Below are some examples of strength training exercises to strengthen the muscles. 
“Health is a great treasure. It is the richest possession mortals can have. Wealth, honor, or learning is dearly purchased, if it be at the loss of the vigor of health. None of these attainments can secure happiness if health is wanting. It is a terrible sin to abuse the health God has given us. Every abuse of health enfeebles for life, and makes us losers, even if we gain any amount of education. Poverty, in many cases, is a blessing; for it prevents youth and children from being ruined by inaction. The physical as well as the mental should be cultivated and properly developed. The first and constant care of parents should be that their children may have firm constitutions, that they may be sound men and women. It is impossible to attain this object without physical exercise. Children, for their own physical health and moral good, should be taught to work, even if there is no necessity as far as want is concerned. If they would have virtuous characters, they must have the discipline of well-regulated labor, which will bring into exercise all the muscles. The satisfaction children will have in being useful, in denying themselves to help others, will be the most healthful pleasure they ever enjoyed. Why should the wealthy rob themselves and their dear children of this great blessing?”
– Christian Education page 16 Physical Exercise: Counsel on Health Page 52 http://text.egwwritings.org/publication.php?pubtype=Book&bookCode=CH&lang=en&pagenumber=52
Physical Culture: Counsel on Health Page 189-192 http://text.egwwritings.org/publication.php?pubtype=Book&bookCode=CH&lang=en&pagenumber=189
Exercise as a restorer: Counsel on Health page 199-200
http://text.egwwritings.org/publication.php?pubtype=Book&bookCode=CH&lang=en&pagenumber=199
Obedient Lifestyle 