Child Strength Study - Supporting Justification - Part B

SUPPORTING JUSTIFICATION – Part B

Child Strength Study funded by CPSC
1. BACKGROUND
The methodology and test fixtures for the proposed study are informed by those used in previous
studies of the strength capabilities of children, described below.

Exertions with Hands and Feet
Brown, Buchanan and Mandel (1973, 1974) conducted a study of strength capabilities of
children ages 2 through 6 years. The intent was to develop standards for toys and products. Two
custom devices were developed: a push-pull and pull-apart tester to quantify hand and grip
strength for a range of postures. The push-pull tester included several attachments: small
diameter knob, narrow lever covered with a rubber sleeve, pull chain connected to a lever,
twister (small diameter knob) mounted on the top and front of a testing device, and a hand
dynamometer. The pull-apart tester included two cylindrical, T-shaped handles. No adaptations
were made in the test rig to account for child anthropometry. For the push-pull measures, the
tester was secured to a table, approximately 20 inches from the floor, and child participants were
encouraged to use innovation, creativity, and volition to achieve maximum performance,
approximating a more normal play condition. For the pull-apart measures, the child participant
held the instrumented cylinder parallel and approximately perpendicular to the shoulders at chest
height and pulled the handles apart, bilaterally and with the left or right hand extended forward.
In addition to verbal encouragement, the testers included a number of colored lights that
illuminated proportional to force exerted to motivate the children. Sample size for this study was
50 children per age group.
Owings et al. (1977) conducted a study of the strength of U.S. children, ages 2-10 years, with the
intention of informing product safety design. The study included 33 isometric exertion measures
conducted on an instrumented reclining chair. Isolated joint strength was measured at wrist,
elbow, shoulder, ankle, knee, hip, and trunk. Torque was quantified about the available degrees
of freedom (e.g., shoulder flexion, extension, adduction, abduction, medial and lateral rotation).
The customized chair included a series of cantilevered beams to form an adjustable exoskeleton
that articulated in at least one plane and aligned to the center of rotation for each joint.
Anthropometric measures were taken to scale the chair to fit each child. Friction contact surfaces
and Velcro straps were used to standardize body posture without causing discomfort. To elicit
maximal voluntary effort, caregivers and research staff provided verbal encouragement and
visual feedback via a graphical display. Criterion for an acceptable measurement was defined as
an exertion sustained for 4-6 seconds that was reasonably repeatable in a test-re-test, and
representative of real-world observations in child strength. Force values were extracted using a
moving average over 1 second during the 3 seconds of the exertion. Sample size for this study
was approximately 20-30 children per age group.
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Grip strength measurements and upper extremity joint strength of children ages 2 to 10 years
were collected as part of a larger study (Owings et al., 1977). Isolated joint strength about the
elbow joint was quantified through a range of angles from flexion through to full extension.
These measures were conducted using the instrumented reclining chair and protocol described
for the whole-body joint torques. Grip strength measures included: 2-pt, 3-pt, 5-pt pinch grips,
lateral grip, and squeeze with different degrees of hand closure. Grip strength measures were
scaled to hand dimensions and performed at a range of one-centimeter increments between 2 and
9 cm. A customized grip transducer with varying handle sets was developed for this study. Force
exertions were quantified as a 2D vector, defined the direction, magnitude, location of the
resultant force in terms of normal and shear components.
Norris and Wilson (1995) compiled CHILDATA, a design resource that accumulated available
references on children to provide guidance to product design. Data on body dimensions, strength,
motor abilities, skills related to specific products, and psychological data were compiled from the
United States, United Kingdom, and the Netherlands. Strength measures included pushing
forward, pushing downwards, pushing sideways, pulling, and lifting up against a range of handle
configurations, in both vertical and horizontal orientations, and in standing and seated postures.
Hand grip strength measures included: hitting force with a fist, wrist twist, opening strength,
squeeze grip, and varying pinches. Sample sizes were relatively small for all of the measures,
typically fewer than 20 per age group.
The United Kingdom Department of Trade & Industry, Government Consumer Safety Research
(2000) conducted a strength study for design safety. Anthropometric and strength measurements
were recorded for 150 participants, ranging from 2 to 90 years of age (n=17 for children ages 25). Hand strength measures included: finger push, pinch-pull, hand grip, wrist-twisting, opening
strength, push and pull strength. Additional measures of whole-body strength were also captured
(DTI, 2002). Measures included: maximal push and pull strength, push with thumb or 2-or-more
fingers, push with shoulder, maximal pull with different grips, wrist twisting and push-and-turn
strength, pull on a can ring-pull, and press and lift with foot. Force targets were presented in a
range of size configurations, orientations and locations, defined by participant anthropometry.
Participants used their dominant hand and self-selected posture during exertions. Visual feedback
was provided throughout the protocol.
Few studies have measured strength in children younger than 24 months. Reus et al. (2013)
presented a pull-strength test based on a simulated play scenario for children 6–36 months of
age. Children were positioned in a chair with an Infant Muscle Strength (IMS) meter attached to
a metal platform with a strength sensor. The chair was adjusted based on the children’s
anthropometric characteristics so that their trunk, shoulders, and hip were fixed and their feet
could not touch the floor. Children pulled on a stiff toy held by the researcher, who provided
counter strength to evoke maximum pulling strength. In the context of that work, the team noted
that no standardized strength testing methods were available for younger children.

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Because the literature review uncovered no systematic studies of exertions with hands and feet
for children less than 24 months old, the methodology and test fixtures for the youngest children
in the proposed study will be adapted from those used for older children, with consideration for
child development patterns and capabilities.

Bite Strength
Within the CHILDATA resource, Norris & Wilson (1995) cite four studies that quantify
maximum bite force. The studies differed in the size, shape, position and material of the device
used to record maximum force. Krogman (1971) recorded bite force at the anterior (incisal) and
posterior (molar) sites within the dental arch for children ages 3-6 years. Garner and Hotwal
(1973) quantified incisive biting force (front teeth) for children ages 10-14 years. Vertical bite
force was measured by Wu (1978) for children that ranged from 18-36 months and 3-8 years.
Bite force was also measured on an instrumented test of a feeding bottle for children ages 2-3
years (CEN 1992).
Lemos et al. (2006) investigated the correlation between chewing performance and maximal bite
force in children aged 7-12 years. Bite force was measured with a pressurized rubber tube
connected to a sensor element. The tube was placed bilaterally between the posterior maxillary
and mandibular teeth. Child participants were instructed to bite the tube with maximum force for
3 repetitions, holding each exertion for 5 seconds.
Mountain et al. (2011) measured bite force in children ages 3-6 years to address a gap in the
literature on primary dentition of young children. Two hundred and five child participants were
asked to bite down for 2-3 seconds on a single tooth force gauge placed between the 1st and 2nd
primary molars and at the central incisors. A customized device was designed to accommodate a
single use parallel bite sensor prong. Results showed substantial variability both between and
within participants, with an overall range of 12 to 350 N.
Maximal occlusal bite force for children in different dentition stages were recorded in a largescale study (Owais et al. 2013). Children were stratified across the range of dentition stages and
ages 3 to 18 years. Two hundred children were measured within each dentition stage. Bite force
was quantified alternatively on the right and left side, positioned at the second primary or the
first permanent molars region. A portable occlusal force gauge consisting of a hydraulic pressure
gauge and bite element encased in a vinyl material was used. Maximal bite forces differed
significantly across the dentition stages, increasing with age. Age, gender and height were found
to be significant predictors of bite force at the later stages of dentition.
Conflicting evidence persists in defining the relationship between bite force and sex, age, size
and physical characteristics of children. The variability of bite force data is considerable with a
large number of factors influencing these results. To address this debate, Singh, Sandhu and
Kashyap (2012) investigated the relationship between bite force and facial morphology,
classified by malocclusion groups, for children ages 12 to 16 years. Bite force was measured at
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intercuspal position and anterior bite position. No significant differences were observed between
the classes of malocclusion.
Recently, Verma et al. (2017) published a comprehensive overview of bite force transducers,
covering both custom devices used in research and commercially available systems. Many of the
systems have limited applicability to child bite force measurement due to the relatively large
sizes (many more than 10 mm thick).
With guidance from the literature, the research team for the proposed study plans to fabricate a
bite dynamometer suitable for measurements with young children. The dynamometer should
enable measurement of maximum and sustained incisal and buccal bite strength, will be capable
of being sterilized, and will be covered with sterile, disposable, non-allergenic material prior to
measurement with each participant.

2. DESCRIPTION OF SAMPLING METHOD TO BE USED
The research team at UMTRI will collect data from approximately 800 children for the study.
A convenience sampling method will be used. The population of interest is children with ages
ranging from 3 months (bite strength) or 6 months (exertions with hands and feet) through 5
years. They will be recruited via their caregivers through the University of Michigan Engage site,
Craigslist, and flyers placed at UMTRI. The participants will be screened via a phone
conversation with the caregiver. Inclusion criteria will include: the targeted age group; no current
illness or injury; age appropriate cognitive and motor development, as reported by the caregiver;
and the caregiver’s ability to understand written and spoken English (older children should also
understand spoken English).

3. DESCRIPTION OF PROCEDURES FOR INFORMATION COLLECTION, INCLUDING
STATISTICAL METHODOLOGY FOR STRATIFICATION AND SAMPLE SELECTION
The target number of participants per age group in the proposed study (50 children) is the same
as that used in Brown, Buchanan and Mandel (1973, 1974) and larger than that that used in
Owings et al. (1977), U.K. Department of Trade & Industry, Government Consumer Safety
Research (2000), Reus et al. (2013), and studies referenced in Norris and Wilson (1995). In
addition, the proposed study has narrow age bands, to allow for more precise assessment of child
strength by age.
The data gathered in this study will be used for a wide range of purposes, some foreseen and
others that will arise in the future. Thus, as an exploratory research study, it is not possible or
sensible to specify a necessary level of precision for any estimates of population distribution
parameters. A calculation of sample size using data from a previous strength study and a
potential confidence bound is shown below, assuming the sample size criteria was met.
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Gajdosik (2005) published data on isometric elbow flexions and extensions, shoulder flexion,
and knee flexion and extension from a sample of 45 children ages 2 through 4 years. In this
dataset, the coefficient of variation (COV) within one-year age bins averaged 0.42. The mean
strength for the exercise with the largest mean (knee extension) was about 40 N. Using a COV of
0.4, the estimated standard deviation within an age cohort is 16 N. Estimation of mean strength
with 95% confidence and ± 2 N precision would require a minimum sample size of 246 children.
When the anticipated COV is high, large sample sizes are needed to obtain high-precision mean
estimates. For the current work, that suggests sample sizes as large as is practicable. Because the
existing data are so sparse, even samples of 25 or more per age cohort will represent a substantial
improvement, however larger samples will improve precision of population parameter estimates.
This study is not designed to be adequately powered for hypothesis testing; however, this is a
controlled, randomized, well-designed study with pre-specified criteria of interest to explore. The
results and information gathered within this study will provide a framework that future
hypothesis testing studies can ideally follow.

4. PROCEDURES
(Note: All time calculations have been included in the burden estimates):
1. At UMTRI, a research assistant will explain and demonstrate the procedures to the
caregiver and child. Written consent will be obtained from the caregivers, who will
remain with the children at all times. Researchers will obtain verbal assent from the
children who are old enough to provide it.
2. Researchers will obtain several standard anthropometric measurements from each child,
including body weight and erect standing height. For bite strength participants,
researchers will record mouth breadth and maximum mandibular opening.
3. Researchers will record the participant’s body shape using a whole-body laser scanner
(VITUS XXL) and a Microsoft Kinect sensor. The research team has used both of these
systems in several previous child studies. The laser scanner captures a high-resolution
image of the subject’s body shape in about 12 seconds. The Kinect will be used to
capture the force-exertion postures. The laser-scan data will be used to create a subjectspecific avatar that is used to aid posture tracking with the Kinect. The posture data are
valuable to characterize the tactics that the children use for each exertion.
4. In the laboratory, the children will perform a sequence of tasks to test maximal exertions
with their hands and feet. For standing tasks, they will grip a padded handle with one or
both hands, as instructed, and push or pull as hard as they can. Each exertion will be
targeted for approximately five seconds, including the ramp-up and release. Feedback
will be provided to the participants via a graphic display that shows their maximum level

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achieved, so that they can be encouraged to go beyond that level, if possible. For bite
strength, children will bite a bite dynamometer fabricated to suit the age cohort.
5. For some trials, the participant will be seated in a specially constructed laboratory chair.
Seated exertions will include pushes and pulls with the hands and pushes with one and
both feet. Conditions will be varied to avoid loading up one part of the body
consecutively. For example, a hand pull might be followed by a hand or foot push.
Researchers will determine the trials to be performed based on the result of previous pilot
testing and the ability of the child. For example, if researchers they have already
established that children 36 months and up can reliably perform standing two-hand
pushes, they will choose different types of exertions with subsequent subjects.
6. Researchers will record video and still images of the trials. The video and images will be
used by the research team to assess the children’s performance, particularly their tactics
for achieving the requested exertions. This information will be valuable for developing
the measurement protocols.
7. Trials will be separated by a minimum of 15 seconds to allow time for recovery. Longer
recovery time is anticipated to be impractical due to a loss of attention from the child, but
switching between limbs and exertion directions will reduce fatigue. The caregivers will
be engaged in every step of the process, including directing and encouraging the children.
8. The number of trials to be performed will depend on the capability and attention of the
child, but the maximum duration of a child’s participation in a single session will be two
hours. A five-minute break will be taken at least every 20 minutes to allow the child to
relax and play. The caregiver and child can take a break or discontinue participation at
any time.
9. The caregiver will be paid an incentive of $40 for up to two hours of participation.
10. Photos and video of the participants will be taken in some conditions to document their
exertion postures. The researchers will de-identify the photos by blurring or obscuring the
faces.

5. DESCRIPTION OF METHODS TO MAXIMIZE RESPONSE RATE AND TO DEAL
WITH NON-RESPONSE ISSUES
To reduce the number of no-shows, researchers will send scheduled participants a reminder letter
and/or call them on the telephone, giving the time of the session and directions to the location.
Researchers will provide $40 compensation for up to two hours of participation, as an incentive
to participate in the experiment.

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6. DESCRIBE ANY TESTS FOR PROCEDURES OR METHODS TO BE UNDERTAKEN
Initial pilot testing has been conducted, and an additional pilot study will be conducted to refine
the data collection procedures and instruments, followed by the full study. Because this pilot is
designed solely to test the study methods and not for analysis of the data, researchers will select
the pilot participants.

Limitations
The study will be based on convenience sampling and is not intended to be a representative
sample to accurately reflect the characteristics of the children living in the United States. The
sampling technique and sample size for the proposed study is consistent with previous child
strength studies. These data have been used to draw inferences about the strength capabilities of
children in the United States.

7. PROVIDE NAME AND PHONE NUMBER OF INDIVIDUALS CONSULTED ON
STATISTICAL ASPECTS OF STUDY DESIGN AND OTHER PERSONS WHO WILL
COLLECT/ANALYZE INFORMATION FOR AGENCY
Matthew P. Reed, Ph.D. (734) 936-1111. Research Professor and Head Biosciences Group,
University of Michigan Transportation Research Institute (Collect and Analyze)

REFERENCES
Brown W.C., Buchanan, C.J. and Mandel, J. (1973). A Study of the Strength Capabilities of
Children Ages Two Through Six. Washington, DC: U.S. Department of Commerce,
National Bureau of Standards. NBSIR 73-156.
Brown W.C., Buchanan, C.J. and Mandel, J. (1974). A Study of Young Children’s Pull-Apart
Strength (An Addendum to NBSIR73-156). Washington, DC: U.S. Department of
Commerce, National Bureau of Standards. NBSIR 73-424.
DTI (2000). Strength Data for Design Safety – Phase 1 (DTI/URN 00/1070). London:
Government Consumer Safety Research. Department of Trade and Industry.
DTI (2002). Strength Data for Design Safety – Phase 2 (DTI/URN 01/1433). London:
Government Consumer Safety Research. Department of Trade and Industry.
Gajdosik, C.G. (2005). Reliability of isometric force measures in young children. Pediatric
Physical Therapy, 17(4):251-257.
Norris B.J. and Wilson J.R., (1995). CHILDATA: The handbook of child measurements and
capabilities – Data for design safety. London, UK: Consumer Safety Unit Department of
Trade and Industry.
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Owings, C.L., Chaffin, D.B., Snyder, R.G. and Norcutt, R. H. (1975). Strength Characteristics of
U.S. Children for Product Safety Design. U.S. Consumer Product Safety Commission,
Bethesda, MD. 011903-F.
Owings, C.L., Norcutt, R.H., Snyder, R.G., Golomb, D.H. and Lloyd, K.Y. (1977). Gripping
Strength Measurements of Children for Product Safety Design (Contract No. CPSC-C76-0119). Prepared for the U.S. Consumer Product Safety Commission, Washington,
D.C. 014926-F.
Reus, L., van Vlimmeren, L.A., Staal, J.B., Janssen, A.J.W.M., Otten, B.J., Pelzer, B.J. and
Nijhuis-van der Sanden, M.W.G. (2013). Objective Evaluation of Muscle Strength in
Infants with Hypotonia and Muscle Weakness. Research in Developmental Disabilities,
34: 1160-1169.

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