Rashi on Shemot 10:22 states:
ויהי חשך אפלה שלשת ימים וגו': חשך של אופל שלא ראו איש את אחיו אותן שלשת ימים. ועוד שלשת ימים אחרים חשך מוכפל על זה, שלא קמו איש מתחתיו. יושב אין יכול לעמוד, ועומד אין יכול לישב.
and there was thick darkness… for three days, etc.: Thick darkness in which they did not see each other for those three days, and another three days of darkness twice as dark as this, so that no one rose from his place. If he was sitting, he was unable to stand, and if he was standing, he was unable to sit.
From the wording, it seems like this inability to stand/sit was because of the thick darkness.
If so: how does darkness effect the ability to stand or sit?
Also, after 3 days standing - wouldn't they have fallen over by then?
If not: was this an additional punishment which came with darkness (i.e. being paralyzed)?