Backdrivability

What it means

Backdrivability describes how readily a load applied at an actuator's output can move the mechanism back toward its input. For a rotary joint, an external torque moves the joint and transmission rather than the motor driving the load. It is a property of the complete mechanical path, so describing the motor alone leaves out important parts of the explanation. [1, 2]

Gearing changes the inertia felt at the output during acceleration. Modern Robotics explains that a motor rotor's apparent inertia is multiplied by the square of the reduction ratio. Friction also resists motion, including at low speed. These effects can make an externally moved joint resist motion even when the rotor itself is small. Backdriving describes output-to-input power flow, not necessarily rotation in the opposite direction. [1, 2]

How it applies to humanoids

Humanoid hands and limbs encounter externally applied forces during contact. Mechanisms that yield to those forces can support compliant interaction, although the response also depends on the controller. Yu and colleagues investigate backdrivability together with torque capability, control bandwidth and tracking accuracy in a wearable hip robot. That combination is more informative than an isolated label. [2]

1X says NEO's low-ratio tendon-driven fingers yield to impacts. This July 2026 update describes the hand assembly; it does not measure every joint in the robot. [3]

Terminology and measurement traps

Backdrivability and compliance are related but describe different questions. Backdrivability asks whether external output motion can drive the mechanism backward. Compliance concerns how a system yields under load, which can also involve an elastic element or active control. The wearable-robot study compares quasi-direct drive with series-elastic actuation rather than treating their mechanisms as identical. [2]

Ask for the measurement behind a backdrivability claim. Breakaway resistance, motion-dependent friction and apparent inertia affect different parts of the response. A slowly hand-moved joint does not establish its behavior during a rapid impact or while a controller is powered. [1, 2]

A high peak torque does not determine backdrivability. The same actuator description should identify the transmission and testing conditions; a strong motor is compatible with very different output behavior. [1, 2]

How to read it on humanoidversus.com

The actuation field records an energy category, and max_joint_torque_nm records a sourced peak joint torque. Neither is a backdrivability measurement. Read actuator details and hand descriptions in the linked sources before drawing conclusions from those fields.

NEO is linked because its record cites the specific hand update discussed here. There is no backdrivability table, and an unknown transmission should remain unknown. The hand claim should not be expanded into an unsourced claim about knees, shoulders or the entire body. [3]

Further reading

Modern Robotics explains reflected inertia and friction. Yu and colleagues provide an experimental actuator comparison, and 1X's hand article gives a current humanoid example with explicit mechanism descriptions. [1–3]