Pistol Grip Power Tool Handle and Trigger Size Effects on Grip Exertions and Operator Preference

HUMAN FACTORS, 1993,35(3),551-569

Pistol Grip Power Tool Handle and Trigger

Size Effects on Grip Exertions and

Opera tor Preference

SEOUNGYEON OH

and

ROBERT G. RADWIN,1

University of Wisconsin,

Madison, Wisconsin

Finger and palmar forces were measured during actual pneumatic nutrunner op-

eration using a strain gauge dynamometer. Eighteen student subjects were as-

signed to one of three categories based on hand length. Two triggers and four

handle spans were presented randomly. Handle span affected maximal and sub-

maximal grip force. Asspan increased from 4 cm to 7 cm, average peak finger force

increased 24%, peak palmar force increased 22%, and average finger and palmar

tool-holding forces increased 20%. When an extended trigger was used, average

peak finger force decreased 9%, peak palmar force decreased 8%, finger tool-

holding force decreased 65%, and palmar tool-holding force decreased 48%. Hand

size affected grip strength (MVC),grip force, and exertion level (force/MVC). Hold-

ing exertion level was maximum for large-handed subjects using a 4-cm handle

and for small-handed subjects using a 7-cm handle. Subjective handle span pref-

erence increased as hand size increased. A similar experiment was performed

using 11 factory workers.

INTRODUCTION

Pneumatic hand-held power tools are

widely used in industry. Power tools reduce

manual force requirements, shorten the time

to accomplish tasks, and improve the quality

of work. The use of power tools, however, is

not without stress. Rauko, Herranen, and

Vuori (1988) interviewed 66 workers from

two companies in Finland. Their investiga-

tion showed that one in five workers (20%)

felt that the most stressful task in their work

Requests for reprints should be sent to Robert G. Rad-

win, Department of Industrial Engineering, University of

Wisconsin-Madison, 1513 University Ave., Madison, WI

53706.

was connected directly to the use of pneu-

matic screwdrivers and nutrunners. There is

also growing concern about designing and se-

lecting power tools for preventing cumulative

trauma disorders (CTDs).

Repetitive motion, forceful exertion, awk-

ward posture, contact stress, cold tempera-

ture, and vibration are risk factors often as-

sociated with CTDs (Armstrong, Radwin,

Hansen, and Kennedy, 1986). The use of

pneumatic power tools may involve one or

more of these risk factors. The best method

currently available for preventing cumula-

tive trauma disorders is to minimize risk fac-

tors associated with tasks, tools, and the

workplace (Armstrong, 1986). Risk factors

SS2-September 1993

associated with trigger and handle design are

the focus of the current study.

A CTn risk factor related to trigger design

is stress concentrations at the volar side of

the fingers. The CTn known as

trigger finger,

stenosing tenosynovitis crepitans

(Bonnici

and Spencer, 1988; Nasca, 1980), is associ-

ated with repeatedly operating the trigger of

a pistol grip power tool when this risk factor

is present. Multifinger-operated triggers have

been suggested as a way of redistributing fin-

ger force over two or more fingers when acti-

vating a power tool and thus reducing con-

tact stress (Lindqvist, Ahlberg, and Skogsberg,

1986; Putz-Anderson, 1988). Use of these tool

modifications, however, may affect how the

tool is handled. It is not known whether tool

operators distribute the required force

among the fingers or whether the sum of the

forces produced by each finger is greater than

the force exerted using only one finger.

Multifinger-operated triggers also leave

fewer fingers available for holding and sup-

porting the tool. The remaining fingers are

often the fourth and fifth digits, the ones that

are the weakest and contribute the least

force. Studies have shown that the index and

middle fingers contribute more to the result-

ant grip force than do the ring or small finger,

and that individual finger contribution is

influenced by exertion level (Amis, 1987;

Radwin, Oh, Jensen, and Webster. 1992).

This study investigates the effect of multifin-

ger triggers on finger exertions during tool

operation.

Previous research in tool handle design has

focused on finding the optimal handle dimen-

sions. Recommendations for handle size were

often based on the span that maximizes grip

strength or minimizes fatigue. Hertzberg

(1955), in an early air force study, reported

that a handle span of 6.4 em maximized

power grip strength. Ayoub and Lo Presti

(1971) found that a 3.8-cm diameter was op-

timum for a cylindrical handle. This was

HUMAN FACTORS

based on maximIzmg the ratio between

strength and electromyographic activity, and

on the number of work cycles before onset of

fatigue. Greenberg and Chaffin (1975) recom-

mended that a tool handle span be in the

range of 6.4 em and 8.9 em in order to achieve

high grip forces. Another study, by Petrofsky,

Williams, Kamen, and Lind (1980), showed

that the greatest grip strength occurred at a

handle span between 5 and 6 em.

Grip strength is also affected by hand size.

Fitzhugh (1973) showed that the handle span

resulting in maximum grip strength for a

95th-percentile male hand length is larger

than the handle span for that of a 50th-

percentile female. Therefore it is hypothe-

sized that a person with a small hand may

benefit from using a smaller handle, com-

pared with a person with a large hand. This

study tests that hypothesis.

The underlying assumption in designing

handles based on maximum grip strength is

that the actual force exerted is independent of

handle size. If the grip force used during tool

operation is the same for all handle sizes,

then the handle span associated with the

greatest grip strength will result in the lowest

exertion level.

Exertion level

in this paper is

defined as the ratio of the actual grip force

used to the maximum voluntary contraction

(MVC)force-generating capacity (force/MVC).

If grip force is affected by handle size, then

the handle span associated with the greatest

grip strength may not be the handle span re-

sulting in the minimum exertion level. This

study investigates the validity of that as-

sumption.

The effects of hand size, handle span, and

trigger type on grip exertions during actual

tool operation were investigated in this study

using an automatic shut-off pistol grip power

tool. A tool handle instrumented with strain

gauges was attached to a functioning power

tool for measuring the forces exerted when

operating the tool. The relationships among

POWER TOOL HANDLE AND TRIGGER SIZE

anthropometry, force, and subjective prefer-

ences were also considered.

METHODS

Experimental Apparatus

An apparatus was constructed for simulat-

ing a pneumatic pistol grip nutrunner in or-

der to measure forces produced during tool

operation. Two strain gauge dynamometers

were installed in a handle for measuring

forces at the finger and palmar sides. The dy-

namometer handle was attached perpendicu-

larly to a modified in-line pneumatic nutrun-

ner motor in the configuration of a pistol grip

power tool and mounted on a rigid frame (see

Figure 1). The motor was a 1000-rpm Inger-

soll-Rand 6W-TM3 nutrunner. This motor

contained an automatic air shut-off torque

control mechanism and was operated at a

6.8-N . m torque setting. The two dynamom-

eters were mounted in parallel on a track so

September 1993-553

the handle span could be continuously ad-

justed between 40 mm and 132 mm.

The aluminum dynamometers were con-

structed so their force sensitivity was inde-

pendent of the point of application. They

were able to add force linearly along the han-

dle length by measuring the shearing stress in

the cross section of a beam using strain

gauges aligned 45 deg to the long axis (Pronk

and Niesing, 1981). Details of the dynamom-

eter construction are provided in Radwin,

Masters, and Lupton (1991). These dynamom-

eters were calibrated by suspending weights,

ranging from 0.5 kg to 10kg, perpendicular to

the handle in the axis of sensitivity. Least

squares linear regression indicated that the

force sensitivity of the beams was 2 mV/N

with a coefficient of determination

0.99.

Two plastic caps were attached to the dy-

namometer beams in order to simulate the

contours of a power tool handle. The finger

side cap was 24 mm wide and the palmar side

ADJUSTABLE SPAN

Figure 1.

Experimental apparatus used for simulating a pistol grip power tool. The

device has an adjustable handle span and strain gauge dynamometers inside the

handle. The inset shows how the apparatus simulates the shape of a pistol grip

power tool.

554-September 1993

cap was 32 mm wide. Both caps were 129mm

long, and the contact surface was curved,

having a radius of curvature of 159 mm. The

circumference of the handle for a span of4 cm

was 12 cm, measured between two points

tangent to the cap contact surfaces. The han-

dle circumference increased an additional 2

cm as the handle span was increased by 1cm.

The tool trigger was mounted on the finger

side cap of the handle, similar to a conven-

tional hand tool (see Figure 1).

Two different trigger types were used. One

was a conventional power tool trigger, acti-

vated using only the index finger. The second

was longer than the conventional trigger and

activated using both the index and middle

fingers. These two triggers are depicted on

power tools in Figure 2. The conventional

trigger was 21 mm long and the extended

trigger was 48 mm long.

leaf spring for con-

trolling tension and a contact switch were in-

stalled inside the triggers. When the trigger

was squeezed, the swi tch tripped a relay and

HUMAN FACTORS

a solenoid valve for supplying air to the

power tool motor. Activation force, measured

using a Chattilon force gauge, was approxi-

mately the same for both triggers. The con-

ventional trigger required 8 N, and the ex-

tended trigger required 11 N for activation.

Ten bolts were contained in threaded holes

in a vertical 13-mm-thick steel panel. The

bolts were 7.3-cm long, 1.6-cm diameter,

number 18 screws with a

lA-em

hex head.

Belleville washers were used for controlling

the torque rate. The washers' outside diame-

ter was 4.0 cm, the inside diameter was 2.0

cm, and the thickness was 0.2 cm. The holes

in the panel were located in two rows of five

across. The vertical location of the screw

panel was controlled using an adjustable

platform that allowed all subjects to hold the

tool at elbow height using a neutral wrist

posture.

An overhead spring balancer was used for

counterbalancing the weight of the simulated

tool. The tool weighed 2.65 kg without the

EXTENDED'"

TRIGGER

CONVENTIONAL ./

TRIGGER

Figure 2.

A conventional trigger and an extended-length trigger on pistol grip power

tools.

POWER TOOL HANDLE AND TRIGGER SIZE

balancer and was 0.25 kg with the balancer

installed. Because the counterbalance force

was proportional to displacement, the dis-

tance between the spring balancer and the

screw panel was kept constant so the coun-

terforce was the same for each subject.

The outputs of the two dynamometers were

sampled using a GW Instruments MacADIOS

II 12-bit analog-to-digital converter. Activa-

tion of the trigger was also monitored by re-

cording the relay voltage. Sample rate was 30

Hz for a sampling period of 50 s. Data acqui-

sition was controlled using an Apple Macin-

tosh II microcomputer and National Lab-

View software.

Subjects

Two groups of subjects were studied. The

first included inexperienced tool operators

from the University of Wisconsin-Madison

campus. The second group included experi-

enced tool operators recruited from two local

automobile assembly plants.

Subjects were arbitrarily divided into three

categories according to their hand length.

Hand length up to 17 cm was classified as

small, between 17 cm and 19 cm as medium,

and greater than 19 .cm as large. A hand

length of 17 cm corresponded to a 21st-

percentile female or a O.3-percentile male. A

hand length of 19 cm corresponded to an

88th-percentile female or a 35th-percentile

male (Greiner, 1991).

University students.

Students were re-

cruited by posting advertisements on bulletin

boards in university buildings or by invita-

tion of the experimenter. Participants had no

history of hand injuries or neuromuscular

disorder symptoms. They were paid for their

participation on an hourly basis.

Participating in this experiment were 18

students: 7 males and 11 females. After the

first 15 subjects were randomly recruited, 4

were classified as having small hands, 5 as

September 1993-555

medium, and 6 as large. Based on their hand

length, two additional small-handed subjects

and one medium-handed subject were re-

cruited to have the same number of subjects

in each size category. The average hand

length was 17.1 cm

(SD

1.0 cm) for the

females and 19.9 cm

(SD

0.8 cm) for the

males. All 18 subjects described themselves

as right-handed and reported no prior expe-

rience using pneumatic power tools. A sum-

mary of student subject anthropometric char-

acteristics is included in Table 1.

Factory workers.

Eleven factory workers

were recruited from two automobile assem-

bly plants in the midwestern United States.

Subjects were selected based on their experi-

ence using pistol grip pneumatic power hand

tools. Of the 11 subjects, 8 were currently us-

ing similar pistol grip pneumatic power hand

tools with shut-off torque between 4.5 N . m

and 6.8 N· m in their jobs. The remaining

three subjects reported that they had previ-

ous experience in using pistol grip pneumat-

ic power hand tools in their jobs for at least

one year.

Eight subjects were male and three were

female. Average hand length was 19.0 cm

(SD

1.2cm) for the females and 19.5 cm

(SD

1.1 cm) for the males. Ten subjects described

themselves as right-handed and one subject

as left-handed. Five subjects were classified

with medium hands and six with large hands.

A summary of the factory worker anthropo-

metric characteristics is listed in Table 2.

Experimental Procedures

Prior to the experiment, a power grip

strength test for the dominant hand was

administered to subjects while they stood in

the same posture used for operating the

power tool. Grip strength was measured us-

ing a strain gauge dynamometer of similar

dimensions as those of the power tool simu-

lator handle. Grip force was recorded for 3 s,

and MVC level was calculated by averaging

SS{r-September 1993

HUMAN FACTORS

TABLE 1

Summary of UniversityStudent Characteristics

Hand Hand Hand

Palm Mean Grip

Hand Size Age Length Length Width Length Strength

Category (years) (em)

Percentile

Gender

(em)

(N)b

Small

22 15.7 1.1

7.5 9.5

236

21 16.2 4.4 F 7.6 9.3

152

20 16.4 7.3 F 7.4 9.5

220

16.6

10.7

F 7.6 9.6

179

20 16.7 13.0 F 7.4 9.4

122

29 16.7 13.0

F 7.4

9.8

181

Medium

17.1

24.7

8.3 10.0

346

17.4

39.2

F 8.0

10.9

195

20 18.1 61.4

8.2 10.7

181

20 18.3 70.7

8.3 10.0

115

31 18.8 27.5

8.7 10.9

382

24 18.9 86.5 F 8.1 10.5

273

Large

25 19.2 43.3

8.6 10.9

286

29 19.8

66.8

M 8.7 11.1 308

23 20.0 73.4

9.9 11.2

525

137

26 20.1 76.4 M 8.4 11.5 433

34 20.8 90.8

9.0 12.2 399

22 20.9 91.9 M 9.5 11.9

492

Mean

24 18.5

8.3

10.5

279

4 1.7

0.7

0.9

133

a Male and lemale hand length percentiles are calculated separately based on Greiner (1991) data.

Average grip strength at spans 01 4 em, 5 em, 6 em, and 7 em

one standard deviation.

for 1.5 s after maximum voluntary contrac-

tion was fully developed. Five different han-

dle spans, ranging from 4 cm to 8 cm, in I-cm

increments, were presented randomly. A

4-min rest period was given between trials.

Because some of the small-handed subjects

were unable to hold the tool handle at the

8-cm handle span, that condition was elimi-

nated from the experiment.

Fifteen minutes were provided for training

and practice operating the power tool. Dur-

ing training subjects had an opportunity to

use the tool simulator with the conventional

trigger and with the extended trigger for a

handle span of 5.5 cm. Subjects were given

more time if they requested it. Numbers were

assigned to each bolt, and subjects followed

the same sequence. Because tool torque was

controlled by an air shut-off mechanism, sub·

jects were instructed to release the trigger af-

ter the tool stopped automatically.

All combinations of two types of triggers

and four handle spans were presented ran-

domly. Subjects were required to tighten 10

bolts in succession in a 50-s period for each

trial. Three replications were performed for

each experimental condition. A I-min rest

was given after finishing each trial. Every

subject operated the power tool for all exper-

imental conditions.

A representative force record during tool

operation is plotted against time in Figure 3.

The peak force at the finger and palmar sides

of the tool handle was determined for each of

the 10 bolts in a single trial, then averaged.

Peak force

was defined as the maximum force

achieved each time the tool was activated.

The average tool-holding force used between

POWER TOOL HANDLE AND TRIGGER SIZE

September 1993-557

TABLE 2

Summary of Factory Worker Characteristics

Experience

Hand Hand Hand Palm

Mean Grip

Using Pistol

Hand Size Age Length Length Width Length Strength Grip Power

Category (years)

(em)

PercentileS

Gender (em) (em)

(N)b

Tools (years)

Medium 46 18.1 63.6

8.7 10.7 221 ± 41

41 18.3

12.7

M 9.2

11.2

411 ± 36 3

52 18.4 15.0 M 8.9 10.3 313 ± 58

18.5 76.9 F

7.8

10.2

259 ± 17 3

51 19.0

35.0

9.7 10.7

315 ± 46

Large 51 19.1 39.2 M 9.6 10.5 328 ± 71 25

19.5 55.4 M 10.1 11.3 277 ± 24 23

49 20.0

73.4

M 9.2 11.6 391 ± 120

20.3 99.2

8.7 11.2

289 ± 52

43 20.8 90.8 M 10.3 11.6 384 ± 83 3

21.2 95.2 M 10.3 11.8

413 ± 151

Mean 48

19.4 9.3 11.0 327 10

SO 6

1.1 0.8 0.6

• Male and female hand length percentiles are calculated separately based on Greiner (1991) data.

Average grip strength at spans of 4 em. 5 em. 6 em and 7 em

one standard deviation.

4 6

ONi

OFF1~

n_------~

I-

that study. Only the third replication for each

experimental condition was used in the data

analysis in order to eliminate learning effects.

After completing all handle and trigger

combinations, subjects were asked which

handle span they preferred for each trigger

type and which trigger type they preferred

overall. They were given another opportunity

TIME (sec)

Figure 3.

Representative finger force and palmar

force measured while handling a tool and during tool

operation.

screws was also computed from the finger

and palmar sides of the handle.

Holding force

was defined as the force used for holding the

tool when the trigger was off. Because a cer-

tain amount of force was required to activate

the trigger, a small lag occurred between

holding the handle and initiating trigger ac-

tivation, and between trigger deactivation

and holding the handle. Both of these lags

were accounted for by eliminating 0.1 s be-

fore and after trigger activation. Exertion lev-

els were reported in terms of percentage of

maximum voluntary contraction (%MVC),

calculated by taking the ratio of the peak

force to grip strength for a given handle span.

Analysis of variance (ANOVA)was used for

testing statistically significant effects for the

full factorial experimental design. Subject

was a random effect variable nested within

hand length. Trigger type, handle span, and

hand length were fixed-effect variables. Be-

cause there were no small-handed subjects in

the worker group, the data were pooled and

the hand length variable was eliminated for

558-September 1993

to operate the tool and readjust the handle

span until they were satisfied with their

selection. The preferred span was then

measured using a Fowler Ultra-cal II digital

caliper.

RESULTS AND DISCUSSION

Grip Strength

Individual subject grip strength is given for

each subject group in Tables 1 and 2. Grip

strength, averaged over the four handle

spans, was 200 N

(SD

70 N) for the female

students and 404 N

(SD

111 N) for the male

students. Average grip strength was 256 N

(SD

46 N) for the female factory workers

and 354 N

(SD

89 N) for the male factory

workers.

Average grip strength is plotted against

handle span in Figure 4. Handle span had a

significant effect on grip strength for student

subjects (see Table 3) and worker subjects

(see Table 4). Grip strength increased signifi-

cantly as span increased from 4 cm to 5 em

for students (see Table 5) and factory workers

(see Table 6).

HUMAN FACTORS

The shape of the strength curve plotted

against handle span was the familiar inverted

U, similar to the results reported in numerous

other grip strength studies (see Figure 4). The

span resulting in maximum grip strength

also agreed with the findings from previous

strength investigations (Hertzberg, 1955;

Petrofsky et aI., 1980). Among student sub-

jects, 17 of 18, and 10 of the 11 worker sub-

jects, recorded their maximum grip strength

for a span of 5 cm to 6 cm in this experiment

(see Figure 4).

Although the handle span resulting in max-

imum grip strength and the grip strength

function agreed with previous findings, the

grip strength magnitude for both student and

worker subjects was less than what has been

previously reported. Schmidt and Toews

(1970) collected grip strength data from 1128

male and 80 female Kaiser Steel Corporation

employee applicants using a Jamar dyna-

mometer. For a handle span of 3.8 em, they

reported an average of 499 N for the domi-

nant male hand and 308 N for the dominant

female hand. Swanson, Matev, and De Groot

(1970) measured the grip strength of 50

400

W 300

I-

200

~ FACTORY WORKERS

~ UNIVERSITY STUDENTS

HANDLE SPAN (em)

Figure 4.

Average grip strength plotted against handle span for university students

and factory workers. Error bars represent standard error of the mean.

POWER TOOL HANDLE AND TRIGGER SIZE

TABLE 3

Summary of Significant ANOVAEffects for University Student Subjects

September 1993-559

Degrees of Freedom

Variable Effect Numerator Denominator

Grip strength

Length

15 11.1 0.001

Span

45 14.1 <0.001

Span x Length

45 3.3 0.009

Peak finger force Type

8.4 0.011

Span

45 8.4 <0.001

Span x Length

2.3 0.051

Peak palmar force Type

1 15 10.0 0.006

Span

8.1 <0.001

Span x Length

45 3.0 0.016

Peak finger exertion level Type

4.9 0.042

Span

45 4.5 0.008

Peak palmar exertion level Type

4.8 0.045

Span

45 4.8 0.005

Average finger holding force Type

46.6 <0.001

Span

45 6.2 0.001

Average palmar holding force Type

15 46.6 <0.001

Span

45 4.9 0.005

Average finger holding

exertion level Type

15 52.6 <0.001

Average palmar holding

exertion level Type

1 15 4.6 0.049

Span

45 8.9 <0.001

Span x Length

6 45 2.5 0.036

females and 50 males using a Jamar dyna-

mometer. Among these subjects, 36 were

light manual workers, 16 were sedentary

workers, and 48 were manual workers. For a

handle span of 6.4 cm, they reported 467 N

for the male dominant hand and 241 N for the

TABLE 4

female dominant hand. All these strength lev-

els exceeded the levels observed in the cur-

rent study.

Differences in maximum grip strength be-

tween previous studies and this investigation

may be attributed to differences in the

Summary of Significant ANOVA Effects for Industrial Factory Worker Subjects

Degrees of Freedom

Variable

Effect Numerator Denominator F

Grip strength Span

3 30

14.4

<0.001

Peak finger force Span

3 30

13.5

<0.001

Peak palmar force Span

3 30 9.2 <0.001

Average finger holding force Type

145.1

<0.001

Average palmar holding force Type

185.2

<0.001

Average finger holding exertion level Type

1 10

134.8

<0.001

Span

3 30 10.1 <0.001

Average palmar holding exertion level

Type

1 10

99.4 <0.001

Span

3 30

8.8

<0.001

560--September 1993

HUMAN FACTORS

TABLE 5

Tukey Multiple Contrasts of Handle Span for University Student Subjects

Span (em)

Grip strength (N)

231 (82) 302 (129) 310 (153) 274 (151)

Peak finger force (N)

106 (45) 131 (74) 131 (75) 131 (75)

Peak palmar force (N)

129 (63) 153 (90) 152 (86) 157 (94)

Average finger holding force (N)

20.1 (12.3) 24.6 (16.3) 24.8 (13.5) 24.1 (14.9)

Average palmar holding force (N)

28.0 (13.8) 33.7 (20.0) 33.2 (15.6) 33.5 (17.0)

Peak finger exertion level (%MVC)

48.9 (20.0) 46.4 (21.7) 45.9 (21.2) 53.1 (22.5)

Peak palmar exertion level (%MVC)

59.7 (26.9) 54.2 (25.6) 54.3 (25.3) 63.7 (27.1)

Average palmar holding exertion level (%MVC)

14.1 (9.5) 11.1 (7.4)

13.2 (9.5)

15.9 (11.2)

One standard deviation is listed in parenthesis. MVC

maximum vOluntary contraction. Underlining indicates no significant difference (p

0.05).

particular handles used and the methods

used for collecting strength data. Grip

strength data often used for handle design are

based on the Jamar dynamometer, Smedley

dynamometer, or similar instruments

(Schmidt and Toews, 1970; Young et aI.,

1989) rather than on a handle representative

of an actual tool. In most previous cases, only

one dimension (handle span) was controlled,

and the other handle dimensions were not

necessarily similar to those of a tool handle.

The Jamar and Smedley dynamometers have

smaller circumferences and narrower widths

than the tool handle used in this study. Also,

TABLE 6

the tool handle curvature is straight, whereas

the Jamar dynamometer has a curved surface

at the grip center. The handle used in this

study closely represented an actual tool han-

dle circumference and width. These size and

curvature differences can affect the position

of the fingers and grip posture. Dimensional

differences must therefore be considered

when designing handles based on strength us-

ing published grip strength data.

Another difference between previous stud-

ies and this study is the method used for mea-

suring grip strength. In the current study sub-

jects were required to exert a maximum

Tukey Multiple Contrasts of Handle Span fqr Industrial Factory Worker Subjects

Span (em)

Grip strength (N)

244 (53) 336 (84) 369 (86) 360 (85)

Peak finger force (N)

142 (34) 190 (38) 193 (59) 184 (47)

Peak palmar force (N)

185 (37)

224 (47) 233 (69) 226 (53)

Average finger holding exertion level (%MVC)

13.0 (5.3)

11.0(5.1) 8.8 (3.1) 8.8 (3.7)

Average palmar holding exertion level (%MVC)

16.8 (6.2) 14.1 (7.2) 11.4 (3.9)

11.4 (4.7)

One standard deviation is listed in parenthesis. MVC

maximum VOluntary contraction. Underlining indicates no significant difference (p

0.05).

POWER TOOL HANDLE AND TRIGGER SIZE

power grip for 3 s. Grip strength was then

calculated by averaging the force for 1.5 s af-

ter a full exertion was developed. Because the

Jamar- and Smedley-type dynamometers rec-

ord peak force, grip strength may seem some-

what greater in studies using those instru-

ments, though great differences are not

expected.

Grip strength significantly increased as

hand length increased for the university stu-

dent subjects (see Table 3). Average grip

strength was 182 N

(SD

46 N) for the small

hands, 249 N

(SD

106 N) for the medium

hands, and 407 N

(SD

114

N) for the large

hands. Figure 5 illustrates the interaction of

Length x Span for the student group. Maxi-

mum grip strength occurred at a handle span

of 5 em for the small- and medium-size hands

and at a span of 6 em for the large-size hands.

Submaximal Forces during Tool Operation

Peak finger and palmar forces, averaged

over the two trigger types, are plotted against

handle span in Figure 6. Peak finger and pal-

mar forces significantly increased as handle

span increased for both subject groups (see

Tables 3 and 4). Both peak finger and palmar

forces were significantly less for a handle

600

500

•....

400

II:

•....

300

a..

HAND LENGTH

([

200

~ LARGE

•••••••• MEDIUM

-<>- SMALL

100

5 6

HANDLE SPAN (em)

Figure 5.

Average grip strength plotted against han-

dle span for the three hand size categories for univer-

sity students. Error bars represent standard error

the mean.

September 1993-561

span of 4 em than for handle spans of 6 em

and 7 em (see Tables 5 and 6). Average peak

finger and palmar force did not significantly

increase

0.05) when increasing handle

span from 5 em to 7 em for either student or

worker groups. Trigger type had a significant

effect on peak finger and palmar forces for the

student group (see Table 3). Average peak

forces were consistently greater for the con-

ventional trigger than for the extended trig-

ger (see Table 7).

The dynamometer used in this experiment

had its sensitivity directed along the horizon-

tal axis, parallel to the tool trigger and spin-

dle. Because the peak forces were less for the

small handle span than for the larger handle

spans, it may be possible that the fingers

could wrap further around the handle, direct-

ing forces away from the dynamometer axis

of sensitivity. However, because subjects

were required to squeeze the trigger in order

to operate the tool. the position of the distal

index finger was controlled throughout the

experiment regardless of the handle span.

This means that it was unlikely that addi-

tional force components were directed away

from the dynamometer axis of sensitivity and

that the decreased forces observed for the

small handle span resulted from force vectors

directed away from the axis of sensitivity of

the force transducer.

In order to replicate these findings, a fol-

low-up pilot study was performed to further

investigate how grip force may be affected by

handle span. Three university students par-

ticipated. They were instructed to grip a

strain gauge dynamometer and lift it off a

platform. A cable was attached to the dyna-

mometer for suspending different weights.

Subjects were allowed to select a finger posi-

tion that was easily reproducible. Distal fin-

ger location at the dynamometer was kept the

same for all handle spans. The locations of

the distal fingers were marked on the dyna-

mometer using a water-soluble marker.

562-September 1993

250

UNIVERSITY

STUDENTS

FACTORY

WORKERS

HUMAN FACTORS

200

150

~"AK

u..

100

PEAK

~ PALMAR

-+-

FINGER

~HOLDING

4 5 6 7

HOLDING

HANDLE SPAN (em)

Figure 6.

Average peak force and holding force plotted against handle span: (A)

university students and (B) factory workers. Error bars represent standard error of

the mean.

Subjects were presented with random com-

binations of two load weights (0.3 kg and 1.3

kg) and two handle spans (4 em and 7 em).

Three replications were made for each exper-

imental condition, and the results were aver-

TABLE 7

Summary of Significant Trigger Effects

aged. The outcome of that study showed that

subjects produced greater grip forces for the

large handle span,

F(1,2)

82.1,

0.01.

The force averaged over two load weights was

130 N

(SD

for a handle span of 4 em

Conventional Trigger Extended Trigger

Subjects

Variable

Finger Palmar Finger Palmar

University

Peak force (N)

130 (71) 153 (85) 119 (66)

142 (83)

students Peak exertion (%MVC)

50 (22) 60 (27)

47 (21) 56 (26)

Average holding force (N)

29 (16) 38 (18)

18 (10) 26 (12)

Average holding exertion level (%MVC)

13 (9)

14 (10) 8 (6) 13 (9)

Factory

Average holding force (N)

38 (11) 47 (13) 26 (9) 35 (12)

workers

Average holding exertion level (%MVC)

12 (5) 16 (6)

8 (4) 11 (5)

One standard deviation is listed in parenthesis. MVC

maximum voluntary contraction .

• p

0.05.

POWER TOOL HANDLE AND TRIGGER SIZE

September 1993-563

HANDLE SPAN

(em)

Figure 7.

(A) Average peak finger force and (B) aver-

age peak palmar force plotted against handle span for

the three hand size categories for university students.

Error bars represent standard error of the mean.

HANDLENGTH

~LARGE

-MEDIUM

-0-

SMALL

HANDLENGTH

-0-

LARGE

_MEDIUM

~SMALL

PEAK FINGER FORCE

PEAK PALMAR FORCE

HANDLE SPAN

(em)

250

200

100

~ 150

u..

why tennis players perceive that they squeeze

harder when the racquet grip is too small.

Exertion levels were computed as the ratio

of rapidly building submaximal peak forces

to fully developed static strength measured

after maximum force build-up has occurred.

The faster a muscle shortens. the less tension

it produces. Kroemer and Marras (1981)

showed that the rate of force build-up is

faster for maximum isometric exertions than

for submaximal isometric exertion. Hence

it is possible that exertion levels reported

250

200

150

u..

100

and 150 N

(SD

47 N) for a handle span of 7

cm. This pilot experiment replicated the re-

sults of the current study using even more

carefully controlled finger positions than

when operating the power tool.

The significant Length x Span. interaction

for the student group for peak finger and pal-

mar forces (see Table 3) is illustrated in Fig-

ure 7. Tukey pairwise contrast tests showed

that the small hands produced significantly

less peak finger force

(p <

0.05) and palmar

force

0.01) than did the medium and

large hands, regardless of the handle span.

Peak finger and palmar exertion levels (de-

fined as the ratio of peak force to MVC).av-

eraged over the two trigger types, are plotted

against handle span in Figure 8. Peak finger

and palmar exertion levels for a handle span

of 5 cm and 6

ern

were less than for a handle

span of 7 cm

0.05) for the student group

(see Table 5). Although hand forces for stu-

dents were at a minimum for a handle span of

4 cm, exertion level was greater for 4 cm than

it was for 5

ern

and 6 cm (see Figure 8). This

was likely because the minimum grip

strength, which was used in the denominator.

occurred for a handle span of 4 cm. Trigger

type also had a significant effect on exertion

level for the student group (see Table 3). On

average, finger and palmar exertion levels for

the conventional trigger were significantly

greater than for the extended trigger (Ta-

ble 7).

Tennis players are often warned not to

choose a racquet grip that is too small

(Brown, 1977; Gothard, 1990). Based on an-

ecdotal evidence. Gothard (1990) explained

that if a grip is too small, it causes more

stress and strain on the arm because the

player has to "squeeze harder" in order to

hold the racquet properly. The results of this

experiment indicated that although subjects

used less grip force, they produced greater ex-

ertion levels for the small (4 cm) handle span

than for any other span. This may explain

564-September 1993

HUMAN FACTORS

UNIVERSITY

STUDENTS

FACTORY

WORKERS

PEAK

--0-

PALMAR

--+-

FINGER

HOLDING

~PEAK

~HOLDING:

...J

ex:

567

4 5 6 7

HANDLE SPAN (em)

Figure 8.

Average finger exertion level and palmar exertion level plotted against

handle span for (A) university students and (B) factory workers. Error bars represent

standard error of the mean.

in this study are underestimated. Fully

developed isometric static strength (MVC)

was chosen as a reference level in the cur-

rent study because it is easy to control and

measure.

Submaximal Forces While Holding the Tool

between Operations

Tool-holding forces, averaged over the two

trigger types, are plotted against handle span

in Figure 6. Finger and palmar holding forces

for the student group were significantly af-

fected by handle span (see Table 3); however,

these effects were small (see Figure 6). Finger

and palmar holding forces for a handle span

of 4 cm were significantly less than holding

force for the other handle spans (see Table 5).

Finger and palmar holding exertion level,

averaged over two trigger types, are plotted

against handle span in Figure 8. Span had a

significant effect on palmar holding exertion

level for the student group (see Table 3). Pal-

mar holding exertion level for a handle span

of 7 em was significantly greater than for

other handle spans (see Table 5). Span had a

significant effect on both finger and palmar

holding exertions for the worker group (see

Table 4). Worker finger and palmar tool-

holding exertion levels for smaller spans were

greater than were exertion levels for larger

spans (see Table 6).

POWER TOOL HANDLE AND TRIGGER SIZE

September 1993-565

HANDLE SPAN (em)

Figure 9.

Palmar holding exertion level plotted

against handle span for university students. Error

bars represent standard error of the mean.

represents the force necessary for holding the

tool in the hand while supporting the weight

of the tool and air line (minus the counterbal-

ancing force of the spring balancer). The dif-

ference between the finger and palmar forces

represents the feed force necessary for keep-

ing the tool socket engaged. The actual forces

exerted by an operator may exceed the mini-

mum forces required for holding and over-

coming reaction forces. The handle span and

trigger effects, therefore, represent the forces

exerted by an operator in excess of the mini-

mum force requirements.

Trigger Preferences

Of the total sample, 11 of the 18 student

subjects (61%)and 5 of the 11factory workers

(45%) indicated that they preferred using the

handle with the extended trigger. Use of the

extended trigger resulted not only in less

peak finger and palmar forces during tool op-

eration but also in less finger and palmar

tool-holding forces exerted between tool op-

erations (see Table 7). The effect of trigger

type was significant for finger and palmar

holding forces for both student and worker

subject groups (see Tables 3 and 4). Average

finger tool-holding force for the student

group was 65% greater for the conventional

trigger than for the extended trigger, and

Finger and palmar holding exertion levels

for the extended trigger were significantly

less than for the conventional trigger for both

student and worker groups (see Tables 3 and

4). Exertion levels, stratified on trigger type,

are included in Table 7. These results indi-

cated that trigger type affected the manner in

which the tools were handled, even when the

tools were not being operated. When using

the extended trigger, operators may have

shifted more of the tool weight over the

crotch of the first and second digits, thereby

reducing the grip force requirements.

The Length x Span interaction (see Figure

9) for the student subjects was significant for

palmar holding exertion level (see Table 3).

Tukey pairwise contrast tests showed that

palmar holding exertion level for a handle

span of 7 cm was significantly greater for the

small-handed group than palmar holding ex-

ertion level for other spans

0.01). Palmar

holding exertion level for the small- and the

medium-handed subjects was significantly

greater than palmar holding exertion level

for the large-handed subjects, regardless of

handle span

0.05).

Comparison between Finger and Palmar Hand

Force Components

Peak palmar force was significantly greater

than peak finger force for students,

F(1,286)

6.4,p

0.0I,andfactoryworkers,F(1,174)

25.8,

0.001. Regression analysis

showed that peak finger force was linearly re-

lated to peak palmar force when regressed

over all experimental conditions for students,

F(1,142)

4467,p

0.001, and factory work-

ers,

F(1,86)

161,

0.00l.

Finger and palmar forces are mostly af-

fected by the force needed to resist the torque

reaction forces produced by the power tool.

This is apparent from the large differences in

peak forces produced during tool operation

and tool-holding forces exerted between tool

operations (see Figure 6). The holding force

...J

5 6

HAND LENGTH

~ SMALL

_ MEDIUM

--G--

LARGE

566-September 1993

average palmar tool-holding force was 48%

greater for the conventional trigger than for

the extended trigger.

Similarly, finger holding force for the in-

dustrial worker group was

45%

greater for

the conventional trigger than for the extended

trigger, and palmar holding force was 37%

greater for the conventional trigger than for

the extended trigger. Because the amount of

operating time spent holding the tool was

76%

in students and 65% in industrial subjects,

using an extended trigger may be consider-

ably beneficial throughout the work cycle.

Five of the seven students and four of the

six workers who reported they preferred the

conventional trigger explained that their de-

cision was based on the concave curvature of

the conventional trigger, rather than on trig-

ger length (see Figure 2). These subjects indi-

cated that if the extended trigger had a con-

cave surface, they would have preferred the

extended trigger. Two of the six workers who

preferred using the conventional trigger said

that their choice was based on the current

tool they operated. Individual finger force

was not measured. Therefore, it is not possi-

ble to see how force distribution among fin-

gers is changed as trigger design changed.

Hand Size and Handle Span Preference

Hand anthropometry was considered as a

factor for determining the optimum handle

TABLE 8

HUMAN FACTORS

span. The size of tennis racquet handles is

sometimes determined by the distance from

the tip of ring finger to the long line that

crosses the center of the palm (Gothard,

1990). In this experiment hand length, palm

length, and hand width were all significantly

related to the subjectively preferred handle

span and to the span resulting in the maxi-

mum grip strength for the student group (see

Table 8).

The relationship between hand length and

preferred handle span for the student group is

illustrated in Figure 10. Because no signifi-

cant difference was observed between pre-

ferred handle spans for the conventional trig-

ger and the extended trigger,

F(1,34)

0.1,

0.7, pooled preferred handle spans were

used for the regression analysis. No anthro-

pometric measurements were related to the

span resulting in the minimum peak exertion

level. No significant relationship was ob-

served between preferred span and the span

resulting in the minimum peak exertion level.

based on either finger force

0.6) or pal-

mar force

0.9), regardless of trigger type

and subject group.

These results suggest that selectable size

handles may be more desirable than having

only a single size handle for power hand

tools. Average exertion level when holding

the tool was less for the large-size hands

than for the small-size hands (see Figure 9).

Summary of Regression Relationships between Hand Anthropometry and Han-

dle Span

Independent

Dependent Variable

variable Slope Intercept

Preferred handle span Hand length

0.48

-3.21

0.59 0.00

Palm length

0.89

-3.85

0.60 0.00

Hand width

0.98

-2.59

0.48 0.00

Span at maximum Hand length

1.41 10.31 0.27

0.03

grip strength

Palm length

0.85 5.73 0.33 0.01

Hand width

0.59 4.96 0.24

0.04

POWER TOOL HANDLE AND TRIGGER SIZE

September 1993-567

HAND LENGTH (em)

Figure 10.

Preferred handle span plotted against

hand length for university students.

Holding exertion level for the large hands

was maximum for the 4·cm handle span,

whereas holding exertion level for the small

hands was maximum for the 7-cm handle

span. Furthermore, the tendency was for

large-handed subjects to subjectively prefer

larger handle spans (see Figure 10).

University Student versus Industrial Factory

Worker Subject Groups

Significant differences were observed be-

tween student and worker groups, including

grip strength, peak forces, exertion levels,

and holding forces.

Apost hoc ANOVAshowed that there was a

significant difference in grip strength be-

tween students and workers, F(1,l08)

4.8,p

0.05. Grip strength, averaged over handle

span, was 279 N

(SD

133 N) for the student

group and 327 N

(SD

90 N) for the worker

group. There were no significant grip

strength differences, however, between stu-

dent and worker groups within each hand

size, F(3,108)

0.5,

0.6.

Peak finger,

F(1,230)

39,

p <

0.001, and

palmar forces,

F(1,230)

48,

0.001, were

less for the student group than for the worker

group (see Figure 6). The difference between

peak palmar force and peak finger force was

--~, --,--,--~, --,--

16 17 18 19 20

significantly greater,

F(1,230)

24,

0.001, for workers than for students (see Fig-

ure 6). Peak finger exertion level,

F(1,230)

13, P

0.001, and palmar exertion level,

F(1,230)

21,p

0.001, were also less for the

student group than for the worker group (see

Figure 8). Finger holding force,

F(1,230)

23,

0.001, and palmar holding force,

1,230)

19,

0.001, were greater for the indus-

trial workers than for the university students

(see Figure 6). No significant differences were

observed between the student group and the

worker group for either finger holding exer-

tion level

0.8) or for palmar holding ex-

ertion level

0.9).

No significant span effects for either finger

or palmar exertion levels were observed for

the worker group; however, there was a sig-

nificant span effect for the student group (see

Figure 8). Furthermore, the effect of trigger

type was not significant (see Table 4) for the

worker group, whereas it was marginally sig-

nificant (see Table 3) for the student group.

Differences between student and worker

groups may result from distinct differences in

their anthropometric characteristics and

strength distribution (see Tables 1 and 2).

There were no subjects with small hands (L ~

17 cm) included in the worker sample,

whereas there were six subjects with small

hands in the student population. Average grip

strength for the workers was 17% greater

than for the students. Of the student subjects,

61% were female, whereas only 27% of the

worker subjects were female. Other differ-

ences were the subject sampling methods

used between the two subject populations

and differences in experience using pistol

grip power tools. Factory worker subjects

were recrui ted based on their experience us-

ing power hand tools and on the similarity

between the tools they had used and the tool

in this experiment. Biased sampling is repre-

sented well by the anthropometric differ-

ences and gender distribution.

•

TRIGGER TYPE

CONVENTIONAL

• EXTENDED

•

.2-

a..

l/)

a..

568-September 1993

It is also possible that the workers' previ-

ous experience operating pistol grip power

hand tools influenced the results. Many oper-

ators were accustomed to push-to-start pistol

grip power hand tools using a conventional

trigger. Their behavior operating these tools

in their jobs may have transferred over to this

experiment. Push-to-start tools require exert-

ing additional palm force in addition to

squeezing the trigger in order to operate the

tool. This would explain the high palmar

forces exerted by the industrial subjects (see

Figure 6). Although 15 min were given for

training and three replications were made for

each condition, this preparation may not be

long enough. Workers' experience may also

have influenced their selection of the pre-

ferred handle span. The range of preferred

handle spans for the worker group was

smaller (4.7 cm to 6.5 cm) than that of the

medium and large hands for the student

group (4.0 cm to 8.2 em). Of 11 workers, 8

commented that their preferred handle size

was similar to the tool handle size they cur-

rently used in their job. Therefore, it is rec-

ommended that future studies provide a

more suitable period of training and time to

become accustomed to these differences.

CONCLUSIONS

This study investigated the effect of han-

dle size and trigger type on forces and exer-

tions produced during pistol grip pneumatic

power tool operation using university student

and industrial worker subjects. The results

of these experiments led to the following

conclusions.

1. Handle span affected sub maximal finger

and palmar forces. Peak finger force in-

creased 24% for the student group and 30%

for the worker group as handle span in-

creased from 4 cm to 7 cm. Similarly, peak

palmar force increased 22% for both the stu-

dent and worker groups as handle span in-

creased from 4 cm to 7 cm. Although peak

HUMAN FACTORS

finger and palmar forces were minimum for a

handle span of 4 cm, grip exertion levels were

minimum for a handle span of 5 em to 6 cm.

Handle span also influenced average finger

and palmar tool-holding forces. Both finger

and palmar holding force increased 20% as

handle span increased from 4 em to 7 cm for

student subjects.

2. Hand size affected grip strength, hand

force, and exertion level. Large-handed sub-

jects produced their maximum grip strength

for a handle span of 6 em, whereas medium-

and small-handed subjects produced their

maximum strength for a handle span of 5 cm.

Small-handed subjects exerted significantly

less peak finger force and palmar force than

did medium- and large-handed subjects, re-

gardless of handle span. Holding exertion

level for large-handed subjects was maxi-

mum when using a 4-cm handle and maxi-

mum for the small-handed subjects when us-

ing a 7-cm handle. Hand size was also

proportional to preferred handle span.

3. Use of an extended trigger affected how

the tool was handled. The extended trigger

may be beneficial in terms of reduced hand

force and exertion levels during tool opera-

tion. Average peak finger and palmar forces

during tool operation for the student group

were 9% and 8%, respectively, and less for the

extended trigger than for the conventional

trigger. Average finger and palmar tool-

holding force between tool operations was

65% and 48%, respectively, and less for the

extended trigger than for the conventional

trigger. Similar effects were observed for in-

dustrial subjects.

ACKNOWLEDGMENTS

This research was supported by a grant from the Inger-

soll-Rand Company. We wish to thank the workers and

plants that participated in this study. Our appreciation is

extended to Edward Gisske for assistance in constructing

the apparatus and to John Dreger for mounting the strain

gauges. Assistance from John Blackburn, Robert Bruno,

John Estep, Steven Grall, Fred Lupton, Ronald Meister,

Daniel Mootz, Steven Thiry, William Waterbury, and

William White is also acknowledged.

POWER TOOL HANDLE AND TRIGGER SIZE

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The Authors

PASCALECARAYON

(Effect of Electronic Perfor-

mance Monitoring on Job Design and Worker Stress:

Review of the Literature and Conceptual Model) re-

ceived an engineering diploma from the Ecole Cen-

trale de Paris and a Ph.D. degree in industrial

engineering from the University of Wisconsin-

Madison, where she has been an assistant professor

of industrial engineering since 1989. She has con-

ducted research in the areas of occupational stress

and health, work organization, VDT ergonomics,

and the sociotechnical effects of new technologies.

Her research interests concern the influence of

technology on work systems, stress, and health.

NEFF WALKER

and

RICHARD CATRAMBONE

(Aggregation Bias and the Use of Regression in Eval-

uating Models of Human Perfonnance)

NEFF WALKER received his Ph.D. in psychology

from Columbia University in 1983. Since then he

has been an assistant professor at the American

University of Beirut and a visiting assistant profes-

sor at the University of Michigan. He is currently

an assistant professor at the Georgia Institute of

Technology.

RICHARD CATRAMBONE received his Ph.D. in

psychology from the University of Michigan in

1988. He is currently an assistant professor of psy-

chology at the Georgia Institute of Technology.

P.A. HANCOCK

and

J. K. CAIRD

(Experimental

Evaluation of a Model of Mental Workload)

P. A. HANCOCKattended Loughborough College

of Education, Leicestershire, England, from 1972

through 1976 and received both teacher certifica-

tion and the Bachelor of Education degree. In 1978

he was awarded the Master of Science degree from

Loughborough University, with his major area of

work concerning computer modeling of physiolog-

ical systems. He received the Ph.D. degree in 1983

from the University of Illinois, where his disserta-

tion research concerned the human perception of

time. From 1983 to 1989 he was an assistant pro-

fessor in the Departments of Safety Science and

Human Factors at the University of Southern Cal-

ifornia. In September 1989 he took an appointment

as an associate professor at the University of Min-

nesota, where he directs the Human Factors Re-

search Laboratory. He is the coeditor, with John

Flach, Jeff Caird, and Kim Vicente, of a forthcom-

ing text on the application of ecological principles

to human factors. His present research concerns

operator strategies in performance under time

stress and the use of virtual reality in human fac-

tors applications.

J. K. CAIRD received a B.S. degree in psychology

from the University of Wyoming, an M.S. degree in

kinesiology from the University of Colorado, and a

Ph.D. in human factors and cognitive science at the

University of Minnesota while on a predoctoral fel·

lowship from the university's Center for Research

in Learning, Cognition, and Perception. He recently

accepted a position at the University of Calgary,

where he is part of the Industrial/Organizational

and Ergonomic program in the Department of Psy-

chology. In addition to research on mental work-

load, he is also interested in perception-action is-

sues, transportation human factors, and interactive

interface development for scientific visualization

and virtual environment systems.

NEFF WALKER, DAVID E. MEYER,

and

JOHN

B. SMELCER

(Spatial and Temporal Characteristics

of Rapid Cursor-Positioning Movements with Elec-

tromechanical Mice in Human-Computer Interaction)

NEFF WALKER. See above for biography.

DAVIDE. MEYER is currently a professor of psy-

chology at the University of Michigan, where he

received his Ph.D. in 1969. Between 1969 and 1977,

he was a member of the technical staff at AT&T

Bell Laboratories, Murray Hill, New Jersey. His

professional activities have included service on a

National Institute of Mental Health review panel

and the editorial boards of several scientific journals.

JOHN B. SMELCER is currently an assistant

professor of management information systems at

the American University. He received his Ph.D. in

computer information systems from the University

of Michigan in 1989.

SALLIE E. GORDON, KIMBERLY A.

SCHMIERER,

and

RICHARD T. GILL

(Conceptual

Graph Analysis: Knowledge Acquisition for Instruc-

tional System Design)

SALLIE E. GORDON is an associate professor in

the Department of Psychology at the University of

Idaho. She received her Ph.D. in psychology in 1982

from the University of Illinois, Champaign-Urbana.

She taught at Wright State University and worked

572-September 1993

for Klein Associates before moving to Idaho in

1985. She has conducted research in applied cog-

nition and instructional design and is publishing a

book on the ergonomic design of training programs

(Prentice-Hall, 1994).

KIMBERLY A. SCHMIERER received her B.A.

degree in psychology from the University of Idaho

and is currently working in Boise, Idaho.

RICHARD T. GILL is an associate professor in

the Department of Mechanical Engineering at the

University of Idaho. He received his Ph.D. in indus-

trial engineering from the University of Illinois,

Champaign-Urbana. He currently teaches a variety

of courses in engineering and human factors and

conducts research on safety, warning label design,

and instructional methods.

RANDY L. SOLLENBERGER

and

PAUL MIL-

GRAM

(Effects of Stereoscopic and Rotational Dis-

plays in a Three-Dimensional Path-Tracing Task)

RANDY L. SOLLENBERGER is currently a

Ph.D. student in the Psychology Department at the

University of Toronto. He received a B.A. from

Shippensburg State University of Pennsylvania

and an M.A. from the University of Toronto. His

research interests include depth perception, hu-

man factors, and computer graphics.

PAUL MILGRAM received a B.A.Sc. in engineer-

ing science at the University of Toronto in 1970,an

M.S.E.E. degree in electrical engineering at the

Technion, Israel in 1973, and a Ph.D. in human fac-

tors/industrial engineering at the University of To-

ronto in 1980. From 1980 to 1982 he was a ZWO

Visiting Scientist and a NATO Postdoctoral Fellow

at the TNO Institute for Perception in Soesterberg,

Netherlands. From 1982 to 1986 he was a senior

research engineer at the National Aerospace Labo-

ratory in Amsterdam. Since 1986 he has been with

the Industrial Engineering Department at the Uni-

versity of Toronto, where he is now an associate

professor. His research interests include human

performance modeling, display issues in telerobot-

ics, stereoscopic video and computer graphics, psy-

chophysiological modeling, and human factors in

medicine, especially anesthesiology. He is also

president of Translucent Technologies Inc., a To-

ronto company that manufacturers and markets

PLATO liquid crystal visual occlusion spectacles,

for visual and psychomotor research.

NAUM YAKIMOFF, STEFAN MATEEFF, WAL-

TER H. EHRENSTEIN,

and

JOACHIM HOHNS-

BEIN

(Motion Extrapolation Performance: A Linear

Model Approach)

HUMAN FACTORS

NAUM YAKIMOFF received his undergraduate

degree in physics in 1968from Sofia University, his

Ph.D. in physiology in 1973 from the Soviet Acad-

emy of Sciences, and his D.Sc. in 1988 from the

Bulgarian Academy of Sciences, where he is cur-

rently a professor and Scientific Secretary General.

Since 1990he has also been a professor of the phys-

iology of vision at the New Bulgarian University.

Since 1992he has been editor-in-chief of

Acta Phys-

iologica et Pharmacologica Bulgarica.

His research

interests include the perception of form, pattern,

and motion.

STEFAN MATEEFF is a research professor at the

Institute of Physiology of the Bulgarian Academy of

Sciences, where he obtained his D.Sc. in 1990. He

received his undergraduate degree in physics in

1968 from the University of Sofia and a Candidate

of Science degree in 1973 from the Soviet Academy

of Sciences. His research interests are psychophys-

ics and human perception and performance.

WALTER H. EHREN STEIN is with the Depart-

ment of Sensory Physiology and Neurophysiology

at the Institut fur Arbeitsphysiologie at the Univer-

sity of Dortmund, where he conducts psychophys-

ical and electrophysiological research in human

perception. He also teaches at the University of

Dusseldorf. He obtained his doctorate in experi-

mental psychology in 1977 at the University of Got-

tingen and has worked at the University of Califor-

nia San Diego, the University of Konstanz, and the

University of Freiburg.

JOACHIMHOHNSBEIN graduated in physics in

1975 at the University of Bochum and received his

doctorate in physiology in 1981 at the University of

Marburg. He is on the research staff of the Institut

fur Arbeitsphysiologie at the University of Dort-

mund and since 1990 has been

Dozent

in physiology

at the University of Bochum. He is a private pilot

and conducts research in visual perception, infor-

mation processing, and human electrical and mag-

netic evoked potentials.

GRAYSONCUQLOCK-KNOPP

and

LESLIE A.

WHITAKER

(Spatial Ability and Land Navigation

under Degraded Visual Conditions)

V. GRAYSON CUQLOCK-KNOPP received a

Ph.D. in engineering psychology from the Univer-

sity of Illinois at Urbana-Champaign in 1983. She

held a postdoctoral research appointment in quan-

titative psychology at Johns Hopkins University in

1984. She is now a research psychologist in the Vi-

sual and Auditory Processes Branch of the U.S.

Army Research Laboratory, Aberdeen Proving

Ground, Maryland. Her chief research interests are

THE AUTHORS

visual performance using indirect viewing devices,

perceptual learning, and off-road navigation.

LESLIE A. WHITAKER received her Ph.D. from

Pennsylvania State University in experimental psy-

chology in 1975. She completed a postdoctoral fel-

lowship at Groton Naval Underwater Medical Re-

search Laboratory. She is currently an associate

professor at the University of Dayton. Her research

and professional interests include decision support

systems, decision making, performance, and cogni-

tive modeling.

CATHERINE BERTHELON

and

DANIEL

MESTRE

(Curvilinear Approach to an Intersection

and Visual Detection of a Collision)

CATHERINE BERTHELON obtained her Ph.D.

in psychology from the University of Provence (Aix-

Marseille I, France) in 1988. Since 1982 she has

been doing research for INRETS (French National

Institute for Research on Transportation and

Safety, Department of Accident Mechanism Analy-

sis) in the field of car driving and the evaluation of

perceptual factors in road accidents.

DANIEL MESTRE obtained his Ph.D. in psychol-

ogy from the University of Provence (Aix-Marseille

I, France) in 1987. Since 1984 he has been conduct-

ing research for the CNRS (French National Center

for Scientific Research). His research interests fo-

cus on the role of visual information during self-

motion. His interests in human factors concern the

relation between subject sensitivity to visual mo-

tion and displacement abilities.

MARK FREEDMAN, PAULZADOR,

and

LOREN

STAPLIN

(Effects of Reduced Transmittance Film on

Automobile Rear Window Visibility)

MARK FREEDMAN received a B.S. degree in

civil engineering from Drexel University in 1970

and an M.S. degree in transportation engineering

from the University of Pennsylvania in 1972. For

one year he worked as a consulting traffic engineer

and since 1973 has been conducting traffic safety

research. Currently, he is manager of transporta-

tion safety research at Westat, Inc. in Rockville,

Maryland. His research has focused on issues of

driver behavior and traffic speed. He has con-

ducted human factors research concerning driver

visual capabilities; visibility of signs, signals, and

delineation; pedestrian safety; and roadway lighting.

September 1993-573

PAULZADORholds a bachelor's degree from Ox-

ford University and a Ph.D. in mathematical statis-

tics from Stanford University. He is currently a se-

nior statistician at Westat, Inc. in Rockville,

Maryland. His research activities include analyses

of the effectiveness of laws concerning alcohol-

impaired driving, motorcycle helmet use, and day-

time running lights; comparisons of the effective-

ness of air bags and seat belts; and evaluations of

driver education programs. He also provides gen-

eral statistical and mathematical support for

Westat.

LOREN STAPLIN holds a Ph.D. in psychology

from Arizona State University. Currently, he is the

director of the Human Factors Division of The Sci-

entex Corporation in Lansdale, Pennsylvania. He is

active on the Transportation Research Board of the

National Research Council in the areas of motorist

information systems, simulation, and aging driver

needs and capabilities. His research interests in-

clude improved vehicle and highway engineering

practices through facilitation of operator percep-

tual and cognitive performance.

SEOUNGYEON OH

and

ROBERT G. RADWIN

(Pistol Grip Power Tool Handle and Trigger Size Ef-

fects on Grip Exertions and Operator Preference)

SEOUNGYEON OH is a graduate student at the

University of Wisconsin-Madison in the Depart-

ment of Industrial Engineering. She has a B.S. in

industrial engineering from the Seoul National

University and an M.S. degree from the University

of Wisconsin. She is currently a research assistant

at the University of Wisconsin-Madison, where she

is pursuing a Ph.D. degree in human factors.

ROBERT G. RADWIN is an associate professor at

the University of Wisconsin-Madison, Department

of Industrial Engineering, where he conducts re-

search in and teaches ergonomics and human fac-

tors engineering. He completed a B.S.E. in electri-

cal engineering from the Polytechnic Institute of

New York, and M.S. degrees in electrical engineer-

ing and bioengineering from the University of

Michigan, where he earned his Ph.D. in industrial

and operations engineering and was a research fel-

low at the Center for Ergonomics. He is actively

studying the causes and prevention of cumulative

trauma disorders in manual work.