Determining index based on a condition

I have a 6001x194 matrix which includes various data from an experiment where subjects walked on a treadmill with 2 force plates (one for each foot). Column 140 and 157 are the force plate data for each of the force plates. I want to determine the time points of toe-offs and heel strikes for the right foot (based on the force plate data from column 140).

Toe-off is considered the first time point where the force plate data is less than 5N after being above 5N in the time frame below. Heel strikes are considered to be the first time point the force plate data is above 20N after being below 20N in the time frame before.

In the end I want 2 separate variables. One would be giving me the index of the toe-off time points and the other giving me an index of the heel strike time points.