Functional Anatomy of the Palmaris Brevis: Grasping for Answers

Introduction The palmaris brevis (PB) is a small, trapezoid‐shaped muscle found in the human hand and the forelimb of quadruped mammals, originating from the flexor retinaculum to insert in the skin of the medial palm. Although it may be considered a subcutaneous vestigial muscle, the function of the PB has been described as deepening the palm of the hand for grasping, or protecting the neurovasculature of the ulnar canal (Guyon's canal). Although the morphological characteristics of the palmaris brevis have been described through cadaveric dissection, the electromyographic (EMG) activity during hand movements has yet to be explored systematically. Recent clinical case studies investigating PB spasm syndrome reported that this muscle may not be under voluntary control, perhaps suggesting the presence of smooth muscle fibers. Thus, the primary objective of this study was to investigate the EMG activity of the PB during simple hand movements, and secondly to explore, histologically, the presence of smooth muscle fibers in the PB of human cadavers. Methods To date, seven healthy participants (6 males, 1 female; ages: 23–32y) were recruited from the University population to participate in this investigation. Palmaris brevis EMG activity was recorded from the subjects’ dominant hand by inserting a pair of intramuscular fine wires via a small diameter (27.5 gauge) hypodermic needle. Subjects were instructed to forcefully contract their hand muscles to achieve the following movements: abduction of the 5 th digit, finger flexion at the metacarpophalangeal joints, and opposition of the 5 th digit to the thumb. Palmaris brevis reflex activity was tested by compressing the ulnar nerve of right palm at the wrist (as described by Montagu 1952. Science. (115)). Integrated EMG analysis of the PB during the hand positions was measured offline using Spike2 software. For the histological analysis, PB samples were obtained from the hands (3 left, 3 right) of three fresh frozen cadavers (ages: 44–88y). These samples were prepared and stained using a typical H & E staining protocol. Results Histological examination of all six PB specimens revealed no evidence of smooth muscle fibers. Integrated EMG activity of the PB was greatest in all subjects during forceful abduction of the 5 th digit. Opposition of the 5 th digit to the thumb produced less muscle activity than observed during abduction (Mean: 59 ± 24% of abduction EMG activity). No measureable PB muscle activity was elicited during the simple finger flexion task in all participants. A palmaris brevis reflex producing dimpling of the skin and EMG activity could be elicited in ~60% of the subjects. Conclusions The proposed protective function of the PB in relation to the ulnar canal is supported by the observation that the EMG activity was greatest during abduction. Movements in which the hand is pressed against a flat surface, such as a push‐up exercise, would rely on abduction of the 5 th digit in order to stabilize and support the hand. As the PB contracts, it provides a protective muscular barrier to the underlying neurovasculature of the ulnar canal. EMG activity recorded during opposition demonstrates the PB provides protection to the ulnar canal neurovasculature during grasping motions such as a cyclist holding their handlebars. Histological evidence demonstrates that the PB is composed of skeletal muscle fibers and can be voluntarily activated during specific hand movements; however, the muscle cannot be contracted in isolation. Support or Funding Information Supported By NSERC

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