external pressure at the hand during object handling and work with tools

ELSEVIER International Journal of Industrial Ergonomics 20 (1997) 191-206

l r t e r national JouHaat ~!

Industrial Ergonomics

External pressure at the hand during object handling and work with tools

Charlotte Hall a,b,* a Department of Ergonomics, National Institute for Working Life, Ekelundsviigen 16, S-171 84 Solna, Sweden

b Department ofNeuroscience, Karolinska Institute, Stockholm, Sweden

Received 1 September 1995; revised 3 July 1996

Abstract

Hand size, pressure-pain threshold (PPT) and hand strength were measured on 15 subjects. External pressure on the hand was measured, using small capacitive pressure sensors, while grasping cylinders (Q~ 10-100 mm) and handling tools. Subjective ratings of cylinder preference, pressure, discomfort and pain were recorded. PPT measured with six years interval differed significantly; however, the PPT was stable over shorter periods. For grasp tasks, the results indicate that hand size is critical when the external force requirement is constant, while there is an interaction between hand size and strength when it varies. When gripping a cylindrical object, the fingers were exposed to the largest pressure. The gripping action was dependent on cylinder diameter, and the subjects strongly preferred cylinders with 30-40 mm diameter. The average pressure during tool handling was 250 kPa or less, except for high-force tools (plate shears). In general, the fingers registered high external pressure and the carpal canal tunnel low pressures. The pain threshold was exceeded during short periods during work with tools. Subjective ratings of pressure and discomfort were significantly correlated with external pressure, but not significantly with pain.

Relevance to industry

Many occupational activities expose the human hand to external pressure. This study describes a methodology for quantifying external pressure to the hand, which may be used for evaluation of hand-object interfaces. The results of this study may also be used tbr future design of hand tools. © 1997 Elsevier Science B.V.

Keywords: Hand ergonomics; Pressure-pain threshold; Grip strength; Tool; External pressure

1. Introduct ion

When gripping an object, the hand exerts a force resulting in a pressure on the hand. This is significant for gripping actions with the fingers which require simultaneous external forces acting upon two

* Corresponding author. E-mail: [email protected], Fax: +46 8 730-9881, Tel.: +46 8 730--9289.

or more segments of each of the fingers as they co-operate to produce the desired function (Amis, 1987).

Increasing mechanical pressure applied to the hand is first perceived as touch, then pressure and finally pain as the pressure-pain threshold (PPT) is exceeded. A map of the hand 's sensitivity to externally applied surface pressure indicated the most sensitive areas to be the thenar area, the skin fold between thumb and index finger and the area around the os

0169-8141/97//$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S01 69-8141 (96)00056-X

192 C. Hall/International Journal of Industrial Ergonomics 20 (1997) 191-206

pisiforme (Fransson-Hall and Kilbom, 1993). In the present study this previous knowledge on the hand's pressure sensitivity is correlated to the pressure ex- perienced while handling objects or tools. The objective for doing this is to investigate if use of common tools implies exposure to painful pressure, and if that is the case, where in the hand the pain arises.

The design and use of tools is of special interest to industry since some of the major causes of work- related ailments and diseases are linked to the use of hand tools (Kadefors et al., 1993b) and poor design of hand tools may result in cumulative trauma disorders (Armstrong, 1986). The surface properties of the tool are of particular interest, since they have a profound effect on the local pressure produced at the hand. Therefore, the surface properties have been identified to be of great importance for the useful- ness of a hand tool (Sperling et al., 1993), and the local surface pressure caused by the tool has been included as a limiting factor in a cube model for classification of work with hand tools (Kilbom et al., 1993b; Sperling et al., 1993). Perceived pain in the hand, caused by high local external pressure, is often a limiting factor during work with hand-held tools (Fraser, 1980; Yun et al., 1992). In addition, high local pressure to the hand may result in blistering (Fraser, 1980; Yun et al., 1992) and the discomfort caused by high pressure may reduce both the effi- ciency of the work and the consumer's satisfaction with the tool (Yun et al., 1992).

The aim of the present study was to investigate externally applied surface pressure to the hand during the handling of objects and tools. Pressure, pressure-pain threshold and subjective ratings were considered for both female and male subjects. To imple- ment this objective, a new methodology for pressure measurements was developed. A second aim was to study the long-term variability in pressure-pain threshold in the hand to investigate if the hand PPT is stable over a long period of time.

2. Materials and methods

2.1. Study design

Ten of the subjects participated in a longitudunal study of PPT. In 1989, two PPT measurements sepa-

Table 1 Characteristics of the participating subjects (mean values, mini- mum-maximum value)

Females (n = 8) Males (n = 7)

Mean min max Mean rain max

Age (years) 44 34 52 43 35 49 Height (cm) 167 161 173 177 173 180 Weight (kg) 65 50 86 79 67 102 B M I ( k g × m 2) 23.34 19.29 30.47 25.35 21.63 32.93 Handlength (mm) 171 163 179 183 176 191 Hand breadth (mm) 80 77 85 89 82 94

Hand length (DIN 33 402, measure 3.15: distance between the tip of the middle finger to the distal point of the radius at the processus styloideus) and hand breadth (DIN 33 402, measure 3.19: distance between the ulnar metacarpal bone and the radial metacarpal bone) were measured at the fight hand.

rated by 1-2 weeks were performed. The second measurement was performed during the main study in 1995, and the last PPT measurement was done four weeks later.

Except for the repeated measurements of PPT six years apart, all other measurements were conducted on the same day for a given subject. The session started with anthropometrical measurements, fol- lowed by an interview about symptoms and hand

No

Fig. 1. Locations at the right hand where the PPT was measured. The pressure sensors were applied to the same locations during the pressure measurements.

C. Hall~International Journal of Industrial Ergonomics 20 (1997) 191-206 193

tool use. Measurement of pressure-pain threshold in the hand, and maximal hand grip strength were perfomed before the pressure sensors were applied to the hand. The pressure was then measured while the subject performed gripping tasks with a range of cylinders and, finally, during work with hand tools. Ranking of cylinders was done during the gripping tasks, and VAS ratings were perfomed for each tool.

2.2. Subjects

Fifteen members of the faculty staff, who all described themselves as right-handed individuals, volunteered to participate in the study. Body height,

weight and hand size were measured (Table 1), and the subjects were interviewed about symptoms and sick leave due to forearm-hand problems and their use of hand tools.

2.3. Pressure-pain threshold

The pressure-pain threshold (PPT) was measured at 15 locations (Fig. 1), using a pressure algometer (Somedic, Algometer type 1) with a 1.0 cm 2 circular aluminium area with rounded edges and 25 kPa X s - pressure application rate. Each measurement point corresponded to the same anatomical position for all subjects. Measurement procedures were used as de-

' 1 0

C 0 U

4 ~

0

1700 -

1600-

1500

1 4 0 0

1300 -

1200 -

1100-

1000

900

800 -

700

600

500

400

3O0

2O0

100

0

~ 6 ~ qg~

c9

J @

J

r ,

(. ;;::;: or

I I , I • I • I I I 9 1 . I ' I ' I I I • I ' I • I

0 1 2 3 4 5 6 7 8 10 11 12 13 14 15 16 17 ' I

18

Sampling time (seconds)

Fig. 2. The graph shows one of the 15 sensors during the calibration procedure. The pressure applied to the sensor (i.e. the 'true' pressure) and the output pressure from the sensor is shown.

194 C. Hall~International Journal of Industrial Ergonomics 20 (1997) 191-206

Table 2 The amplitude and distribution of pressure in the right hand were investigated for six tools

Tool Specification Task Subjects

Pen BIC Copying manuscript 8 female + 7 male Nippers LindstrSm 7190 Cutting plastic coated copper wire, 0 1,20 mm 8 female + 7 male Saw Handsaw, unknown trademark Sawing a 23 mm wooden plate 8 female + 7 male Screwdriver (hand tool) Sandvik 800, A4 Driving M5 wooden screws into a plate 5 female + 6 male Drill (power tool) Black and Decker Drilling 0 3,00 mm holes in a wooden plate 8 female + 7 male Plate shears; low force unknown trademark Cutting 1,00 mm thick aluminium plate 8 female + 7 male Plate shears; medium force a unknown trademark Cutting 0.70 mm thick steel plate 8 female + 2 male Plate shears; high force a unknown trademark Cutting 2,00 mm thick aluminium plate 3 male

a The subject first tried to make cuts through 2.00 mm aluminium. If this was not possible, the subject cut through 0.70 mm steel instead.

scribed in Fransson-Hal l and Ki lbom (1993). A sub- group of the subjects, five females and five males,

had part icipated in an invest igat ion six years ago where the PPT was measured at 64 locations at the

fight hand. Since the same defini t ions for localisat ion

of the pressure areas were used, the PPTs could be compared be tween 1989 and 1995.

2.4. Maximal hand grip strength

Maximal voluntary hand grip strength (MVC) was

measured for the fight hand using a strain gauge hand grip dynamometer , adjusted to the preferred

individual hand grip span.

2.5. Pressure measurements

2.5.1. Equipment The pressure at 15 locations at the right hand was

measured us ing a commerc ia l ly avai lable electronic

pressure recording device, the Micro-Emed Sys tem (Novel GmbH, Munich , Germany) . The system con- sists of hardware components (i.e. sensors, ampli- fiers, and recording equipment) and accessory PC

software. In the present study the 'Mic ro -Emed Ba- sic Vers ion ' was used together with 6 × 6 X 1 mm single sensors and required software. The sensors are

capacitive, i.e. two parallel metal f i lm capacitor plates

separated by an elastic, dielectric, material. When pressure is applied, the elastic material is compressed and the electric capaci tance changes. The recording

equ ipment of the system converts the change in capaci tance into a voltage output. In the present study, the voltage output was sampled at 10 Hz.

Before each measurement , the output signals of the unloaded sensors were recorded for subsequent sub-

traction from the sensors ' output signals. By this

procedure, a possible variabil i ty in offset voltage and sensor compress ion due to the tape used for attach-

ing it was controlled. Each of the sensors was calibrated separately

using a pressure algometer (Somedic, Algometer type

1). Since the pressure application area of the algometer was 1.0 cm 2 and the sensor area was 0.36 cm z,

the pressure at the sensor was calculated:

Pressure at the sensor =

( 1 / 0 . 3 6 ) X Pressure at the algometer area. ( 1 )

Table 3 Rated perception of perceived pressure, discomfort and pain on the 10 cm visual analogue (VAS) scales. The subject was instructed to rate each sensation independent of the other

Rated sensation VAS-scale

Left-side anchor Right-side anchor

Pressure; not painful a Pressure; painful a

Discomfort Pain

No pressure Just about painful pressure No discomfort No pain

Just about painful pressure Intolerable pressure Intolerable discomfo~ Intolerable pain

a Pressure was rated on one of these two scales.

c. Hall / International Journal of Industrial Ergonomics 20 (1997) 191-206 195

The pressure was applied to the sensor at a constant rate. During this time, both the sensor and algometer output signals were sampled at l0 Hz (Fig. 2). A third-order polynomial calibration curve was fit for each sensor using linear regression. An example, showing one of the sensors, is shown in Eq, (2).

y = 3.3 + 0 .7x + 4 .5e - 5x 2 + 6 .3e - 7x 3,

R 2 = 0.999 (2)

When attaching the sensors to the right hand, a surgical glove (Perry) was first put on the hand. The sensors were then affixed to the hand with surgical tape (Blenderm, 3M), and a second surgical glove was put on to secure the sensors and all connecting wires.

The pressure measurement locations in the hand were chosen based on the following criteria: the

locations were expected to be exposed to pressure

during object handling and work with tools, the locations corresponded to previously defined locations for measurement of PPT (Fransson-Hall and Kilbom, 1993), and the locations were distributed over the entire hand area to reflect the pressure at different sections of the hand.

2.5.2. Grasp tests on a range o f cylinder diameters

Nine cylinders, with diameters ranging from 10 to 100 mm, were investigated. All cylinders were coated with the same type of textile material to provide similar friction and texture. Two grasp tasks were studied. For each of the cylinders, the subjects first held the cylinder using a transversal volar grip and increased the grip force at a steady rate without jerking until no further increase could be registered. Secondly, the cylinder was used as a horizontal

Normalized pressure (%

800 -

700

600

500.

400-

300

200

1 O0 Vmo082kp 90 (95 kPa)

0 1 , 70 (151 kPa) : 6 0 (141 kPa)

50 (168 kPa) Cy l inder 9 lO\1 .q 40 (182 kPa)

30 (267 k P a ) d i a m e t e r Measurement 12 1: ' 20 (230 kPa)

l o c a t i o n 15 10 (185 kPa)

Fig. 3. Pressure amplitude (average for 15 subjects) at the right hand during maximal exertion using cylinders with diameters ranging from 10 mm to 100 ram. 'Measurement location' refers to locations in the hand described in Fig. 2. The measurement values have been normalized; the average value for all 15 locations for each cylinder diameter is denoted 100%. The average pressure amplitude (in kPa) appears within parentheses.


Table 4 Pressure-pain threshold (PPT) measured at 15 locations at the right hand. The locations are defined in Fig. 1

Location Females (n = 8) Males (n = 7)

Mean [kPa] 95% c.i. [kPa] Mean [kPa] 95% c.i. [kPa]

1 358 267-449 556 364-748 2 396 288-503 500 321-678 3 401 284-517 622 404-840 4 371 273-469 603 422-784 5 364 251-477 512 336-689 6 302 226-378 405 300-510 7 307 235-380 417 339-495 8 322 250-394 463 333-593 9 315 249-381 389 318-460

10 374 267-482 592 435-750 11 342 243-440 475 328-623 12 420 285-554 551 358-745 13 406 271-542 644 471-817 14 436 335-537 651 466-836 15 452 321-584 608 447-770

handle for pulling along the forearm with 90 ° elbow flexion and vertical upper arm at a constant force of 200 N during 15-30 seconds using a transversal volar hand grip. The subjects were provided visual feedback to be able to keep the pulling force at a steady level and they were instructed to distribute the force evenly between the left and right hands. The pressure exerted to the right hand was measured during both conditions. The reason for choosing two grasp tasks was that with existing euipment, external force could only be measured during the pulling task. Since the subject rested for three minutes between each trial to avoid effects of fatigue, the maximal volontary contraction in the first situation was as- sumed to be contstant for all cylinder diameters. Prior to the cylinder tasks, the subject was instructed to rank the comfort of the cylinders. The ranking, from 'the most comfortable' to 'the least comfortable', was done after the cylinders had been tested

450

4 0 0

35C

30(

25q

20

15

lC

O0 (60 kPa) (59 kPa)

80 kPa) kPa)

;Pa) Cyl inder ') d iameter

. . . . . . . . . l b 1U (bb KPa)

Fig. 4. Pressure amplitude (average for 15 subjects) at the right hand when pulling 200 N, using cylinders with diameters ranging from l(I mm to 100 ram. 'Measurement location' refers to locations in the hand described in Fig. 2. The measurement values have been normalized; the average value for all 15 locations for each cylinder diameter is denoted 100%. The average pressure amplitude (in kPa) appears within parentheses.

C. Hall / International Journal of Industrial Ergonomics 20 (1997) 191-206 197

and the subject was allowed to try the cylinders again during the ranking procedure.

2.5.3. Work with tools Six commonly used hand tools were investigated

(Table 2). They where chosen to represent both low- and high-force tools. Also, the use of these tools implied a selection of different hand grips. The amplitude and distribution of pressure in the right hand was investigated during use of the tools; average during the entire working time (WT), average during a typical work cycle (TYP) e.g. during cutting with the plate shears and the maximal pressure (MAX) were recorded and calculated. The tools were presented in a random order, and each task lasted for about 30 seconds. All subjects were given a short introduction on the use of the tools unfamiliar to them, and practised handling the tool before the measurements started. The subjects worked at a rate chosen by themselves, i.e. the experiments were not paced. After each task, the subject rated the perceived pressure, discomfort and pain of the right hand on a l0 cm VAS-scale (Table 3). The areas

where pressure, discomfort and pain were experi- enced were also indicated on a drawing of the right hand.

2.6. Statistical methods

Analysis of variance (ANOVA) was used to study (1) the influence of gender, individual and location on PPT, (2) the influence of cylinder diameter, location, individual and gender on pressure amplitude when gripping a cylindrical object, (3) the influence of tool, situation, location, individual and gender on pressure amplitude during work with tools, and (4) the influence of location, individual and gender on the ave rage re la t ive pressure (pressure amplitude/PPT) during work with tools. A paired one-tail t-test was used to compare the PPT measured in 1989 and 1995 for ten subjects. The agreement between the subjects in ranking the preference of the cylinders was tested by the Kendal coefficient of agreement, u, for paired rankings. The association between hand size and hand strength and average pressure was calculated by the Pearson correlation

kPa 1 0 0 0

9 5 0 9 0 0 8 5 0 800 750- 700- 650- 600- 550 ~ 50O ~

450 ~ 400 ~

Screwdriver

I I Total working tJme~ Typical work-cykle / Maximum value J

Nippers

350 ~, r 3 o o t -r . /

T

. . . . . . . ~. ~.J.~ . . . . . . . . . . . . . . -~. . ~ = - ~ . ~ ; - ~ . ~. 3 4 5 6 7 8 9 10 11 12 13 14 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 lS

Location Location

Fig. 5. Example of pressure recorded at the right hand (location 1-15) during work with two tools. Average value and 95% c.i. for 15 subjects (8 females + 7 males), except for screwdriver (5 females + 6 males). Black bars display the total working time, i.e. the average pressure during the entire working time with the tool. Striped bars display the pressure during a typical work-cycle, e.g. during a cut with the nippers. White bars display the maximal pressure recorded for each tool.

198 C. Hall~International Journal of Industrial Ergonomics 20 (1997) 191-206

Table 5 The subjects ranked the cylinder diameters from most preferred (lst) to least preferred (9th). The table shows the number of subjects by ranking order and cylinder diameter

Diameter 1st 2nd 3rd 4th 5th 6th 7st 8th 9th

10 2 5 4 3 1 20 1 5 4 5 30 5 6 4 40 8 7 6 50 2 1 4 2 60 5 3 7 70 4 11 90 1 14

100 1 14

coeff icient . The associat ion be tween re la t ive pressure

(%PPT) and V A S rating was calculated by the

Spea rman rank corre la t ion coeff ic ient , correc ted for

ties. S ince the number o f samples was larger than 25,

a normal approximat ion with z = ,/(N - 1) was used.

Di f fe rence be tween females and males in V A S rating

was tested by W i l c o x o n - M a n n - W h i t n e y rank sum

statistics, corrected for ties. For all calculat ions, p <

0.05 was chosen for statistical s ignif icance.

3. R e s u l t s

3.1. Questionnaire

No subject had been on sick leave during the last

6 months due to f o r e a r m / h a n d symptoms. Four

subjects (2 m + 2 f) reported symptoms from the

f o r e a r m / h a n d during the last 12 months, but only

one male had symptoms the last 7 days. No subject

reported nocturnal numbness or low hand force,

Normalized pressure (%)

800 -

700-

600 -

500-

400-

300

200

I 00.

0- :1 0 1

~ i ~ f ~ ~ O ~ [ ] ~ ~ ~ ' ~ | ~ 1 - P l a t e - s h e a r s , medium force (45 kPa) 2 ~ ' ~ l ' ~ . ~ q V ~ " ~ J ~ l ~ l m |)"rb,4Vl-Plate-shears, low (19 kPa)

3 5 ' ~ r ~ - - - / ~ ~ . . . O ~ ~ ~ " D r i l l (29 kPa) b 7 8 I ~ " ~ / , , ~ , J ~ ~ l ~ / S c r e w d r i v e r (28 kPa)

. . . . . . . - 9 i n ' ~ . ~ l n ~-~: Saw (83 kPa) M e ~ , _ ~ , l , . ~ r / N i p p e r s ' ~ 1 2 kPa)

lOCal:Ion 13 14 Pencil (9 kPa) 15

Fig. 6. Pressure amplitude during work with tools; average during the entire working time. Fifteen subjects participated, except for screwdriver (11 subjects), medium force plate shears (10 subjects) and high force plate shears (3 subjects). 'Measurement location' refers to locations in the hand described in Fig. 1. The measurement values have been normalized; the average value for all 15 locations for each tool is denoted 100%. The average pressure amplitude (in kPa) appears within parentheses.

c. Hall / International Journal of lndustrial Ergonomics 20 (1997) 191-206 199

whereas two females often had white fingers. No females used tools at work, and five never used tools during the leisure time. Hand and power tools were used ' se ldom' by two females and ' somet imes ' by one female during the leisure time. No males used power tools at work, and hand tools were never used by five and ' se ldom' used by two males. During leisure time, three males used hand tools and five used power tools to a varying extent.

3.2. Pressure-pain threshold (PPT)

The PPT measured on 15 subjects in 1995 differed significantly between the investigated locations ( F = 7.02, df: (14,98)) and females had lower PlY[ (average 67%) than males ( F = 66.35, df: (8,112)) (Table 4).

The long-term PPT measurements were performed on ten subjects. No significant difference was seen for PPT measurements separated by 1 - 4 weeks, however, there was a significant difference between measurements conducted in 1989 and 1995 ( F =

9.21, df: (1,4)). For one subject, the PPT was higher

in 1995, for one subject there was no difference between the two occasions and for the remaining eight subjects the PPT was significantly lower in 1995 as compared to 1989, resulting in the average PPT at the second occasion comprising 79% of PPT at the first occasion. A significant interaction was obtained between localisation and occasion ( F = 4.28, df: (14,56)), i.e. the decrease was largest at the fingers (second occasion compared to the first: fingers 71%, thenar area 83% and palm 87%). There was also a significant interaction between occasion and gender ( F = 27.14, df: (5,300)); for females the average PPT at the second occasion was 70% of the first, while for males it was 87%.

3.3. Maximal hand grip strength

Males had significantly higher MVC (average: 512 N, 95% c.i.: 442 -582 N) than females (average: 352 N, 95% c.i.: 302-402 N).

. . . . . . . . ,5 T o t a l F i n g e r s T h e n a r P a l m

ILm EI I I I 'L. I l lh| II |I _ _ ll' o ~ I ~ ~ I

e n Nip Saw Scr Ori Psi Psm iPen Nip Saw Scr Dri Psi Psm Pen Nip Saw Scr Ori Psi Psrn iPen Nip Saw Scr O r l P s l Psm

,5'oP~ . . . . . . . . . . . . . t i

1,;:1 , I , s 1 11 o°t i I i

ii ' ' ~o I I I n . _ ~ _ m r - ~ I o ~ i ~ - ~ ~ ~ - ~ I ~ i . - ~ ~ - ~ I

Pen Nip Saw Scr Dri Psi Psm Pen Nip Saw Scr Dri Psi Psm Pen Nip Saw Scr Dri Psi Psm Pen Nip Saw Scr Dri Psi Psm i

Fig. 7. Pressure in relation to pressure pain threshold (PPT) during work with tools. The average pressure at all fifteen locations (total), the six ringer locations (ringers), the four thenar area locations (thenar) and the five palmar locations (palm) are shown for the 7 investigated tools (Pen: pencil, Nip: nippers, Scr: screwdriver, Dri: drill, Psl: plate shears, low force and Psm: plate shears, medium force). The top row displays the relative pressure (% PPT) during the entire working period, and the lower row shows the highest pressure recorded during the measurement in relation to PPT.


3.4. Pressure measurements

3.4.1. Hand size and grip strength in relation to pressure

Neither hand size, nor grip strength, were linearly correlated to average pressure recorded during grasp around cylinders or work with tools. However, a multiple correlation between hand size and average pressure recorded during pulling at a constant rate was significant:

Pressurez00N pun(kPa) = 287 - 0.86 × hand length

- 0.69 X hand width,

r 2 = 0 . 5 8 , df = 14, F = 8.32, p < 0.01.

When MVC was included in the multiple correlation, it was still significant for pressure recorded during pulling at a constant rate ( r e = 0.58 df = 14, F = 5.12, p < 0.01) but not for the other pressure measurements.

3.4.2. Grasp tests on a range of cylinder diameters When grasping the cylinders, the highest pressure

was recorded at the fingers. During maximal force exertion, the average pressure at the fingers was in the range of 100-500 kPa for women and 100-800 kPa for men, depending on cylinder diameter and location. Corresponding pressures at the thenar and palmar area were 0 - 2 0 and 10-300 kPa for females and 0 - 2 0 0 and 20-100 kPa for males, respectively. The quotient between pressure recorded at female and male hands varied depending on location ( F = 3.52, df: (98,784)), but in average the pressure recorded at female hands was lower (grand average 87%) than for males ( F = 4.61, df: (7,56)).

When the cylinders were used for pulling 200 N, the average pressure at the fingers was in the range of 10-300 kPa for both women and men, depending on cylinder diameter and location. Almost no pressure (0 -20 kPa) was applied to the thenar area. At the palmar area the average pressure ranged between 10-200 kPa. The quotient between pressure recorded at female and male hands varied depending on location ( F = 5.18, df: (98,784)), but in average the pressure recorded at female hands was higher (grand average 156%) than for males ( F = 5.27, df: (7,56)).

For both the investigated situations, the pressure varied significantly according to the cylinder diame-

ter (Fmaxima I force = 27.2; F20o N p u l l = 10.74, df: (8,48)). Also, there was an interaction between cylin-

der diameter and investigated location (Fmaxima I f o r c e

= 7.97; F200 N pull = 4.98, df: (112,672)), implying that the pressure distribution at the hand is dependent on the diameter of the handled object (Fig. 3 and Fig. 4).

There was a strong agreement between the subjects in ranking the preference of the cylinders (u = 0.80, c 2 = 154.89, f = 10.94). Cylinder diameters of 40 and 30 mm was most preferred, and the largest diameters were least preferred (Table 5).

3.4.3. Work with tools

3.4.3.1. Pressure amplitudes during work with tools. An example of pressure amplitudes during work with tools are shown in Fig. 5. The average pressure

Table 6 Visual analogue scale (VAS) ratings of pressure, discomfort and pain during work with tools. The values describe the distance (in ram) from the left anchor (see Table 3) to the mark

Tool Females Males

Median Min Max Median Min Max

a. VAS ratings of pressure Pen 20 Nippers 14 Saw 34 Screwdriver 36 Drill 7 Plate shears, low force 31 Plate shears, medium force 77 Plate shears, high force -

b. VAS ratings of discomfort Pen 0 Nippers 0 Saw 9 Screwdriver 28 Drill 0 Plate shears, low force 17 Plate shears, medium force 45 Plate shears, high force -

c. VAS ratings of pain Pen 0 Nippers 0 Saw 0 Screwdriver 0 Drill 0 Plate shears, low force 0 Plate shears, medium force 11 Plate shears, high force -

2 71 8 0 19 0 29 l0 2 20

10 99 27 13 54 6 83 28 17 92 0 21 11 5 28 2 93 25 9 41

18 98 60 31 78 82 60 100

0 26 0 0 2 0 13 0 0 0 0 53 0 0 24 2 38 10 0 26 0 19 0 0 4 0 58 4 0 62

16 73 36 8 79 25 9 51

0 9 0 0 0 0 14 0 0 1 0 40 0 0 9 0 13 2 0 52 0 13 0 0 2 0 34 0 0 26 0 41 5 0 49

0 0 78

C. Hall/International Journal of Industrial Ergonomics 20 (1997) 191-206 201

during the entire working time (WT) did not exceed 250 kPa. The average pressure during typical work cycles (TYP), e.g. when actually cutting with the plate shears, was higher but it still did not exceed 250 kPa, except for medium- and high-force plate shears for which the typical work cycle pressure was 300 kPa or higher. The maximal pressure (MAX) differed more between the investigated tools. It was 250 kPa or less for the pen, nippers and drill, 350 kPa or less for the saw and low-force plate shears, about 600 kPa for the screwdriver and up to 1000 kPa for medium- and high-force plate shears. The

recorded pressure differed significantly between the investigated tools (FwT = 47.00; FTV e = 12.46; FMA x = 33.69, dfwT and MAX: (5,30); dfTvP: (5,30)), and there was also a significant interaction between tool and investigated location (FwT = 8.08; FTv P = 6.85; FMA x = 8.01, dfwT a,d MAX: (70,240); dfTv P (42, 252)). This is reflected in the pressure distribution (Fig. 6).

There was no difference between females and males in average pressure during the entire working time or during a typical work cycle. However, the maximal pressure was somewhat lower (grand aver-

Table 7

Association between recorded relative pressure (%PPT) and visual analogue scale (VAS) ratings calculated by Spearman rank correlation coefficient, corrected for ties

a. For the total work time and typical work cycle, the correlation was calculated both including the verage of all pressure locations at each

area, and after excluding locations with zero pressure

VAS rating

Pressure Discomfort Pain

Situation Area n p n p n p

Total working time: Fingers 87 < 0.001 88 < 0.001 88 ns

all locations Thenar 88 < 0.001 89 < 0.001 89 0.013

Palm 88 ns 89 ns 89 ns

Total 87 < 0.001 88 < 0.001 88 0.045

Total working time: Fingers 85 0.0013 86 < 0.001 86 ns

locations with zero Thenar 55 ns 55 0.023 54 ns

pressure excluded Palm 67 ns 65 ns 68 ns

Total 88 < 0.001 89 < 0.001 89 ns

Typical work cycle: Fingers 52 0.(109 52 < 0.001 51 0.029

all locations Thenar 47 0.(102 47 0.010 46 ns

Palm 52 0.1)12 52 ns 51 ns Total 52 0.012 52 < 0.001 51 0.046

Typical work cycle: Fingers 50 0.017 50 0.004 49 ns

locations with zero Thenar 89 < 0.001 85 < 0.001 86 < 0.001

pressure excluded Palm 85 < 0.001 90 < 0.001 86 < 0.001

Total 86 < 0.~101 86 < 0.001 90 < 0.001

Maximum: Fingers 88 < 0.001 89 < 0.001 89 0.013

locations with zero Thenar 88 ns 89 ns 89 ns

pressure excluded Palm 87 < 0.001 88 < 0.001 88 0.045

Total 85 0.0013 86 < 0.001 86 ns

b. The correlation between the average recorded relative pressure (%PPT) at areas where the subject indicated pressure, discomfort and pain on a drawing and the VAS rating of the same sensation

Sensation n Z

Pressure 76 3.96

Discomfort 19 ns Pain 10 ns

2 0 2 C. Hall/International Journal of Industrial Ergonomics 20 (1997) 191-206

age 94%) for females (FMA x = 2.58, dfMAX: (7,35)). For all three situations, there was a significant interaction between the gender of the subject and investigated location (FwT = 1.49; FTy e = 1.84; FMA x = 2.16, dfwT ~,d MAX : (98,490) dfvvp: (98,294)).

3.4.3.2. Relative pressure during work with tools. The relative pressure, i.e. pressure amplitude in relation to pressure-pain threshold (PPT), varied between the investigated locations (Frelative p . . . . . . . WT = 3.88;

Frelative p . . . . . . . MAX = 4.02, df re la t ive p . . . . . reWT and MAX:

(14,84)), and the fingers were the most exposed (Fig. 7). The highest relative pressure, about 35% PPT, was recorded for sawing. Cutting with medium force plate shears and driving screws implied the highest relative pressure, 100% PPT and higher especially for the fingers (Fig. 7). Generally, females were exposed to higher relative pressures as compared to

males (Fre la t ive p . . . . . . eWT = 8.66; Frelative p . . . . . . . MAX =

10.20, dfrelative p . . . . . . eWT and MAX: (1,6))(Fig. 7). The objective for studying relative pressure is that when the applied pressure exeeds the pressure-pain threshold, the subject feels pain in the hand. Hence, it is a methodology to investigate if a tool is likely to induce pain during use.

3.4.3.3. VAS ratings in relation to relatice pressure. VAS ratings of pressure, discomfort and pain (Table 6) did not differ between females and males. The highest correlation between subjectively perceived sensation (VAS rating) and relative pressure (%PPT) was obtained for the thenar and palmar areas for typical work cycles where the locations with zero pressure had been excluded in the calculation, i.e. where only locations in contact with the tool were considered (Table 7a). There was a significant correlation between perceived pressure at areas indicated on the drawing of the hand and recorded relative pressure. However, the ratings of discomfort and pain at areas indicated on the same drawing were not associated to relative pressure (Table 7b).

4, Discussion

4.1. Pressure-pain threshold (PPT)

The lower PPT for females as compared to males has been described previously (e.g. Brennum et al.,

1989; Bystrtim et al., 1995; Fransson-Hall and Kil- born, 1993; Fransson-Hall et al., 1994). The differ- ences in sensitivity between disparate areas of the hand has been reported previously, but the PPT amplitudes recorded in this study are a bit lower than corresponding areas for other populations, although the difference is not significant (BystriSm et al., 1995; Fransson-Hall and Kilbom, 1993; Fransson- Hall et al., 1994).

For nine out of ten subjects, the PPT recorded with six years interval differed significantly, however the PPT was stable over a shorter period. This result suggest that a subject sets up a standard for determining the threshold between pressure and pain that is stable over a period of some weeks, but that this standard might change over a period of several years. Previous investigations on the PPT variation report high agreement between measurements separated by one week (Kosek et al., 1993), while contra- dicting results haven been obtained for measurements separated by a number of weeks. Kosek et al. (1993) reported an increased PPT after 9-12 weeks and Jensen (1986) documented an increase in the course of five repeated measurements at weekly intervals; but Ohrbach and Gale (1989) reported no major difference between sessions up to 8 weeks after the initial session. It appears unlikely the long- time difference in PPT over several years is related to ageing, since a recent (cross sectional) measurement of PPT of a large population with a wide age span showed no significant difference in PPT for young as compared to older subjects (Fransson-Hall et al., 1994). Jensen (1986) found no correlation between age and PPT at the temporal region, but Theorell et al. (1993) report lower PPT at the trapezes muscle for older subjects when dichotomising the populations. Low PPT at the hand is known to be strongly associated to subjective symptoms from the hand, especially for women (BystriSm et al., 1995). It is however not likely that the difference in PPT between 1989 and 1995 is due to hand problems, since no females and only one man (palpation pain at the elbows due to a fall some days prior to the measurement) reported recent symptoms from the forearm/hand. All reported symptoms during the last 12 months occurred at least three months ago, and affected the wrist (one subject) or elbows (2 subjects).


4.2. Hand size and grip strength in relation to

pressure

During constant force exertion to a cylinder, the pressure amplitude was higher for females compared with males while the opposite was true during maximal exertion. This was expected due to the relation between hand size and strength: in average female hand strength comprised 69% of male strength, and a rough estimate of female hand area (length X breadth) comprised 84% of the estimated area for males. Hence, at equal external force to the hand, the predicted pressure to the smaller female hands would be (force/(0.84 × area))/(force/area) = 1.19 times the male pressure. However, when applying maximal force to a cylinder, the predicted pressure at a female hand would be ((0.69 × f o r c e / ( 0 . 8 4 × area)))/(force/area) = 0.78 times the male pressure. The difference between the estimated and measured values probably indicate that the estimate of hand area is too rough as an estimate of the pressure distributing area of the hand.

During work with tools, males were exposed to higher maximal pressures than females, probably due to higher maximal force application since males MVC exceeded female by about 45%. Yet, the average pressure during the entire working time and during a typical work cycle did not differ between females and males. Since the female force distribution area is smaller (se discussion above), this implies that females must perform the tasks at a lower force than males. This has been shown for plate shears, and the difference between the genders increased with increasing force requirements (Kilbom et al., 1993a).

In conclusion, calculations of correlation between hand size and MVC and the pressure amplitudes indicate that hand size is the critical factor when the external force requirement is constant, while there is an interaction between hand size and strength when the external force varies, e.g. during maximal con- tractions or free paced work with tools.

4.3. Grasp tests on a range o f cylinder diameters

When gripping a cylindrical object, the fingers were exposed to the largest pressure, while only negligible pressure was applied to the thenar area.

The pressure distribution was dependent on the diameter of the cylinder as well as the task performed. During maximal exertion around the cylinders, the pressure was evenly distributed between the :fingers for small diameters. With increasing cylinder diameter, the distribution shifted towards increasingly higher proportion of the pressure applied to the thumb and distal phalanx of the index finger. The practical consequence is a shift from a transversal volar type of grip towards a grip more similar to a pinch grip. This transition, from a multisegmental gripping action to a cantilevered finger tip action, has previously been observed during measurement of finger forces in grasps on cylinders (Amis, 1987; Radhakrishnan and Nagaravindra, 1993). The distribution of pressure has been shown to shift: under application of dynamic loads (Gurram et al., 1993; Gurram et al., 1995). During pulling of a handle using constant force, the pressure was mainly applied to the middle phalanges of fingers II-IV. For this situation, the significant interaction between pressure distribution and cylinder diameter reflects that the pressure is more evenly distributed over fingers and palm for diameters in the middle of the investigated range while it is mainly concentrated to the fingers at the smallest and largest diameters.

A strong agreement was seen between the subjects in ranking the preference of the cylinders. The optimal cylinder diameter seemed to be somewhere between 30-40 mm, while diameters larger than 60 mm and smaller than 30 mm were regarded as not preferred. The present results are in line with previous findings which maintain that diameters between 30 and 40 mm were the most preferred (Cisneros and Armstrong, 1994; Drury, 1980) The preferred cylinder diameter also agrees with the diameter suggested as optimal for overall gripping force, 37 mm (Ayoub and Lo Presti, 1971).

4.4. Work with tools

The subjects participating in the present study were all laymen with regard to work with tools. One of the male subjects often used tools during his leisure time, but the majority of the subjects never, or seldom, used tools during their leisure time. This might affect the way the tools were handled; it is possible that a professional worker, with everyday

204 C. Hall / International Journal of' Industrial Erg~momics 20 ( 1997) 191-206

use of hand tools, has different pressure amplitudes as well as a different distribution of the pressure in the hand. The potential difference between professional workers and laymen remains to be investigated. However, the tools investigated in the present study were well-known for the majority of the subjects, except for the plate shears.

The average pressure during the entire working time and typical cycles was 250 kPa or less, except for plate shears for which the average pressure exceeded 300 kPa during a typical work cycle. Peak pressure differed more between the investigated tools; is was below 250 kPa for three of the tools, but reached 1000 kPa during work with plate shears. The recorded pressure amplitude and distribution differed between the investigated tools since the tools operated at different force levels and dissimilar part of the hand was used to accomplish the required force. A general similarity between the tools was however that the fingers were exposed to a large proportion of the external pressure. During writing, external pressure was applied only to fingers I, lI and III due to holding the pen and, for some subjects, to the hy- pothenar area due to leaning the hand against the table. During cutting with cross-action tools, such as the nippers and the plate shears, external pressure was primarily applied to the fingers and the thenar area, since the fingers are used to close the tool and the thenar area is used for opposing this force. Previous experience shows that the most common areas of discomfort during cutting with plate shears are between the thumb and the index finger (Kilbom et al., 1993a; Oster et al., 1994) and at the radial part of the palm (Ayoub and Lo Presti, 1971). During work with the power tool drill, forces needed to support the tool is reflected as pressure distributed mainly to the fingers and palm, while the force required for pushing the trigger shows up as pressure to the middle phalanx of fingers III and IV. During sawing, the fingers are used for pulling the saw towards the body while the centre of the palm is used for pushing it away from the body, which both reflects in the pressure amplitude and distribution.

The external pressure to the transverse carpal ligament, limiting the carpal canal on the palmar side of the hand (location 10), was negligible for all the tools investigated in this study. This result suggest that the high intra carpal canal pressures recorded

during routine work tasks (Armstrong et al., 1991) is more dependent on wrist posture and the position of the finger joints than externally applied pressure.

In analogy to pressure amplitude, the relative pressure (pressure amplitude/PPT) varied between the investigated locations and the highest relative pressure was found at the fingers. Generally, the average relative pressure did not attain the PPT, but the pain threshold was exceeded momentarily, especially for the fingers during cutting with plate shears and sawing. Since female PPT generally is lower than male, females were regularly exposed to higher relative pressure than males. This did however not show up in the VAS-ratings. The somewhat low PPT amplitudes for this population affect the relative pressure (pressure amplitude/PPT). The described relative pressure might therefore be a bit high, but the distribution of relative pressure would still be proportional.

VAS-ratings of pressure and discomfort were pos- itively correlated to external pressure. For pain, however, the correlation was low. This is reasonable, since the external pressure only exceeded PPT during short periods for some of the investigated tools. The drawing of subjectively perceived distribution of external pressure, discomfort and pain in the right hand showed only moderate correspondence to ob- jectively measured values. Pressure seemed to be the easiest sensation to describe, while discomfort and pain appeared not to be perceived as localised sensa- tions, but influenced by other factors, such as fatigue and sensory afference from other parts of the body than the palmar aspect of the fight hand. Hence, subjective ratings of discomfort and pain in the hand, previously used as a substitute for objective measurements (e.g. Kadefors et al., 1993a) should be analysed with precaution. The difficulty in localising pressure, discomfort and pain during work with tools might be affected by the work situation itself, since previous investigations show that skin sensitivity is modulated by exercise (Paalasmaa et al., 1991).

Acknowledgements

The author wish to thank Professor Asa Kilbom and Dr Gi~ran H~igg for valuable comments on the manuscript and Ronny Hall for helpful technical


suggestions. The partial support by the Swedish Work Environmental Fund is gratefully acknowledged.

Appendix A. Equipment for pressure measurements

Pressure is closely related to force, since pressure equals force per unit area. Therefore previous studies of pressure and force sometimes overlap; the same equipment have been used for both types of measurement. An estimate of the grip pressure distribution at the hand-handle interface has been measured using the Emed system (Gurram et al., 1993; Gurram et al., 1995). Resistive sensors (Interlink Electronics, CA, USA; with or without encasement) have been used for measuring the pressure applied to the hand (Pax et al., 1989; Yun et al., 1992). Ultrasonic force sensors (Schoenberg, 1983), piezoresistive force sensors (Neuman and Buckett, 1982) and capacitive force sensors (Neuman and Buckett, 1982) have also been described for measurements of hand or finger force. However, the by far most widely used tech- nique for force measurements is resistive sensors (Interlink Electronics, CA, USA; with or without encasement) (e.g. Bishu et al., 1993; Fellows and Freivalds, 1991; Jensen et al., 1991; Radwin and Oh, 1992). The sensor principle for a 'sensor glove', used for describing the force distribution in the hand (Sato, 1992) is not described.

Transducers used for pressure measurements must not be encumbering to normal movement. Thus, they should be of low mass, small size and mounted in such a way to avoid limiting the range of movement or degrees of freedom of the hand. These constraints imply that the sensor must be thin and flexible and easily adjust to the individuals hand. The capacitive pressure sensors used in the present study were primarily chosen because of their softness and small design (6 × 6 X 1 mm). The sensors were not perceived to be disturbing or alternating the natural hand movements during the measurements. For comparison, the most commonly used force sensor, Inter- link, is available in different shapes and sizes. The smallest have an effective sensing area of 5 X 5 X 0.3 mm but the overall dimension is 13 X 7 X 0.3 mm or larger (if customer suited enclosing is used). A second feature with the Emed system was the possibility

to record and store up to 7040 samples in a battery operated internal memory for subsequent transfer to a PC, making the system portable.

In the present study the pressure sensitive sensors were affixed to carefully defined locations in the hand by surgical tape. A surgical rubber glove was used closest to the hand, under the sensor, to prevent the tape from detaching due to hand sweating, and a second glove was used to gather the wires. Since the sensors were tightly applied to anatomically defined locations in the hand, each measured pressure could be rendered precise to a restricted area. In some previous studies of pressure or force the sensor has been applied to the hand, either by tape (e.g. Jensen et al., 1991; Radwin and Oh, 1992) or by fastening it to a cotton-knit (Yun et al., 1992) or tightly fitting (Sato, 1992) glove. Others (e.g., Fellows and Freivalds, 1991; Gurram et al., 1993; Gurram et al., 1995) have attached the sensors to the tool or handle. An apparent risk with not attaching the sensors firmly, such as using a loose-fitting glove or applying them to another object, is that the sensors may slide during hand use implying that the location of pressure or force application is not under control.

Previous investigations of capacitive sensors show that localised load generates a larger output than the same load uniformly distributed over the whole area of the transducer (Miyazaki and Ishida, 1984). This effect is however of greater importance for large sensors, e.g. transducers which measure the pressure under the heel or forefoot during gait analysis, and may probably be neglected for the small sensors (6 × 6 mm) used in the present study. Highly localised loads can be present in the hand during work with tools, e.g. for handles designed with grooves for the fingers, but no such tool was included in the present investigation.

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external pressure at the hand during object handling and work with tools

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