Externalist (machine/muscle-heads) cannot keep their arm stationary, without flexing the muscles; and explain it without using Qi.
Internalist use Qi for the 'Unbendable Arm'.
See:
http://forum.kungfumagazine.com/foru...ad.php?t=60755
Here is how to test:
1) muscle use will show the muscles enlarged with blood,
the arm-hand will be rigid and/or spring back reactively.
2) when using Qi, the arm will have a little flexibility, but not much: like rattan.
3) the tissue response from being pressed down is more resilient,
than when limp yet has more give than when contracted muscularly.
If it were just the joints locking, without muscle tension, no explanation is given as to how this done consciously. Even if this were to be explained, it does not explain why the muscle tissue is neither limp nor flexed.