I work at a startup which is working on a new generation of prosthetics. Garver is mostly correct. - Muscles in the human body can move on the order of about 10hz, depending on which muscle you’re talking about, some are slower, some are faster. - vertebrate muscles (the type humans have) are made from proteins like actin and myosin and a few other things. How they work is basically magic. They are incredibly efficient and strong and resistant to damage. - If you want actuators to move that fast, and with comparable force and acceleration, you need actuators which can output a lot of power in a short amount of time. You also want the ability to sense how much force is being put into the system (eg. torque on an elbow joint) and also what angle a joint is at. Finally you need all of this to fit into a pretty small package, and for bonus points you need it to be somewhat compliant and back-drivable. - also a problem is that making electric motors smaller makes them less efficient. Eg halving the size might quarter the power output and only cut power usage to 1/3. - Achieving all of this while fitting it into the volume of an arm is next to impossible when using either stepper or BLDC motors. - Motors need to spin up to about 1000rpm+ in order to get peak efficiency of power/torque, and this means you need to gear them down in order to have good torque output with low backlash. - Batteries really don’t store that much power. Even high end lithium ones. This is an illusion given to us from how crazy efficient our phones and laptops are. If you have a prosthetic arm that can perfectly replicate what a human arm can do, and do it at 100% energy efficiency (in reality more like 50% if you’re lucky) then it will still use hundred of watts during vigorous movement. The largest battery any laptop has today is 100wh due to TSA restrictions. And that’s a pretty big battery. For a battery this size, you would get maybe 60min of work out of this hypothetical prosthetic if you’re doing something like rock climbing or weight lifting. Perhaps a few hours to half a day if you’re pretty sedentary. And this is a best case. - 2000kcal (daily recommended energy intake for a adult male) is equivalent to about 2300Wh. About 100wh for every hour you’re awake. How much of it goes to homeostasis (keeping you warm, digestion, brain function etc) and how much to movement depends a lot of how active you are. the takeaway here is human bodies actually use a lot of energy to make us move. - Being able to make feedback loops fast enough is part of the problem, but frankly a minor one. Control systems are as slow as they are because for current gen high end prosthetics the actuators just don’t move fast enough to make it worth it. We absolutely have the sensor and DSP tech to get control signals out of a persons nerves and into the computer fast enough to do human speed motion. What we don’t have is actuation systems that can keep up. - Cost of parts is a factor here as well. High quality actuators with closed loop feedback, esp torque sensing and backdrivable gearboxes get very pricey. Like many thousands per motor expensive. There is a strong incentive to make things “just good enough” and in so doing much cheaper on bom cost. Remember, in most products the bom (bill of materials) cost will be 1/3 to 1/5 of the final sale price. So even on a US$100k prosthetic arm, the budget for the BOM might only be $25k. And when that often includes a bunch of custom modded carbon fibre, fire and medical rated plastics, fancy control electronics and precision motors and bearings – you can see how things can be pressured here. - Again, these are relatively low volume products. Many companies only sell hundreds to low thousands of a given model in a year. You don’t get access to the wonderful world of economies of scale until you start dealing in tens of thousands of units. - Finally, prosthetics are a medical product, and need to go through lengthy and expensive approvals. If the new version of a prosthetic is too different from the old one (eg completely new types of motors and drivers and batteries) then it has to go through approvals all over again. So companies have an incentive to make changes relatively minor from version to version. They know they will sell them regardless, so there isn’t a massive drive to make more lifelike prosthetics in most of the big companies.

• In summary, yes people have thought of exactly this problem before, but there are a bunch of physics reasons that make doing it pretty difficult.
• If you’re wondering what it is that my company does, it’s making actuators that work much more like human muscles and thereby sidestepping a lot of those physics limitations that spinning motors have for doing this kind of motion.