Avoiding fatigue from flexion

For discussion of hand designs.
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Avoiding fatigue from flexion

Postby peridotclover » Mon Jun 15, 2015 9:20 pm

Hi! My name is Iris Lin, and I am a Biomedical engineering intern / Second year medical student at the University of Virginia. I have been working with a team here on printing and assembling E-nable designs. After assembling our last hand, a Raptor Reloaded, I brought the completed assembly to get some input from an Orthopedic surgeon we are working with, and from that meeting, I had a design consideration that I was wondering if anyone has found a solution to?

Under the current design of the Raptor Reloaded, in order to grasp something, the user flexes the wrist, which leads to flexion of the fingers. This means that in order to pick up something for an extended period of time (e.g. holding a spatula while cooking), one must keep the wrist flexed during the duration of the grasping action. The problem is, I would imagine that the extended period that the user must exert force to keep the wrist flexed could easily lead to fatigue or soreness in the wrist / forearm.

I experimented on switching the tension cords on the Raptor reloaded, so that the resting position of the hand would be a grasping position, and flexion of the wrist would result in extension of the fingers; however, this resulted in a pretty weak grip strength. Has anyone designed solutions that might incorporate either a locking mechanism that would allow the user to lock the hand in the grasping position, or a way for the maintenance of the grip to require less than continual force from the user?

Thanks! :)

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Re: Avoiding fatigue from flexion

Postby AdamArabian » Wed Jun 17, 2015 6:28 am

You, my friend, have hit upon one of our holy grails.

A voluntary open hand would be excellent but it would demand an enormous amount of strength in the forearm muscles in order to have any reasonable grip strength.

Locking mechanisms have been tried (if you search on the google+ group you will find some). We have tried a rolling latch, slide latch and a latch at the palm/bracer joint. Slide or rolling latches are the most likely to work but are tricky and tend to let the cable slide. The ratchet at the palm/bracer works but it forces you to have your hand at an awkward angle for a prolonged period, which is also sub-optimal.

If you can get this figured out you get a big virtual trophy though!

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Re: Avoiding fatigue from flexion

Postby Bocaot » Mon Jun 22, 2015 11:46 pm

Usually you have more grip strength when wrist is extended about 20 degrees. Also the normal functional mobility of the wrist/hand is tenodesis which is when your wrist is flexed the digits extend and when the wrist is extended the digits flex. So maybe the tension on the wires have to be changed

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Re: Avoiding fatigue from flexion

Postby mhobsoncnm » Fri Jul 10, 2015 11:09 am

Hi Iris - I just joined this forum hoping to find someone at UVA who might be able to help my daughter get a hand, and yours was literally the first post I clicked on. She is 14 and was born missing her right hand - she has a shortened forearm, wrist bones, small palm, and nubbins. I adopted her in China when she was 2 years old, and she has always been able to do everything she has attempted. We worked with the UVA prosthetics department who made her a prosthetic arm to play the cello, and she has never wanted any other device until now. She saw the Talon hand on the e-nable website and thinks it looks cool. If you would be interested in meeting please email me at mhobsoncnm@gmail.com. My daughter's name is Lally and we live here in Charlottesville. Thanks!

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Location: Falmouth, KY

Re: Avoiding fatigue from flexion

Postby GreggDennison » Fri Jul 10, 2015 1:30 pm

Here is something I worked on a while ago. I would love to get this developed better. The first is a spring loaded design that keeps the hand open all the time but could cause fatigue while holding an object. The second video shows how we reversed the spring to keep the hand closed all the time, you just flex to open and grab something. Depending on the strength of the spring affects the grip strength. I've been trying to drum up interest in this for over 6 months now. There are no strings or elastics and we did use one strap of velcro in the palm bit that could possibly be eliminated with a thermomesh palm.



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Re: Avoiding fatigue from flexion

Postby ARKtest » Fri Jul 10, 2015 1:58 pm

I'm new to the hand designs but if I understand the issue right is it not that the amount of grip being directly related to the amount the wrist is flexed down past a resting, straight, orientation? So to relieve the awkward tension of holding things for a long period of time, would it not be better to push the rotation back so that opening the hand is done by pulling the wrist up, and closing the hand by pushing it back to normal. Forcing it past normal would be trying to squeeze the hand shut as hard as possible.

Like I said, I'm new to the designs so let me know if this has already been tried/failed.

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Re: Avoiding fatigue from flexion

Postby archinaut » Mon Sep 14, 2015 1:04 am

Hello, my name Is Dario Ameni and I'm just starting to work on this project, and I had the same thoughts...so to deal with this I'm working on a design that Keeps the hand, by default, in a closed grip that opens when the wrist is flexed. Has anyone here tried this approach before?

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