Comparing elbow torque for iLimb Ultra Revolution, passive arm (new, old), body powered arm (hook alu, hook steel, Becker hand) [tech]

I measured weights, center of gravity and thus lever arm length, and from these, I obtained torque estimates for my elbow (reminder: I am talking about that elbow, yes, THAT elbow, contained in the tag "below elbow amputee").

The elbow thus carries the weight and moves the prosthesis in a major way. It is the last joint that I have on that arm before it ends. The results are interesting and explain what I have already observed about my posture and handling of these prosthetic options.

Results - elbow torque


Prosthetic arm Distance stump end to elbow joint D Center of gravity distal to D Weight [g] Force of distal half [N] Torque@180deg [Nm] Torque@90deg [Nm] Torque@60deg [Nm]
iLimb Ultra Revolution 0,23m 0,02m 1,050kg 5,15 1,54 1,09 0,77
Body powered, Becker hand 0,23m 0,002m 0,870kg 4,26 1,20 0,85 0,60
Body powered, steel hook 0,23m 0,002m 0,760 3,72 1,05 0,74 0,52
Body powered, alu hook 0,23m 0,002m 0,620kg 3,03 0,85 0,60 0,42
Passive arm (old) 0,23 0,005 0,488 2,39 0,68 0,48 0,34
Passive arm (new) 0,23 0,005 0,374 1,83 0,52 0,37 0,26

Explaining the results

The iLimb's presence on one's arm is undoubtedly hard to ignore.

While its absolute weight is not even so extremely high, it can neither grab very hard, nor can it grab too precisely, nor does it provide a great means to push (such as when operating switches or buttons). Really it is a bit of a dead weight as far as function is concerned, but a lovable type of dead weight. Given that, one wonders why they do not totally abolish grip function and make it a puppet hand just for gestures. Or, why not slap on 2-3 kg and make it a massive grip monster. It would probably look cool to have the DLR hand in red, smoke cigars and wear Hellboy shirts.

As it is, however, the iLimb as such causes great mechanical strain when one moves around the office even without actual physical work; this is the stump with friction rash and blisters a few hours after I stopped wearing the iLimb for simply waving it around the office a bit:


The real reason that this arm causes such strain is its passive mechanical property: relatively bad center of gravity, relatively high weight, relatively bad suspension choices. The actual trade-off here is not function. Cannot be function, otherwise I am sure I would have stumbled across it by now. No, the trade off is that this is a very cool gadget that one ends up schlepping around.

Interestingly, physics seems to match my experience.

Just typing for a few minutes causes my elbow and shoulder to pain when wearing the iLimb. For typing, the best thing to wear is the cosmetic or passive arm. Second best, a body powered arm with an aluminum hook.

And if I still get stuck typing with the iLimb, best to pull up the shoulder and flex the elbow to reduce torque - by reducing the angle. In fact that is what many amputees do. Actually, the reduced angles of elbow and shoulder, these are the dead giveaway that someone wears one of these anchors! That is what messes up our posture with these myoelectric or "bionic" arms.

Really, all these beautiful demo videos we are being shown with perfect posture? Those are all staged. I saw one professional arm mechanic wearing a Michelangelo hand, himself. When he thought no one was looking, guess what he did. He held the cables of the computer that he was setting up with his mouth.

The body powered arm is quite versatile - Becker hand.

Of course, wearing a durable and affordable prosthetic hand with an adaptive grip has its price. The Becker hand does generate sufficient torque to make long typing a bit uncomfortable.

While it is still significantly less than an iLimb, it costs only a mere fraction, has a repetitively reliable precision grip, it can be fitted with any normal work glove, it actually does deliver a useful grip strength, and it holds up for years without repairs.

Hooks may be underestimated.

While people generally and some specifically take it upon themselves to make hook wearers look bad, these devices are functional in a way that can be explained, shown, demonstrated and experienced.

One aspect of a good Hosmer aluminum hook is that it generates just about as much torque on an extended arm than an iLimb generates when one bends the arm almost fully (that is, as much as a prosthetic socket probably allows), at the elbow. Obviously, wearing a hook then saves a lot of energy and reduces strain, measurably, in numbers.

That is one of the hard reasons why people, that talk us out of wearing hooks, or that look down on hooks generally, are dumb. Actually, some of them may have 'vested' interests.

Passive arms are even more underestimated.

The comfort that one gets from wearing and having correct limb proportions - relevant for typing - and ever so little weight to handle becomes palpable when considering these results here. The cosmetic arm is a superb prosthetic solution for typing, for example, and for eating out.

So to summarize

  • To save on wear, tear, strain and overuse, wear a passive arm unless you need grasp function.
  • If you need grasp function, wear a body powered arm with an aluminum hook.
  • If you must wear a functional hand, reconsider the Becker hand. For the function and durability, its price is unbeatable.

The 'Paradrom Rathausen' tip - trying out extra asymmetric torque to work on empathy skills

To let you in on how these extra Kilos feel, you may want to wear a 1kg bracelet around one of your wrists (no, not around the other one). Now imagine all these people gassing your head full about how good for your posture that is! And, how cool and "bionic" you now are. If you do not get a serious friction rash after 6 hours, try adding some vinegar under the skin for extra feeling. Do that for 6 weeks, wearing it for 14 hours a day. Tell us then how grateful you are. And, yes, my elbow and shoulder feels probably about the same.

Keywords - Center of Gravity, COG, Socket comfort, Weight

Cite this article:
Wolf Schweitzer: - Comparing elbow torque for iLimb Ultra Revolution, passive arm (new, old), body powered arm (hook alu, hook steel, Becker hand) [tech]; published 22/06/2014, 20:32; URL:

BibTeX: @MISC{schweitzer_wolf_1656511464, author = {Wolf Schweitzer}, title = {{ - Comparing elbow torque for iLimb Ultra Revolution, passive arm (new, old), body powered arm (hook alu, hook steel, Becker hand) [tech]}}, month = {June}, year = {2014}, url = {} }