Is it considered a sin not to carry out a father's will to train and build muscles after his death?

The father's will to train and build muscles is not obligatory to fulfill, because the legally binding will (wasiyyah) pertains only to money, and not to recommended or permissible matters. Nevertheless, it is beautiful to fulfill the father's will if it does not result in harm and does not violate the Sharia, because that is considered a form of showing righteousness to parents after their death, such as praying for them, seeking forgiveness for them, and fulfilling their covenants.