I have a question regarding exercising on Shabbat. I was wondering if I am allowed to attend a gym class on Shabbat? I wouldn’t be using any of the machines, I would just simply be doing some cardiovascular exercises.

It is forbidden to exercise on Shabbat (Shulchan Aruch Orach Chayyim 328 42) other than the exercise one gets by doing what people do even when they are not specifically exercising- like walking, climbing stairs etc. (Shulchan Aruch Orach Chayyim 301 2; Mishna Berura 301 5-7). You can, therefore, take a long walk on shabbat for the cardio-vascular benefit, since there are those who walk for other than medicinal/health reasons. The prohibition against exercise on shabbat is included in a general prohibition against taking medicines on Shabbat when a person's illness (even one that is not life-threatening) does not require taking them. In addition, one should refrain on Shabbat from sport activity inappropriate to the spiritual atmosphere of Shabbat. A casual game of ping-pong or shooting a few hoops (where there is an eruv) is permissible; a full game of basketball would not be.