The contribution of the palmaris longus muscle to the strength of thumb abduction.

The palmaris longus muscle (PLM) is described as a weak flexor of the wrist and a tensor of the palmar aponeurosis, but not a thumb abductor. The PLM is believed to aid thumb abduction through its insertion onto the thenar eminence. Two groups, both right hand dominant, were selected from 1,200 sampled participants. The first group comprised of 38 subjects with unilateral presence of the PLM and was used to determine the strength of thumb abduction. The second group comprised of 30 subjects, with bilateral presence of the PLM, and it was used to calculate the effects of hand dominance. A significant number of subjects with bilateral absence of the PLM were observed and undocumented. Using a dynamometer in subjects with unilateral presence of the PLM, the force of thumb abduction was significantly greater on the hand with a PLM than the one without it (P = 0.014), irrespective of hand dominance. In the second sample with bilateral PLM, thumb abduction on the dominant hand was 10% stronger than on the nondominant hand and was similar to the universally accepted average of 10% increase in grip strength of the dominant hand. Thus, 10% was deducted from all the dominant hands, and the force of thumb abduction remained greater on the hand with PLM than the hand without it (P = 0.049). The results of this study demonstrated the PLM to be involved in thumb abduction, and the authors therefore recommend that this action of the muscle be universally accepted by anatomists and hand surgeons.