|MadSci Network: Anatomy|
Why do the fourth and fifth fingers bend together? When you flex your fingers (bend them forward to form a fist) or extend them (straighten them out) the fingers move at three joints. The joint closest to the hand is the metacarpophalangeal joint, next is the proximal interphalangeal joint and last is the distal interphalangeal joint. There are different sets of muscles to control these joints in different ways. When you extend your fingers you use the extensor digitorum muscle, which tries to extend the fingers at all three joints simultaneously. To flex the fingers at only one joint you need to selectively overpower the extensor digitorum muscle. General finger flexion is controlled by three groups of muscles – flexor digitorum superficialis and flexor digitorum profundus and the lumbricals. Flexor digitorum profundus crosses all three finger joints and so, when acting alone, will flex all three joints simultaneously. Flexor digitorum superficialis crosses only the first two joints and so cannot flex the distal interphalangeal joint. The lumbricals arise from the tendons of the flexor digitorum profundus muscle but insert onto the common extensor sheath of the extensor digitorum muscle. When a lumbrical contracts in isolation it causes the finger to flex at the metacarpophalangeal joint and extend at the proximal and distal interphalangeal joints. People have different abilities to control the independent movement of their fingers – some of these abilities are learned and some are natural – but they are basically the function of these three muscles. This interplay of muscle function basically explains how you can move your middle and ring fingers. Try an experiment. Grasp the middle of your right forearm tightly between the index finger and thumb of your left hand. Now wiggle the fingers of your right hand, make a fist, move each finger independently. When you do this you should be able to feel both the extensor and flexor muscles acting in your forearm. The index finger and pinky are special cases. Both fingers have distinct and independent extensor muscles (extensor indicis and extensor digiti minimi). The action of these muscles allows you to fully extend your index finger or pinky while the other fingers are flexed. Notice that you cannot make a fist and then independently and fully extend your ring or middle fingers. When you make the rude gesture with the middle finger the other fingers are not fully flexed, you actually need to use your thumb to keep the other fingers flexed against the palm. The flexor digitorum muscles are power muscles, without much independent control. When you flex your ring finger you are using these flexor muscles, which to some extend are acting upon all the fingers simultaneously. Notice carefully, in most people when they flex just the ring finger they are still producing some movement in the other fingers, especially the middle finger. The pinky won’t move much, if at all, because it has the extensor digiti minimi muscle to keep it extended. The middle finger doesn’t move much because it is a longer finger and the “slack” that forms in the extensor digitorum tendon by flexing one of the other fingers will not affect it as much. However, when you flex the pinky at the proximal interphalangeal joint you need to overcome the power of extensor digitorum to keep the other joints extended. Because the ring finger does not have an independent extensor muscle it must flex along with the pinky. There are some people that do have independent extensor muscles for the middle and ring fingers. Having one for the middle finger is fairly common – about 1 in 20 people. Having an independent ring finger extensor is less common. However, if you can find someone who can keep their ring finger straight while they flex their pinky you will have found one of those people with the independent ring finger extensor muscle.
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