The ulnar (medial side of Flexor Digitorum Profundus- FDP) and median (all of FD superficialis) nerve are both involved in finger flexion, so how to differentiate in testing?

The Anterior Interosseous Nerve (AIN) is a subdivision of the median nerve which innervates deep ant. forearm flexors including the remaining lateral part of the flexor digitorum profundus (for DIP flexion).

Have a look at this mechanical test in the picture above. Here you ask patient to use both hands to make and “Okay” sign by forming a circle with thumb and index finger (similar to Froments sign). Median nerve palsy may make one hand produce a ‘pinched circle’ due to weakness in DIP finger (FDP), and also the hyperextension in the MCP1.