Finding that his health was failing, because of the assiduousness which a proper discharge of the duties of the position demanded the year round, he acquainted the President with his desire to withdraw from public duties.
Someone who is assiduous works hard or does things very thoroughly. Podulski had been assiduous in learning his adopted language. = diligent. very careful to make sure that something is done properly or completely = meticulous assiduous in (asiduus, from assidere; ASSESS)