Degenerating muscles: A paradox

Palmaris longus and flexor digitorum brevis are both phylogenetically degenerating muscles.Palmaris longus was well developed in animals with good brachiation and prehensile function.In man, it has lost its functional significance, as brachiation is not used as a means of locomotion.In concordance with its evolutionary degeneration, it is frequently absent and its agenesis ratio in various populations is on the rise.Flexor digitorum brevis is well developed in animals with arboreal locomotive habits.In humans, due to adoption of bipedal posture and gait, several changes have taken place in the foot-ankle mechanism.Flexion of toes is more dependent on the long flexor tendons rather than the short tendons, making flexor digitorum brevis functionally less significant and prone for evolutionary degeneration.The little toe is less active and muscles acting on it are more liable for degenerative changes.Hence the tendon slip going to the fifth toe is frequently absent.In such a scenario where the muscle bellies are expected to be absent, it is interesting to note a double palmaris longus and a supernumerary muscle slip of flexor digitorum brevis going to the little toe.An analysis of development of these anomalous muscles with reference to evolution is made.Palmaris longus is widely used as an autogenous tendon graft making it imperative to know its morphological variations.Similarly, knowledge of variations of flexor digitorum brevis is required for tendon grafting and reconstructive surgeries.