Ki and mechanics, a story:
by Avni Klein
When I first started Aikido a bunch of years ago, I too, learned unbendable arm. Project Ki-feeling, energy, water gushing from your arm as from a firehose, and above all relax. And it worked. Skinny-armed 120 lb me, had unbendable arms.
Then I went to medical school, while still going to my college dojo (kokikai -- not totally fruity, but by no means "ki is crap") when I could. In medical school, I learned about a special class of nerve fibers (spindle afferents, I think -- its been a while), whose job it was to measure stretch of the muscle. They give positive feedback to the muscle, enhancing the muscle's activity. Now things started making sense to me. So, if you have a relaxed, but partially bent arm (triceps relaxed, biceps partially contracted), and gentle pressure is exerted from outside to unbend the arm, the biceps will start to stretch. The bicep's nerve fibers will reflexively cause a little bit of extra contraction in the biceps which will oppose the unbending force. Note: All of this is reflexive! It happens in the spinal cord, doesn't get to the level of consciousness, and it is much faster than conscious acts. So you just can stand there with a relaxed mindstate, and you won't even realize that your muscle is taking up all of the slack for you! [Which is what it feels like to me. I always need to ask, "Are you pushing hard?" b/c I feel so much like I'm just not even trying.] This is because its happening at a spinal level.
On the other hand, there are other nerve fibers that sense when the ligament (which holds bone to muscle) is being stretched. This nerve is inhibitory to the muscle. So, if you actively tense the muscle as much as you can, the muscle will be unable to stretch, and the force will make the ligament stretch. This sets up another spinal reflex, one which relaxes the biceps. So, you tense up your arm, someone pulls against it, and inevitably your muscle fails. (To an extent. It would depend how tightly one is tensing the bicep in the first place. Some people may have enough "stretch" left in their muscle to tense it up somewhat and still resist your puny pulls.)
So, there is unbendable arm from the eye's of a first year medical student. And, being so young and naive (at the time), I ran to class that Thursday night, and we were doing unbendable arm, and I told Sensei, "We learned about neuromusculature and I think I understand how unbendable arms works!" And proceeded to tell him. Sensei smiled politely and said "Your unbendable arm's not relaxed enough. Practice."