From a reader…
I work in an office for 8 hours each weekday. Any physical activity I can get is a welcome relief. Am I able to work out on Sundays without violating the third commandment?
Let’s turn first to the Catechism of the Catholic Church:
2185 On Sundays and other holy days of obligation, the faithful are to refrain from engaging in work or activities that hinder the worship owed to God, the joy proper to the Lord’s Day, the performance of the works of mercy, and the appropriate relaxation of mind and body. Family needs or important social service can legitimately excuse from the obligation of Sunday rest. The faithful should see to it that legitimate excuses do not lead to habits prejudicial to religion, family life, and health.
So, provided that you are still performing your religious duties according to the 1st Commandment of the Church (i.e., attend Mass on Sunday and Holy Days of Obligation), you can work out.
In general, we should avoid strenuous work on Sundays. A “work out” is, by definition, “work”, but of a different kind than most labor. First, though it is hard, we derive a real good and satisfaction from it. By working out, you are caring for your body, which is your responsibility before God to do.
It is also possible to pray, for example, the Rosary while working out or running, etc.
That reminds me of the old chestnut about whether or not it is permitted for priests to smoke a cigar while reciting their breviary. The answer is, of course, No, it is not permitted. However, it IS permitted to pray the breviary while smoking a cigar. I think a Jesuit came up with that one. An oldie but goodie.
If some activity hinders you from fulfilling your obligations or if, after examining your conscience you sense that is lessened Sunday as an important time to rest in the Lord, as the Lord Himself rested on the seventh day, then leave that activity aside on Sunday.