Learn Icelandic: Stífur vöðvi getur aukið sársaukann í bakinu. - A stiff muscle can increase the pain in the back.

In Icelandic, the auxiliary verb geta (can or to be able to) is special because it takes the supine form of the main verb, not the infinitive. The supine is the neuter singular form of the past participle, which is why we see aukið.

We are learning about root nouns with irregular plurals, like fótur becoming fætur. Do vöðvi and bak have i-umlaut vowel changes in the plural too?

No, they do not. Vöðvi is a standard weak masculine noun, so its plural is simply vöðvar. The neuter noun bak (back) does have a vowel change in the plural, but it takes a standard u-umlaut to become bök, not an i-umlaut.

How does the ending on sársaukann work?

It comes from the weak masculine noun sársauki (pain). Because it is the direct object of the sentence, it takes the accusative case, changing to sársauka. When you attach the masculine accusative definite article (the) as a suffix, it merges to become sársaukann.

Why do we use the dative form bakinu after the preposition í instead of the accusative bakið?

The preposition í takes the dative case when describing a stationary location or state, like a pain that is already located in the back. It would only take the accusative case if the sentence described physical motion directed into the back.