Learn Spanish (Spain): Vuestras piernas son fuertes y rápidas.

Vuestro is used in Spain for "your" when you are talking to multiple people (like "you all"). Possessive words must match the noun they describe. Since piernas (legs) is feminine and plural, the word becomes vuestras.

Can I use sus instead of vuestras?

Yes, you can use sus if you are speaking formally to a group (using ustedes) or if you are speaking Spanish from Latin America. In Spain, however, vuestras is the standard way to say "your" when talking to a group of people you know informally.

Why does the sentence use son (from ser) instead of están (from estar)?

We use the verb ser for traits and characteristics. Because being strong (fuertes) and fast (rápidas) describes the permanent or fundamental qualities of the legs, we use son. If you were saying the legs were temporarily tired, you would use están.

Vuestras, piernas, and rápidas all end in "-as". Why doesn't fuertes end in "-as" too?