Upper Limb Strength of Young Limb Deficient Children as a Factor in Using Body Powered Terminal Devices-A Pilot Study


Purpose of the Study

Preschool aged limb deficient children using body-powered terminal devices often find that their terminal devices do not provide enough grip for good function. The problem of poor grip is especially evident when children are just learning to operate controls of a body power system, but it persists through much of the toddler period. With voluntary opening (VO) terminal devices, young children cannot overcome the resistance of enough loading on the terminal device to give good grip, and with voluntary closing (VC) terminal devices children need to learn to exert continuous force to hold the terminal device closed. Usually, by the time the children start school, they are able to get good grip from VO or VC terminal devices, and the extreme simplicity, light weight, reliability and low cost of body power would make this system an attractive option for young children, if some way could be found to improve grip in the early years.

Previous efforts to increase grip in children's body powered terminal devices have resulted in some improvements, such as the use of frictional compressible materials to enhance grip as a substitute for heavier terminal device loading. Still, the underlying problem has not been solved. The investigators considered it important to analyze and quantify all underlying factors before proceeding with more technology development. The approach in this investigation is to compare "power in" (body power available for control) and "power out" (power requirements for prehension). Then, interventions and/or a research and development program could be targeted at correcting problem areas.

The investigators initiated a study to learn more about the power-in/powerout equation under a Pediatric Rehabilitation Engineering Center grant from NIDRR (National Institute for Disability Rehabilitation and Research) to Los Amigos Research and Education Institute in Downey, CA. A review of reports and published literature yielded no studies that included objective measures of young limb deficient children's body power sources for operating a terminal device. The investigators initiated a pilot study to measure the amount of body power that young children with unilateral congenital below-elbow limb deficiencies have available for operating a terminal device. These studies are underway at the Child Amputee Prosthetics Projects (CAPP) at UCLA and Shriner's Hospitals, Los Angeles. The findings of that pilot study on children's strength are reported here. The "power out" studies on terminal devices, cable and harness systems are underway at Packard Children's Hospital at Stanford and will be reported elsewhere.

Study of Children's Strength

As a first step, the investigators searched for data on normal children's arm and shoulder strength. They reasoned that if they could measure arm and shoulder strength of a small sample of limb deficient children and show that it coincided with that reported for normal children, they could use the data on normals for the "power in" portion of the equation. The expectation, based on clinical observations of limb deficient children, was that their strength was near or close to normal, especially on the sound side.

The investigators found two studies that included specific measurements of normal children's strength. Sykanda2 reported standards for isometric muscle forces of shoulder (humeral) flexion in percentiles in relation to height, and Backman1 reported standards for isometric muscle force for shoulder (humeral) abduction as means and standard deviations in relation to children's ages. Both investigators report strength data separately for boys and girls. The strength data are based on measures of shoulder (humeral) flexion and abduction with the arm (humerus) at ninety degrees of flexion and ninety degrees of abduction respectively. Since prostheses are usually controlled with motions close to the body, the investigators measured these two shoulder motions in both the ninety degree and zero degree positions. Besides the studies mentioned here, no others were found with data on normal children's strength.


The investigators selected four motions with potential for operating a body powered terminal device: shoulder (humeral) flexion, shoulder (humeral) abduction, shoulder girdle elevation and shoulder girdle protraction (abduction). Fourteen children (seven boys and seven girls) with unilateral congenital below-elbow limb deficiencies between the ages of three years, three months and six years, six months were located in the CAPP Clinic lists. Children were excluded if they had other neuromuscular limitations, including other limb deficiencies. No other selection criteria were included except willingness to participate in the study. Eight of the fourteen children (six girls and two boys) have prostheses. Although some children under the age of three years might have been able to cooperate with the study protocol, no standardized strength data were available for comparison, and early attempts to get these children to cooperate with the protocol convinced the investigators that most two-year-old children would not understand the task well enough to provide reliable data.

The studies reporting strength standards were conducted using a Penny & Giles Myometer and following a measurement protocol described in the reports by Sykanda3 and Backman.1

The myometer is a portable electronic dynamometer. The force transducer is hand held, and the peak force appears as a digital display in kilograms on the measuring unit. The readings are given to one decimal place with accuracy to ±0.3 Kg. of force. The child was seated in an appropriately sized straight chair with a back. The child's trunk was immobilized in the chair using a posture vest. Each measure was repeated three times and the average of the three measures was reported. For example, the measure of humeral flexion required the child to hold the arm in ninety degrees of flexion at the shoulder, the therapist stabilized the applicator pad of the myometer at the distal end of the humerus and the child was told to sustain the position. The therapist said, "don't let me push your arm down, hold it there." The therapist used the myometer to maximally resist the force exerted by the child (Fig. 1. ). The myometer registered the peak force of the child's contraction in kilograms. To measure shoulder girdle motions the investigator stabilized the applicator pad of the myometer over the tip of the acromion for shoulder girdle elevation and over the anterior aspect of the head of the humerus for shoulder girdle protraction (abduction). During all measurements, the therapist observed the child's position to avoid measuring substitute motions. Strength measures of shoulder (humeral) flexion and abduction of the fourteen subjects (Table 1 ) were compared to the standard strength data appropriate for each child's age and height (Table 2 ).1 The comparison is graphically illustrated in figures 2 ,3 ,4 and 5 .


Strength of shoulder (humeral) flexion and abduction for the fourteen children (seven girls and seven boys) was much lower than that of normal children on both the limb deficient and sound sides (Table 1 ). In shoulder (humeral) flexion on the limb deficient side, 11 children (six girls and five boys) had strength below the fifth percentile of the standardized data. For this motion on the sound side, nine children (six boys and three girls) had strength below the fifth percentile of the standardized strength data. For those children who reached the fifth percentile, none reached as high as the fiftieth percentile on either side of the body. Among girls, the two tallest, and oldest, had almost the same amount of strength in shoulder flexion on the sound side as the shortest (youngest) girl. Among boys, the tallest, and oldest, boys were also strongest and had shoulder flexion strength at or near thefifth percentile on both sides of the body.

In shoulder (humeral) abduction on the limb deficient side most children's strength was less than -2SD (minus two standard deviations) below the mean. Only one boy was strong enough to reach the mean in shoulder abduction on the limb deficient side. On the sound side, strength of four children (two girls and two boys) reached slightly above the - 2SD level, but no one reached the mean.


For most children, the differences between strength at ninety degrees and zero degrees in shoulder (humeral) flexion and abduction were relatively small. No consistent relationship was evident between strength measurements at zero degrees and those at ninety degrees; some children were stronger at zero degrees and some were stronger at ninety degrees. Therefore, it seems reasonable to accept the strength measures at ninety degrees as reflective of strength at zero degrees.

While no standards were available for comparison of shoulder girdle strength measurements, shoulder girdle elevation was the strongest of all motions measured, and eight children (57%) had greater strength in this motion on the limb deficient than on the sound side.

It was interesting to examine findings in relation to prosthesis wear. Two thirds of children who were stronger on the limb deficient side in shoulder flexion and shoulder girdle abduction, and all children who were stronger on the limb deficient side in shoulder abduction, do have prostheses. This suggests that wearing and using a body powered prosthesis may help to strengthen these motions. It was not possible to see if this effect was more pronounced among boys or girls since all but one girl has a prosthesis and only two boys have prostheses.


In summary, comparison of findings with standards for shoulder (humeral) flexion and abduction at ninety degrees shows that the majority of the children's strength is much less than normal on both the limb deficient and sound sides, with the few exceptions noted in the analysis above. Very small differences were demonstrated when strength was measured at zero degrees of flexion and abduction. The analysis of motions also suggests that increased strength may be associated with having a prosthesis. The pilot study clearly showed that standardized data may not be used in the power-in, power-out equation to analyze body powered systems. The pilot study raised several questions that need further study:

  1. The pilot study included only fourteen children. While the study does suggest that limb deficient children have much less strength in the arms and shoulders than normal children, the measurements did not seem sufficient to warrant conclusions on the amount of power that actually is available in various upper limb motions for operating a body powered system. The overall population of limb deficient children is relatively small, and the investigators have access to a limited number of very young limb deficient children who do not have other problems that might invalidate strength measurements. Nevertheless, the investigators believe that measurements are needed on at least twenty children before the data may be confidently used as the basis for further research on terminal devices.
  2. Some investigators have expressed concern about the use of standardized strength data that included only populations in Canada and Scandinavia. It was recommended that we gather additional strength data from normal populations of children in the Southwestern USA (geographical location of this study). Also, we need to standardize data on shoulder girdle strength to make our measurements of limb deficient children more meaningful.
  3. It is possible that some factors related to amounts and types of play activity may account for differences in strength among limb deficient children compared to other children, this needs further study. It may be possible to increase young children's strength through an intervention. If we knew more about young limb deficient children's play and the effects of an intervention to increase strength, we could describe clinical procedures that would promote greater strength at an early age.

Future Plans

The investigators have started a second phase of the study to answer the questions raised above. The study has three objectives: (1) to gather strength data on at least twenty young limb deficient children; (2) to study the types and amounts of limb deficient children's play activities that might affect upper limb strength and to learn whether increasing strenuous activities that affect the shoulders will increase strength; (3) to establish strength data standards on normal preschool-aged children in California. A group of University of Southern California graduate students in Physical Therapy is measuring strength of all the motions of interest on a large number of young preschool children in Southern California.

The strength measurement data from these studies will be incorporated into the "power in," "power out" equation and made available to researchers who may use it to establish design criteria for improvement of existing body powered terminal devices or for new systems. The information on children's play and the outcome of the study on increasing strength through play will be provided to clinicians with recommendations for patient care. Overall, the study findings may offer additional insights that will be useful in making decisions on prescription of terminal devices for young limb deficient children.


This study was funded by a grant from the National Institute on Disability and Rehabilitation Research (NIDRR), U.S. Department of Education, to Los Amigos Research and Education Foundation for a Rehabilitation Engineering Center on Technology for Children with Orthopedic Disabilities.

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  1. Backman, E, Odenrick, P, Henriksson, KG, Ledin, T: Isometric muscle force and anthropometric values in normal children aged 3.5 and 15 years. Scandinavian Journal of Rehabilitation Medicine, 21:105-114, 1989.
  2. Sykanda, AM, Armstrong, RW: Standards for Evaluating Muscle Strength in Children Using the Myometer. Poster Session, APTA/CPTA Joint Meeting, Las Vegas, 1988.
  3. Sykanda, AM, Armstrong, RW: Measurement of muscle strength: Development of norms in children of school age using the myometer. Unpublished report, British Columbia Children's Hospital, Vancouver, BC, Canada, 1989.