An arm that is unbendable has many uses with in the martial arts, for Aikido its helpful for conserving energy and momentum whilst rolling (after being thrown), its also an important tool to help soft block or redirect strikes and can be extended to holding weapons such as the bokken correctly.
As an example of 'Ki' development a practitioners arm is presented whilst a partner applies a force to try and bend it. During a successful test the practitioner remains calm whilst the partner strains to bend the arm with little success. The figure below demonstrates the test pictorially with the practitioner.
In the diagram (courtesy of Westbrook & Ratti, "Aikido and the Dynamic Sphere") an Aikido student (wearing black hakama pants) presents an arm for bending, his partner then tries to bend the arm by applying downward force at the elbow and upward force at the hand. Using extension of 'Ki' the practitioner keeps his/her arm unbendable.
The test is first performed with the arm tensed (to provide a strength baseline) and completely collapsed (no mind baseline), in both these cases the arm is usually easier to bend than when practicing 'Ki extension'. A successful test is where the arm is harder to bend in the final example. This test is often used in demonstrations of 'internal energy' and can look quite spectacular. In many cases children are able to pass this test with adult partners.
"'Ki' is universal energy" say the Aikido and 'Ki' master, "Allow the force to flow through you" says Yoda, "Root the energy of your opponent to the ground" say the Tai Chi and Kung Fu masters. Visualization is an important key to passing this test and the above are good examples of that. A favorite teaching analogy is to imagine your arm as a fire hose, completely flexible when empty but when energised with water ('Ki') rushing out the nozzle (your fingertips) it becomes really strong and difficult to bend.
Use this technique to energize your arm and imagine spraying an object some meters distant. Now when your partner tests maintain this visualization to pass the test, in so doing you learn to cultivate your 'Ki' extension through your arm and finger tips to make your arm unbendable.
At its most basic level students are tested slowly to help the student pass, at higher levels the test is more sudden or applied without warning. The intent of the higher level tests are to see how well Ki extension is maintained during stress.
There are several underlying physical principles that make this test work, the most obvious are all related to our muscular and skeletal systems.
b) The triceps, which extends the forearm.
These muscles work in opposition with each other to control the arm effectively. Hence when an arm is tense both the biceps and triceps are activated, in effect the biceps are helping someone bend the unbendable arm. When relaxed however the bicep no longer aids the applied force - only the triceps become engaged, the net effect is your relaxed power is stronger.
Many students initially pass the test but when there is a small bend in the arm, suddenly become tense, imagining they are failing the test... and of course they do. Muscles however have considerable elasticity in them and so this small movement is a natural part of our physical makeup and hence when the small movement is ignored (rather than focused on) the test can be passed.
This paper has introduced the concept of Ki as practiced in the traditional martial art of Aikido. Unbendable Arm, a specific example of Ki development as practiced in Aikido, was described and an alternative using basic science was presented. While the analysis cannot be generalized across all internal arts practice it nonetheless has demonstrated that some aspects of Ki can be explained using science for examples that are sufficiently simple.