  Polymeric filament like type IV Pilus (TFP) can transfer forces in excess of
100pN during their retraction before stalling, powering surface
translocation(twitching). Single TFP level experiments have shown remarkable
nonlinearity in the retraction behavior influenced by the external load as well
as levels of PilT molecular motor protein. This includes reversal of motion
near stall forces when the concentration of the PilT protein is lowered
significantly. In order to explain this behavior, we analyze the coupling of
TFP elasticity and interfacial behavior with PilT kinetics. We model retraction
as reaction controlled and elongation as transport controlled process. The
reaction rates vary with TFP deformation which is modeled as a compound elastic
body consisting of multiple helical strands under axial load. Elongation is
controlled by monomer transport which suffer entrapment due to excess PilT in
the cell periplasm. Our analysis shows excellent agreement with a host of
experimental observations and we present a possible biophysical relevance of
model parameters through a mechano-chemical stall force map
