What Your Patient Is Not Telling You
By Marins Inc.
They will not use the word proprioception. They probably will not say the rubber band is doing the work instead of them. They will not frame it as a design tradeoff.
What they will say, if they say anything at all, is something like:
I keep dropping things. I watch it constantly. My shoulder is tired by noon. I just do not trust it with anything fragile.
These are not complaints about motivation or adjustment. They are descriptions of a system that was designed to hold without feedback.
Voluntary opening devices solve the hold problem well. The rubber band maintains grip without sustained cable tension, which matters for endurance. That is a real advantage, and it is part of why VO became the fitting default.
But the feedback loop is not there. The user cannot feel force through the cable the way a voluntary closing user can. Hichert et al. (2017) established this formally -- VC systems provide extended physiological proprioception, meaning cable movement and tension directly inform the user about grip parameters in real time. Berning et al. (2014) showed users reported meaningfully better grip control with VC systems in daily activities. Hichert and Plettenburg (2019) confirmed that VC users can reproduce target forces accurately without visual input.
Without that feedback, users compensate. They watch the device instead of the task. They grip harder than necessary as a margin of safety. They avoid objects they are not certain about. They adapt their life around the device's limitations rather than the other way around.
That adaptation is quiet. It does not always surface in a clinical appointment. But it shows up in what your patient stops attempting. And it starts before the first follow-up ever happens.
The traditional counterargument for VO over VC has always been fatigue. Sustained VC grip requires sustained cable tension, and over time that costs the user. It is a legitimate concern. For a long time it was the deciding factor.
The question the field has not revisited often enough is whether that is still the only choice available.
The ProHensor® was designed for exactly this tradeoff. It is a voluntary closing device with an auto-locking mechanism that maintains grip after closure -- without requiring continuous cable tension to hold it. The VC feedback loop is preserved. The fatigue penalty is not. Fatigue and control are no longer in opposition, which means the fitting decision no longer has to be either.
When that tradeoff is removed at the point of fitting, the quiet adaptations may never take root. The narrowed task list, the compensatory watching, the objects left untried... those outcomes are not inevitable. They are the downstream cost of a design constraint that now has a different solution.
The follow-up appointment is the right place to ask what your patient has stopped trying. But the fitting appointment is the right place to change what they never have to give up.
References
1. Hichert M, Abbink DA, Kyberd PJ, Plettenburg DH. High cable forces deteriorate pinch force control in voluntary-closing body-powered prostheses. PLoS ONE. 2017;12(1):e0169996. doi:10.1371/journal.pone.0169996
2. Berning K, Cohick S, Johnson R, Miller LA, Sensinger JW. Comparison of body-powered voluntary opening and voluntary closing prehensor for activities of daily life. J Rehabil Res Dev. 2014;51(2):253-261. doi:10.1682/JRRD.2013.05.0123
3. Hichert M, Abbink DA, Vardy AN, van der Sluis CK, Janssen WGM, Brouwers MAH, Plettenburg DH. Perception and control of low cable operation forces in voluntary closing body-powered upper-limb prostheses. PLoS ONE. 2019;14(11):e0225263. doi:10.1371/journal.pone.0225263