Thread by @KarlZelik

Excited to share our new #biomechanics publication on the effect of low-profile elastic #exosuits on back muscle fatigue. #exoskeleton #exosuit

nature.com/articles/s4159…

Challenging experiment, but we learned a lot in the process. Here are the top 4 lessons I took away....

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First, huge kudos to lead author @lamers_erik who completed his PhD last month!

During his time @CREATEatVandy he completed a series of studies on quasi-passive wearable assistive devices spanning from foot prostheses to back-assist exosuits

Super proud of the work he did!

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Next some background: When I talk to scientists they often want to know how much exosuits reduce muscle activity, or joint torque, or metabolic rate, or about the optimal assistance levels, specific design features, etc.

And I love this technical aspect of research, but...

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The last few years I've also been spending a lot more time talking with, observing, and getting to know non-scientific stakeholders -- in this case workplace safety officers, managers & workers who are potential exosuit end-users.

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They want to know about important practical issues: Will an exo be comfortable? Will it get in their way? Or help make them less tired at the end of their shift? Or help their back feel less fatigued and sore each day? How will it affect their life on and off the job?

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These experienced have steered & broadened my interest in wanting to monitor outcomes more relevant to end-users questions/concerns.

They motivated our study of exo comfort & our interest in assessing the effect of exosuits on back muscle fatigue.

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journals.plos.org/plosone/articl…

Until a few years ago our lab had not conducted muscle fatigue studies before. So we had to learn... and it was unexpectedly difficult.

Huge credit to @lamers_erik and the rest of the @CREATEatVandy team for their perseverance & diligence in developing the test protocol.

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We spent nearly a year just learning about muscle fatigue test protocols & doing pilot testing. I don't think we touched an exo for first 6 months. Just focused on how to measure back fatigue reliably using surface EMG, and validating repeatability across people and muscles.

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But once we got all of these details worked out, the cool thing is that by monitoring the median frequency (MDF) of the EMG signals we could track the rate at which each back muscle was fatiguing during a physically strenuous task (bending forward & holding a heavy weight).

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So now let's the dive into the 4 key lessons I took away from this study:

LESSON #1 Modest torque assistance leads to fairly large reductions in back muscle fatigue.

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The exosuit prototype in this study applied 12-16 Nm of torque, which is probably only ~10-15% of the total torque about the lumbar spine during the task we tested.

However, on average, we observed reductions in back fatigue on the order of 29-47% (N=6).

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This reflects a non-linear relationship between muscle loading and fatigue, which is also evidenced in the scientific literature.

Interestingly, there is also a non-linear relationship between tissue loading and microdamage (which leads to overuse injury).

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What this suggests is that we don’t need to design exos that do all the work for the user. In fact we generally don’t want to do this. Rather we can develop exos that provide partial/modest assistance & these can lead to meaningful & tangible relief for overburdened workers.

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To put this in perspective: we found that wearing the exosuit made holding a 35-lb. weight (avg weight 4-year-old child) less tiring on the back than holding a 24-lb. weight (avg weight of an 18-month-old baby) without the exosuit

As parents know -- this a huge difference!

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LESSON #2
The lat muscles act kind of like an #exosuit for your low back muscles when they get tired. This was fascinating & unexpected, since the lats are primarily used to adduct/extend the shoulder. This behavior was something we stumbled upon during pilot testing.

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We found that when a participant’s low back muscles become over-strained and fatigued, they would occasionally summon extra assistance from their lats.

When they did, the activity in the primary low back extensor muscles would noticeably decrease.

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Elastic bands in our exosuit work in a similar way: they both apply forces across the entire thoracolumbar spine, they both have a mechanical advantage vs. underlying lumbar muscles, & when engaged they both relieve back strain & fatigue, which helps users sustain endurance.

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LESSON #3
Validate. Validate. Validate. I don't care if the method was from a prior publication. Validate that you can actually measure what you care about!!

One of the most interesting observations had nothing to do w/ the exosuit but with reliably measuring back fatigue.

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Before exo test, we did measurement validation in which we had each subject hold a 24 lb mass (less tiring) vs. a 35 lb mass (more tiring) for 90 sec. This enabled us to check for each subject that EMG frequency metrics were sensitive enough to observe differences in fatigue.
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Surprisingly, only half the participants tested during this measurement validation test showed changes in EMG median frequency between the two weight conditions.

Meaning for half the subjects we could not reliably/confidently monitor muscle fatigue using conventional metric

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If we had not done this critical validation step then we would have been trying to understand exosuit effects in a population in which we could not even reliably monitor the primary outcome metric for 50% of the subjects.

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Individuals that were validated (i.e., for whom we could confidently measure differences in back muscle fatigue rate via EMG when they held diff weights) then went on to test leaning & holding a 16 kg weight with vs. without the exosuit on a separate day.

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LESSON #4
Single-subject study design was better & more appropriate for this stage of device eval (in almost every conceivable way) than traditional group comparison (e.g. t-test).

Sadly, few studies in our field seem to use this kind of study -- but probably should IMO.

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Let’s be honest: In early-stage R&D on assistive devices there is a lot we don't know. We don’t know if all users are going to adapt the same way to a device. We don’t know exactly how much training time is enough. We don’t know the precise rest time needed between trials…

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We don’t know if device will fit everyone the same way -- they usually don’t they're prototypes. We don’t know if prototype is equally comfortable for all. We don’t know if users will adopt similar muscle patterns, e.g., with back where there are so many different muscles.

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Given all these unknowns in early stage device R&D, usually our first experiments aren't trying to establish broad generalizability across all users & scenarios.

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So to me -- often group comparison study designs seem misaligned with the actual (realistic) goals of this early stage research (e.g., to assess feasibility, or gain deeper understanding of how different individuals use a device, or range of effects observed).

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Single-subject study designs (e.g. A-B-A) often seem better suited for answering questions we have in early stage R&D given the numerous unknowns.

These allow individuals to serve as their own controls, & allow researchers hone in on effects of an intervention on each user.

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These involve testing a baseline condition, then introducing an intervention, then removing the intervention to ensure outcome metric returns back to baseline.

I’ve found these studies insightful & I've been more confident in my interpretations for early-stage device tests

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We were fortunate to use this ABA study design here b/c we found inter-subject variability in the number of muscles that benefited, which specific muscles benefited, & the degree to which each muscle benefited (percentage reduction in fatigue rate) from wearing the exosuit.

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For instance, subject 2 and subject 6 each benefited from the exosuit yet showed reductions in fatigue for a different (non-overlapping) subset of lumbar muscles.

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Diff people use exos in diff ways to relieve diff muscles. Just like any other tool.

We should keep this in mind in the development of industrial standards, & when trying to compare across different studies (which tested different users); until sample sizes are much larger

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I'll end by saying that I appreciate how this study pushed me/us to learn new methods, and really think deeply about applied/translational research -- to identify metrics that connected our #biomechanical science with outcomes that were of importance to end users.

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The interdisciplinary & translational parts of research & engineering, & continual learning are part of what I love about this job.

I was appreciative to have engineering as well as clinical collaborators involved -- shout out to @AaronYangMD & undergrads who contributed.

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I’ll note this muscle fatigue study was completed in 2018 & helped motivate us to found HeroWear.

You can follow @HeroWearExo to learn more about field tests & Apex #exosuit that came out from this research.

Excited to learn more from real workers using these #exosuits.

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Link to the full open access article on #exosuits & muscle fatigue is here: 
nature.com/articles/s4159…

@ExoskeletonRep

#exoskeleton #exosuit #wearables #wearabletech #ergonomics

And...

... more recent #exosuit field test research was discussed in this webinar by @MatthewYandell & @MatthewCMarino during #WearRAcon earlier this year:


Technology & research updates will also be discussed at #WeRob2020 next month (werob2020.org)