Metacarpophalangeal Joint (Knuckle)

Bones: The metacarpal bone (of the hand) and the first phalanx (bone of the finger).

Cartilage: There is a small portion of articular cartilage on the ends of each bone.

Ligament: There is a transverse metacarpal ligament that wraps around the end of each bone, but leaves a small groove on the top of the knuckle to allow for the connection of a tendon.

Tendon: There are several tendons that connect at this joint this way multiple muscles can work together to create the fine movements we can make with out fingers.

Muscles/Mechanics

• Flexor digitorum superficialis: This is one of the main muscles that helps flex the finger (bringing it in towards a fist). It connects to the underside of the joint that way when contracted it will pull the finger down, and it originates at the elbow connected to the radius, ulna, and humerus. The bulk of the muscle is by the elbow joint, if you place your finger by the medial condyle of the humerus (the "knob" of bone on the in side of the elbow joint, also referenced as the funnybone) and clench a fist you can feel the muscle contract.
• Flexor digitorum profundus: Is a muscle set on the bottom of the forearm beneath the flexors of the wrist and make um a decently large chunk of the meat on the forearm. They run up through the tendons of these muscle run up through the hand to flex the fingers and make the main force on a strong grip or fist.
• Lumbricals: There are four of them, one for each finger, they are on the underside of the hand and assist in flexion, all slightly different but same overall function. Interestingly they connect at the first and second phalanx flexing the first and extending the second, this is mostly used when creating an upstroke in writing.
• Palmar interossei: They are between the metacarpals and assist in flexion.
• Extensor digitorum: This muscle is on the top of the forearm and a main muscle for extension of the fingers and wrist. The tendon splits off to each finger and pulls the fingers upwards and slightly outwards, then it will start to assist in extending the wrist. This muscle is near the surface and can be felt at the middle of the forearm on the top as the hand opens.
• Extensor indicis: This muscle connects directly and only to the index finger, similarly the Extensor digiti minimi connects solly to the pinky finger. This is what enables these two fingers to move more independently than the rest. Because of this degree of movement it makes them easy targets for breaking on a grabbing attacker, also since there is an additional muscle there is one more tendon to rip and cause greater pain to get the attacker off of you.

Range of motion: Flexion at this joint is able to pull the first phalanx and the rest of the fingers downward at about 90 degrees. Extensors are able to pull the fingers open at around 35 degrees upward.

Manipulation: During a grab or grope the finger can be grabbed and extended till the joint snaps.

Injury: This is THE joint to use when throwing a punch and should be the only point of contact as if the next joint down makes contact the metacarpophalangeal joint will over flex and could rip/pop out of place. Punching impact also causes the metacarpals to expand, hence why boxers wrap their hands.

Stretching: Stretching the the fingers down and back (consistently) will make this joints be more limber and react better under impact with less injury (not to say there wont be any), impact being both from a punch and to any random finger bending injury.

For information on additional joints please refer back to the Joints page.