Q. how would I use commas in this sentence: {The participant stands unassisted on one leg and is timed in seconds from the time one foot is flexed

Full disclaimer: I have not had grammar since maybe the 4th grade. Knowing my weakness, I consulted another librarian who is a former 8th grade English teacher. We recommend this ... 

"The participant stands unassisted on one leg and is timed in seconds from the time the foot is flexed off the floor to the time when the foot touches either the ground or the standing leg or an arm leaves the hip."

I think the idea is that while you have three items in your list, two are "equal" (i.e. actions of the foot) and therefore constitute one unit. Your second unit is the action of the arm. Because of this, you don't need a comma since you only have two items. In Boolean terms it is ...

(torch floor OR touch leg) OR (arm leaves hip).

Have a good weekend!