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this post was submitted on 06 Jul 2026
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thats not really true at all. these are features of physics not biology, so robotics have the same tradeoff. in robotics its even worse though because we currently have no actuators that are capable of matching biological muscle in terms of combined speed, force, and precision, let alone in such a small package. if you make a robotic limb capable of the force of a human hand it will be incredibly stiff and slow, or require actuators that are too large and heavy to be feasibly carried by a person, to say nothing of the energy storage. in practice robotics capable of functioning as prosthetics are either sufficiently strong but totally lacking in any dexterity, or have dexterity but are very weak and/or slow compared to biological hands.
the actuator problem is pretty much the single biggest barrier to creating prosthetics that rival the functionality of human hands. people have been working on it for decades, and while there have been some interesting results, nothing has come close to being even a poor replacement for biological muscle. when we finally figure it out, which i think is inevitable if we continue to survive as a species, the robotics we make with them will operate under the same physical constraints as biological bodies.
And as durable. People don't really know how much force you are working with as a body in motion. Ground reaction force for a small jump can be 7x someone's body weight. So a person weighing 200lb can have 1400lbs of mechanical load being distributed by the feet and ankles. I build and fit prosthetics, it would surprise most people how general use can trash materials like reinforced carbon fiber and titanium. You do the same to your body, but your body repairs itself.
Yep, the actuators in prosthetic limbs are typically located in the wrist, or in the actual terminal device, which makes them pretty clunky. People have tried making ones that are housed further up the limb, but then you are working against the length of the lever arm. Your biceps brachii insertion can exert about 10x more force than what you can curl, it just seems weaker because you have a leverage disadvantage.
They used to have a operation where they attached a terminal device to a trans radial prosthetic directly to the bicep, but those guys could close the terminal devices(claw) with anywhere from 350-500lbs of pressure.
It's one of the problems, though I don't think it's the main one. The main problem is mimicking anything close to the propeceptive abilities that humans have. The mind body connection that you have to your hands is just eons away from anything we currently have. The current tech most advanced limbs utilize is myoelectrics, which have been around since the late 70s and are for the most part unchanged. There have been some pretty cool improvements in nerve reintegration, but it's not really a feasible technology atm as it takes dozens of hours for a team of surgeons and a special candidate for a patient.
I would say the second biggest hurdle, especially for lower limb prosthetics is energy storage, it's not fun to lug around a heavy ass lithium battery that you have to recharge every few hours. Upper limb uses less energy, but only because there's only so much they can do atm.
I know to a lot of you I'm killing your cyberpunk dreams, but you will never have a limb that even comes close to the quality and utility as your current healthy limb. The technology just isn't anywhere close to where a lot of people believe it is.
the neural interface to prosthetics is particularly fascinating to me. i read a couple papers a while ago that showed that human muscle control is essentially highly orchestrated bangbang control. it was always confusing to me why we have such over actuated limbs, with thousands of muscle fibers per individual muscle that our brains actuate individually. what they showed was that each cluster connected to a particular nerve is either on or off, with our brains sending complex sequences of on and off to each fiber to perform motion, and our tendons essentially acting like filters to hold the tension and smooth out the motion so we arent super jerky.
the big takeaway to me though is that if we want to make the kinds of prosthetics that come close to replicating the utility of human limbs the actuators should be highly redundant bangbang systems the way our muscles are. my understanding is current mechatronic prosthetics use electric motors with continuous control of monolithic motors for each joint, which is very much not even close to how our brains control our bodies. i think the translation from what our brains do to that causes a lot of issues that would be solved by providing our brains with something closer to bangbang actuators.
one of the papers also showed that while we do use feedback from sensor neurons in our limbs, its actually pretty slow compared to some of the activities people perform, to the point where the action starts and ends before the feedback even reaches the brain. so basically our brains just perform those rapid actions open loop, and only use the feedback for learning after the event. isnt that fucking cool! that suggests that if we can just make actuators that respond similarly to muscles and connect them with existing nerves we could leverage this system to get pretty far along the way to people having fine motor control over prosthetics, even without all the feedback, and potentially completely analog with no microcontrollers required. people just might need to look at their limb while theyre doing stuff, especially during the early neural training when theyre figuring out how to move it.
Part of the reason for this is that most prosthetics are controlled by just a few myoelectric sensors reading the controlled activity of what healthy muscle groups the patient has left. Most upper limb amputations are from trauma or something like cancer, where there're not a lot of options when it comes to healthy residual tissue.
To control the actions in the terminal device the patient has to learn how to flex those individual muscle groups to activate them. Not only does this take a lot of practice and concentration, but it can cause a lot of fatigue in muscles that are usually atrophying.
Yeah, we have a couple ways to get around motor planning perturbation. Our cns can anticipate predictable events and start prepping the action before it happens on an open loop as you discussed. Or it can take a reflexive shortcut. When dealing with unpredictable stimuli that over stretches muscle spindle, or over stimulates nociceptors, the peripheral nervus system will skip communication with the brain all together and just communicate with the spine.
I'm not sure if we could ever get to super fine motor control. Really fine motor control isn't performed quickly, it's dependent on the constant feedback of propiceptors for all the small adjustments we make.
What you are absolutely correct about is that nerve integration would make unconscious and even some reflexive movements possible, and it doesn't even really require a new type of actuator, though that would help.
When I was in college I helped build the shoulder and chest housing for a neutral interface for someone who had an operation for targeted muscle reintegration. The surgeons moved his nerve to reintegrate into an area in his chest where they cut the muscle into a grid, so when you touched a part of the grid, it felt to him like you were touching part of the missing limb.
You build a prosthetic that has haptic feedback so when the prosthetic touches something the interface on his chest would create stimuli to the corresponding area on his chest. I think I have a video somewhere of part of the neural training where he was stacking blocks. At one point he knocked the block over and reflexively caught it before it dropped. Which I believe was one of the first times a powered limb showed signs of unconscious reflexive movement.
Couldn't find the video of him with the blocks, but this is the guy I >helped out on. This is more advanced control than pretty much any limb on the market nowadays and this is from the early 00s. Unfortunately there aren't a lot of people doing Targeted muscle reintegration anymore, it's just too costly. So I think most of it is done at Walter Reed now for research purposes.
Fair enough. You seem to be more knowledgeable about this than me. I get that its an inherent physics thing though I just meant that, theoretically at least, you could simply brute force grip strength in a way that you can't brute force dexterity - at which point it becomes sensible to optimize for dexterity. I guess we're not as close to that point as I assumed though.
A lot of it is limited by the amount of control an amputee would have. It's hard to create something that is both stronger than the human grip with the same amount of control over how you exert that strength.
Some of the older myoelectric pincer type terminal devices had a slip reflex built into them. Where if the sensors in the pincer felt an object slipping, it would crank down harder. It had an average grip strength, but because of the nature of how pincers work, all that power was exerted into a much smaller inflexible area.
It worked fine on solid objects, but if you ever tried to use it on a person and that person tried pulling away.....you could give someone a really really unpleasant pinch.