When we flex our fingers by 
making a fist, the muscles 
responsible have to pass in 
front of the wrist and so tend to 
contract that joint too. The 
extensors of the wrist have to 
contract to offset this tendency 
and keep the wrist stiff.

If you make a fist, the muscles 
in the front of your forearm (on 
the palm side of the hand) 
contract, as you can feel if you 
put your other hand on your 
forearm. (Most people probably 
think that the muscles that 
flex the fingers are in the 
hand. The hand does contain 
some muscles, but they happen 
not to be finger flexors.) As the 
diagram to the left shows, the 
forearm muscles that flex the 
fingers attach to the three 
bones of each finger by long 
tendons that can be seen 
threading their way along the 
front of the wrist. What may 
come as a surprise is that in 
making a fist, you also contract 
muscles on the back of your 
forearm. That might seem quite 
unnecessary until you realize 
that in making a fist you want 
to keep your wrist stiff and in 
midposition: if you flexed only 
the finger flexor muscles, their 
tendons, passing in front of the 
wrist, would flex it too. You 
have to offset this tendency to 
unwanted wrist flexion by 
contracting the muscles that 
cock back the wrist, and these 
are in the back of the forearm. 
The point is that you do it but 
are unaware of it. Moreover, you 
don't learn to do it by attending 
9 A.M. lectures or paying a 
coach. A newborn baby will 
grasp your finger and hold on 
tight, making a perfect fist, 
with no coaching or lecturing. 
We presumably have some 
executive-type cells in our 
spinal cords that send 
excitatory branches both to 
finger flexors and to wrist 
extensors and whose function 
is to subserve fist making. 
Presumably these cells are 
wired up completely before 
birth, as are the cells that 
allow us to turn our eyes in to 
look at close objects, without 
thinking about it or having to 
learn.