Mechanical Work Efficiencies of Body-Powered Prehensors for Young Children



The Child Amputee Prosthetics Project (CAPP) at UCLA and Shriners' Hospital in Los Angeles is conducting a research project as part of a Rehabilitation Engineering Center grant to Rancho Los Amigos from the National Institute on Disability and Rehabilitation Research. The purpose of this biomechanical study is to improve the grip strength of children's prehensors. The study initially is being conducted in two parts, one to look at what strength young children can generate and the other to look at what strength is required to operate body-powered prostheses. As part of the determination of what strength is required for operation, the mechanical efficiencies of the prostheses must be assessed. Obviously, any inefficiencies must be overcome with greater effort by the young children wearers. As a first step in this assessment, five body-powered prosthetic prehensors currently available for young children were tested for mechanical efficiency.

Prehensors Tested

The prehensors selected for testing in the study are listed and shown in Figure 1.


Some very fine work was done by Corin, et al2 and reported in an article on "Mechanical Comparison of Terminal Devices". Among other prehensors, testing was done with the CAPP I Terminal Device, the Hosmer 10X Hook and the Steeper 2.0 Hand for young children.4,5 They did not test the NYU Child Size Hand or the Adept C II or F II prehensors. 4,5 Also, they calculated "force efficiency" (percentage ratio of the force of closing divided by the force of opening, or "force out" over "force in") but did not calculate the work in versus the work out.

Mechanical work is defined by the action of a force moving through a distance. For the simple case where the force is constant (e.g., lifting a weight vertically), the work is simply the force multiplied by the height raised. However, with prehensors, neither the input nor gripping forces are constant with excursion. In that case, the work is defined by W = F(x) - dx where W = work, F = force and x = excursion. Graphically, the work is represented by the area under the curve of force versus excursion.

Since both force and excursion are important in characterizing prehensor operation, this study calculated work efficiencies rather than force ratios.

Determining the work in and the work out is a matter of measuring the force versus excursion curve for the opening and closing of each prehensor and calculating the area under the curves. For the CAPP I Terminal Device, the Hosmer 10X Hook and the Steeper 2.0 Hand, the basic data was acquired by Corin, et al and used in this study to calculate the work in and work out and thereby determine the mechanical work efficiencies of these three prehensors. For the NYU Child Size Hand and the Adept C II and F III prehensors, no basic data existed so these prehensors were tested according to the same protocol as Corin, et al and mechanical work efficiencies determined accordingly.


See Table 1 for the summary of the testing and calculation of mechanical work efficiencies for all five prehensors. There are no big surprises. The Hosmer 1OX Hook has the highest efficiencies and the two mechanical hands (Steeper 2.0 and NYU Child Size) with gloves have the lowest efficiencies, with the CAPP I Terminal Device in between but closer to the values for the hook. All four of the above prehensors are voluntary opening (VO) in operation. The Adept is the only voluntary closing (VC) in operation and must be analyzed differently than the VO devices.


The body-powered prehensors are spring actuated in one direction and cable controlled in the other direction. A VO prehensor is opened by cable pull and closed by the spring force to grasp. A VC prehensor is opened by a light spring force and closed by cable pull for grasp. The force and excursion curves for VO and VC mechanisms can be idealized as shown in Figures 2-3.

In the case of the VO mechanism, it takes more force to open the prehensor than is returned in gripping force. There is a hysteresis or loss of force due to friction which is the inefficiency of the mechanism. With a VO mechanism the maximum gripping force is constant because it is determined by the rubber bands or metal springs for closure.

In the case of the VC mechanism, the forces of opening and closing are very low until the fingers of the prehensor contact an object. Then gripping force is exerted by cable pull to the extent desired by the wearer. The efficiency of the mechanism is not easily calculated in this case. With the VC mechanism, the gripping forces are variable at the discretion of the wearer and can be quite large if necessary for secure grasp.

Because high prehension forces can be applied, VC prehensors are becoming more popular, at least with young children using body-powered prostheses. The Adept prehensors are being used more, and Liberty Mutual (the US distributor of Steeper products) reports that they are getting requests for the Steeper 2.0 Hand to be converted from VO to VC for better function.6


The work efficiencies of hooks is much higher than hands, with the implication that mechanical hands are a prime target for redesign effort to improve operating efficiency and functional use.

There are tradeoffs between VO and VC prehensor mechanisms. The VC has the advantages of variable prehension force and low energy requirements for initial operation. The VO has the advantages of being able to hold an object without exertion and the prehensor fingers staying closed when not in operation.

The VC prehensor mechanism is a prime target to be "re-visited", compared to the VO for function, and both VO and VC analyzed for possible design improvements for optimum use by limb-deficient children.


This work is supported in part by Grant #H133E00015 from the National Institute for Disability and Rehabilitation Research (NIDRR), US Department of Education to Rancho Los Amigos Medical Center with Mark Hoffer, MD as PI and Donald McNeal, PhD as Co-PI. Opinions expressed in this article are those of the authors and should not be construed to represent the opinions or policies of NIDRR.

Journal of the Association of Children's Prosthetic-Orthotic Clinics

Vol. 27, No. 3, Winter 1992


  1. Carlson, LE, Long, MP: Quantitative Evaluation of Body-Powered Prostheses, Modeling and Control Issues in Biomechanical Systems, American Society of Mechanical Engineers, DSC-Vol. 12, BED-Vol. 11, 1-16, 1988.
  2. Corm JD, Holley, TM, Hasler, RA, Ashman, RB: Mechanical Comparison of Terminal Devices, Clinical Prosthetics & Orthotics, 11:4, Fall, 1987.
  3. The Adept prehensors are available from TRS, Inc., 1280 28th St., Suite 3, Boulder, CO 80303-1797.
  4. The CAPP I Terminal Device, Hosmer 1OX Hook and NYU Child Size Hand are available from Hosmer Dorrance Corporation, P.O. Box 37, Campbell, CA 95008.
  5. The Steeper 2.0 Hand is available from Liberty Mutual, 71 Frankland Rd., Hopkingtom, MA 01748.
  6. Williams, T., Walley, III, Liberty Mutual, personal communication, 1992.
  7. Packard Children's Hospital, Rehabilitation Center 94304 Welch Road, Palo Alto, CA