Hola Rosemary

We generally drop the definite article when the nouns are seen as "general nouns" as opposed to nouns that we identify and are specific. This is what we are doing here in:

"En el gimnasio hay aparatos para hacer ejercicio, por ejemplo, bicis y cintas de correr."

We are not referring to any specific bikes and treadmills, they are not things that we have mentioned before or have identified; they are seen as general things, so among those machines there are bikes, treadmills, etc.. 

If the text continued and the narrator wanted to give more information about those machines, then the definite article may have been used as we would "then" have already identified those. 

So, for example, it could say:

"En el gimnasio hay aparatos para hacer ejercicio, por ejemplo, bicis y cintas de correr. Yo siempre me monto en las bicis y las cintas porque son mis aparatos preferidos."

I hope this clarifies it. The omission and presence of articles in Spanish is a topic that creates a lot of discussion and unfortunately there is no clear cut rule to say exactly when we drop it and when we use them. 

Saludos

Inma