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NBSIR 73-424

A Study of Young Children's Pull-Apart

Strength (An Addendum to NBSIR73-156 - A

Study of the Strength Capahilities of

Children Ages Two Through Six)

W. C. Brown

C. J. Buchanan

Consumer Product Systems Section

Measurement Engineering Division

Institute for Applied Technology

and

J. Mandei

Institute for Materials Research

April 1974

This is a final report

U. S. DEPARTMEWT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 73-424

A STUDY OF YOUNG CHILDREN'S PULL-APART

STRENGTH (AN ADDENDUM TO NBSIR73-156-A

STUDY OF THE STRENGTH CAPABILITIES OF

CHILDREN AGES TWO THROUGH SIX)

W. C. Brown

C. J. Buchanan

Consumer Product Systems Section

Measurement Engineering Division

Institute for Applied Technology

and

J. Mandel

Institute for Materials Research

April 1974

This is a final report

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

Introduction

Each year, thousands of young children are injured as a result of

playing with unsafe or potentially hazardous toys. In order to aid in

developing reliable and realistic test methods for use in establishing

standards for children's toys, a study was conducted by members of the

staff at the National Bureau of Standards to determine the strength

capabilities of children. The main objective of the Child Strength Study

was to determine what forces young children ages 2 through 6 are capable

of exerting when pulling, pushing, twisting and squeezing. The results

of this study are found in a report "A Study of the Strength Capabilities

of Children Ages Two Through Six (NBS I R73- 1 56) . Another phase of the

child strength study involved an investigation of the forces exerted by

children when attempting to pull objects apart. It is important to have

some knowledge of these forces in order to establish design parameters

and select appropriate materials for the manufacture of safe toys. Many

accidents result when toys and other objects are pulled apart and expose

hazards, such as sharp points and jagged edges which would normally be

i naccess i b 1 e.

The child pull-apart strength study was conducted with approximately

500 children, ages two through six.-- The children participating in the

study attended rural, suburban and inner-city schools which vvere selected

at random from the various child institutes in the Washington Metropolitan

area.

^Definition of age groups studied are given in "A Study of the Strenqth

Capabilities of Children Ages Two Through bix", page 1.
- 2

-

The test device used to measure the forces exerted by the children

was a prototype instrument, designed and constructed at the National

Bureau of Standards.

Prior to beginning the Child Pull-Apart Strength Study, consultations

were held with psychologists from the National Bureau of Standards and

the National Institute of Child Health and Human Development, National

Institutes of Health. These disucssions were instrumental in designing

the test procedures used in this study.

Instrumentation Description

The pull-apart tester (Figure 1) was designed to measure the force

that children can exert when pulling an object apart. The tester consisted of an instrumented cylinder, with a T-shaped handle protruding

from each end, and a display box, which housed a series of multi-colored

lights. These lights lit up sequentially as increasing tension was

applied to the T-shaped handles. The lights served as a motivational tool

to induce the children to perform to the maximum capability.

One of the T-shaped handles was attached to an extension spring and

coupled to a rotary potentiometer. When the T-shaped handles were pulled

apart, the change in the resistance of the potentiometer, which had a

constant voltage applied across it, resulted in an electrical output

proportional to the force exerted. This output was recorded on a strip

chart recorder and later converted to units of force.

The pull-apart tester and recording system was calibrated to an

accuracy of 2.2N (0.5 lb). The calibration was checked periodically to

Insure that the data collected was as accurate as possible throughout

the testing period.
No attempt was made to adapt the instrumented cylinder to correspond

to the size of each child tested.

Test Description

Prior to being tested, the children were divided into small groups

according to age. Each group of children was tested separately and a

brief demonstration of the procedure to be used during testing was

presented by the test instructor.

Two pull-apart methods were used in the Child Pull-Apart Strength

Study. The first method consisted of holding the instrumented cylinder

parallel to the shoulders at chest height, and pulling the T-shaped

handles apart. The second pull-apart method consisted of holding the

instrumented cylinder approximately perpend 1 cu 1 ar'> to the shoulders, at

chest height, and pulling the T-shaped handles apart with either the left

or right hand extended forward.

A minimum of nine trials was allowed in the child pull-apart

strength tests. Three trials were allowed when using the "chest pullapart" method and six trials (three with the left hand extended and three

with the- right hand extended) were allowed when using the "perpendicular

pul 1-apart" method. The children were encouraged to watch the multicolored

lights illuminate sequentially as they tried to pull the handles apart

as far as possible.

"During testing the children were allowed to vary slightly from the

perpendicular plane to a position most comfortable for them. The amount

of variation did not exceed + kS° right or left and up or down.
- k

-

Test Results

The results of the pull-apart strength tests are presented In

Tables I through 4, Tables la and lb contain averages, 95th percentMes+

and 5th percentiles for each age and sex group tested in both the chest

pull-apart and perpendicular pull-apart modes. Table 2 contains standard

deviations, showing the variability among children within each group,

and coefficients of variation expressed in percentages.

in terms of the ultimate purpose of the child strength study, the

distinction between chest pull-apart strength measurements .and perpendi-

/ cular pull-apart strength measurements is of secondary importance.

Therefore, the data were condensed in the following way. For each child,

the maximum pull-apart force value was selected, regardless of whether

exerted in the chest pull-apart mode or perpendicular pull-apart mode.

A compilation of the averages, 95th and 5th percentiles was then made,

for each age and sex group tested, from the condensed pull -apart

strength data. The results of the condensed pull-apart strength data are

summarized in Figure 2. It can be seen that for the 4 through 6 year old

children, the males exerted greater forces than the females. The reverse

situation observed for 2 and 3 year old children may be the result of

sampling fluctuations. '

A comparison of the pull-apart forces exerted in the two performance

modes, by children of various age groups, leads to an interesting observation. These results are presented in Table 3 which shows the percentage

+The 95th percentile is a value such that 95 percent of the entire population of children in a particular age and sex group, when measured by a particular test, will have a pull-apart strength capability less than or

equal to this value. The definition of the 5th percentile is determined

by interchanging the numbers 95 and 5 in the preceding definition.
- 5

-

difference of the averages in the two performance modes in each age and

sex group. Children in the 2 through k year age groups exerted significantly greater force, overall, when pulling the handles apart in the

perpendicular pull-apart mode as opposed to the chest pull-apart mode.

This result contrasted markedly with the performance of the 6 year old

children whose pull-apart strength test results were just the opposite

(i.e. the 6 year old children exerted greater pull-apart force in the

chest rather than perpendicular mode). The 5 year old pull-apart strength

performance was essentially the same for either pull-apart mode.

Correlation coefficients (Table k) were calculated to show the

relationship between the two modes of the pull-apart test, as well as

the relationship between either of these modes and the strength tests

described in the report on initial phases of the child strength study

(Reference 1 )

.

Cone 1 us ions

1. As was the case with the previous child strength study which dealt

with twist, pull, push, and squeeze forces, the pull-apart strength

study yielded useful results on the strength capability of

children. These are summarized in tables of averages, 95th percentiles, and 5th percentiles for children ages 2 through 6, of both

sexes

.

2. With the exception of the 2 and 3 year old children, males showed

higher strength than females. The reversal for the lower age

groups is statistically not significant, i.e. it might be due to

sampling fluctuations.
- 6

-

3. An unexpected result was obtained in the relation between the two

modes of performing the test. For children of the lowest age group

(2 years old) the test result was significantly smaller when the pull

was exerted parallel to the shoulders than when it was exerted perpendicular to the chest. The difference decreased, however, as the

ages of the children increased, and reversed for children of age 6,

for which the direction parallel to the chest yielded larger values

than the perpendicular direction. We have no explanation for this

observation.

4. The correlation between the two modes of the pull-apart tests is

very high (correlation coefficient of 0.99), and the correlations

between either of the two modes with the strength tests carried out

in the previous child strength study (Reference 1) are all high

(correlation coefficient 0.92 or better). While these large correlation coefficients reflect in part the increase in the strength of a

child with an increase in age, they lend support to the hypothesis

that all the tests included in the child strength study measure the

basic characteristic of physical strength. Thus the strength

characteristic measured by the pull-apart test appears to be closely

related to the strength characteristics measured by the other tests.
References

Brown, Willie C; Buchanan, Jane, and Mandel , John. A Study

of the Strength Capabilities of Children Ages Two Through Six,

Report No, NBSIR 73-156, National Bureau of Standards,

Washington, D.C., U.S. Department of Commerce, 1973-

Cimons, Marlene. A Tug-of-War to Make Toys Safer, Los Angeles

Times News Service, October 1972.

Ikai, Michio, and Steinhaus, A.H. "Some Factors Modifying the

Expression of Human Strength", Journal of Applied Psychology 16:

157-163, 1950.

Jones, H.E. "The Relationship of Strength to Physique", American

Journal of Physical Anthropology 5: 29, March 19^7.

Krogman, W.M. The Manual and Oral Strengths of American White

and Negro Children, Ages 3-6 Years, Philadelphia Center for

Research in Child Growth, p. 14, September 1971.

Mandel, John. "A New Analysis of Variance Model for NonAdditive Data", Technometrics 13: 1 - 18, 1971.

Royce, J. and Smith, Leon. "Muscular Strength in Relation to

Body Composition", Annals of the New York Academy of Sciences

10: 809, September I963.

Table la. Force Values for Pull-Apart Test

Expressed in Newtons

95th Percent i les

Age (years) Chest Perpend i cu 1 a r Fema 1

e

Male Fema 1

e

Male

z 5^.7 42.7 CZi 7. / 4/. /

3 54.7 48.9 oU . U bU . (J

k 70.3 81 .8 68.

1

79.2

S 87.2 92.5 81.8 84.5

6 112.

1

122.8 94.3 100.

1

Averages

Age (years) Chest Perpend ii cu 1 ar

Fema le Male Fema le Male

2 24.9 21.4 31.1 28,

9

3 34.2 33.8 42.3 41.8

h 46.7 49.8 51.6 53.8

5 59.6 67.2 60.5 67.6

6 81 .8 85.8 73.8 79.2

5th Percent! les

Age (years

)

Chest Pe rpend

I

r. . .. cu 1 ar

Fema 1

e

Male Feme 1

e

Male

2 10.7 10.7 13.3 15.6

3 17.8 22.2 22.2 22.2

k 27.6 22.2 27.6 35.1

5 35.1 42.7 45.8 45.8

6 54.7 54.7 54.7 52.0
Table lb. Force Values for Pull-Apart Test

Expressed in Pounds

95th Percent i les

Age (years) Chest Perpend i cu 1 ar Female Male Female Male

2 12.3 9.6 12.3 9.6

3 12.3 110 13.5 13.5

k 15.8 18.4 15.3 17.8

5 19.6 20.8 18.4 19.0

6 25.2 27.6 21.2 22.7

Averages

Age (years) Chest Perpend i cular

Female Male Female Male

2 5.6 4.8 7.0 6.5

3 7.7 7.6 9.5 9.4

4 10.5 11.2 11.6 12.1

5 13.4 15.1 13.6 15.2

6 18.4 19.3 16.6 17.8

5th Percent i les

Age (years) Chest Perpendicular

Female Male Female Male

2 2.4 2.4 3.0 3.5

3 • 4.0 5.0 5.0 5.0

^ 6.2 5.0 6.2 7.9

5 7.9 9.6 10.3 10.3

6 12.3 12.3 12.3 11.7
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0 - 9 9 1

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k -8.2 -5.6

5 -0.9 -1.3

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"Chest pull apart strength minus perpendicular

pull apart strength, expressed as a percent of

the average of 2 modes.
Key for Table 4

TT Top twJst tester

FT Front twist tester

PSH Push

PUL Pul 1

Ch Chain .

K Knob

R Rubber sleeve

SQZl Squeeze, one hand

SCIZ2 Squeeze, two hands

LL Large (2") knob, left d i rect i on

LR Large (2") knob, right di rection

ML Medium (1 1/2") knob, left direction

MR Medium (I 1/2") knob, r i ght di rection

SL Small d") knob, left d i rect i on

SR Small d") knob, right d i rect i on

F Fema le

M Male
Table k. Correlation Matrix for Pull-Apart Test"

Chest Mode Perpend icul

FTSL .98 .99

FTSR .98 .98

FTML .98 .99

FTMR .98 .99

FTLL .98 .99

FTLR .98 .99

TTSL .98 .99

TTSR .93 .95

TTML .96 .98

TTMR .92 .94

TTLL .98 .99

TTLR .96 •97

KPSH .96 .98

KPUL

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RPSH .97 .98

. ^0 qft • ^0

CHPL .98 .98

SQZl .98 .99

saz2 .98 .98

Chest Mode 1 .00 .99

Perpendicular Mode .99 1 .00

"This matrix shows the correlation coefficients between

the chest and perpendicular pull-apart tests and all other

tests in the child strength study for the 10 age and sex groups studied.
Figure 1. Pu] 1 Apart Tester . This device is used to measure the

pulling forces that children are capable of exerting when

pulling an object apart. The pull apart tester (unit on the

right front) features a "T" shaped handle, which can be

pulled and twisted simultaneously, located at each end of an instrumented cylinder. A series of lights, mounted on a display box (unit at right rear) illuminate sequentially as increasing force is applied to the handles. These lights serve to motivate the test subject to perform to his maximum

capability. A strip chart recorder (unit on the left) provides a permanent record of the forces exerted by the test

subject.
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NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUMLK ATION OR REPORT NO. NBSIR 73-424

2. Gov't Ac< < Ksi<)n No.3. Rcc ipic nl 's Acc ession No.

4. TITLE AND SUHHTLl-;

A Study of Young Children's Pull-Apart Strength

(An Addendum to NBSIR73-156 - A Study of the Strength

Capabilities of Children Ages Two Through Six)

5. PubliLation F3atc April 197^

6. Performing Organization Code

7. AUTHOR(S)

W r Brnwn* fcjl WWII) CIJ i JW Burhanan* i-jLit.. iiniinii| J ' Mandel ^ _

8. Performinc Organ. Report No.

9. PHRFOR.MING ORGANIZATION NAME'AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. ^^60142

1 1. Contract/Grant No.

12. Sponsoring Organization Name and Complete Addrt-ss (Street, City, Stale, ZIP)

National Bureau of Standards

Department of Commerce

Washington, D.C. 2023'»

13. Type of Report & Period Covered

Fi nal

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The Child Pull-Apart Strength Study was conducted to provide information which

can be used to develop reliable and realistic standards and test methods for

children's toys. The study was conducted with over 500 children in the Washington

Metropolitan area, and included both black and white children from varying economic

and social backgrounds.

The pull-apart test device used is a prototype model designed and constructed

at the National Bureau of Standards. This test device was designed to measure the

force that children can exert when pulling an object apart.

The results of the pull-apart strength study are consistent with those obtained

in the previous child strength study which dealth with twisting, pulling, pushing,

and squeezing (NBS I R73- 1 56) . The study provided quantitatively precise and useful

information about the effects of age and sex on tlie pull-apart strength capability

of children ages two through six. The results are exhibited in tables of averages,

standard deviations, coefficients of variation, and 95th and 5th percentiles.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Children; Children's Strength; Pull-apart; Safety; Strength; Test Methods;

Toys; Toy Safety

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