Flexion/Extension and Abduction/Adduction

Two basic joint motions: flexion/extension is bending and straightening, abduction/adduction is spreading apart and drawing together.

These are anatomical terms for joint motion, carried over into dexterous hands, exoskeletons, and human motion capture. Flexion is the bending motion that decreases the angle between two adjacent segments, as in making a fist; extension is the opposite, straightening back out. Abduction moves a part away from the midline of the body or hand — for fingers, this means spreading apart; adduction moves it back toward the midline, i.e. bringing the fingers together. In the human hand, the metacarpophalangeal (MCP) joint at the base of each finger can both flex/extend and abduct/adduct, giving it two degrees of freedom, while the interphalangeal joints further out (PIP, DIP) can only flex and extend. Chinese-language dexterous-hand spec sheets commonly label abduction/adduction as 'lateral swing' and flexion/extension as 'curling' — for instance, the Linker Hand's specs list 'thumb lateral swing' and 'four-finger curling.' Some dexterous hands simplify their design by giving the four fingers flexion/extension only, while the LEAP Hand specifically designed MCP joints that can abduct/adduct across a range of poses, to make retargeting human hand motion easier.

ExampleSpreading all five fingers apart into a fan shape is abduction; bringing them back together into a flat hand is adduction. Making a fist is flexion of each finger; flattening the palm out is extension.

Also called

Lateral Swing, Flexion/Extension, Abduction/Adduction

Related

Dexterous Hand · Metacarpophalangeal / Proximal & Distal Interphalangeal Joints (MCP / PIP / DIP) · Thumb Opposition · Degrees of Freedom (DoF) · LEAP Hand · Motion Retargeting

Sources

Wikipedia: Anatomical terms of motion
Shaw, Agarwal, Pathak: LEAP Hand（universal abduction-adduction mechanism）
灵心巧手 Linker Hand 官网（产品参数：拇指侧摆、四指弯曲） (Chinese)