1. Boston dynamics pioneered hydraulic actuators (for their humanoid Atlas) and some really complex motors for their quadrupeds (I don’t think they ever explained their design)
2. There was a whole research line on Series Elastic Actuators that was pursued in the quadrupeds from Europe, famously Anymal from ETH Zurich.
The main problem they were trying to solve was high torque density with back drivability. If you wanted large torque, you could use a high gear ratio but then you lose back drivability and the walking becomes really stiff. MIT Cheetah basically custom built a motor with a really large radius that just output a really large torque. So to summarize, we had hydraulics, a really complex motors + spring mechanism, a really complex large motor to solve this problem and they all required huge amounts of money and complexity ( arguably MIT cheetah was simple but I heard thermal management was a big headache, and the entire robot cost millions)
Ben Katz, then at MIT, showed that you could use regular BLDC Motors, heavily used in drones with a small gear ratio, like say 1:10 or 1:20 and get a really good torque density, backdrivability tradeoff. It started around the above blogpost, in the coming year as a part of his masters thesis, he build the MIT Mini Cheetah, that was also the first quadruped I saw that performed a backflip, (maybe Boston dynamics was earlier and so it was the second then), and that basically started the goldrush into his actuator tech that we call Quasi Direct Drives.
Looking now 10 years later, in some ways he was vindicated, all humanoids and quadrupeds are made with this actuator now, Unitree is basically QDDs through and through. But the same faults we saw then, still hobble it now. It just does not generate enough torque, its position control isn’t milli degree precise, and so it never took off in industrial robot arms, though many tried (Berkeley Open Arms for one).
A minor correction: both the original Cheetah and Mini Cheetah motor designs were QDDs and were using BLDCs. The innovation was from using a off-the-shelf BLDC model as much as possible instead of a custom-design/modify one. Moreover the placement of electronics and gearbox was much more compact, leading to the modern day design. If any thing, the original Cheetah motors were actually simpler than those in Mini Cheetah.
https://www.militarytimes.com/industry/techwatch/2026/08/18/...
Thesis: A low cost modular actuator for dynamic robots
https://dspace.mit.edu/entities/publication/b85069e2-f1cd-47...
https://www.youtube.com/watch?v=nmViQ1T03Rk
[2] Unitree, LIMX Dynamics, and other Chinese robotics companies may also have referenced or been influenced by his actuator design in the evolution of their own actuators.
https://www.militarytimes.com/industry/techwatch/2026/08/18/...
[3] Ben, tear down a Chinese version of the Mini Cheetah.
https://robot-daycare.com/posts/2022-11-02-mini-cheetah-clon...
https://www.youtube.com/watch?v=t5SHUWb6kH4&list=PL8H6WijZPO...