Prehensor Grip for Children:
A Survey of the Literature
Julie Shaperman, MSPH, OTR
Maurice LeBlanc, MSME, CP
ABSTRACT
A survey of literature was conducted on
hand grip strength and grip strength
with prosthetic prehensors of children 2
to 4 years old. The survey's purpose was
to set grip strength requirements for a
pediatric body-powered prosthetic
hand. The survey included studies of
grip strength in normal hands, grip provided by children's voluntary opening
and voluntary closing prehensors and
structural factors that enhance grip, such
as friction, resilience and prehensor
geometry. After comparing findings
from studies in the literature survey,
minimum grip force levels of 2 lbs. for 2-year-olds and 4 lbs. for 3- and 4-year-olds were incorporated into hand design
specifications. Additional grip force, beyond the minimum, is considered highly
desirable. Incorporation of grip-enhancement features into prehensor
structures also is recommended.
Introduction
A three-phase study was conducted on
ways to increase grip in pediatric body-powered prehensors. The study was
part of a project for the Rehabilitation
Engineering Research Center on technology for children with orthopedic
disabilities. The first phase studied existing prosthetic systems and quantified
inefficiencies in prehensors, cable control systems and harnesses. The second
phase measured children's strength to
find out how much arm and shoulder
force they can produce for operation of
a body-powered prosthetic system. The
third phase defined how much grip a
young child needs in a prehensor.
These findings helped establish design criteria for an improved body-powered hand for young children. The
studies from phases one and two are
the subject of another publication (1).
This article describes the literature survey for phase three to define minimum
grip forces young children need in a
prehensor. This literature survey on
grip strength is one part of a larger
study and should be viewed within that
context.
Survey of the Literature on Grip
The literature review's objective was to
find out the minimum amount of grip
that 2- to 4-year-old children need to
perform play and self-care activities.
The researchers assumed that additional amounts of grip strength, beyond the
minimum, would be helpful and desirable, but it was necessary to first identify the minimum to provide a baseline.
Five questions guided the literature
survey on grip force:
- How much grip force do normal 2to 4-year-old children possess?
- How much grip force must prosthetic prehensors provide to hold the
objects children use for play and other
daily activities?
- How much grip force is reported in
manufacturers' specifications for externally powered prehensors, and how
much is measured in the laboratory on
young children's voluntary-opening
(VO) body-powered prehensors?
- How much grip force do young
children achieve when using voluntary closing (VC) prehensors?
- How important are factors that enhance grip even though they are not included in grip force measures (e.g., friction and resilience of prehensor surfaces, prehensor opening span, depth
and shape)?
Grip Force in Normal Hands
Very few studies were found that reported hand grip strength of 2- to 4-year-olds. Brown et al. (2) measured
hand grip strength using a commercially purchased dynamometer. Twenty-five children ages 2 to 4 years used their
four fingers and their thumbs to
squeeze a stirrup-shaped handle. The
grasp pattern appears similar to cylindrical grip. Brown reported an average
of three maximum squeezes (see Figure
1
). Forsberg et al. (3) measured palmar
pinch between the thumb and index
finger of 150 children ages 2 to 4 years
using a specially constructed apparatus
that resembled a pinch gauge and
recorded force at each surface using
strain-gauge transducers. The grasp
pattern appears similar to a two-point
palmar pinch. (It is important to note
that grip strengths shown in Figure 1
are maxima and are unlikely to be required every time a child holds an object.)
Grip Force Required for Holding Objects Used in Play and Other Daily Activities
Gottlieb (4) assembled objects used by
children of each age and used a strain-gauge instrumented hook to determine
the amount of prehension force necessary to "just hold" and to "use" these
objects. Grip forces could be adjusted
in 0.5-lb. increments. "Just hold" prehension forces were lower than "use"
prehension forces. Gottlieb reported
that 2-year-olds can hold most objects if
they have at least 2 lbs. of grip force; 3and 4-year-olds require 4 lbs. of grip
force for their activities.
Carroll (5) reported that Dorrance
VO hooks used regularly by 2- to 4year-olds had an average of 1.25 to 2.25
lbs. of grip force depending upon the
child's level of amputation. She reported that children were successfully
wearing and able to use their prostheses while performing four test tasks but
gave no indication if this amount of
grip force was adequate for the majority of daily activities.
Grip Force of Prehensors as Measured
in the Laboratory and Reported by
Manufacturers
Grip forces of three pediatric body-powered prehensors were measured in
the laboratory using a pinch gauge. (6).
b
The Dorrance lox hook was measured with two rubber bands; the
CAPP Terminal Device IC had a regular spring, and Steeper Mechanical
Hand #2 was measured as shipped.
Figure 2
shows results of grip force
measures of these four body-powered
prehensors and grip force specifications
provided by manufacturers of three externally powered prehensors (7).
It also shows that the Dorrance hook
with two rubber bands has 3 lbs. of grip
force, but it is unlikely that a 2- to 4year-old child can operate a hook with
this much rubber band loading. Figure
2 also shows that externally powered
prehensors provide a range of grip
forces from 4.5 to 10 lbs. No report was
found indicating that children need 10
lbs. of grip to perform most of their activities. Therapists who observe patients using the VASI externally powered hands have stated that 4.5 lbs. of
grip force is very adequate for most activities (8).
Grip Forces with Body-Powered Voluntary-Closing (VC) Prehensors
Occupational therapists in four clinics
(9), where VC body-powered prehensors are often prescribed agreed to
measure the maximum amount of grip
their patients could achieve. The children used Adept F and VC prehensors or Steeper Hands modified for
VC operation. Grip force was measured with a pinch gauge and recorded
along with the child's age and prehensor type (see Figure 3
). Two-year-olds
achieved 1 to 2.3 lbs. of grip force while
some 3-year-olds achieved 3 to 5 lbs. of
grip force. Therapists noted that the
Adept F prehensor has some flexibility
in the "fingers," so it was difficult to accurately measure its grip force.
Additional Factors that Enhance Grip
Factors such as friction on gripping surfaces, resilience and compressibility of
prehensor surfaces, prehensor opening
span, depth and shape are not reflected
in grip force measures, but they may influence security of grip on objects.
- Friction of the prehensor grasping
surfaces can enhance grip without requiring increased operating force (4,1011). To some extent, the friction of a
hand prehensor's cosmetic glove enhances grip. The friction of the Kraton(r) pads of CAPP Terminal Device
and the neoprene lining of Dorrance
hooks also serve this function.
- Resilience of materials in prehensor grasping surfaces and object surfaces allows their shapes to conform
and lessens reliance on grip force.
When an object is held, the flesh of the
finger/palmar areas of the human hand
is deformed to conform to the object's
shape and lessen the amount of muscle
force needed to hold the object securely(l2).
- The amount of surface contact area
is another important factor that enhances security of grip (4,12). Therefore, the wide surface contact area of
the CAPP Terminal Device should provide greater enhancement of grip than
the narrow hook fingers of the Dorrance hook. The small surface contact
area between fingertips and thumb tip
of most prosthetic hands and the objects they hold fails to take advantage
of this feature. Prehensor design often
cannot take full advantage of this feature because wide surface contact areas
may obstruct the wearer's visibility of
the objects held in the prehensor, and
good vision of objects held is important
to good function.
- Geometry of the prehensor (e.g.,
span and depth of opening, and shape
of the prehensor's holding area) as well
as configuration of the object held influence the amount of grip force needed for secure hold (4,12). If the object
shape and size fit the grasping area of
the prehensor, there is more contact
and less chance that it will slip or drop
out.
It is difficult to mathematically calculate the effects of specific amounts of
friction, resilience and surface contact
area in reducing the amount of grip
force needed to hold different types of
objects in various prehensors. Clinicians and researchers acknowledge that
increasing the above-mentioned factors
lessens dependency on grip force alone
for secure hold on objects (11).Yet clinicians and researchers still rely mainly
on measured grip force as an indicator
of good grip.
Discussion
Findings from all of the studies described in this article were reviewed to
find an answer to the original question:
How much grip must a prehensor provide for a child at the age of 2, 3 or 4
years? To answer this question, it was
necessary first to determine the minimum acceptable level of grip force that
a prehensor must provide. The minimum establishes a baseline for evaluating the data in the studies described
above. The minimum also is important
to incorporate into design requirements for a pediatric prehensor.
Gottlieb (4) and Carroll (5) reported
minimum levels of grip force needed to
perform age-appropriate activities, but
Carroll's patients were constrained by
limited strength for operating VC prehensors while Gottlieb measured grip
force requirements independent of
problems with operating force. Also,
Carroll's patients had less grip force
than Gottlieb identified as the minimum required for performance of activities at ages 2 to 4 years. Since limitations in operating force had already
been incorporated into the design specifications for the prehensor under consideration in the authors' study, the
Gottlieb data appeared more relevant
to the goal of identifying grip force
needs independent of operating force
abilities.
Next, the Gottlieb data (2 lbs. for age
2 and 4 lbs. at ages 3 to 4) were compared with findings on children wearing
VC prehensors (9) and shown in Figure
3. Those children achieved less grip
force at age 2 and equal or greater grip
force at ages 3 and 4 than the amounts
recommended by Gottlieb.
The basis for the work to improve
grip in pediatric body-powered prehensors is that very young children currently wearing and using them have insufficient grip for functional activities.
The findings of both the study on grip
strength of children wearing VO prehensors (5) and VC (9) prehensors
showed that very young children are
getting less grip with these prehensors
than Gottlieb (4) reported as the minimum required for activities at this age.
Based on these comparisons, the
Gottlieb data were selected as minimums of grip force that should be provided by a body-powered prehensor for
children between 2 and 4 years of age.
Grip forces beyond these minimums
were considered highly desirable. Also,
grip force requirements can be lowered
through the use of friction, resilience
and effective prehensor geometry.
Conclusion
Grip force requirements for a 2- to 4year-old child's prosthetic hand were
explored through a survey of literature.
Studies provided data on normal hand
grip as well as grip force of pediatric
prehensors. Findings of a study by Gottlieb (4) showed that activities of 2- to
4-year-old children require minimum
grip force of 2 lbs. for 2-year-olds and 4
lbs. for 3- to 4-year-olds. These findings
are independent of limitations in operating force that have been identified
previously. These minimum grip forces
are being incorporated into design
specification for a pediatric body-powered hand.
Acknowledgments
This work is supported in part by Grant
#H133E0015 from the National Institute
for Disability and Rehabilitation Research
(NIDRR), U.S. Department of Education,
Rancho Los Amigos Medical Center with
Donald MeNeal, PhD, and Mark Hoffer.
MD, as co-principal investigators of the Rehabilitation Engineering Research Center
on Orthopedic Technology for Children.
Opinions expressed in this article are those
of the authors and should not be construed
to represent the opinions of NIDRR.
JULIE SHAPERMAN, MSPH, OTR,
works in the Rehabilitation Engineering Research Center on technology for children
with orthopedic disabilities at Rancho Los
Amigos Medical Center She formerly was
affiliated with the Child Amputee Prosthetics
Project at UCLA and Shriner's Hospital for
Crippled Children, Los Angeles.
MAURICE LEBLANC, MSME, CP is
director of research of the Rehabilitation
Engineering Center at Packard Children's
Hospital at Stanford and is a technical consultant on the project described in this article.
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