IF I'm reading your question correcttly, I think the answer is habit and
efficiency.
If you 'block' the other joints, you'll see that they do move
independently.
This is easily demonstrated by simply laying the hand on a table with the
palm up. Move the desired joint. The table blocks the others.
Easier yet is to block by grasping a finger with the other hand just below
the joint and bend.
Try a 'come here' motion with your index finger, concentrating on bending
only at the middle.
You do have to concentrate, and often looking at the finger while you do
it helps - I presume to override habit.
I do hundreds of these a day as therapy to train nerves after losing a
fight with a circular saw.
See also Current Diagnosis and Treatment - Orthopedics 4th ed - H. Skinner
(ed) (McGraw-Hill, 2006), chapter 10.
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Hi, I'm a mechanical engineer presently working on a project probably
more suited to a biomechanical engineer in the sense that it involves
knowledge of anatomy or human mechanics that I have no knowledge of.
I am trying to understand something about the motion of fingers that
perhaps someone here can offer insight into. What I do not understand
is why when you curl a finger, all 3 segments/links of the fingers
move together rather than, for example, the finger tip rotating all
the way until it cant rotate anymore, then the middle segment moving
until it cant anymore, and then the last segment moving until it can't
anymore (and the finger completely curled up). Instead of movement
occuring as I describe, when a finger curls, all three joints rotate
the same amount simultaneously.
The reason this goes against what I would think would happen is that
fingers really have only one degree of freedom in the sense that their
only input is a tendon that attaches to the last segment/fingertip --
there is only one tendon pulling -- there are not three. For example,
in robotic arms, in order to get this kind of movement there have to
be three separate actuators -- one for rotation about each joint.
I can understand why finger curling occurs the way it does when the
there is not externally applied force/the finger is not applying any
force to something external. However, for example, lets say when you
pick up a bag, I see nothing in the mechanism to cause all the
segments to curl equally rather than only the fingertip curling until
it cant anymore, followed by the next doing the same thing, etc.
Is my question clear?
There has to be something in the anatomy of fingers that is designed
to create this "distributed curling". However, I do not know what it
is. I wqould immensely appreciate any insights you may have.
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