What i have seen, by studying the sutta’s, is that kamma is listed as 1 of 8 possible causes for sufferings.
When kamma is listed as 1 of the 8 possible causes for suffering what is meant by kamma-as-a-possible-cause?
I belief this refers to the specific mechanism that a bad kamma beeja that one has accumulated due to bad intentional actions in mind, speech and body, can ripen at a certain moment and experienced as sufferings.
This aspect of a ripening kamma-beeja is essential, i belief, when kamma in the sutta’s is mentioned as 1 of 8 causes for sufferings.
So, the sutta’s teach, i belief, this specific ripening mechanism of a kamma beeja, that can (sometimes) explain someones unpleasant feelings, discomfort and illnesses. And in that case one can say ‘it is due to kamma’.
Pain arising from standing on one leg, although one decides to do that, although intentional, i belief it is better not to say this is due to kamma.
Siebe