Is curling the fingers considered digit flexion? Is curling the fingers considered finger flexion and straightening the fingers considered finger extension? I feel like this is correct in everyday language, but I am not sure if it is correct in anatomical terms.

In the vertical plane, bending of the fingers is indeed referred to as _flexion_ , while straightening is called _extension_. In the horizontal plane, spreading the fingers is called abduction, and approximation is called adduction (Fig. 1).

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Fig. 1. Finger movements and terminology. source: US Dept. of Veteran Affairs