Characteristics of extensor indicis and index extensor digitorum during finger extension: an electromyographic and kinematic study

INTRODUCTION: Functional differences between the extensor indicis proprius and extensor digitorum communis of the index finger remain poorly understood. We quantified these functional differences by measuring metacarpophalangeal joint angles and muscle activities during extension tasks. METHODS: Fifteen healthy participants performed index metacarpophalangeal joint extension under two conditions: a single-finger extension task, primarily activating the extensor indicis proprius; and a four-finger extension task, extending all four fingers, primarily activating the index extensor digitorum communis. Joint kinematics were recorded using a three-dimensional motion analysis system, and muscle activities were measured via surface electromyography. RESULTS: During the single-finger extension task, electromyography showed significantly higher extensor indicis proprius than index extensor digitorum communis activity. In contrast, there was higher index extensor digitorum communis activity during the four-finger extension task. Angle waveforms of the index metacarpophalangeal joint showed comparable flexion/extension patterns between the single-finger extension task and four-finger extension task, but greater adduction (mean 14° vs. 1°) and less internal rotation (-1° vs. 5°) were observed in the single-finger extension task compared with the four-finger extension task, indicating that the extensor indicis proprius has an adduction and external rotation action on this joint. CONCLUSION: These findings suggest that anatomical differences between the extensor indicis proprius and index extensor digitorum communis underlie their distinct kinematic roles, and that index finger adduction/abduction and rotation may be altered following extensor indicis proprius tendon transfer.