Flexor pollicis brevis muscle innervation zone location in healthy individuals of both sexes

The flexor pollicis brevis (FPB) is a muscle of the thenar eminence that plays an important role in thumb function. There is data about its innervation zone (IZ) distribution and sex differences. Knowing the location of the ZI of FPB could be helpful in treating spastic hand, serving to define the site of injection of the botulinum toxin. The aim of this study was to describe the IZ location in the FPB using surface electromyography (sEMG), and also make a comparison between male and female subjects. Thirty young healthy volunteers participated in this study (15 males: 21.5 ± 2.6 years, 70.7 ± 7.2 kg y 175.0 ± 5.5cm. 15 Females: 19.9 ± 1.4 years, 57.9 ± 11.1 kg y 161.9 ± 6.6 cm). The IZ was identified by recording the action potentials of the FPB motor units, using a sixteen-electrode array. The action potentials were recorded during isometric contractions at 10% of maximum voluntary contraction. The location of the IZ was expressed as absolute and relative values in relation to a reference system constructed in the palm of the hand, based on anatomic references. There were not significant differences in the location of the IZ between male and female subjects. Of all the subjects, the IZ of the FPB was found at the 41.9% of the distance between the most distal and medial palpable extreme of the metacarpophalangeal joint line of the thumb and the line which rises at the palpable apex of the proximal side of the first phalanx of the third finger passing through the radial longitudinal fold.