Backdrivability

A robot joint is backdrivable when external force on the output can move it through the transmission without the motor actively commanding the motion. Direct-drive and low-gear-ratio architectures are highly backdrivable; high-gear-ratio transmissions (harmonic drives, planetary stages) resist back-drive proportionally to the ratio.

The distinction matters because backdrivability is the dividing line between cobot-safe humanoids and traditional industrial arms. A backdrivable arm absorbing a collision with a human can yield rather than transmit the impact through the structure. Apptronik's Apollo, Agility's Digit, and Figure's hand designs use compliant actuators specifically for this. Boston Dynamics' Atlas uses hydraulic and high-output electric architectures that aren't fully backdrivable — relying on impedance control and exteroceptive sensing for safety instead. The architectural choice cascades through the rest of the design.