We don't need exoskeletons, we need exo-muscles

[Mize]

We already have skeletons and, in fact, we need them to be frequently and dynamically loaded with our weight.

So why are so many focused on exoskeletons for SCIs instead of exo-muscles-systems?

But, what's an exo-muscle-system?

To be honest, it's just an idea. I'm sure someone had it before I did, but I haven't found any research.

Here's the idea:

1. Hard contact points at the heels, knees and hips (like knee pads only harder).
2. Motors inside these hard points pull (rotary winding likely) on strands that run between the hard points.
3. Protective material runs under the strands so that they don't compromise skin (abrasion, etc.)
4. Strands run for both extension and flexion (so both sides of each muscle group/joint)

Any ReWalk or other folks on here who might comment on any work along these lines?
Something like this could be worn under clothes and in a car, etc.

[Tman9513]

When I was doing grad school work in a project management course, I developed a hypothetical "FES BioSuit" project. The concept is a body suit, such as a speedskater's, that had embedded FES electrodes all throughout and would fire the muscles in accordance with gait mannerisms. For it to actually work, a hairless body with good skin contact on the electrodes, muscles/skeleton that could support the weight, and a minimal backpack/CPU board with battery pack in it. Much like an FES bike in concept except for the entire paralyzed portion of the body would get zapped. The suit would need to account for sweating (or not sweating) and a few other things.

I should probably patent that and start a GoFund Me...

[Matt Bellman]

Mize, great idea. There are a handful of groups working on "exosuits" (see Harvard's Wyss Institute), which differ from "exoskeletons" in that they are primarily soft and flexible. The main idea is to mimic the compliance and actuation of muscle, allowing for a more natural fit, feel, and function.

The main problem with exosuits (or exo-muscle-systems), is that we don't currently have actuators that can perform like muscles. Muscle is powerful, lightweight, flexible, and dense. The closest we have are hydraulic actuators, but these need large fluid reservoirs, high pressures, and a lot of energy. Nothing really comes close to the specs needed to make a successful exosuit. Exoskeletons can take advantage of existing robotics technologies and don't have to deal with the complexities of exosuits. Some day though.

To Tman's point -- it may be possible to substitute for some of the deficiencies in current actuation technologies by supplementing them with FES. The biggest challenge there is muscle atrophy. Most people with SCI have had it for a while, and their muscles have atrophied to the point that they're really weak, so they wouldn't be able to support activities like walking. Each person would have to go through months of intensive, full-body, FES-based muscle strengthening before they could use the system normally. Then the question becomes -- who's going to pay for that therapy? The ParaStep went through all this and failed, even after getting insurance reimbursement, though their fall was partly due to the poor performance of the system.

[Mize]

Plus FES using skin electrodes results in rapid muscle fatigue. For extended use the electrodes need to be implanted at the nerve junctions.

[tomsonite]

Plus FES using skin electrodes results in rapid muscle fatigue. For extended use the electrodes need to be implanted at the nerve junctions.

Theoretically, wide-pulse stimulation using skin electrodes would improve the fatigue factor, possibly as good or even better than electrodes implanted at nerve junctions.

[Matt Bellman]

Theoretically, wide-pulse stimulation using skin electrodes would improve the fatigue factor, possibly as good or even better than electrodes implanted at nerve junctions.

What makes you say that wide-pulse stimulation would improve the fatigue factor?

[Mize]

Theoretically, wide-pulse stimulation using skin electrodes would improve the fatigue factor, possibly as good or even better than electrodes implanted at nerve junctions.

Would love more information on this!

[tomsonite]

What makes you say that wide-pulse stimulation would improve the fatigue factor?

Would love more information on this!

https://www.ncbi.nlm.nih.gov/pubmed/17620928

This article is a good starting point - I'm not sure if it's public access since I automatically get access through my university account.

In a nutshell, FES stimulates a motor axon directly - you stimulate a motor axon, bring it above threshold, and the muscle fibers the axon innervates (the motor unit) contract. Once the muscles or the axon fatigues, it stops firing, and can't be recruited again for a while. Everyone in this thread so far seems like they would know that when you put FES pads on, you only recruit the motor axons that are directly within the electrical field the pads create - you either fire all of them at once, or don't fire any at all, and you don't get any of the axons that are too far away from the pads. This is a very unnatural way to recruit motor units - our nervous system typically does a much better job of firing only the motor units we need to do a certain task, and if a motor unit fatigues, our nervous system recruits another one automatically.

Enter wide-pulse stim. Its the same physical set-up as FES - pads over a muscle. But, now, you selectively stimulate only the sensory neurons using very low amplitudes, and much higher frequencies and pulse widths. Bringing a sensory neuron above threshold will set off a sensory volley - the sensory neuron stimulates a spinal interneuron, which then stimulates a motor neuron - and you indirectly get recruitment of a motor unit, and firing of a muscle. Interneurons are quite good at regulating how many and which motor units get recruited to complete a task - i.e., they help regulate the fatigue issue caused by traditional FES. So while it is not perfect and certainly not as efficient as a fully-functioning nervous system, wide pulse stim should do a much better job of mitigating fatigue, and offers the added benefit of spinal cord excitation.

Wide-pulse stim was approved by the FDA this year for Restorative Therapies to use in one of their devices. Frankly, I don't know why we haven't seen every single motor rehabilitation researcher pounce on it.

[Matt Bellman]

Frankly, I don't know why we haven't seen every single motor rehabilitation researcher pounce on it.

Correct me if I'm wrong, but it works based on a noxious sensory stimulus. Regular FES is already physically uncomfortable for most people -- wide pulse is orders of magnitude beyond. It's only going to work for those without sensation of the stimulation.

[Mize]

How high a frequency and how long a pulse width?