Does Palmaris Brevis Structure Allow for Protection Against Prolonged Overlying Compression? An Ultrasound and Immunohistochemical Investigation

The palmaris brevis (PB) is small hand muscle located superficial to the hypothenar eminence in the medial palm. Although this muscle may have variant morphology, its complete absence is considered rare (<3%) compared to its counterpart, the palmaris longus, whose absence is more frequent (14%). From cadaveric studies, PB function has been postulated to protect the neurovasculature of the ulnar tunnel from prolonged overlying compression; however, it is unknown whether the PB has the gross histological structure and fatigue resistance necessary for prolonged contraction. Therefore, the initial objective was to determine the fiber‐type composition of the PB by staining for myosin heavy chain (MHC) type I (slow‐twitch) and type II (fast‐twitch) isoforms in cadaveric specimens. When the PB contracts, it has the potential to create a muscular barrier to protect the neurovasculature of the ulnar tunnel; however, the extent of change in PB dimensions from rest to maximal contraction remains unknown. Previous PB electromyographic investigations recorded the greatest muscle activity during maximal abduction of the 5 th digit, but it is unknown whether significant changes in muscle architecture (length and thickness) occur during this contraction. Thus, the second objective was to investigate and quantify the dynamic changes in PB morphology using ultrasound imaging in young participants. Methods To date, fiber‐type proportions have been quantified from 5μm cross‐sections of 4 PB muscles (3 left and 1 right) from 3 cadaveric specimens (2 Females, 1 Male; age: 80 ± 1 y) using immunohistochemistry by staining for MHC type I and MHC type II. Type I and type II fibers were manually counted using ImageJ software from 3 sites per slide from each specimen. Architectural changes in PB muscle length and thickness were investigated in the left and right hands of 8 male participants (age: 27 ± 4 y). Ultrasound images were taken at rest and during maximal abduction of the 5 th digit. Results The cadaveric PB specimens displayed a homogenous fiber‐type distribution with a mean fiber ratio of 71% slow‐type (range: 62–82%) to 29% fast‐type (range: 18–38%). Upon PB muscle contraction, a 26% and 28% decrease in mean muscle length was observed in the left and right hands, respectively. A concomitant increase in mean PB muscle thickness of 40% and 48% was also observed in the left and right hands respectively. Conclusion The preponderance of MHC slow‐type isoform relative to fast‐type in the PB render the whole muscle with fatigue resistant contractile properties that would allow the muscle to remain contracted during prolonged grasping tasks. Considering the muscle fiber type proportions along with the significant changes in muscle length and thickness observed with ultrasound imaging supports the supposition that the PB is capable of acting as a thick muscular barrier to the contents of the ulnar tunnel during a movement known to elicit the greatest EMG activity. Support or Funding Information Supported by NSERC