Question

he lumbricals flex the fingers at the metacarpophalangeal (knuckle) joints, and extend the fingers at the interphalangeal joints. On a model, examine the path of these muscles and speculate why they act as flexors at one joint and extensors at another.

Answer 1

LUMBRICAL	FORM	ORIGIN	INSERTION	NERVE SUPPLY
FIRST	Unipennate	Origin from the radial side of the most radial tendon of the flexor digitorum profundus (Corresponding Index Finger)	Passes posteriorly along the radial side of the index finger to insert on the extensor expansion near Metacarpophalangeal Joints.	MEDIAN NERVE
SECOND	Unipennate	Origin from the radial side of the second most radial tendon of the flexor digitorum profundus (Corresponding to Middle Finger)	Passes posteriorly along the radial side of the middle finger and inserts on the extensor expansion near the metacarpophalangeal joint	MEDIAN NERVE
THIRD	Bipennate	One head originates from the radial side of flexor digitorum profundus (corresponding to Ring finger) Other head originates on ulnar side of the tendon of middle finger	passes posteriorly along radial side of ring finger and insert on its extensor expansion	ULNAR NERVE
FOURTH	Bipennate	One head originates from the radial side of flexor digitorum profundus(corresponding to Little finger) Other head originates from the ulnar side of the tendon of ring finger	Passes posteriorly along the radial side of the little finger and insert on its extensor expansion	ULNAR NERVE

The lumbricals permit a dynamic interaction between flexors and extensors.Their attachments transmit their force to both the Flexor Digitorum Profundus Tendon and extensor mechanism.

1. Increases Passive Tension in the Extensor Mechanism
2. Decreases Passive Tension in Flexor Digitorum Profundus Tendon's distal portion

MAIN REASON: The reason for opposite reactions is that because the tendons cross the Metacarpophalangeal joints on the palmar side but distally insert at the dorsal side of the finger and these are combined movements for a strong hand grip.

BIOMECHANICS: Basically, the Lumbricals are not primary Metacarpophalangeal Joint flexors they are Interphalangeal Joints Extensors and not dependent on Metacarpophalangeal Joint position

1. As the muscle attaches proximally to flexor digitorum profundus and distally to lateral band of extensor expansion, when the lumbrical contracts it creates tension on the lateral band leading to the extension of Interphalangeal Joint extension
2. While doing the process of lumbrical contraction at the same time of Interphalangeal Joint extension it also contracts the Flexor Digitorum Profundus distally by releasing the passive flexor tension which causes Metacarpophalangeal Joint Flexion and also helps in Interphalangeal joint extension
3. According some other studies, the Interphalangeal Joints didnot get extended until the tension within the lumbricals is equal to the tension in the Flexor Digitorum Profundus tendon.This causes Metacarpophalangeal Joint Flexion and Interphalangeal joints extension