I would like to give a definition of tznius. Tznius means covering halachically mandated areas, and not deliberately acting in a way which is an invitation for others to join in immoral activity. (This is a fine line, and only the RBSH knows what’s in her heart.)
It does not include anything innocent that may happen to attract a man’s interest. That is his problem. Otherwise, a girl could not get orthodonture, as that might make her too beautiful. She would not be allowed to wear jewelry, or any nice dress, and would be obligated to dress like a shlump. She could not walk down the street because maybe somebody will take improper interest.
As a matter of fact, probably many of our daughters who are pure and innocent, when they hear they cannot wear something that might be too tight, never ever would have thought of themselves as objects, and this is the first time they are hearing it. It is giving them the wrong idea and stirring up a yetzer hara in them they may never have had to begin with. Just like we don’t allow — education in yeshivas, since it will have the opposite effect, this constant focus on their bodies for tznius purposes may be no better.
Years ago it was the norm that frum weddings and events and kiruv programs had no mixed dancing, but no mechitzah either. The ruchnius of the singing and dancing prevented anybody from looking at the opposite circle for improper purposes. After all it was separate, and each side was so involved with their hisorirus and dveikus.
To tell a girl that she cannot rollerblade because it may attract attention from men is wrong, since it is an innocent activity and she is dressed properly. (Maybe wear pants under the skirt for extra measure.) The same with horseback riding. The chumash tells us that our imahos rode on animals (Vatipol rivkah me’al hagamal).
A man’s yetzer hara is his problem. For many men, there is nothing a woman can wear that will get rid of the man’s yetzer hara. Still the Torah does not obligate women to torture themselves for the sake of a man’s yetzer hara. Lo hinachta bas l’avrohom avinu. It is not their fault that they are so beautiful, the RBSH made them that way for his own reasons. They probably don’t even realize how attractive they are. Just like a flower doesn’t know how attractive it is.
It is the Muslim approach that we torture women because a man may do something improper. They must wear burkas, they cannot go to school etc. etc.
The Torah’s approach is that innocent girls are innocent girls, and if they cover the halachic areas and don’t deliberately try to entice people, they have fulfilled their obligations. Of course this kind of dancing on a machine is pretty much deliberately enticing men, and would not be tzniusdig. However, rollerblading or horseback riding is simply innocent and healthy physical exercise. It is dangerous not to get exercise, and even the Rambam wrote this in Hilchos Dayos. To insist that a woman must do all her exercise indoors in a women’s only gym denies her the benefits and enjoyment of fresh air which is a big component and joy of exercising. Running outdoors and seeing the beautiful scenery is far different than on a treadmill. People are far more motivated to run outdoors in beautiful weather, and very few will maintain a routine on a tedious treadmill. They will give it up sooner or later. There is nothing like running by a beautiful lake or pond and seeing beautiful trees and flowers and scenery along the way. (Also, try running in a rich neighborhood with mansions, and see all the beautiful houses.)
Let men worry about their own obligations.