Get access

Origin of surface motor unit potentials in hypothenar motor unit number estimation


  • This study was supported by Grants-in-Aid for Scientific Research (19591011 and 23591285) from the Ministry of Education, Science, Sports and Culture of Japan, and by Health and Labor Sciences Research Grants for the study of intractable diseases (CNS Degenerative Disease Research Committee and Neuroimmunological Disease Research Committee) from the Ministry of Health, Labour and Welfare of Japan.

Correspondence to: M. Sonoo; e-mail:


Introduction: Far-field potentials (FFPs) from muscles other than the abductor digiti minimi (ADM) may interfere with motor unit number estimation (MUNE) from that muscle. Methods: We identified the origin of each surface motor unit potential (SMUP) during hypothenar MUNE using the multiple point stimulation method in 20 control subjects by recording from individual ulnar-innervated muscles with a common proximal reference (pref). Results: ADM SMUPs comprised 39.0% of the accepted SMUPs, followed by those from the fourth dorsal interosseous muscle (14.0%), the fourth lumbrical muscle (9.2%), and the second and third palmar interosseous muscles (8.8% each). The percentage of ADM SMUPs varied from 18% to 73% of accepted SMUPs among individual subjects. Accepted non-ADM SMUPs were usually much smaller than ADM SMUPs, and many more non-ADM SMUPs were excluded due to their small size. Conclusions: A large contribution from non-ADM or non-hypothenar SMUPs obscures the meaning of the MUNE value. Muscle Nerve, 48: 185–190, 2013