Siebe said: “.When i decide to stand on one leg, and as a result feel pain, is this pain due to a bad kamma-beeja ripening, or has it just a physiological cause.”
I have explained in detail why the decision to stand on a leg is NOT a kamma vipaka. Please read above and let me know the points that you don’t agree with.
Let me try another way to explain this, since this is important to understand.
This is exactly what one needs to do in Satipatthana. In this particular case of moving a body part it is Kāyānupassanā. When a thought comes to the mind to stand on one leg, one needs to think about the consequences of that action. If it is a beneficial act then one needs to do it; if it is a detrimental act one should not (as I explained in detail above).
This is no different than deciding to hit someone. When such a thought comes to the mind, one needs to realize that it is a bad bodily action.
Many people tend to focus on lobha and dosa as immoral. But moha is immoral too. The decision to carry out an act that one KNOWS will lead to suffering for oneself or others is immoral.
See the post on Kāyānupassanā for more details:
Maha Satipatthana Sutta
P.S. As Akvan said, it is possible that the thought to stand on one leg MAY come to the mind as a kamma vipaka. But the ultimate decision is up to the individual. Otherwise, working of kamma will be deterministic and no one will be able to attain Nibbana.