  We introduce a general framework to study the processivity of molecular
motors moving along a polar filament and discuss the average time spent
attached to the filament as a function of a tangentially applied load. Our
study of specific models suggests that the attachment time of a motor decreases
with increasing ATP concentration and that double-headed motors such as
kinesins lose their processivity under forcing conditions while processive
single-headed motors are less sensitive to tangential forcing.
