In response to my last posting on this topic, someone wrote to check out a couple of sources. The first of these is what Maimonides wrote (משנה תורה, הלכות רוצח ושמירת הנפש יא:ד):
The second is a direct quote from the aforementioned by Rabbi Yosef Karo (שו"ע, חו"מ תכ"ז:ח):וכן כל מכשול שיש בו סכנת נפשות מצות עשה להסירו ולהשמר ממנו ולהזהר בדבר יפה יפה שנ' השמר לך ושמור נפשך, ואם לא הסיר, והניח המכשולות המביאין לידי סכנה, ביטל מצות עשה ועבר על לא תשים דמיםAnd, similarly, there is a prescriptive commandment to remove every stumbling-block in which there is danger and to be careful with it really well, as it is said, "Guard yourself and protect your body." And if you do not remove it, you will be placing stumbling-blocks which cause danger, you are losing out on this prescriptive commandment and transgressing on the proscriptive commandment of not placing blood.
However, while these texts do quote the verse about guarding one's body, this is in regards to removing harmful things to one's body. It can be easily said that this can serve as a reason to engage in regular cardiovascular exercise, however, it may be more difficult to serve as a reason to engage in weight-lifting/strength training. However, strength-training also serves to aid one's health. I think this topic needs more looking in to, however, I don't think one can say that there is any imperative to engage in weight-lifting.וכן כל מכשול שיש בו סכנת נפשות, מצות עשה להסירו ולהשמר ממנו ולהזהר בדבר יפה, שנאמר: השמר לך ושמור נפשך. ואם לא הסיר והניח המכשולות המביאים לידי סכנה ביטל מצות עשה ועבר בלא תשים דמים
While I'm on the topic, it is interesting to note Rabbi Moshe Rivkash's note to Rabbi Karo's inclusion of this line (באר הגולה, חו"מ תכ"ז:כ), wherein he says that Maimonides derives this obligation from the pious man story I discussed in a previous posting, However, that's a bit odd as I wrote there that "this verse is used in the context of guarding oneself - one's body, even - nevertheless, it is used by a gentile general in somewhat of a polemical context." Indeed, Rabbi Eidels said that "these verses are not dealing at all with guarding one’s body itself from danger", but rather, with learning. Anyways, it is clear that Rabbi Karo directly quoted from Maimonides, but it is not clear how Maimonides derived this.