Revisiting the functional anatomy of the palmaris longus as a thenar synergist.

Surgical studies describe the palmaris longus (PL) as a synergist in thumb abduction, which may facilitate its use in restoring thumb function using opponensplasty. However, beyond morphological descriptions and isometric thenar abduction strength measures, the evidence supporting the PL as a thenar synergist in-vivo is limited. The purpose here was to determine whether the PL provides synergistic contributions to thenar musculature by: (1) recording PL muscle activity using indwelling electromyography (EMG) during thumb movements; and (2) quantifying changes in PL muscle architecture using ultrasonography. In 10 healthy males, PL muscle activity was recorded during maximal thenar muscle contractions (abduction, flexion, opposition, adduction, and extension) with the wrist secured in a neutral position. The PL EMG was normalized to its maximal EMG recorded during isometric wrist flexion. Dynamic changes in PL muscle thickness (MT ) were determined during abduction and adduction using ultrasound imaging. The results indicate that the PL is activated during thenar movements with greatest relative PL EMG recorded during thenar abduction (46%), flexion (35%) and opposition (37%). Compared to rest, PL MT significantly increased (21%) during maximal thenar abduction. With direct measures in vivo, this study supports morphological and surgical observations indicating the PL acts as an extrinsic hand muscle in enhancing thenar muscle actions. Knowledge of the synergistic relationship between the PL and thenar musculature may allow for further development of surgical opponensplasty approaches using the abductor pollicis brevis and PL as a functional digastric unit. Clin. Anat. 31:760-770, 2018.