Influences of span and wrist posture by shames

ARTICLE IN PRESS

International Journal of Industrial Ergonomics 35 (2005) 527–536 www.elsevier.com/locate/ergon

Inﬂuences of span and wrist posture on peak chuck pinch strength and time needed to reach peak strength Yuh-Chuan Shih , Yu-Chin Ou Graduate Institute of Logistics Management, National Defense University, P.O. Box 90046-15, Chung-Ho, Taipei 235, Taiwan Received 16 October 2003; received in revised form 18 November 2004; accepted 6 December 2004 Available online 20 January 2005

Abstract Evaluation of pinch strength is useful in alleviating carpal tunnel syndrome (CTS), and when peak strength is reached (denoted as TMVC), may be a useful and interesting index for evaluating strength generation and the relationship between resistance and response time. This paper intends to investigate the effects of pinch span and wrist posture on how much time is required to reach MVC (TMVC), as well as MVC. Thirty right-handed subjects including 15 males and 15 females volunteered for this experiment. A nested-factorial design was employed with four fixed independent variables of gender, subject (nested within gender), span (2, 4, 6, and 8 cm) and wrist posture (neutral, maximal extension and maximal flexion). The ANOVA results indicate that male MVC is greater than female MVC at any given span or wrist posture. On average, male MVC is 89.6 N, and female MVC is 53.6 N, nearly 60% that of males. For both genders and under four spans, neutral MVC is the greatest and maximal-flexion MVC is the least. Additionally, both neutral and maximal-extension MVCs increase as span increases, but maximal-flexion MVC increases up to 6 cm, and then decreases at 8 cm. On the other hand, only the gender effect on TMVC is significant. Average male TMVC is 1.828 s, and it is 1.346 s for females. Notably, the effects of span and wrist posture are not able to affect TMVC at all, even if they affect MVC pronouncedly. Relevance to industry Present data provide useful information about the effects of grip span and extreme wrist posture on tool/task design requiring pinch strength. The TMVC possible becomes the upper limit of response time to overcome resistance in tools/ tasks. r 2005 Elsevier B.V. All rights reserved. Keywords: Maximum volitional contraction; Maximum acceptable sustained time; Splints; Pinch

Corresponding author. Tel.: +886 2 22222137; fax: +886 2 22250488.

ARTICLE IN PRESS 528

Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

1. Introduction Pinching is still common in different workplaces, even in the service industry. Tasks requiring higher pinch strength have been proved to be highly associated with carpal tunnel syndrome (CTS) (Armstrong and Chaffin, 1979; Eastman Kodak, 1986), and pinch strength was then considered in a checklist for evaluating CTS (Lifshitz and Armstrong, 1986; Keyserling et al., 1993). Therefore, evaluating pinch strength is useful in alleviating CTS. In the workplace and in daily life, the location and the size of devices, tools, and equipment commonly change the posture of the wrist and fingers, which can change the muscles–tension relationship and the moment arm of these muscles, thus resulting in a reduction in pinch strength (e.g. Kraft and Detels, 1972; Fernandez et al., 1991, 1992; Hallbeck and McMullin, 1993). More specifically about wrist position, Hallbeck and McMullin (1993) investigated the five wrist postures of neutral, 451 and 651 flexion, and 451 and 651 extension and concluded that larger wrist deviation was connected to more diminution in strength, but flexion 451 and extension 651 were indifferent. Dempsey and Ayoub (1996) also studied the impact of the five wrist postures of neutral, maximal extension (ME), maximal flexion (MF), maximal radial deviation, and maximal ulnar deviation on pinch strength. They pointed out that significant wrist deviation could reduce the force output, especially at maximal flexion, and that the remaining three deviation postures were not significantly different from each other. Lamoreaux and Hoffer (1995) evaluated the effects of wrist deviation on grip and pinch strengths and indicated that maximal radial/ulnar deviation reduced grip strength, but did not influence pinch strength. Shih et al. (2003) also investigated the three wrist postures of neutral, flexion 301, and extension 301 with different pinch types and concluded that the effect of wrist posture on pinch strength was not significant. Furthermore, researchers have indicated that chuck pinch strength was not affected by wrist extension, even up to 301 (Anderson, 1965; Kraft and Detels, 1972), but a decrease in chuck pinch strength was found at

wrist flexion 151 (Kraft and Detels, 1972). Carey and Gallwey (2002) revealed that subjects perceived the most discomfort at maximal flexion. In a summary, flexing the wrist has more influence on the pinch strength. As for the evaluation of grip span, Imrhan and Rahman (1995) assessed pinch spans ranging from 2 to 14 cm at an increment of 1.2 cm and showed that there was a tendency of decrease in strength as the spans became larger for three pinch types: chuck, pulp2, and lateral. The indifferent range was 2–5.6 cm for chuck, 3.2–5.6 cm for lateral, and 2–9.2 cm for pulp2. Dempsey and Ayoub (1996) evaluated pinch strengths of chuck, lateral, pulp2, and pulp3 pinches at spans of 1, 3, 5, and 7 cm. The results demonstrated that pinch span affected strength significantly, that the strength generally increased and then decreased as the spans got larger, and that all four types had the greatest strength at 5 cm. On the contrary, Shivers et al. (2002) concluded the opposite, with results in which the lateral pinch strength increased as the span increased from 0% to 100% of maximal distance between the interphalangeal joint of the thumb and the distal interphalangeal joints of the index finger. Furthermore, the data on how much time is needed to reach peak strength (denoted as TMVC hereafter) may be a useful and interesting index for evaluating strength generation. Tsaousidis and Freivalds (1998) indicated that the corresponding TMVC values for the peak grip, pinch, and torque strength under bare hand were 2.04, 1.42, and 2.14 s, respectively. Jung and Hallbeck (2004) used TMVC as one of selected criteria to quantify the effects of instruction type, verbal encouragement, and visual feedback on static and peak handgrip strength. They indicated that TMVC among three instruction types—free instruction, fast contraction and immediate release, and fast contraction and maintain—were insignificant (around 1 s), and faster than those of slow contraction and immediate release, and slow contraction and maintain (around 2 s). In addition to the above, TMVC may be an index of resistance-response time, that is, how much time is needed to overcome a given resistance. Unfortunately, information on these data under various wrist postures and gripping

ARTICLE IN PRESS Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

spans seems to be still limited in quantity. Hence, the objectives of this paper are to investigate whether wrist posture and pinch span cause variations in TMVC from a relaxation status, as well as the peak strength.

Hinge

529

Force Load cell

Pad Span

2. Methods

Pad

2.1. Subjects

Stick

Thirty right-handed students, 15 males and 15 females, volunteered for this experiment. All were healthy and free from musculoskeletal disabilities, and their anthropometric data are presented in Table 1.

Force

2 cm

2.2. Apparatus and materials A fabricated pinch gauge, as shown in Fig. 1, embedded with a load cell (SENSOTEC, Model:

5 cm

Fig. 1. The pinch gauge used in this experiment.

Table 1 Subjects’ anthropometric data (15 males and 15 females) Items

Age (yrs.) Weight (kg) Height (cm) Hand length (cm) Palm length (cm) Palm breadth (cm) Circumference of forearm (cm) (relax) Circumference of wrist (cm) Circumference of grip (cm) Circumference of palm (cm) Thumb–Index Distancea Thumb–Middle Distanceb 2 cm/Thumb–Index Distance (in %) 4 cm/Thumb–Index Distance (in %) 6 cm/Thumb–Index Distance (in %) 8 cm/Thumb–Index Distance (in %) 2 cm/Thumb–Middle Distance (in %) 4 cm/Thumb–Middle Distance (in %) 6 cm/Thumb–Middle Distance (in %) 8 cm/Thumb–Middle Distance (in %)

Male

Female

Mean

Std

Min

Max

Mean

Std

Min

Max

21.1 67.0 172.2 18.4 10.7 8.2 26.3 16.2 18.8 20.6 13.2 15.1 15.3 30.5 45.8 61.1 13.3 26.6 40.0 53.3

1.8 5.5 4.5 0.7 1.2 0.4 1.1 0.6 0.7 0.8 0.8 1.1 0.9 1.9 2.8 3.8 1.0 2.1 3.1 4.1

19.1 58.0 163.0 17.1 9.5 7.6 24.8 15.0 17.0 18.7 11.8 12.9 13.6 27.2 40.8 54.4 11.8 23.7 35.5 47.3

26.3 80.0 180.0 19.6 14.4 9.0 29.0 17.4 19.5 21.8 14.7 16.9 16.9 33.9 50.8 67.8 15.5 31.0 46.5 62.0

25.1 53.7 162.5 17.4 9.9 7.3 22.3 14.9 17.9 17.8 12.2 14.3 16.5 33.0 49.5 66.1 14.1 28.1 42.2 56.2

3.6 6.6 4.3 0.6 0.6 0.5 1.2 1.8 0.6 0.9 0.8 0.9 1.0 2.1 3.1 4.2 0.9 1.7 2.6 3.5

19.0 43.0 157.0 15.8 8.5 6.8 20.2 13.3 17.0 16.2 10.9 12.7 14.7 29.4 44.1 58.8 13.0 26.0 39.0 51.9

30.2 66.0 174.0 18.7 11.2 8.7 24.8 21.0 19.0 19.2 13.6 15.4 18.3 36.7 55.0 73.4 15.7 31.5 47.2 63.0

a and b mean the maximal distance between the interphalangeal joint of the thumb and the distal interphalangeal joints of the index ﬁnger and the middle ﬁnger, respectively.

ARTICLE IN PRESS 530

Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

13/2444-03, capacity: 100 lb, compression) was designed to measure the force output, and several aluminum cuboids measuring 2 4 1 cm3 (Length Depth Height) were used as attachments on the pads of the pinch gauge to adjust the pinch span. The hinge was designed to allow the pads to move inside smoothly. When the pads were pushed oppositely, an iron stick contacting the load cell would press it and transmit the force through a 12-bit A/D converter card to a personal computer, which was used to record the data. Because the moment arms, defined as the distance between force applied and the center of the hinge, were not equal between the pinch exertion on the pads (7 cm) and the counter force of the stick acting on the load cell (2 cm), the linearity of the load cell was calibrated with several known weights at sites both 2 and 1 cm away from the right-side edge of the pad, on which the area finger pads made contact and exerted. The average slope was adopted for strength measurement. 2.3. Experimental design A nested-factorial design was employed with four factors of gender, subject (nested within gender), pinch span (2, 4, 6, and 8 cm), and wrist posture (neutral (N), MF, and ME). The span was set at absolute value, and it was relative for the wrist posture. The neutral wrist posture was the same as that defined by Hallbeck and McMullin (1993): the long axis of the third metacarpal inline with the long axis of the ulna. The extreme wrist deviations (MF and ME) were selected due to the interest of their influence on TMVC, given their obviously negative effect on peak strength generation. There were, therefore, 12 random combinations (4 spans 3 wrist postures) in total for each subject, and three replications were conducted on different days. All data from the three replications were for data analysis. The dependent variables were MVC (maximal volitional contraction) and time needed to reach MVC, termed TMVC. 2.4. Experimental procedure and data acquisition The experimenters explained the goals and procedures to all the subjects first. To facilitate

the study of how soon MVC could be achieved, an instruction on fast contraction and maintain of a type employed by Jung and Hallbeck (2004) and Tsaousidis and Freivalds (1998) was adopted. The reasons for this choice are that it is able to generate a greater strength output, has less variation in the strength output, and reaches the peak strength the fastest (Jung and Hallbeck, 2004). That is, subjects were asked to exert as fast as possible, but without jerking, from an initial state of relaxation, and then to maintain this maximum effort for 3–5 s, until the program automatically stopped recording. Several trials were performed to familiarize the subjects with how to perform the task, and then 10-min rest was scheduled before formal measurements were taken. The subjects performed the task with their self-reported dominant hand with a chuck pinch due to its greater force generation (Imrhan and Loo, 1989; Imrhan, 1991; Fernandez et al., 1991, 1992; Imrhan and Rahman, 1995; Dempsey and Ayoub, 1996). All runs were conducted from a sitting posture with the body adjacent to the table and the upper arm extended with an angle (y) of about 25176.21 to enable the whole forearm to rest on the table; the elbow angle was near 1101–1201 and the palm was toward the medial position (see Fig. 2). When the formal measurement started, the subjects ﬁrst had their wrists in the given posture, N, MF, or ME; the experimental assistant then gave the pinch gauge, which had been set at the desired span, to the subjects. Subjects put their distal interphalangeal joints adjacent to the outside edges of the pads of the gauge and then pushed them together. This was to make sure that the subjects could keep their wrist postures and contact area as constant as possible when the grip span varied. The pinch strength was recorded as soon as the gauge was zeroed. A 2-min rest was scheduled between successive trials to avoid muscle fatigue. If necessary, a longer rest period was allowed. The force output was acquired by a computer program with a sampling rate of 1000 Hz, and the load cell was zeroed prior to each measurement. Subjects were asked not to exert until a tone was produced by the computer, and data were recorded immediately after that. In order to determine when exertion started, a 95%

ARTICLE IN PRESS Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

confidence interval for the first 10 data points was calculated. When five consecutive points after these 10 data points exceeded the upper bound of this confidence interval, the first of them was considered to be the start of the force development. The first maximum strength over the duration was considered as MVC as force generation is not monotonic. The consumed time from

531

exertion start to reach MVC is, therefore, TMVC. The program STATISTICA was used for data analysis.

3. Results and discussion The ANOVA and descriptive data are in Tables 2 and 3, respectively. The results for MVC and TMVC are demonstrated separately as follows. 3.1. MVC

θ°

The ANOVA results for MVC in Table 2 show that all but the highest-order interaction is signiﬁcant. Therefore, the signiﬁcant interactions of gender span, gender wrist, and span wrist are shown in Figs. 3, 4, and 5, respectively. As shown in Fig. 3, gender span interaction, MVCs for both genders generally increase as spans become larger, but the increment becomes less as grip span gets wider for both genders. The result of Duncan’s multiple range test (MRT) further indicates that all are mutually statistically different with the exception of female MVCs at spans of 4, 6, and 8 cm. That is, the positive effect of grip span on pinch strength for females seems to lessen as the span increases past 4 cm. This fact enlarges the MVC gap between genders from 3.1 to 4.2 kg

(90°+θ°)

Table

Fig. 2. The sitting posture in this experiment.

Table 2 ANOVA results for MVC and TMVC Sources of variation

Responses TMVC

MVC

Subject(sex) Sex Span Wrist Sex Span Sex Wrist Span Wrist Sex Span Wrist Error

28 1 3 2 3 2 6 6 1028

Total

1079

M.S.

F-value

p-value

M.S.

F-value

p-value

49.48 3646.55 62.44 153.77 16.72 7.99 9.89 1.52 1.49

33.14 2442.40 41.82 103.00 11.20 5.35 6.62 1.02

0.0000 0.0000 0.0000 0.0000 0.0000 0.0049 0.0000 0.4138

6.50 62.70 0.98 0.11 1.20 1.91 1.11 0.26 0.68

9.49 91.58 1.43 0.16 1.75 2.79 1.63 0.37

0.0000 0.0000 0.2324 0.8512 0.1555 0.0618 0.1367 0.8958

ARTICLE IN PRESS Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

532

Table 3 The descriptive data for MVC (N), TMVC (s) and the rate of force development (N/s) Factor

Gender

Span

Wrist

Level

TMVC

MVC

M F

Rate (MVC/TMVC)

Mean

Std

F/M (%)

Mean

Std

F/M (%)

Mean

F/M (%)

89.6 53.6

19.9 15.3

22 29

59.8

1.828 1.346

0.973 0.855

53 64

73.6

49.0 39.8

81.2

2 cm

M F

80.7 50.1

18.7 15.3

23 31

62.1

1.798 1.288

1.027 0.854

57 66

71.6

44.9 38.9

86.8

4 cm

M F

87.6 53.9

18.5 14.7

21 27

61.5

1.842 1.471

0.946 0.895

51 61

79.9

47.6 36.7

77.1

6 cm

M F

93.0 54.7

19.1 15.0

21 27

58.9

1.724 1.336

0.928 0.855

54 64

77.5

53.9 41.0

76.0

8 cm

M F

97.1 55.6

19.7 15.8

20 28

57.2

1.946 1.287

0.986 0.810

51 63

66.2

49.9 43.2

86.4

M F

90.3 54.3

19.9 13.5

22 25

60.1

1.778 1.404

0.950 0.858

53 61

78.9

50.8 38.7

76.1

M F

97.1 58.1

20.5 15.9

21 27

59.9

1.813 1.390

0.999 0.914

55 66

76.6

53.5 41.8

78.1

M F

81.4 48.3

16.0 14.9

20 31

59.4

1.891 1.243

0.972 0.783

51 63

65.7

43.0 38.9

90.3

11.0 10.0 9.0 8.0 7.0 6.0 A

2 cm

4 cm

6 cm

8 cm

8.2

8.9

9.5

9.9

5.5

Male-Female (kg)

5.1 3.1

3.4

5.6 3.9

4.2

Female/Male (%)

62.1

61.5

58.9

57.2

5.0 4.0 Male (kg) Female (kg)

5.7

Fig. 3. Gender Span interaction (male–female: MVC difference between genders; female/male (%): female MVC based on male MVC).

(refer to ‘‘male–female’’ in Fig. 3) and decreases the ratio of female exertion over male from 62.1% to 57.2% (refer to ‘‘female/male (%)’’ in Fig. 3),

which seems to imply that the contribution of an increase in span is more signiﬁcant to males than to females. It may be due to the females having

ARTICLE IN PRESS Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

533

12.0 10.0 8.0 6.0 4.0 2.0 0.0 Male (kg) Female (kg) Male-female (kg) (posture/Neutral) (%)-male (posture/Neutral) (%)-female

8.3 4.9 3.4 84 83

9.9 5.9 4.0 100 100

9.2 5.5 3.7 93 93

Fig. 4. Sex wrist interaction (male–female: MVC difference between genders; (posture/neutral) (%): MVC of different posture based on neutral).

10.0 9.0 8.0 7.0 6.0 5.0

2 cm

4 cm

6 cm

8 cm

N (kg)

7.2

7.8

8.2

8.5

MF(kg)

6.4

6.6

6.9

6.6

ME(kg)

6.5

7.2

7.6

8.3

N-MF(kg)

0.8

1.2

1.3

1.9

N-ME(kg)

0.7

0.6

0.2

MF/N (%)

89.1

84.4

83.6

78.1

ME/N (%)

90.9

91.6

92.3

97.6

Fig. 5. Span wrist interaction (ME/N (%) and MF/N (%) are ME and MF MVC based on N MVC in percentage, respectively).

signiﬁcantly smaller hand dimensions (po0:05) in the present paper. In gripping greater spans, females must lengthen the muscles and tendons more, which decreases strength generation. If the spans are expressed relative to hand dimensions, they are equal to 15.3–61.1% and 16.5–66.1% of the Thumb–Index Distance for males and females,

respectively, and 13.3–53.3% and 14.1–56.2% of the Thumb–Middle Distance for males and females, respectively, referred to in Table 1. Here, the ﬁnding of a positive effect of pinch span is the same as that concluded by Shivers et al. (2002), who speciﬁed the span from 0% to 100% of the maximal distance between the interphalangeal

ARTICLE IN PRESS Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

534

Table 4 Results of Duncan’s MRT for span wrist interaction Rank

Combinations (cm)

Mean

Group* 1

1 2 3 4 5 6 7 8 9 10 11 12

8 8 6 4 6 4 2 6 4 8 2 2 *

N ME N N ME ME N MF MF MF ME MF

83.1 81.1 80.3 76.9 74.1 70.5 70.1 67.1 64.9 64.9 63.7 62.5

x x x x x x x x

x x x x

Marked with ‘x’ represents indifferent in means (a ¼ 0:05).

joint of the thumb and the distal interphalangeal joints of the index finger. However, the impact of span on pinch MVC is different from what was concluded by Imrhan and Rahman (1995) and Dempsey and Ayoub (1996). Although the gender wrist-posture interaction is significant and Duncan’s MRT further demonstrates that all six combinations are different mutually, as seen in Fig. 4, the influence of wrist posture seems not to be dependent on the gender. That is, for both genders, N MVC is the greatest, followed by ME MVC (about 93% of N MVC), and MF MVC is the least (about 84% of N MVC), and average female exertion is about 60% that of males for all three specified wrist postures. This unobvious interaction seems due to the wrist postures having been set according to individual conditions, and not to absolute conditions. Furthermore, the finding of the effect of wrist posture on pinch strength is consistent with past studies (Imrhan, 1991; Fernandez et al., 1991, 1992; Dempsey and Ayoub, 1996). Moreover, as illustrated in Fig. 5, the plot for span wrist–posture interaction, the rank in MVC among the three wrist postures under any given span is the same as that in Fig. 4: N4ME4MF: There is a positive effect of span augmentation on both N and ME MVCs, but for MF posture, the MVC initially increases and then decreases at 8 cm. Additionally, according to Duncan’s MRT (see

Table 4), there seems to be less of an influence of span increase on MF MVC than on that of the other two postures. If the span is small, say 2 cm, the difference in MVC between ME and MF becomes limited. If the span becomes larger, it is able to diminish the force gap between N and ME postures, but it thus increases the gap between N and MF postures, according to data provided in Fig. 5. As for the gender effect, Figs. 3 and 4 reveal that, no matter which span or wrist posture was applied, any one of the male MVCs is greater than any female MVC. It means that, for MVC, the gender effect seems to dominate the other two evaluated effects of span and wrist posture. In general, male MVC is 89.6 N, and female MVC is 53.6 N, nearly 60% of male. Lower pinch MVC for females is supported by past studies (Swanson et al., 1970; Berg et al., 1988; Imrhan and Loo, 1989; Hallbeck and McMullin, 1993; Dempsey and Ayoub, 1996). The above findings may be due to males having a greater amount of muscle area than females, thus enabling them to generate a greater amount of force (McArdle et al., 1986). 3.2. TMVC Table 2 reveals that the gender effect is the only significant effect on TMVC. Male TMVC (1.828 s) is longer than that of female (1.346 s); female TMVC

ARTICLE IN PRESS Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

is about 73.6% of male. The average TMVC ratios for female to male are from 66% to 80% for different spans or wrist postures (see Table 3). Human skeletal muscle consists of three fiber types of slow-oxidative, fast-oxidative, and fast-glycolytic fibers. The motor units of slow- and fastoxidative fibers are thinner and are activated more quickly than those of fast-glycolytic fibers. The fast-glycolytic fibers usually are activated when the muscle tension is beyond 40% of the total muscle tension (Vander et al., 1998). Consequently, the cause of the gender difference in TMVC could be that the cross-section of male muscles is greater than that for females, and it implies that the fastglycolytic fibers of males are more numerous and thicker. As a result, males take more time to reach MVC than females, and, of course, males generate more force than do females. Of interest to note is that the effects of span and wrist posture are not able to affect TMVC pronouncedly, nor are first-order interactions, even though they affect the MVC pronouncedly. This seems to imply that the posture change due to grip span and wrist extension/flexion is not able to alter the muscular cooperation, even if it results in a significant diminution in MVC. Furthermore, the average pulp2 TMVC of 12 males and 3 females reported by Tsaousidis and Freivalds (1998) was 1.42 s, with the pinch performed by a neutral hand at a 2-cm span. It falls between the male TMVC (1.828 s) and female TMVC (1.346 s) obtained in this study. The large coefficients of variation in Table 3, ranging from 50% to 70%, seem to present an unstable state when MVC is reached. Table 3 additionally presents the force development rates derived from mean MVC over mean TMVC. The mean rates for male and female are 49.0 and 39.8 N/s, respectively; the female rate is 81.2% that of the male. As for the rates at different spans, both genders have greater rates at 6–8 cm. On the other hand, both genders have faster rates at neutral posture, and the slowest is at MF. Therefore, if pinching is necessary in any equipment, device, tool, or task design or operation, or operator selection/screen, keeping the wrist neutral is necessary. If not, flexing the wrist, especially flexing at larger angles, should be

535

avoided as much as possible owing to not only decreased MVC but also to the higher carpal tunnel pressure induced (Rojviroj et al., 1990). Despite the fact that TMVC seems to be another indicator useful in evaluating pinching, the strength applied in real workplaces or daily life is often less than MVC. For example, here, female MVC is about 60% that of males, and we really want to know how much time it should take if the resistance is the same for both genders. Thus, more detail on the force–time relationship, such as the time needed to reach 25%, 50%, and 75% MVC, and so on, is necessary.

4. Conclusions Aforementioned results indicate that no matter which span or wrist posture is applied, any one of the male MVCs is greater than any female MVC. In general, male MVC is 89.6 N, and female MVC is 53.6 N, nearly 60% that of males. As expected, regardless of gender and selected spans, the maximal MVC occurs at neutral wrist posture and the minimum at MF. An interactive inﬂuence on MVC exists between span and wrist posture. N and ME MVCs increase as the span increases from 2 to 8 cm, but it ﬁrst increases and then decreases for the MF posture. In addition, this increase in span reduces the MVC gap between N and ME, but enlarges the gap between N and MF. On the other hand, the gender effect affects TMVC (time needed from initial to reach MVC), but the effects of wrist and span do not. Average male TMVC is 1.828 s, and it is 1.346 s for females.

Acknowledgement This paper presented the results from the project sponsored by the National Science Council. The Project no. was NSC91-2213-E123-001. References Anderson, C.T., 1965. Wrist joint position inﬂuences normal hand function. Master’s Thesis, University of Iowa, Iowa City, IA, unpublished.

ARTICLE IN PRESS 536

Y.-C. Shih, Y.-C. Ou / International Journal of Industrial Ergonomics 35 (2005) 527–536

Armstrong, T.J., Chaffin, D.B., 1979. Carpal tunnel syndrome and selected personal attributes. Journal of Occupational Medicine 21 (7), 481–486. Berg, V.J., Clay, D.J., Fathallah, F.A., Higginbotham, V.L., 1988. The effect of instruction on finger strength measurements: applicability of the Caldwell regimen. In: Aghazadeh, F. (Ed.), Trend in Ergonomics/Human Factors. pp. 191–198. Carey, E.J., Gallwey, T.J., 2002. Effects of wrist posture, pace and exertion on discomfort. International Journal of Industrial Ergonomics 29, 85–94. Dempsey, P.G., Ayoub, M.M., 1996. The influence of gender, grasp type, pinch width and wrist position on sustained pinch strength. International Journal of Industrial Ergonomics 17, 259–273. Eastman Kodak Company Ergonomics Group, 1986. Ergonomic Design for People at Work. vol. 2. Van Nostrand Reinhold, New York. Fernandez, J.E., Dahalan, J.B., Halpern, C.A., Viswanath, V., 1991. The effect of wrist posture on pinch strength. In: Proceedings of the Human Factors Society 35th Annual Meeting. Human Factors Society, Santa Monica, CA, pp. 748–752. Fernandez, J.E., Dahalan, J.B., Halpern, C.A., Fredericks, T.K., 1992. The effect of deviated wrist posture on pinch strength for female. In: Kumar, S. (Ed.), Advance in Industrial Ergonomics and Safety IV. Taylor and Francis, London, pp. 693–700. Hallbeck, M.S., McMullin, D.L., 1993. Maximal power grasp and three-jaw chuck pinch force as a function of wrist position, age, and glove type. International Journal of Industrial Ergonomics 11, 195–206. Imrhan, S.N., 1991. The influence of wrist position on different types of pinch strength. Applied Ergonomics 22 (6), 379–384. Imrhan, S.N., Loo, C.H., 1989. Trends in finger pinch strength in children, adults, and the elderly. Human Factor 31 (6), 689–701. Imrhan, S.N., Rahman, R., 1995. The effect of pinch width on pinch strengths of adult males using realistic pinch–handle coupling. International Journal of Industrial Ergonomics 16, 123–134. Jung, M.-C., Hallbeck, M.S., 2004. Quantification of the effects of instruction type, verbal encouragement, and visual

feedback on static and peak handgrip strength. International Journal of Industrial Ergonomics 34, 367–374. Keyserling, W.M., Stetson, D.S., Silverstein, B.A., Brouwer, M.L., 1993. A checklist for evaluating ergonomic risk factors associated with upper extremity cumulative trauma disorders. Ergonomics 36 (7), 807–831. Kraft, G.H., Detels, P.E., 1972. Position of function of the wrist. Archives of Physical Medicine and Rehabilitation 53, 272–275. Lamoreaux, L., Hoffer, M.M., 1995. The effect of wrist deviation on grip and pinch strength. Clinical Orthopedics and Related Research 314, 152–155. Lifshitz, Y., Armstrong, T., 1986. A design checklist for control and prediction of cumulative trauma disorders in hand intensive manual jobs. In: Proceedings of the Human Factors Society 30th Annual Meeting. Human Factors Society, Santa Monica, CA, pp. 837–841. McArdle, W.D., Katch, F.I., Katch, V.L., 1986. Exercise Physiology: Energy, Nutrition, and Human Performance, second ed. Lea and Febiger, Philadephia, PA. Rojviroj, S., Sirichativapee, W., Kowsuwon, W., Wongwiwattananon, J., Tamnanthong, N., Jeeravipoolvarn, P., 1990. Pressures in the carpal tunnel: a comparison between patients with carpal tunnel syndrome and normal subjects. Journal of Bone Joint Surgery 72B, 516–518. Shih, Y.C., Chen, W.L., Ou, Y.C., 2003. A study of the effect wrist splint, gender, pinch type, and wrist posture on the peak pinch strength. Journal of the Chinese Institute of Industrial Engineers 20 (4), 411–421 (in Chinese). Shivers, C.L., Mirka, G.A., Kaber, D.B., 2002. Effect of grip span on lateral pinch grip strength. Human Factors 44 (4), 569–577. Swanson, A.B., Matev, I.B., De Groot, G., 1970. The strengths of the hand. Bulletin of Prosthetics Research Fall, 145–153. Tsaousidis, N., Freivalds, A., 1998. Effects of gloves on maximum force and the rate of force development in pinch, wrist ﬂexion and grip. International Journal of Industrial Ergonomics 21, 353–360. Vander, A., Sherman, J., Luciano, D., 1998. Human physiology—the mechanisms of body function, seventh ed. McGraw-Hill, New York (Chapter 11).