Objective: To investigate which hand muscles were used for interphalangeal joint (IP) extension when holding the metacarpophalangeal joint (MCP) in 90 flexion, 0, and full hyperextension.
Material and Method: Fifteen volunteers extended IP when MCP was manually maintained in 90 flexion, 0, and full hyperextension for 5 seconds with and without maximal manual resistance twice each. Maximal resistance was ensured and maintained manually by the actions of both researcher and subject. By using needle electromyography, the root mean square(RMS)* values representing the muscle function of the extensor digitorum communis(EDC), lumbrical, and interosseous muscles of the middle fingers of the dominant hands were recorded and averaged from the two tests conducted on each subject.
Results: In 87% of the subjects, EDC had the highest RMS value in 90 MCP flexion without resistance, and in 100% of them, it had the highest value in both 90 MCP flexion and 0 with resistance. There were no clear differences between the three muscles when extended in all other positions. When the same muscles were compared, it was found that EDC had the highest value in 90 MCP flexion, especially without resistance. Its RMS values were significantly increased when the resistance was applied in all positions. It was also found that the lumbrical and interosseous muscles had the highest RMS values in full hyperextension.
Conclusion: EDC had the highest muscle activity for IP extension in 90 MCP flexion, especially when the resistance was applied. However, the activities among the three muscles could not be clearly differentiated when extended in other positions.

Keywords: EMG activity, Intrinsic hand muscle, Extrinsic hand muscle, Interphalangeal joint, Metacarpophalangeal joint