If one drops tefillin without their cover, they must fast or give charity (Kitzur Shulchan Aruch - Yalkut Yosef, Siman 44:2). If one is wearing tefillin and fell with them on, must one fast or give charity? Maybe, if the person fell and not the tefillin, they would not be required to.
Would the halacha change if the tefillin itself touched the ground when the person fell?
The Beis Halevi quoted in the Piskei Teshuva (Chelek 2 : Siman 157) discusses the question of when there are two people heading for the same entrance. We have a Halacha that applies to seforim and respectable people and holy objects in that that we allow them to enter first (kind of like ladies first. See Brachos 47a). The Beis Halevi's asks, what would the halacha be if one person was holding a pair of teffilin and one person was wearing the teffilin, who goes first? Basing himself on the Mishna in Negaim (chap. 13) which states that a person who entered a Beis Hamenugah - a house inflicted with tzaraas while holding objects, the objects become tamei. However, if a person was wearing those objects only he is tamei but the objects remain tahor. The Beis Halevi rules that the one holding the teffilin goes first instead of the one wearing the teffilin. He explains that wearing something is not considered as if you are holding it. Thus, based off this Beis Halevi, it follows that when falling, your not "dropping" the teffilin rather YOU are falling. And even if the teffilin touched the floor, for the teffilin are considered tafel - secondary and subsumed to your body
Quoted Beis Halevi:
As far as I know (and it's not too far), if someone is sick, G-d forbid, he is allowed to put tefilin sitting or layed down. Maybe falling would fit in this category, of a quick sickness in wich it is allowed to wear tefilin during that short period of time. But ask your rabbi for a truly reliable answer.
