown to affect force magnitude generation. Flexion force generation is reported to be greater than that for extension (Thompson, 1981; Norkin and Levangie, 1983; Brand, 1985). The findings
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
of studies by Jorgensen et al (1989) and Hallbeck et al (1990) indicated that the direction of power grasp force relative to the hand also appears have a significant effect on the magnitude of the force exerted. Wrist-dedicated muscles
Static positioning of the hand and wrist requires balanced action of many synergistic muscle sets (Brand, 1985). The muscles that control the wrist serve two functions in the hand. They provide the fine adjustment of the hand to its functioning position and, once this position is achieved, they stabilize the wrist to provide a stable working platform for the hand and digits (Hazelton et al, 1975). If wrist movement is considered in relation to digit movement, two independent actions are found to be possible. When the wrist-dedicated muscles stabilize the joint, the extrinsic and intrinsic digit muscles can alter the position of the digit(s). Conversely, when digit posture is stabilized, the wrist can be positioned over a large range of motion (ROM) (Flatt, 1961). Muscles whose tendons merely cross the wrist (secondary wrist movers) en route to the digits cannot be relied upon for their wrist-moving and stabilizing characteristics, as wrist function may at times conflict with their primary digit function(s) (Brand, 1985). This dual innervation may be bypassed when subjects are instructed to extend the wrist with maximal effort while their fingers are loose and flaccid (Ketchum et al, 1978). Wrist position
Wrist position affects finger position (Pryce, 1980; Woody and Mathiowetz, 1988). Grip strength is greater in the extended wrist than in the flexed wrist, owing to increased length of the flexor tendons (Linscheid and Chao, 1973). The angles at which the extrinsic digital flexors approach the digits change with wrist position (Bunnell, 1944; Brand, 1985). The capacity of the musculotendinous units to generate force is dependent in part upon the effective functional length (Hazelton et al, 1975). Thus the highest grasp or pinch strength would occur where the length-tension relationship is maximized. Digit, hand, wrist and elbow position influence peak pinch and grip strengths (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975; Pryce, 1980; Mathiowetz et al, 1985b; Woody and Mathiowetz, 1988; Hallbeck and MeMullin, 1991; McMullin and Hallbeck, 1991). As wrist position deviates from normal, both pinch and power grasp forces decrease with a larger decrement in flexion than extension (Hallbeck and McMullin, 1991, 1993); however, the effect of wrist position on the wrist-dedicated muscles was not found in the literature. Gender
Human skeletal muscle can generate approximately 3-4 kg of force per square centimetre of cross-sectional muscle (Astrand and Rodahl, 1986). Males possess a greater amount of muscle area than females and therefore they can generate a greater amount of force (McArdle et al, 1986). On average, female strength has been found to be approximately two-thirds that of males; however, there is a large overlap in gender
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strength distributions, and there are many females stronger than males (Roebuck et al, 1975; Chaffin and Andersson, 1984). Female grip strength has been found to be approximately 50-60% of male grip strength by several authors (Kellor et al, 1971; An et al, 1983; Mathiowetz et al, 1985a), but was found to be 74% that of males by McMullin and Hallbeck (1991). Females can also be expected to exert less pinch force than males. The three-jaw chuck pinch strength that females can exert has been reported as 50% (McArdle et al, 1986), 60-63% (Kellor et al, 1971), 67% (Grandjean, 1982; Mathiowetz et al, 1985a), 70% (Berg et al, 1988), 74% (An et al, 1983), and 78% that of males (Hallbeck and McMullin, 1991, 1993). An el al, (1986) found that females could generate 60% of the torque that males could generate in maximal wrist flexion and extension. Measured flexion a n d extension forces
The 'strength' of each muscle/tendon unit has been studied in cadavers. However, the contraction combinations that control the wrist movement mechanism cannot be examined without internal innervation; thus these 'strengths' are of little use to the industrial ergonomist (Gilford et al, 1943; Brand, 1985). The forearm complex has been optimized for strength in flexion, not extension; the hand contains more flexors than extensors, and they are more powerful (Thompson, 1981). When the physical capacity of the wrist muscles is assessed, the wrist flexors have more than twice the capacity of the extensors (Norkin and Levangie, 1983). Only one study was found in the literature that measured torque generation in wrist flexion and extension; it employed static measurement of the intrinsic and extrinsic digital muscles and the wristdedicated muscles (An et al, 1986). In this study An et al (1986) found that on average the flexion torque was twice that of extension. This finding supports previous findings in the literature. Summary
The forces that can be generated by the wrist-dedicated muscles alone have not been studied. Force generation has been hypothesized to be a function of the musculature employed, wrist position, gender, and direction of force exertion (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Hazelton et al, 1975, Roebuck et al, 1975; Pryce, 1980; Thompson, 1981; Norkin and Levangie, 1983; Chaffin and Andersson, 1984; Brand, 1985; Mathiowetz et al, 1985a; An et al, t986; Sanders and McCormick, 1986; Woody and Mathiowetz, 1988; Hallbeck and McMullin, 1991, 1993; McMullin and Hallbeck, 1991). To evaluate the effect of these variables on force generation of the wrist-dedicated muscles, a study was performed, with flexion and extension forces measured at a number of wrist positions for both male and female subjects.
Method Apparatus
A flexion-extension force measurement apparatus (FEFMA) was designed to measure the force generated in flexed and extended wrist positions. The basic
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
elements of the FEFMA were adapted from a similar device employed by the Mayo Clinic Biomechanics Laboratory (An et al, 1986). The FEFMA consisted of a box that secured the forearm and ensured constant upper limb posture, a pair of force transducers, and a handle (proximal to the metacarpophalangeal joint) connected to the force transducers. The forearm was secured in the metal box between adjustable pads, which held the distal forearm midway between pronation and supination. The proximal forearm was positioned by packing the area between the walls of the box and the forearm with foam padding. The restraint ended far enough proximal to the wrist to allow full range of motion, and the padding was not so tight as to affect the muscular contraction nor to obstruct blood flow. This padded restraint box secured the forearm into a standardized posture and allowed no appreciable motion of the forearm during the measurements. By restraining the forearm, the measured output force was only that force which could be generated within the forearm. In addition, this restraint system would limit the effect of upper arm isometric contraction on the measured force and limit shoulder force (with a 'stiffened' wrist), which may artificially inflate the measured force at large angles of wrist flexion or extension. The handle structure consisted of two plates designed such that one plate rested on the palm and the other on the dorsal side of the hand, both proximal to the metacarpophalangeal joint. The plate separation was adjusted for each subject so that maximal force exertion was not limited by hand slippage or by pain. The entire handle was padded with a thin layer of foam to cushion pressure points along the metacarpals to prevent slippage and pain. The dorsal side of the handle had a distal extension plate, which was parallel to the long axis of the third metacarpal. Force generated in the wrist was transmitted via turnbuckles from the handle to either of the force transducers (LeBow load cells, 136 kg capacity, Model no. 3397). These load cells were mounted on a pivoting arm to keep the line of force application perpendicular to the load cell, as well as perpendicular to the long axis of the third metacarpal, for each measurement position. This pivoting arm was fixed in each measurement position. One force transducer measured extension, the other flexion force, in each position. The transducer voltage output was amplified and converted into a digital signal utilizing a Metrabyte ® A/D converter. This signal was then stored on an IBM PC, as a function of time. Subjects
Sixty right-hand-dominant subjects (30 male and 30 female) between 20 and 30 years of age voluntarily participated in this study contingent upon positive palpation of the palmaris longus muscle of the subject. Each subgroup was kept as homogeneous as possible by limiting the age range and selecting only right-handdominant subjects having all six wrist-dedicated (carpi) muscles. Procedure The subjects read and signed an informed consent agreement, which included the statement that they
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were not limited by any orthopaedic dysfunction, especially within the upper limb. Range o f motion. The active range of motion (ROM) in flexion and extension for a no-load condition was measured by the experimenter using a plastic goniometer. The active R O M was defined as the angle included between extreme active flexion and extension that could be achieved with no discomfort. Force measurements. Each subject was seated in a chair with the upper body in a standardized upright position. This position specified that the upper arm of the zubject should be relaxed, hanging with no shoulder abduction, the elbow joint flexed with a 90° included angle. After the active R O M was measured on an individual, a series of positions were located at 15° intervals, beginning at the neutral position, up to and including the extreme flexion and extension points. Thus the number of wrist positions tested was dependent upon each individual's ROM. Fifteen degree increments were selected, in lieu of percentages of each individual R O M , for ease of comparison and future use in design of equipment. The hand and arm of each subject were then placed in the F E F M A for force measurements. After securing the forearm of the subject midway between pronation and supination, the hand was inserted into the handle. Flexion and extension forces generated by only the wrist-dedicated muscles (with the digits relaxed) were measured in each of the wrist positions. To eliminate any order effects on the force measurements, the wrist position order was randomized by computer. Once the wrist was positioned in a specific wrist position in the FEFMA, the subject was instructed to maximally flex or extend the wrist, using only those muscles directly affecting the wrist. Each was told to relax the fingers, not to stiffen, flex, or hyperextend the digits as Ketchum et al (1978) had instructed, and subjects were visually monitored by the experimenter to verify relaxed fingers during the exertion. The relaxation minimized recruitment of the digital muscles and thus the force was generated by the wrist-dedicated muscles. A photograph of a subject performing the exertion is shown in Figure 1. The force generation measurements followed a modified Caldwell regimen (Caldwell et al, 1974) developed by the Virginia Polytechnic Institute and State University Industrial Ergonomics Laboratory in 1987. This protocol, modified for the smaller number of both muscle groups and fibres involved, called for a slow build-up to peak force (MVC), sustaining that peak for 3 s, with a slow return to a relaxed state (Berg et al, 1988). During the 3 s sustained exertion phase, the measured peak force was required to lie within a _+10% band about the average sustained force for each exertion as defined by Caldwell et al (1974). If this requirement was not met, the static exertion was performed again until this criterion was met. Two static force exertion measurements (flexion and extension) were taken at each position; a 1 min rest was provided between each exertion as suggested by Chaffin (1975). The time required to reposition the wrist was at least 1 min; thus fatigue should not have affected the MVC values.
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
generated in flexion and extension were utilized in further analyses. This lack of significance also illustrates that the exertion can be considered a static exertion, where the average force is not significantly different from the peak; thus the _+10% criterion was effective To evaluate all possible effects of gender, wrist position (13 levels, between 90° flexion and 90° extension), and direction of force exertion (flexion and extension) on the average force that was generated, an A N O V A procedure was performed. The ANOVA summary table (Table 1) shows that gender, wrist position, direction, and the interaction of wrist position and direction effects were statistically significant.
Gender Females were able to exert less force than males in all wrist positions, as shown in Table 2. Females averaged 76.3% of the male flexion force and 72.4% for extension. When the forces were summed over all wrist positions and both directions, average female force exertion was 74.2% that of males.
Wrist position Post-hoc testing was performed on wrist position, and the Tukey test results are shown in Table 3. Mean force~ at wrist positions grouped by vertical lines do not differ significantly at et = 0.05. Flexed wrist positions Table 1 Force ANOVA summary table
Figure 1 Photograph of flexion-extension force measurement apparatus (FEFMA)
Experimental design To verify that a 'static' exertion was achieved by the testing procedure, the peak value in the 3 s window and average forces over the 3 s exertion, for both flexion and extension, were compared using the critical ratio test. The result of no significant difference between peak and average force would verify the testing procedure for static exertion; thus average forces could be utilized in further analyses. The measured static force for the 60 subjects was the dependent variable for an analysis of variance (ANOVA) procedure. The independent variables for the 2 × 30 X 13 X 2 mixedfactor full-factorial design were gender, subjects, wrist position (WP) and direction of force exertion respectively. There were 13 levels of WP corresponding to the 13 possible positions from 90° flexion to 90° extension in 15° increments. At each WP, each subject performed static flexion and extension exertions, which were the two levels of the direction factor. Post-hoc (Tukey) tests were performed on all significant effects.
Results Comparisons between the peak value within the 3 s exertion window and the average of the 3 s exertion forces were performed for flexion and extension forces, and the combination of flexion and extension forces. There was no significant difference between the mean and peak force for either flexion or extension. Therefore, only the averages of the 3 s exertion forces
382
Source
df
Between subject G e n d e r (G) Subjects (S/G)
MS
F
103 984.9 4 853.7
21.42 0.0001
58 12 12 597 1 1 58 12 12 597
5 675.8 98.9 161.9 38 689.3 30.9 650,0 3 457.6 122.7 89.7
35.06 0.000l 0.61 0.8339
1
Within subject Wrist position (WP) W × G W × S/G Exertion direction (D) D × G D × S/G WP × D WP × D × G WP × D × S/G Total
P
59.52 0.0001 0.05 0.8279 38.56 0.0001 1.37 0.1764
1361
Table 2 Male and female flexion and extension forces by wrist position Male force (N)
Female force (N)
Wrist
position
Flexion Extension Flexion Extension
90 ° flexion 75 ° flexion 60 ° flexion 45 ° flexion 30 ° flexion 15° flexion 0 ° (neutral) 15 ° extension 30 ° extension 45 ° extension 60 ° extension 75 ° e x t e n s t o n 90 ° extension Avg over all WP Avg over WP and dir
104.24 68.54 101.05 58.86 76.99 51.79 81.60 64.62 81.06 66.77 76.36 64.44 73.12 63.16 74.78 68.35 73.21 68.26 65.74 60.35 57.45 52.24 47,54 54.29 60,55 76.87 73.15 62.29 67.72
75.23 45.49 79.89 46.12 58.05 40.72 59.76 46.49 63.16 47.68 57.15 45.56 53.53 45.40 58.25 46.90 54.01 47.01 48.28 42.31 43.06 37.46 39.11 40.46 47.05 59.03 55.42 45.05 50.23
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
had higher mean force exertions than extension. The surprising finding is that extreme flexion wrist positions showed higher force magnitudes than those in a neutral wrist position.
Direction The average extension force was found to be significantly different from the average flexion force for both males and females. The A N O V A results have been further summarized by tabulating the average flexion force, extension force, the ratio of extension to flexion and the exact probabilities that the flexion and extension forces exertions have the same mean at each wrist position (Table 4). At each wrist position a significant difference was shown between average flexion and extension forces. In all but the two extreme extension wrist positions, flexion force was greater than extension force. The mean flexion force was 62.09 N, significantly higher than the extension force of 52.73 N. Thus extension is 84.9% of flexion force overall. Male flexion forces over all wrist positions were 73.24 N and extension was 62.29 N, with extension representing 85.1% of flexion. Female flexion forces over all wrist positions were 55.42 N and extension was 45.05 N, with extension representing 81.3% of flexion.
Interaction between wrist position and direction The interaction of wrist position and direction of force exertion was significant for the dependent variable of force exerted. Thus, a post-hoc test was performed for flexion force by wrist position (Table 5) and a second text was performed for extension force by wrist position (Table 6). The order of the magnitudes of force differ by direction of force exertion. As shown in Table 5, flexion force in flexed wrist positions was significantly better than in most extended wrist positions. Extension force did not have such a clear pattern, with larger groupings and a different wrist position order, as shown in Table 6. The mean forces at each wrist position (categorized by gender, Table 2) were calculated and plotted for flexion and extension. These values were then plotted separately for males and females, with Figures 2 and 3 for flexion and extension respectively. A distinct pattern over the range of wrist positions is seen. The pattern for flexion differs from that of extension; yet the male and female data form the same pattern, which may account for the non-significant interaction of gender with position. Discussion a n d conclusions The mean, over subjects, of the 3 s average force did not significantly differ from the peak force within the
Table 3 Post-hoe test for force by wrist position
Wrist position
Mean force
Table 5 Post-hoc test for flexion forte by wrist position
Grouping
(N) 90 ° flexion 75* flexion 30 ° flexion 45* flexion 15" extension 15" flexion 30 ~ extension 90 ° extension 0 ° (neutral) 60 ~ flexion 45 ° extension 60° extension 75 ° extension
76.92 70.07 64.67 63.12 62.07 60.88 60.37 59.21 58.80 56.62 54.17 47.08 44.95
I
Wrist position
Mean force
90 ° flexion 75 ° flexion 30° flexion 45 ° flexion 60° flexion 15 ° flexion 15° extension 0~ (neutral) 30 ° extension 45 ° extension 90~ extension 60 ° extension 75 ° extension
93.69 88.71 72.11 70.68 67.18 66.76 66.52 63.33 63. I0 57.01 52.37 49.70 43.02
Grouping
!
Vertical lines indicate that there is not a significant (p ~< 0.05) difference between means
Table 4 Flexion and extension forces, their ratio, and probability of difference between flexion and extension
Vertical lines indicate that there is n o t a significant (p ~< 0.05) different between means Table 6 Post-hoc test for extension force by wrist position Wrist position
Average force
Mean force
Grouping
t~
(N) Wrist
lmeitlon
Flexion Extension Ext/fiex%
90 ~ flexion 75 ° flexion 60~ flexion 45 ° flexion 30 ~ flexion 15 ° flexion 0 ° (neutral) 15 ° extension 30~ extension 45 ° extension 60 ~ extension 75 ° extension 90 ~ extension
93.69 88.71 67.18 70.68 72.11 66.76 63.33 66.52 63.10 57.01 49.70 43.02 52.37
60.16 51.43 46.06 55.56 57.22 55.00 54.28 57.62 57.64 51.33 44.46 46.88 66.06
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64.2% 58.0% 68.6% 78.6% 79.4% 82.4% 85.7% 86.6% 91.3% 90.0% 89.5% 109.0% 126.1%
p 0.0061 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 0.0055 0.0003 0.0001 0.0014 0.0001
90 ° 90 ° 30° 15° 30° 45 °
extension flexion extension extension flexion flexion 15 ° flexion 0° (neutral) 75 ° flexion 45 ° extension 75 ° extension 60° flexion 60 ° extension
66.06 60.16 57.64 57.62 57.22 55.56 55.00 54.28 51.43 51.33 46.88 46.06 44.46
Vertical lines indicate that there is not a significant (p ~< 0.05) difference between m e a n s
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Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
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0 Fgo 05 F60 Fi5 F30 F15 N/:U Ei5 E30 El5 E~'E-75 EgO Wrist Position (Degrees) f --m'- Male ----A~Female l
Figure 2 Plot of male and female flexion forces by wrist position 110"I
and physical condition (self-reported as good to excellent). Prior research (Anderson, 1965; Skovly, 1969; Kraft and Detels, 1972; Linscheid and Chao, 1973; Hazelton et al, 1975; Pryce, 1980; Mathiowetz et al, 1985b; Woody and Mathiowetz, 1988; Hallbeck and McMuilin, 1991, 1993; McMullin and Hallbeck, 1991) shows the effect of joint angle upon MVC force for grip and pinch. Thus, significance of wrist position for force generation was expected. As each joint position changes, each bone comprising that joint shifts with respect to others, displacing all tendons and ligaments in contact with the bone (Weber, 1988). Wrist position may affect the magnitude of forces that can be generated in the carpus by changing the tendon length and moment arms of the bony configuration. The pinch and grasp findings cited above lead to the hypothesis that neutral or slightly extended wrist positions would allow a greater force exertion than flexed wrist positions. However, the p o s t - h o c test on the main effect of wrist position on mean force illustrates that, in general, mean forces in flexed wrist positions are significantly higher than those performed with an extended wrist. The direction of the force was also expected to be a significant factor in the force-generation magnitude. Flexion forces were hypothesized to be greater than those of extension, as the distal upper limb contains more flexors than extensors and they are more powerful (Thompson, 1981). Ketchum et al (1978), Brand et al (1981), Norkin and Levangie (1983), Brand (1985), and A n et al (1986) found that flexion forces were twice those generated in extension. These flexion and extension force values were for hand strength, a combination of wrist and digital muscles. There are three wristdedicated flexors and three extensor muscles, and while the flexors have a larger physiological cross-sectional area, it is not twice the area of the extensors (Brand, 1985). Thus the wrist-dedicated muscles, when acting alone, may be more balanced than those of the hand. This study demonstrates a smaller difference between flexion and extension forces than those in previous studies. The mean over all wrist positions demonstrates the extension forces are 85.6% of those in flexion for males and 81.2% for females. The effect of the interaction of wrist position and direction on force exertion has not been found reported in the literature. Conceptually, one would assume that flexion force in a moderate to extremely flexed wrist position would be less than in a neutral or slightly extended position. This reduction of flexion force could be due to limitation of the contractile mechanism at extremely flexed wrist positions and a passive (recoil) mechanism aiding the flexion force in moderate to extreme extended wrist positions. The opposite would be assumed to be true for extension force: namely, higher extension forces at neutral to flexed wrist positions than in extremely extended wrist positions. This was not found to be the case, as demonstrated in Tables 5 and 6 for flexion and extension forces respectively. One explanation for the high forces at extreme postures is forearm movement with a 'stiffened' wrist. The apparatus was designed to minimize the movement potential, but deformation of the skin over the forearm may have allowed enough movement to cause significant force differences. However, visual examination of
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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0 FgO F75 F60 F~I5 F30 FlS NEU EiS ~ E,4.5E60 E'75 EgO Wrist Position (Degrees) { --=- Male -~- Female ] Figure 3 Plot of male and female extension forces by wrist position 3 s window for either males or females. This finding was an artefact of the static testing procedure. The static force measure was rejected if there was a single peak point outside a + 10% band around the 3 s average for that exertion. As the peak and average forces were not significantly different, these forces could be termed a static exertion. Thus one of the underlying assumptions of this study was confirmed to be statistically true. The ANOVA procedure demonstrates that the main effects of gender, wrist position and force exertion direction are statistically significant, as is the wrist position interaction with direction. Based upon the literature, these results were expected. It has been stated that female strength is significantly lower than that of males. Female forces range between half and two-thirds of that which can be generated by males (Roebuck et al, 1975; An et al, 1986). The average forces measured in this study are significantly different for males and females. The female forces generated in this study are 76.3% and 72.4% of those generated by males in flexion and extension respectively. The subject selection may have influenced these results, as well as the subjects' age (20-30 years of age)
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Applied Ergonomics 1994 Volume 25 Number 6
Flexion and extension forces generated by wrist-dedicated muscles: M.S. Hallbeck
the raw data revealed that all subjects followed a very similar pattern for force over the wrist positions. It is unlikely that all subjects were able to enhance their performance in this manner. However, arm movement within the fixture does not explain the findings in the less extreme postures. If one looks at the effect of wrist position on flexion force exerted, even disregarding extreme wrist positions, flexed wrist positions allow higher force exertion than in neutral and significantly higher forces than in most extended wrist positions. This pattern is totally reversed for extension forces. In most extended wrist positions, again disregarding extreme wrist positions, extension force is higher than in a neutral wrist position and significantly higher than in most flexed wrist positions, but not significantly. Thus further study of this phenomenon is warranted. In summary then, the results of this study are similar to findings of other researchers for the main effects of gender and direction of force exertion. The main effect of wrist position on combined flexion and extension forces utilizing the wrist-dedicated muscles had not previously been studied, and yielded results that differed from those expected. This contradiction of expectation was even more apparent when the wrist position-exertion direction interaction was examined. Further study of this interaction is necessary in future experiments to explain these findings. Acknowledgements I would like to thank Dr K. H. E. Kroemer who acted as major professor for this dissertation work, as well as supporting me during my stay at VPI&SU. I would also like to acknowledge the members of my committee: Drs M. M. Ayoub, R. D. Dryden, R. C. WiUiges and H. L. Snyder. References An, K. N., Clmo, E. Y. and Asken, L. J. 1983 'Functional assessment of upper extremity joints' in IEEE, Frontiers of Engineering and Computers in Health Care IEEE, pp 136-139 An, K. N., Askew, L. J. and Clmo, E. Y. 1986 'Biomechanics and functional assessment of upper extremities' in Karwowski, W. (ed.) Trends in Ergonomics~Human Factors II1 Elsevier Science Publishers, pp 573-580 Anderson, C. T. 1965 'Wrist joint position influences normal hand function' Unpublished masters thesis, University of Iowa, Iowa City, IA ~strand, P. O. and Rodahl, K. 1986 Textbook of Work Physiology 3rd edn, McGraw-Hill, New York Berg, V. J., Clay, D. J., Fathallah, F. A. and Higginbotham, V. L. 1989 'The effects of instruction on finger strength measurements: applicability of the Cadlwell regimen' in Aghaeadeh, F. (ed.) Trends in Ergonomics~Human FactorsVElsevier Science Publishers, pp 191-198 Brand, P. W. 1985 Clinical Mechanics of the Hand C. V. Moshy, St Louis, MO Brand, P. W., Beach, R. B. and Thompson, D. E. 1981 'Relative tension and potential excursion of muscles in the forearm and hand' J Hand Surg 6(3), 209--219 Bunneil, S. 1944 Surgery of the Hand 2nd edn, J. B. Lippincott, Philadelphia, PA Caldwell, L.S., Chaflln, D . B . , Dukes-Dobos, F . N . , Kroemer, K. H. E., Laulmch, L. L., Snook, S. H. and Wasserman, D. E. 1974 'A proposed standard procedure for static muscle strength testing' Am lnd Hyg Assoc J 35(4) 201-206 Chaffin, D. B. 1975 'Ergonomics guide for the assessment of human static strength' Am lnd Hyg J 36, 505-511 Chaflin, D. B. and Andersmn, G. 1984 Occupational Biomechanics Wiley and Sons, New York, NY
Applied Ergonomics 1994 Volume 25 Number 6
Flatt, A . F . 1961 'Kinesiology of the hand' Instructional Course Lectures, 18, 266-281 Gilford, W . W . , Bolton, B.M. and Lambrinudi, C. 1943 'The mechanism of the wrist joint, with special reference to fractures of the scaphoid' Guy's Hospital Report, 92, 52-59 Grant[jean, E. 1982 Fitting the Task to the Man: An Ergonomic Approach Taylor & Francis, London Halibeck, M. S. and MeMuHin, D. L. 1991 'The effect of gloves, wrist position, and age on peak three-jaw chuck pinch force: a pilot study' in Proc Human Factors Society 35th Annual Meeting Human Factors Society, Santa Monica, CA, pp 753-757 Hallbeek, M. S. and MeMulfin, D. L. 1993 'Maximal power grasp and three-jaw chuck pinch force as a function of wrist position, age, and glove type' lnt J lnd Ergonomics 11(3), 195--206 Halibeck, M. S., Stonegpher, B. L., Cochran, D. L., Riley, M. W. and Bishu, R. R. 1990 'Hand-handle coupling and maximum force' in Proc Human Factors Society 34rd Ann Mtg Human Factors Society, Santa Monica, CA, pp 800-804 Hazelton, F. T., Smidt, G. L. Flatt, A. E. and Stephens, R. I. 1975 'The influence of wrist position on the force produced by the finger flexors' J Biomech 8, 301-306 Jorgensen, M. J., Riley, M. W., Coehran, D. J. and Bishu, R. R. 1989 'Maximum forces in simulated meat-cutting tasks' in Proc Human Factors Society 33rd Ann Mtg Human Factors Society, Santa Monica, CA, pp 641--645 Kelior, M., Frost, J., Siiherberg, N., Iverson, I. and Cummings, R. 1971 'Hand strength and dexterity' Am J Occup Ther 25 (2), 77-83 Ketchum, L. D., Brand, P. W., Thompson, D. and Poeack, G. S. 1978 'The determination of moments of extension of the wrist generated by muscles of the forearm' J Hand Surg 3(3), 205-210 Kraft, G. H. and Detels, P. E. 1972 'Position of function of the wrist' Phys Med Rehabil 53, 272-275 Linscbeid, R. L. and Chan, E. Y. 1973 'Biomechanical assessment of finger function in prosthetic joint design' Orthop Clinics North Am 4(2), 317-330 Mathiowetz, V., Kashman, N., Volland, G., Weber, K., Dowe, M. and Rogers, S. 1985a 'Grip and pinch strength: normative data for adults' Arch Phys Med Rehabil 66, 16-21 Mathiowetz, V., RenneBs, C. and Donahoe, L. 1985b 'Effect of elbow position on grip and key pinch strength' J Hand Surg 10A(5), 694-697 McArdle, W . D . , Katch, F . I . and Katch, V.L. 1986 Exercise Physiology: Energy, Nutrition, and Human Performance 2nd edn Lea and Febignr, Philadelphia MeMuliln, D. L. and Hallbeck, M. S. 1991 'Maximal power grasp force as a function of wrist position, age, and glove type: a pilot study' in Proc of Human Factors Society 35th Annual Meeting Human Factors Society, Santa Monica, CA, pp 733-737 Norkin, C. C. and Levangie, P. K. 1983 Joint Structure and Function: A Comprehensive Analysis F. A. Davis, Philadelphia, PA Putz-Anderson, V. 1988 Cumulative Trauma Disorders: A Manual for Musculoskeletal Diseases of the Upper Limbs Taylor & Francis, London Pryce, J. C. 1980 'The wrist position between neutral and ulnar deviation that facilitates the maximum power grip strength' J Biomech 13, 505-511 Roebuck, J . A . , Kroemer, K. H. E. and Thomson, W . G . 1975 Engineering Anthropometry Methods Wiley and Sons, New York, NY Skovly, R. C. 1967 'A study of power grip strength and how it is influenced by wrist joint position' Unpublished masters thesis, University of Iowa, Iowa City, IA Terrell, R. and Purswell, J. L. 1976 'The influence of forearm and wrist orientation on static grip strength as a design criterion for hand tools' in Proc Human Factors Society 20th Ann Mtg Human Factors Society, Santa Monica, CA, pp 28-32 Thompson, D. E. 1981 'Biomechanics of the hand' Perspect Comput 3(3), 12-19 Weber, E . R . 1988 'Wrist mechanics and its association with ligamentous instability' in Lichtman D. M. (ed.) The Wrist and its Disorders W. B. Saunders, Philadelphia, PA, pp 41-53 Woedson, B. 1981 Human Factors Design Handbook McGraw-Hill, New York, NY Woody, R. and Mathiowetz, V. 1988 'Effect of forearm position on pinch strength measurements' J Hand Ther 1(3), 124-126 Zellers, K. K., Gandini, C. and Loveless, T. 1992 'The effect of arm position on pinch strength measurements: a pilot study' in Kumar, S. (ed.) Advances in Industrial Ergonomics and Safety IV Taylor & Francis, London, pp 709-712
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