- Extensor apparatus of index finger receives contributions from 5 tendons: Extensor digitorum, Extensor indicis, 1st dorsal interosseous (inserts laterally), 1st palmar interosseous (inserts medially), 1st lumbrical (inserts laterally).
All of the above blend into the extensor apparatus, which continues distally, trifurcating into a central slip (which inserts onto the middle phalanx) and 2 lateral bands (which insert onto the distal phalanx).
All 5 muscles act to extend the interphalangeal joints, while the interossei & lumbrical muscles also flex the metacarpal-phalnageal joint due to their line of pull in relation to the axis of rotation at the MP joint. The interossei also provide for abduction (dorsal) and adduction (palmar) of the index finger.