Complex sentence: These muscles are unusual in that they do not attach to bone, instead attaching proximally to the tendons of flexor digitorum profundus and distally to extensor expansions on the dorsal surface (back) the hand.
Split into simpler sentences: These muscles are unusual in that they do not attach to bone. Instead they attach proximally to the tendons of flexor digitorum profundus and distally to extensor expansions on the dorsal surface (back) the hand.