Body powered arms [technical design overview]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Body powered arms [technical design overview]; published November 21, 2013, 07:05; URL: https://www.swisswuff.ch/tech/?p=2371.

BibTeX: @MISC{schweitzer_wolf_1571591116, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Body powered arms [technical design overview]}}, month = {November},year = {2013}, url = {https://www.swisswuff.ch/tech/?p=2371}}

Body powered arms are not the same. Despite everyone saying they understand what these are, these arms are not the same.

My setup explained, a generic setup explained. To show similarities and differences. For those in need to learn about this, technical differences to myoelectric arms are explained.


Compared to a generic body powered arm (left), my own setup (right) differs in relevant aspects.

A. Shoulder anchor

Normal body powered arms are typically equipped with a prosthetic manufacturer's average type harness. Otto Bock's standard plastic sheath and fabric strap contain three serious problems, though: firstly, they compress the brachial plexus and arm nerves. To the point where that hand goes numb. Secondly, they sell you some plastic that starts to stink real bad of sweat. Trying their recommended cleaning solution does not make things any better. Thirdly, to extend the arm and to get the terminal device to open, one must extend the wrist for about 10-12 cm at least. That makes for awkward movements.

My body powered arm contains a custom made shoulder anchor that is wide, soft to deformation but stiff to elongation. With that, the open/close control range for the prosthetic terminal device is 4,5 to 5 cm. That allows for far more relaxed, tighter control with considerably less awkward motions. Also, that shoulder device does not compress the nerves or brachial plexus. And it is made of material that does not pick up the smell of sweat.

Its development was motivated by me suffering from first signs of numbness on my left arm and hand. A neurologist and me figured that the standard issue harness was the problem. Disability insurance funded my prosthetists to develop a new way for mounting the cable on my left shoulder. The critical parts were choice of materials (the shoulder component now is made of around 3 different materials or so: a type of plastic, a type of stiffer material and metal) and shape (which took time to figure out).

(A myoelectric arm has no shoulder anchor or cable. The shoulder anchor and cable of the body powered arm's control allows for time-wise unlimited, body powered, finely tuned, very precise, limited touch- and force-feedback enabled grip function no myoelectric arm can ever attain. The requirement to wear a body powered arm thus are a sufficiently long stump, sufficient muscle quantity, sufficient motor control, and the interest and willingness to use extremely intuitive and direct motion to control the prosthesis.)

B. Cable and cable sheath

Normal body powered arms have a tube system mounted on the body somehow. Otto Bock systems contain a plastic string. The problem of that string is that it rips real fast and with that, it cannot be fixed by the amputee as these parts are all "special". They are relatively easy to mount as the technician just squeezes some parts together, so the overall work is rather affordable. Metal strings as well as plastic cables are protected by fixed frame mounted tubes or sheaths. Cables then rip against the sheaths. That causes them damage. In extreme cases, I got cables that were repaired to break - minutes after the repair, still in the prosthetic technician's shop. Normally, my cables would break after 4-14 days of usage. With the cable tubes being screwed or bolted to the socket, one always had to take the whole arm in for repairs.

My body powered arm contains a different cable system that I figured out myself, after consulting with a range of cable specialists throughout industries. The cable itself is a 1-2mm steel cable. My cable sheath or tubing is set up in a very peculiar non-obvious way as it protects the cable through a complex force vector distribution, thus avoiding stationary damage to both tube or cable. With my cable setup, I can use massively higher cable forces (and thus, grip forces). I also get some 6-9 months or more of service free cable operation out of this. Also, the assembly is modular, so I can easily repair it, or swap parts, myself.

C. Socket

Normal body powered systems are fully wired up. All parts are laminated and bolted together to yield one continuous appearance. Originally, I had been equipped with Ossur liners that ended up causing me congestion eczema at the tip of my stump.

My socket contains a modular setup. So I can isolate and swap the whole cable setup fast. I wear Alpha gel liners that are heat dilatable to adjust their shape.

(Myoelectric arms require that the electrodes are precisely placed over the stump regions where they work best. For that, usually a hard socket is used. To suspend a hard socket, usually an elbow hinge or Munster type suspension is employed which in other words means that the arm hangs on the elbow. If your elbow is a sensitive area, like in my instance, wearing a myo arm spells out as pain. Also, I cannot lift more than very small weights with such a socket. Usually, academic researchers love myoelectric technology for some reason, and they do not care about pain in their test subjects, so that is that, and it needs to be mentioned here. Also, opening and closing the prosthesis requires activation of flexor and extensor stump muscles. My flexor and extensor tendons are joined over the bone ends of my stump and moving them a lot ends up being very painful there. Academic researchers do not care about that a lot. They also do not believe that, as I am an amputee and what do amputees know, and when I participated in a myoelectric arm research project once, my stump end became highly irritated which boosted stump pains to very high levels. The researchers did not believe that being the researchers they were, and then asked a neuroscientist, who did not believe that either. They then asked an orthopedic surgeon who said it was indeed a problem, and I should rest the stump. Since then I am critical towards academic researchers as they often lack knowledge that I find very critical. What would be a sentence with the word 'inconsiderate' in this context?)

D. Wrist unit

Normal body powered arms contain stock components such as Otto Bock wrists. One of these - the Otto Bock Movowrist - jammed irrversibly with my very first test socket while still at the prosthetist's premises. That was quite embarrassing. So I was equipped with another Otto Bock wrist that dilated over a number of weeks.

My body powered arm now contains a quick lock mount that was custom built from scratch. It works like drill hammer quick locks or chucks. In fact, it was designed and custom built with one of the most experienced custom chuck manufacturers we have in the area. It remains stable for years, despite thousands of hard bangs everyday using industry strength keyboards, and despite heavy weight pulls.

E. Terminal devices

Normal body powered arms may come with no particular or unbranded terminal devices. Even Otto Bock hooks were known to fall apart faster than anyone would have liked due to their construction. The Otto Bock Movohook 2Grip would contain a joint that had raw metal parts scraping on a whitish plastic disc (no ball joint aspects whatsoever), and that had a metal lever that over the course of weeks to months would indent the hook body to the point of minimal failure to fully close the hook, so thin papers would fall out. The cheapest solution was shortening the spring and getting brass joint inlays custom made. Later I abandoned these for better hook devices. Otto Bock voluntary opening hands would decay relatively rapidly to a point where the spring mechanism would fail; a repair then would cost as much as a new hand, and with that the question was whether any of that was worth it given that no regular normal glove would fit an Otto Bock hand.

I now prefer other hook devices (Hosmer, Toughware PRX) and Becker hands. I found these to cover my requirements a lot better than other parts. Hosmer hooks are extremely nice and durable. Becker hands are very durable, have a great look, work very well with everyday tasks due to their well set up adaptive grip. Also, Becker hands fits most standard gloves, protective gloves or work gloves found in hardware shops or anywhere else. The V2P is easy to tweak, and with tweaking its surface, the grip characteristic can be improved a lot. For bicycle riding, I now use a Mert hand.


If one was to assume that component manufacturers and prosthetists have a full insight into the technical aspects of what they are doing, it is not hard to argue that normal body powered arms are systematically built to fail.

In fact, we read this [1]:

While it used to be fact that cable operated systems were indeed more reliable than their electrically powered counterparts, it can now be argued that the incidence of repairs for both types of systems are nearly equal and the disparity now rests on the cost of repairs, rather than the frequency. This is extremely important to note, as proximity to a prosthetic facility was once a defining factor in deciding the use of a control system. As it is being discovered that cable operated systems break down frequently under heavy use and that even a minor failure will typically result in a visit to the local prosthetist, more of the focus should be on providing a secondary prosthesis in the advent of such an event, rather than confining an otherwise perfect candidate for electric power to an alternative control system. Of course, exposure to severe impact or immersion in water are to be avoided as with most electronic devices and electrical interference, although not as significant as in previous designs, still poses a problem in some cases. (...) Cognitive ability is certainly an area of concern with the use of electrically powered prostheses, particularly with more complex systems incorporating strategies that require precise timing or strength of contractions, or that involve multiple contractions to achieve a desired result. Battery management is also an issue for the cognitively challenged and activities such as inserting and removing batteries (a challenge for most prosthetic users of externally mounted battery systems), safe and effective storage and charging and keeping track of a battery's charge state are all tasks that demand a certain level of cognition in order to assure a successful prosthetic outcome. Unfortunately, there is no absolute standard
or tool for objectively determining whether a particular individual has sufficient apperception to successfully utilize an electric prosthesis other than simple observation of specific tasks.

It is far more apparent that cognitive skills and experience outside some small time tinkering are indeed necessary in order to build a well working and functionally good body powered arm. Not just anyone can walk in and do that. It is a fact that prosthetists nowadays push myoelectric arms also because these can cap all amputees' experiences to low expectation levels, hence making life easier for prosthetists. Secondly, a +30% surcharge is often charged on any hardware, and selling body powered parts (1500 CHF) or myoelectric hardware (45000 CHF upwards) may make all the difference to motivate prosthetists to state anything these days, as long as it makes them generate income.

If one assumes that component manufacturers or prosthetists lack full insight into what they are doing, we should seriously question the overseeing bodies - for manufacturers, those are insurances and label issuing firms - and the people that train them.

It certainly took me about three years to see to the bottom of these issues, and to try to organize solutions, or come up with solutions, or to interact with others towards finding a solution.

Now my body powered arm works well, and it remains intact, for considerable amounts of time. Thank you for everybody that helped with it.

Update 2018 - we recently wrote up the experiences with comparison and tweakings, of both body powered and myoelectric technology. Check here: publication [link]

[1] [doi] R. D. Alley and H. H. Sears, "Powered Upper Limb Prosthetics in Adults," in Powered Upper Limb Prostheses, A. Muzumdar, Ed., Springer Berlin Heidelberg, 2004, pp. 117-145.
booktitle={{Powered Upper Limb Prostheses}},
editor={Muzumdar, Ashok},
title={{Powered Upper Limb Prosthetics in Adults}},
publisher={Springer Berlin Heidelberg},
author={Alley, R.D. and Sears, H.H.},
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