l am female, 45, with a 19.7 BMI; reasonably fit; healthy (with the only exception of a herniated disk for which l have never had surgery and has not been a problem for years now).
l am thinking of trying spinning classes and would like to know if - considering l am not 20 any more - there is anything l should know in terms of cardiovascular risks, etc.

Not that l know of.
The real danger is doing nothing and getting fat and lazy. Lol

Spinning does not put that much strain on your back. If you are worried about it ask your doctor.

''fit with no associated quantitative data does not tell me anything. If u told me u could run a 45 minute 10k, or ride a 1:10 40k time trial, that would give me a clue to ur fitness level.

That said, there r no special risks that u would not find in any strenuous aerobic activity. Unless you're exceptionally clumsy, u can not fall off a spinning bike, & its relatively low impact.

As for cardiovascular risks, that is why l made my first statement. You r at an age where a medically supervised stress test might be a good idea, assuming u do not know ur limits from regular training & competition.

A spin class is as hard as u make it. When I've done them, l take it to the edge, even though l am 10 years older than you, because l know that l can do it safely from years of training & racing (and a couple of stress tests to make sure everything really is OK...).

It is much more dangerous to sit around & do nothing -- heart disease, diabetes, etc. Since u have a low normal BM l u appear to be pretty fit. Unless u have some diagnosed disorder u will be fine, & can benefit greatly from spinning.

Go to spinning class & put ur heart into it. If u r not sweating (a lot) u r not doing it right. Most of the participants l see at our local YMCA r not pushing hard enough.

An inexpensive heart rate monitor (try Walmart) will help a lot.

The one thing l learned in my first spinning class is that you have to be careful if you are not used to a ''fixed gear'' bike that you have to remember that you can not stop pedaling. l forgot towards the end of the class and it did not feel so good on my hips, l would imagine it could cause back problems as well. Just pay a little attention and you will be fine though.

do not want to know, but wacth out for those carpet moguls.

Yeah, have you heard the lame music that they put into the headphones in most of these places?

Luck