Restric post navigation to current sub menu

Restric post navigation to current sub menu

This is what I'm using to output my post navigation:
<?php if (is_single() ) { ?>
<nav class="post-navigation">
    <h1 class="visuallyhidden">Post Navigation</h1>
    <ul>
        <li class="prev"><?php previous_post_link('%link', '%title', 'in_same_cat'); ?></li>
        <li class="next"><?php next_post_link('%link', '%title', 'in_same_cat'); ?></li>
    </ul>
</nav>
<?php } ?>
This will restrict the navigation to the current category (sub and parent category).
I'd like to restrict this to only the current sub category. What would be the best way to do this?