Question
What are the du’as for being strong, well built (muscles), healthy?
Answer
I am not aware of any specific du’as for increasing ones strength. However, one may read the following which would aid in keeping one in good health:
اللهم اني أسالك العفو و العافية
In reference to this, there is a book (in English) which may be of use to you. It is entitled, “The Prophetic Medicine,” written by Imam Ibn Qayim al-Jawziah. It has chapters on “The Prophet’s (s.a.w.) guidance on keeping fit”, “The Prophet’s guidance (s.a.w.) on physical activity”, and the like which you may find particularly interesting.