When to use definite/indefinte articles; and when to use possessive adjectives

Question

It seems I have considerable confusion over when to use a definite/indefinite article before a noun. I know it is needed when using gustar (e.g., me gusta el chocolate),
 but I have confusion in other contexts. For example, for the prompt "you need to have strong legs",  I wrote necesitas tener las piernas fuertes, but the correct answer was "necesitas tener piernas fuertes" --- without the article. What's the rule on this?  My other confusion appears to be with the use of possessives. The prompt was "I'm going to exercise my muscles". I wrote voy a ejercitar los músculos but the correct answer was "voy a ejercitar mis músculos". It seems I'm confusing the rule of not using the possessive adjective as is required in such phrases as "My head is hurting"  --- "Me duelo la cabeza" , or "I raise my hand" -- "Me levanto la mano".  Please help me to know the difference for when I can (have) to use the possessive adjective versus when it is not used.Thank you and I look forward to some clarity (finally) on these two issues.Regards,Pati Ecuamiga

Accepted Answer

Hola Pati,
there are lots of doubt in general in this topic: when to use or omit the article
In some specific cases and with specific verbs like "gustar" the rule to apply is "always use the article" (it is incorrect if you don't use it), but there are other many cases where there doesn't seem to be any clear rule. There is a general rule that says that if you are talking about things in general you can drop the article and if you are more specific you tend to use the article, for example:
Hoy he comido arroz  
 This is general, you are not talking about any specific rice.
vs
Hoy he comido el arroz con pollo de mi madre, que está riquísimo. 
Here we are being more specific; we are talking about a specific rice dish, we are referring to that specific one - use the article.
Having this in mind, the sentence about the legs in the exercise could be applying this; you can either use las or omit it (both are correct, and the other option will be added to the exercise as also correct). If we say:
Necesitas tener piernas fuertes. 
Here we are saying that you need to have strong legs "in general" - we are seeing it as a more abstract object. But if we say:
Necesitas tener las piernas fuertes. 
Here we are referring to "your legs" so the object is more specific.
With this same effect we could have used the possessive "tus piernas fuertes". 
If we had used the indefinite article "unas" instead (also correct):
Necesitas tener unas piernas fuertes.
This would be talking again with a more general meaning, more undefined/unspecific (the same way as when it is omitting the articles).
The problem with this is that it is quite difficult to see whether the speaker is referring to something more general/abstract or something more specific. 
There have been more questions related to the omission or presence of the articles in the forum, as it is something that doesn't always coincide with the English and there is no magic rule. But we keep searching!
As per your example using the possessive in:
Voy a ejercitar mis músculos.
You are right at thinking that you need the definite article instead "los músculos" because it is obvious that it is "your" muscles the ones you need to work, so the possessive would be redundant. But sometimes we actually use the possessive so that we "emphasise" it. This is why we used it here. (The option with "los" will also be added to the exercise as correct).
I hope this clarified somehow your doubts.
Un saludo
Inma