Re: If I bend my pinky, why does my ring finger bend too?

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.