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Myoelectric prosthetic arms do not "really" function - so whom to sell them to? [cynical economic considerations]

Cite this article:
Wolf Schweitzer: Technical Below Elbow Amputee Issues - Myoelectric prosthetic arms do not "really" function - so whom to sell them to? [cynical economic considerations]; published February 2, 2019, 15:11; URL: https://www.swisswuff.ch/tech/?p=9304.

BibTeX: @MISC{schweitzer_wolf_1568643031, author = {Wolf Schweitzer}, title = {{Technical Below Elbow Amputee Issues - Myoelectric prosthetic arms do not "really" function - so whom to sell them to? [cynical economic considerations]}}, month = {February},year = {2019}, url = {https://www.swisswuff.ch/tech/?p=9304}}


Myoelectric prosthetic arms do not "really" work, we all know that, and it has been clear for decades.These factual aspects are difficult to make that problem go away like, poof. With that the more interesting question is: whom do we sell these to?

While selling to people that are gullible [link] seems to have a lot going for it, it is risky. A more sustained approach may base on taking actual risk factors for myoelectric failure into account:

  • Sweat
  • Weight
  • General error rate

General error rates for myoelectric control are very high

Due to the high degree of error rates inherent to myoelectric control, use of a myoelectric arm for actual grips in everyday life as well as work related activities constitutes a risky behavior.

Alternatively, these devices may be worn but not actually used for grips, which lowers the risk of damage both of the device and the objects handled considerably. Since myoelectric arms are associated with advanced hopes and are hyped up medially, they may appeal more to people that are risk takers, thrill seekers and novelty seekers.

The technical basis for myoelectric control failure, as control error rates, has/have not changed in forty years [link].

What is the ideal group of users?

Sweat [1]: below ~14 years of age

The age to ideally use myoelectric arms based on least sweat interference is below ~14 years of age for boys based on a study that examined the development of sweat volumes [1]. This means, in other words, that myoelectric technology appears to least suffer from excessive sweat in prepubescent boys (and probably girls, even though the study only addressed males). After that age, average sweat volumes leap up disproportionally in relation to age to approximate average hourly adult sweat volumes. Even in sedentary professions, myoelectric control will deliberately fail a normal careful adult [link: myoelectric dialog], leave alone a physically demanding adult user [link: JNER].

Development of sweating pattern. Chloride concentration [Cl-] in sweat and sweating rate in different age-groups of male teenagers who exercised at moderate intensity (heart rate 160-170 beat/min) on the cycle ergometer. Climatic conditions were 29°C, 60% relative humidity. [Cl-] was determined in sweat sampled from the chest, sweating rate by changes in body weight. Each point is a mean of eight boys. The arrow indicates the age at which pubertal changes were first noted. Data from Araki et al. (1979) cited and illustrated in [1].

Weight / strength [2]: below ~15 years of age

Already a relatively low object weight of 12 kg appears to be a maximum of what a myoelectric arm with hard socket suspension is able to lift comfortably and reliably.

Extrapolating data from [2] towards younger ages, the capacity to lift increases dramatically after puberty onset in children. A myoelectric arm will however not be able to sustain more than 10-15 kg of weight, realistically, which caps the ideal age for use at ~15 years, Here is a diagram that shows this (including data from [2] extrapolated to lower ages):

So clearly, a user group that does not by themselves routinely lift stuff that is heavier than 10-15 kg is of great advantage to selling prostheses with serious physical limitations such as the typical 3D-printed or myoelectric prosthesis.

Conclusion

The ideal candidate for a myoelectric arm, based on physical realities of coming of age, therefore appears to be a child, up to the age of ~14-15 years, or any other person whose activities both in terms of sweat and force generation do not exceed that of a child aged ~14-15 years of age. Both handled weights and average sweat per hour are disproportionately lower for children up to ~14-15 years of age than older people, so technical issues of real concern appear to be minimized there. With that, adults wearing myoelectric arms may face serious lifestyle restrictions; actual applications are gesturing or using the prosthesis as costume [link], or attempting to embody a Cyborg [link].

That idea has been successfully exploited so far by a range of actual / current projects, where the market of <15 year old novelty seekers has been identified as particularly interesting for myoelectric arms possibly combined with 3D-printing [link], without any apparent closer consideration of better socket fit, grip quality or material strength. These projects with such a focus contain (but are not restricted to) the Open Bionics project and the Bionic Man project.

[1] O. Bar-Or, "The growth and development of children’s physiologic and perceptional responses to exercise," in Children and sport, Springer, 1984, pp. 3-17.
[Bibtex]
@incollection{bar1984growth,
  title={The growth and development of children’s physiologic and perceptional responses to exercise},
  author={Bar-Or, O},
  booktitle={Children and sport},
  pages={3--17},
  year={1984},
  publisher={Springer}
}
[2] J. Lowndes, R. L. Carpenter, R. F. Zoeller, R. L. Seip, N. M. Moyna, T. B. Price, P. M. Clarkson, P. M. Gordon, L. S. Pescatello, P. S. Visich, and others, "Association of age with muscle size and strength before and after short-term resistance training in young adults," Journal of strength and conditioning research/National Strength & Conditioning Association, vol. 23, iss. 7, p. 1915, 2009.
[Bibtex]
@article{lowndes2009association,
  title={Association of age with muscle size and strength before and after short-term resistance training in young adults},
  author={Lowndes, Joshua and Carpenter, Richard L and Zoeller, Robert F and Seip, Richard L and Moyna, Naill M and Price, Thomas B and Clarkson, Priscilla M and Gordon, Paul M and Pescatello, Linda S and Visich, Paul S and others},
  journal={Journal of strength and conditioning research/National Strength \& Conditioning Association},
  volume={23},
  number={7},
  pages={1915},
  year={2009},
  publisher={NIH Public Access}
}
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