From the "Carrying Capacity" section of the rules, specifically the section on "Lifting and Dragging" (emphasis mine):
Lifting and Dragging: A character can lift as much as his maximum load over his head. A character’s maximum load is the highest amount of weight listed for a character’s Strength in the heavy load column of Table: Carrying Capacity.
A character can lift as much as double his maximum load off the ground, but he or she can only stagger around with it. While overloaded in this way, the character loses any Dexterity bonus to AC and can move only 5 feet per round (as a full-round action).
A character can generally push or drag along the ground as much as five times his maximum load. Favorable conditions can double these numbers, and bad circumstances can reduce them by half or more.
I have not seen any specific rules for what happens if a creature becomes so weak they can no longer support their own body weight. RAW, they would need a Strength score of 0 before this would happen (and they would become unconscious).
See the Pathfinder SRD for a table that describes low strength scores. The only one that mentions a penalty to movement is a Strength score of zero (and that is because you would be unconscious).
I have not seen any rules on equipment crushing a character as they become too weak to carry it.