ADL learning and body powered prosthesis control [paper review]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - ADL learning and body powered prosthesis control [paper review]; published October 9, 2016, 11:42; URL: https://www.swisswuff.ch/tech/?p=6484.

BibTeX: @MISC{schweitzer_wolf_1571505689, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - ADL learning and body powered prosthesis control [paper review]}}, month = {October},year = {2016}, url = {https://www.swisswuff.ch/tech/?p=6484}}

Learning to use a body-powered prosthesis: changes in functionality and kinematics. Laura H. B. Huinink, Hanneke Bouwsema, Dick H. Plettenburg, Corry K. van der Sluis and Raoul M. Bongers. Journal of NeuroEngineering and Rehabilitation 2016 13:90.

Abstract [1]

Background: Little is known about action-perception learning processes underlying prosthetic skills in body-powered prosthesis users. Body-powered prostheses are controlled through a harness connected by a cable that might provide for limited proprioceptive feedback. This study aims to test transfer of training basic tasks to functional tasks and to describe the changes over time in kinematics of basic tasks of novice body-powered prosthesis users. Methods: Thirty able-bodied participants and 17 controls participated in the study, using a body-powered prosthetic simulator. Participants in the training group were divided over four groups and practiced during a 2-week-period either direct grasping, indirect grasping, fixation, or a combination of these tasks. Deformable objects with different compliances had to be manipulated while kinematic variables and grip force control were assessed. Functional performance was measured with the Southampton Hand Assessment Procedure (SHAP) prior to and after the training sessions, and after 2 weeks and 3 months retention. The control group only performed the SHAP tests. Results: All four training groups and the control group improved on the SHAP, also after a period of non-use. Type of training had a small but significant influence on the improvements of the SHAP score. On a kinematic level movement times decreased and hook closing velocities increased over time. The indirect grasping group showed significantly shorter plateau times than the other training groups. Grip force control only improved a little over training. Conclusions: Training action-perception couplings of body-powered prosthesis in basic tasks transferred to functional tasks and this lasted after a period of non-use. During training movement times decreased and the indirect grasping group showed advantages. It is advisable to start body-powered training with indirect grasping tasks but also to practice hook-object orientations. Keywords: Upper-limb prosthesis, Body-powered prosthetic

Testing in general: context

The timing of this paper is cool in that I just yesterday started to intensify my own use of the VC voluntary closing TRS prehensor that Bob Radocy (TRS) had used yesterday in his Cybathlon winning performance. There, Bob as only user of a body powered arm won with clear distance over the myoelectric arm users [link] without hurry but with great focus. The same gripper is featured in this scientific paper.

The result of that Cybathlon prosthetic arms race was clearly to be expected. I know that also because I use my own, enhanced body powered technology for a work environment that makes all myoelectric arms fail quite comprehensively. Also I use my body powered arm around the house, like, all the time. From that, the pure daily activity performance of body powered technology is without question.

But then, as I never get tired to say: to apply single dimensional ratings for prosthetic arms can be bad business, as our gadget-happy aficionados now may find particularly hard to swallow. Not only had I told them that a single metric would be not helpful under rehabilitation consideration and that the world would be lot better without medals or single wins, but instead, a within-individual comparison of each one's performance across various approaches within the same individual (various prosthetic options, no prosthesis arm on) with both qualitative and quantitative metrics (I will probably have to slaughter my kitchen or washing room, or possibly the garden, for a proof of concept some time) would yield far more informative data. But then, it is a cognitive thing to appreciate multivariate data for their beauty and for the fact that there is usually not a single best but only an optimization within that data space, and to realize that depending on  your data and how you tilt and swivel your parameter space different optima may (or may not) emerge. And yesterday was not set up so much as a cognitive thing.

Despite a body powered arm being superior in many ways to a myoelectric arm, it depends on the situation, use and context. Conversely it is my personal deep conviction that a myoelectric gadget hand such as an iLimb may have a lot going for it - it is just that house work, or kitchen work, or even real work is not such a big part of that - and it may never be, given the many constraints, where positive properties also cancel each other out in relevant constraint constellations.

Much rather, the success of "bionic" gadgets may lie in the emotional realm of those that are so inclined and so I still wait for the first socio-psychological study that studies the advantages of a similarly gloved iLimb over a Becker hand (as there lies a considerable price difference with the product trying to accomplish the same: first look like a hand, secondly, grasp, but not primarily act as predator type gripper device that slaughters all bimanual tasks with top reliability). Actually, their quality of grip may just require so much extra time that any hurry is a problem for using an iLimb to perform work procedures, and so the times when I do practice using the iLimb, I will tell people off for hurrying me.

Maybe there are studies too, where one learns how to stand up for oneself to defend usage of an under-performing but lovable device such as the iLimb. As I say, you have to apply trash culture where it is applicable, and not confuse conversation pieces with manual performance jackhammers.

The Cybathlon was a great exercise to see how people get confused. As in all good circuses, the "art of illusionism" is combined with the "taming of the screw" to a complex layer cake that now waits to be milled to pieces for details.

Review and comments of the study

In this study, they used intact subjects with a simulator arm. Now that means their arm was too long. I feel a considerable difference in my shoulder muscle and neck muscle strain when my prosthetic arm is built too long. As I have a long stump, the technician needs to take special technical steps to stack my connector parts together to achieve a minimal length overhead which then will just suffice to make the prosthetic arm the correct (but not excessive) length.That was not always the case, and that is how I know, unfortunately. So one wonders whether that plays into all testing with subjects that are intact, have two arms and hands, and that wear a prosthetic arm simulator.

This study shows that body powered arm usage proficiency improves over time, with accentuated acceleration in a group that obtained structured training, but also in the group that was just exposed to the test runs. That means that body powered control is so intuitive and fast to learn that being exposed to a few test runs is enough to deliver a good performance. I read a paper a while back whereas the result was that simply using the prosthetic arm in unstructured household work would not result in a significantly lesser performance than when following a structured occupational therapy type training. I am unable to identify that study now, which is a pity, but it was at a time when we discussed whether I needed extra ergotherapy or not and we concluded that it was absolutely enough to just wear it with making very conscious steps to also use it.

With that, I started to assign a few household areas a tag called "hook zone". I have a few things, that I force myself doing with a hook, just as much as possible if ever. Kitchen clean up including dish washer emptying, laundry handling in the cellar, particularly pulling laundry out of the machine and hanging it as well as all floor cleaning are typical examples.

The study restricted testing to smaller objects. Now, I do not exempt myself from handling objects that may be outside the league of the device I am wearing, using the prosthesis. The objects that are present in everyday life are on a very continuous scale with regard to their likeness to be grasped with my prosthetic hook, whatever model I am wearing. So to stop using the prosthesis for larger diameter items pre-emptively will always mean that I then have to use my human arm for these and to tell the truth, it is my particular aim to alleviate the use of my human arm from work with mid range to heavy objects. Overuse reduction is a very clear aim, for one, and it is a heavily weighted parameter in my more complicated cost function that determines my prosthetic use. So that bit of struggle is part of the plan.

So I do realize only now that I apply a more complex "cost function" to my prosthetic use, and after Jon Sensinger's talk on Thursday at the Cybathlon Symposium I feel I need to lay out in more detail what goes into that. I will write that up and link that here when I get around to it. So far, I used gut feeling and atmospheric vibrations to estimate whether to use this or that prosthetic device or whether to apply this or that grip to an item - like, hold a tray or support it, for example. But there are so many more constraints that can be seen as part of a cost function. And one clear cost function trade off that goes into the ADL grips that I apply is to reduce my left arm overuse, while possibly increasing slip risk for objects that are my own and not precious. In other words, I will most definitely handle a salad bowl that is a cheap heavy super market product while being far more careful with holy clay based bowls that tend to break a lot faster and need to be washed manually because they are not dish washer resistant. So I perform as many direct grasps as ever possible under a complex cost function; unloading a washing machine with wet, tethered and heavy clothes, trying to pull out two entangled jeans, for example, is just such an example. This is quite relevant as it definitely skews my application and thus training/exposure profile. If anything, I feel that to alert a new amputee to the very existence of that being a possible problem space may be of great help - but then, we all converge to also becoming proficient without prosthetic arm, some of us managing to avoid asymmetry issues (!). I start to consider an indirect grasp with transfer from prosthesis to hand in instances where I would have to use repeated pronation of my left intact arm: repeated pronation under load really tends to increase the chance of elbow tendon issues whereas supination of my left hand and forearm seems to work with considerable less overuse. My very clear focus on overuse reduction and reduction of asymmetry problems is also the reason why I end up using body powered technology so far more often: every weekend starts with me giving the myoelectric arm another go. Everytime it fails somewhere in the hook zone application range under given constraints. Every time I switch back to body powered.

VO and VC hook used to empty dish washer

The below video from this morning shows me trying to empty a dish washer with the VC TRS prehensor. I have not used  voluntary closing technology much (I still prefer VO but currently revising that) and so I am currently picking up speed.

The voluntary closing (for me, new) and opening paradigm are not too different with regard to a few relevant aspects.


Here is a video of the use of my voluntary opening hook for the same task:

The VC hook requires a far more thought out positioning of body and prosthetic arm than the VO hook.

Particularly, VO means to move my body away from the prosthesis, then grasp, and then relax - which is position wise easy. The VC device requires me to conduct the body motion while the gripper remains stable in position and that is different: there, elbow bending to the side or scapular abduction or other means to not shift the gripper play a bigger role.

VO and VC usage for scrubbing

While scrubbing surface with a sponge never had occurred to a technical university graduate attempting to test or evaluate prosthetic arm so far in medical or technical literature, it is my daily reality that wiping but also scrubbing is part of cleaning.

If ever a scientific paper resorts to using scrubbing as test for prosthetic arms, I will insist that the test be called "Wolf test for scrubbing" (WTFS), as ill defined as it is given just these indications.

While wiping a surface smoothly and gently is no problem for my (human) arm and hand, hard pressure for actual scrubbing or deep wiping will require the prosthetic arm to be used as I feel these activities to definitely contribute to epicondylitis or carpal tunnel on my left arm; as I systematically keep the ball low for all these activities for my left arm, I have not needed physiotherapy in 7 years and not needed carpal tunnel surgery in 5 years since these problems started to exacerbate first. In other words, prosthetic arm use here as the declared goal to keep me healthy.

Here is the deal: scrubbing involved the application of high pressure, and that spells out as kilogram per area. The smaller the area however, the quicker the sponge may get perforated so you see that I have more experience with scrubbing using prosthetic hook devices than you (admit it: you had no idea).

So I will probably use my V2P prehensor for VO scrub works and now also the VC TRS prehensor for VC scrub works.

Here, an occasional problem with the VO system is that it does not allow for an increase of the closing force when applying extra shear forces onto the sponge, with the effect that the sponge falls out or slips every now and then.  Conversely, the claws may be pushed open when hitting some edge structure or when forces exceed the rubber band or spring.

Toilet scrubbing to remove calcification using a V2P prehensor, a voluntary opening VO device:

The voluntary closing VC TRS prehensor however offers a far more organic and smooth control compared to the VO hook system.

Of course you cannot just make non-disabled volunteers wear body powered arm simulators to scrub surfaces in an effort to see how they will be doing. For a number of reasons. But come back here, and I tell you from the front.

Hard/soft grip constraints

One trade off that we all have with these devices is that the more rubbery or deformable a gripper surface is, the faster it will wear down and require replacement.

With that, a moderately deformable plastic will last a lot longer, which is good, while underperforming on hard slippery cone shaped objects, which is bad: the wet glasses shown in this task movie (Mobius socket mount camera) that slip out of the grip clearly illustrate this constraint or dilemma. At the same time the TRS prehensor contains small protrusions at its tips that must exceed a minimal mechanical stiffness and that greatly aid in grasping handles or other parts of object structures.

Of course one could cut up a nitrile fingered work glove and equip the hook with covers, or, wear a standard Hosmer hook and pull cheap silicone tubing over the claws. At one time, I used several layers of sheet rubber and Gecko tape to massively boost the grip performance at extremely low compression forces for a V2P prehensor (whose flat surfaces lend themselves to that) - but a more interesting (because resource and time saving) approach is to suffer through the frustration of this moment and learn different tricks: after a while, I figured that the best way to still get the slippery heavy glasses out of my dish washer was to turn them around first. And I might just flip them over with my left hand and still be able to carry out the weight bearing part of that exercise (to save on overuse of my left arm) with the prosthetic gripper.

Only after spending a long time figuring out such ways *under consciously placed constraints* will one be able to "perform" fast, and yet, still then it is more healthy for my overuse prevention to not be too fast - because under full speed constraints I will always tend to use / overuse my left human hand - but to focus on constraints and grips in what I designated my "hook zones" and quite possibly expand these then.


The playful or systematic, irritated or patient twiddling that one may call training is in many aspects a function of one's own internally placed "cost function". In the setting of this paper, it becomes clear that two groups learn almost similarly well despite one not obtaining extra training.

What really pushes my training experience is the conscious aspect of paying attention to all parts of the activity: how to portion it into segments, how to attribute what segment to which side (left, right hand) and how to maybe classify various approach options for different object shapes and surface properties (dry, wet, soapy, etc.).


[1] [doi] L. H. B. Huinink, H. Bouwsema, D. H. Plettenburg, C. K. van der Sluis, and R. M. Bongers, "Learning to use a body-powered prosthesis: changes in functionality and kinematics," Journal of NeuroEngineering and Rehabilitation, vol. 13, iss. 1, pp. 1-12, 2016.
author="Huinink, Laura H. B.
and Bouwsema, Hanneke
and Plettenburg, Dick H.
and van der Sluis, Corry K.
and Bongers, Raoul M.",
title="Learning to use a body-powered prosthesis: changes in functionality and kinematics",
journal="Journal of NeuroEngineering and Rehabilitation",
abstract="Little is known about action-perception learning processes underlying prosthetic skills in body-powered prosthesis users. Body-powered prostheses are controlled through a harness connected by a cable that might provide for limited proprioceptive feedback. This study aims to test transfer of training basic tasks to functional tasks and to describe the changes over time in kinematics of basic tasks of novice body-powered prosthesis users.",
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