Go to content Go to navigation

Note: This text is a work in progress. If you find any inaccuracies or material that needs to be included, please contact us.


Due to their reliability, durability, and relatively low cost, body powered hooks and hands are the most popular upper extremity prostheses. They are controlled by a harness that captures an unrelated movement of the user’s body, such as a shrugging of the shoulders, and transfers this movement through a cable system to a terminal device such as a hand or hook. Body power is also used to control elbows and other motions in hybrid electronic/mechanical systems.

There are two major types of body powered terminal devices. A voluntary opening device is held closed by springs or rubber bands and the user moves to open it. This mechanism makes it easy to hold and move objects without effort, but has a grip strength limited to the strength of the springs. A voluntary closing device is held open by springs and the user moves to close it. This allows for a strong grip but requires effort to hold and move objects.

To get a feel for the range of available terminal devices, see the following manufacturer catalog pages:

An interesting prototype hook, the vector prehensor, is a voluntary opening device with highly adjustable grip force. It works by adjusting the angle between the spring and the hook. The authors filed for a patent but the prior art search turned up a similar 1954 German patent. Authors Lawrence Carlson and Dan Frey have generously agreed to let us publish their design on this website. It looks like a great product, and we can’t wait to see it become available for use. Carlson and Frey did get patents on some other interesting ideas: a locking mechanism and a hand that grips by rotating the thumb.

Concept development

One way to achieve the advantages of voluntary opening and voluntary closing mechanisms in the same device is to somehow switch between the two modes. This could be done manually, but we feel a simpler method would be to have the device use part of the user’s motion for one mode and part of it for the other. The following illustrates one simple way of doing this:

Slotted cam mechanism for mode switching

  1. The cable is relaxed and the hook is completely closed.
  2. The cam pushes the hook open against the closing spring’s tension, operating exactly like a voluntary opening device.
  3. The cam begins to pull the hook closed with the closing spring and return spring almost balanced, operating exactly like a voluntary closing device.

Apart from the illustrated pin and slot, this method relies on two opposing springs: a closing spring which tries to pull the hook closed and a return spring which tries to pull the cable back to its neutral position. The return spring ensures that when the cable is released the device always returns to its initial state even if it has to stretch the closing spring to do it. If this is unclear, please see the videos of one of our prototypes doing this.

This mechanism has some interesting possibilities, such as shaping the slot and/or making the cable wind onto a shaped pulley to control the mechanical advantage at any given point in the cycle. However, there are some practical problems when implementing it. The main problem is one of space: the slot neccessarily takes up a lot of longitudinal room. If the hand is made to be interchangable with others on the market, it has a certain maximum length, and the parameters of its motion are fairly fixed. Specifically, it needs to provide a four inch opening using two inches of cable pull. If the cable pull is split between opening and closing, only one inch of cable pull has to produce a four inch opening. All of these constraints make the mechanism difficult to pack into the five or six inches of available length.

We are working on some alternative mechanisms which might fit into the space better. One of our ideas is to allow the user to manually switch between voluntary opening mode and voluntary closing mode, possibly in combination with a closing force adjustment like the vector prehensor. We came up with a number of mechanisms to do this, but most needed large wheels or levers to provide the correct mechanical disadvantage. Our most successful idea so far employs a combination of bevel gears and a pulley:

A mode switching mechanism using bevel gears

  1. Voluntary opening mode – The cable turns a pulley which drives the moving finger through the pinion on the right.
  2. Voluntary closing mode – The shaft slides so that the moving finger is driven by the pinion on the left.

The parts in the illustration are scaled to deliver the proper mechanical advantage and a to-scale penny is included to indicate the potential compactness of the mechanism. We have not yet determined whether gears this size will bear high enough loads to last, but the basic idea seems promising.

Other Ideas


Our first prototype is made with LEGO® TECHNIC® parts and uses a cam sliding over a bar to get the bidirectional motion:

Cam side of hook prototype 1Spring side of hook prototype 1

The second prototype is a refined LEGO version which works more smoothly and can actually lift small objects:

Closeup of cam side of hook prototype 2Cam side of hook prototype 2
Closeup of spring side of hook prototype 2Spring side of hook prototype 2

Here are some videos of the second prototype operating:

All content and designs on this site are in the public domain, and we place no restrictions on their use. We encourage any derivative works, but all designs are registered periodically so that our work cannot be kept from the public by patents.