No, theologians would not classify exercise and working out as "servile work".

Aside from general considerations about going to the gym at all, i.e. whether it's an occasion of sin or scandal ... there's no really necessity for a gym to stay open on Sundays, since you can work out just fine Monday - Saturday.

So the issue is that you're participating in an immoral activity.

Just flip things around.  Let's say you OWN a gym.  Would you be justified in keeping it open, asking employees to come in to work in order to support it?  Perhaps the only justification might be that if you believe that your gym would go out of business if you kept it open, since people would not want a membership there but would go to a competitor.  In reality, though, that really doesn't seem to happen.  Places that have had the courage to close on Sundays have actually thrived.  Depending on the nature of the operation, though, staying open might entail little more work than sitting at a desk, but then usually there work cleaning the bathrooms, the showers, taking care of the trash, generally cleaning up, washing towels if you hand them out.

So, generally speaking, apart from some extenuating circuмstance, it would be wrong for you to keep a gym open on Sundays.  Therefore, if you GO to an open gym, you're participating in a sinful enterprise.

Now, here's where I disagree with the assessment of many of the moralists these days where they might say you're only a material participant in the sin if you go there, since you don't "agree with his staying open", where their notion of being a formal accomplice to sin means that in your mind you agree with the sinful activity.  I hold that to be nonsensical.  If you are a cause for the sinful activity, you're a formal participant, regardless of whether you "agree with it" or not.

If I hold a loaded gun up to someone's head, pull the trigger, even if I sit there the entire time THINKING "I don't want him to do die", if you will the cause, then you will the reasonably-expected (or even inevitable) effect.  If you will to be a cause of the sinful action, you're FORMALLY involved with the sin.  Period.

So if we go to a place of business that has no legitimate reason to be open on a Sunday, we're formal accomplices in the sin.  Now, perhaps the sin would be venial, since we play a relatively small role in it and are only a partial cause, but it's a sin IMO nonetheless.  If you are part of a mob that's all together are moving to push someone off a cliff, then you're a formal accomplice, even if you didn't just by yourself cause it.

So ... my personal opinion is that it would constitute light matter and therefore venial sin barring any sufficient justification.  Let's say it's a Sunday and you're going to be way or busy all week and it's the only day you could get the activity in, and it's very important for your health (perhaps you have something going on).  Generally speaking, I would avoid it unless there's a reasonable justification, and it is my opinion that it would constitute venial sin without said justification.

Places like gas stations, food stores, restaurants, etc. ... they might have sufficient justification to stay open since they provide services that may be necessary for people.  So, for instance, how would a Traditional priest travel around offering Mass, or perhaps medical professionals drive to help patients, etc. ... if gas stations were nearly all closed?  Perhaps in a Catholic society, one might designate a certain number of gas stations near highways to stay open, and now nearly all gas can be purchased with "self serve", where you just swipe your card, get your gas and move on, and then just minimally keep an attendant on duty, which you have to do in case something happens ... gasoline spills, things malfunction etc.  But there's no need for gyms to stay open.

24/7 gyms don't have workers on sundays

You have to look at the spirit of the law more than just the letter of the law. If, by work out, you mean beat your brains out for physical and competitive reasons, I’d say no. If you mean to engage in exercise mostly for leisure, ie. stretching, running for fun or play, going to the gym on Sunday may be permitted if it is not for a long period of time. Some people cannot go at other times and do not have a suitable place to engage in physical activity for relaxation. Example, a parent who lives in a small studio apartment in a city takes his child to the gym for fun. Compare with a man who frequents the gym for the purposes of competitive body building, no. Let Sunday be a day of rest. I said man, but the same applies to women although in general, women tend to be less frequent gym-goers. 

The same principle applies to non-servile activities. A person should not perform what he does for a living on Sunday unless it is out of necessity or charity demands it. Non-servile work can be just as tiring, even more tiring than servile work. If I write computer programs 40+ hours per week, I shouldn’t do it on Sunday. God did not make us to be able to work days on end without a break. Ask anyone who works rotating shifts or has to work seven days to pay the bills. 

Sure, if we had truly Catholic states, these conditions wouldn’t exist and gyms would be closed or very limited in available activities. But we do not have Catholic governments and must do the best we can to honor God under actual, not fictitious circuмstances. 

Not wanting to totally derail the thread, but is it Catholic to go a gym, a restaurant, an amusement park on a Sunday, where people are obliged to work for our (unnecessary) amusement?

How is this different to a jew having a shabbos goy to press buttons and drive him around on a Saturday.

Right.  Even if the activity is licit for YOU, if it requires other people to work, then you're a cooperator in their sin.  That's what I address above.  Now, there are exceptions for necessary services and for charity that have always been there ... since Our Lord overturned the excessively legalistic view of the Pharisees.  But if one day I do become wealthy, I might consider hiring a Sunday Jew to take care of the problem ... even if working out can't be done vicariously.

To me the big question has always been eating out at restaurants on a Sunday. By using restaurants , you are encouraging them to remain open ( Sunday is a big day for the restaurant industry) and causing the employees to work on the Lord's day. However many employees need to work on such a busy day, and Sunday business may have a lot to do with a restaurant being in the black so to speak.
For many of us , eating out at a restaurant on Sundays is a "blessed" relief from cooking, which can be pretty labor intensive and a large part of your day if you are cooking for a lot of people. 

So SOMEONE  is doing the work SOMEWHERE.

I still haven't figured this one out.

I really would advise people to have gyms/equipment in their own house if possible. Too much stuff is passed around in those places...

On a side note ... yes.  If you have a bit of room somewhere, perhaps a basement you can get really good cage or squat rack with bench for about $300 and then actual weights for about $300, probably for about as much as a year of gym memberships.  With those racks, if you have some of the pulleys, there's almost nothing you can't do, and the compound exercise are much better.  Don't waste time on this isolation machines (hey, this machine does my bicepts, etc.)  Not only is that a waste of time, but it's unnatural.  If you're pressed for space, you can get these dia-a-weight systems where you can get all the free weights you need and just change the weight on a single set.  That's my setup, a squat cage, bench, set of dial-a-weigts ... probably total cost of $1,000 -- which is about three years of gym membership at the cheaper places like "Planet Fitness", or one year for 3 people.  Given that I have 8 people in my family, if 6 of them would want to use it, it would pay for itself in 6 months and then it's free after that.

I really would advise people to have gyms/equipment in their own house if possible. Too much stuff is passed around in those places...

Staph
Herpes
MRSA

etc.

Very common these days.

How do get staph, herpes and Mrsa from gym.  I wish I saw this before joining a gym.  

For many of us , eating out at a restaurant on Sundays is a "blessed" relief from cooking, which can be pretty labor intensive and a large part of your day if you are cooking for a lot of people.

Buy a crockpot (or 2, if you have a large family).  They are not that expensive new, often on sale, and frequently found in good working order at thrift shops.

Saturday put quartered potatoes, sliced onion, carrots, celery, and whatever else (Worcestershire Sauce, garlic, bay leaves ...) in the crockery.  Add a beef or pork roast, apply a rub if you wish, put the crockery in the fridge.  Sunday before leaving for Mass put the crockery in the crock pot, sit it on low, dinner's ready when you return.  An extra ten minutes and you can make gravy from the juice.  A salad could be made up the day before also.  Pop dinner rolls in the oven while everyone changes their clothes.

A variation on this, if dinner doesn't need to be ready as soon as the family gets home:  Mix all of the above in a roasting pan, a whole chicken can also be substitute for the beef or pork roast, keep in refrigerator.  Right when you get home Sunday put it in the oven at 350 degrees Fahrenheit and dinners ready in an hour.

If the oven was turned on to bake/warm the dinner rolls or to cook the roast and fixings, a pie can be baking or warming while dinner is eaten.

Rinse dishes and cutlery, put in dishwasher.  Put the crockery or roasting pan in the sink to soak.  Do the dishes in the morning.

Since St. John Paul the Great declared that Saturday afternoons are the Sabbath, I conclude that going to the gym on Saturday is an even more heinous sin than going to the gym on Sunday.   When you go to the gym on Saturday the sim is compounded by the antisemitic hate crime of not respecting all things тαℓмυdic.


Really!