Assalamu'alaikum, is it true in zad Al Maad Ibn Al Qayyim said that enlargening the muscles of the body weakens one's mind? Can we prefer researches by modern non muslim doctors over Ibn Al Qayyims research regarding medicine?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

We did not find the words that you attributed to Imam Ibn Al-Qayyim in his book Zad Al-Ma'ad. In addition, there is no doubt that contemporary medical researches are more accurate than the ancient physicians' medical efforts which are based on experiment and observation. Imam Ibn Al-Qayyim in his book Zad Al-Ma'ad mentioned medical matters that are established in the Sunnah of the Prophet, and there is no doubt about their truthfulness as they are from the Prophet . Imam Ibn Al-Qayyim also mentioned medical matters about the ancient non-Muslim doctors, such as the Greek doctors Galen, Plato, and other doctors. Thus, not everything that Imam Ibn Al-Qayyim mentioned in his book Zad Al-Ma'ad is from the prophetic medicine established in the Sunnah.

Allah knows best.